Rail power conditioners based on modular multilevel converter in AC railway networks

Universidade do Minho Escola de Engenharia Mohamed Tanta Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks November 2020 UMinho | 2020 Mohamed Tanta Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Doctoral Thesis Doctoral Program in Electronics and Computer Engineering Thesis submitted under the supervision of: Professor João Luiz Afonso (University of Minho) Professor António Pina Martins (University of Porto) November 2020 Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho i COPYRIGHTS AND TERMS OF USE BY THIRD PARTIES This is an academic work that can be used by third parties under the internationally accepted rules and good practices with regard to the copyright of the authors. This work can be used under the terms underlined in the license below. If the user needs permission to be able to use of this work under not provided conditions in the indicated licensing, the user should contact the author, through the RepositóriUM of the University of Minho. The license granted to users of this work Attribution-NonCommercial-NoDerivs CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/ DIREITOS DE AUTOR E CONDIÇÕES DE UTILIZAÇÃO DO TRABALHO POR TERCEIROS Este é um trabalho académico que pode ser utilizado por terceiros desde que respeitadas as regras e boas práticas internacionalmente aceites, no que concerne aos direitos de autor e direitos conexos. Assim, o presente trabalho pode ser utilizado nos termos previstos na licença abaixo indicada. Caso o utilizador necessite de permissão para poder fazer um uso do trabalho em condições não previstas no licenciamento indicado, deverá contactar o autor, através do RepositóriUM da Universidade do Minho. Licença concedida aos utilizadores deste trabalho Atribuição-NãoComercial-SemDerivações CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/ Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho ii ACKNOWLEDGEMENT First, I would like to express my sincere gratefulness to my Ph.D. supervisors, prof. João Luiz Afonso, prof. António Pina Martins and prof. Adriano Silva Carvalho, for their continuous support during the Ph.D. research, for their immense knowledge, inspiration, motivation and the endless patience. Their helpful supervision, explanations and comments helped me to complete and submit of this Ph.D. thesis, as an outcome after four consecutive years of research and development. I would like to thank my fellow colleagues in the Group of Power Electronics and Energy – GEPE (in Portuguese: Grupo de Eletrónica de Potência e Energia – GEPE), especially to Eng. José Cunha for his massive support regarding the implementation part. I would also like to thank Dr. Gabriel Pinto, Dr. Vítor Monteiro, Dr. Bruno Exposto, Dr. Delfim Pedrosa, Eng. Luis Cardoso, Eng. Tiago Sousa, Eng. Luis Barros, Eng. Luis Machado, Eng. Manuel Silva, Eng. Ana Rodrigues and Eng. Catia Oliveira for their availability, knowledge shared, providing a great work environment of respect and friendship and dealing with all needs in the laboratory during the past years. I am also thankful to the technicians of the Department of Industrial Electronics and Computer Engineering at the University of Minho, especially to Carlos Torres who always supports with great sympathy and commitment. I am grateful to all the people not mentioned, who helped directly and indirectly in the submission of this doctoral thesis, including my family, my friends and my relatives who have been beside me in the past years for the support and stability provided throughout this academic journey. A special thanks to Dr. Helena Barroco, the diplomatic adviser of the Portuguese ex-president Dr. Jorge Sampaio, for the encouragement and the moral support provided during this academic journey. And last but not least, the Portuguese foundation for science and technology (in Portuguese: Fundação para a Ciência e a Tecnologia – FCT), which allowed me to continue my studies with the PD/BD/127815/2016 Ph.D. scholarship under the Innovation in Railway Systems and Technologies Doctoral Program – iRail . Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho iii STATEMENT OF INTEGRITY I hereby declare having conducted this academic work with integrity. I confirm that I have not used plagiarism or any form of undue use of information or falsification of results along the process leading to its elaboration. I further declare that I have fully acknowledged the Code of Ethical Conduct of the University of Minho. DECLARAÇÃO DE INTEGRIDADE Declaro ter atuado com integridade na elaboração do presente trabalho académico e confirmo que não recorri à prática de plágio nem a qualquer forma de utilização indevida ou falsificação de informações ou resultados em nenhuma das etapas conducente à sua elaboração. Mais declaro que conheço e que respeitei o Código de Conduta Ética da Universidade do Minho. University of Minho, November, 08, 2020 Nome: Mohamed Tanta Assinatura: ________________________________________________________________________ Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho iv Abstract Railway systems have progressively been developed since James Watt presented a technique of converting steam power into a circular movement back in 1763. With the novelty of steam engines at that time and the increasing of railway networks, railway industry quickly became an economic catalyst throughout the world due to the advantages of passenger and freight transport. In 1879, Siemens & Halske company introduced the world’s first electric train in the city of Berlin, consisting of a locomotive and three wagons, and supplied via an insulated third rail with 150 V direct current (DC). From that time, the world has begun to recognize the important transition from steam power to electric power, and the potential of the electrified railway as a mode of mass transport. Due to the plenty of fuel in the last century, Diesel trains were not only common, but they also dominated the railway sector for a few decades. Consequently, the development in the infrastructures of electric trains decelerated, and the path to having fully electrified trains was long enough. In this context, electric trains have introduced progressively, in which Diesel and electric power have been combined to create hybrid locomotives. However, and with the increased demand for transportation and the higher fuel prices in the last decades, electric trains can substantially offer lower operating costs and lower emissions compared with the Diesel-powered trains. Nowadays, most of the high-speed electric trains use alternating current (AC) power supply for their traction power systems, which provide better performance under long-distance power transmission than DC power supply. However, as the need for railway transportation increases due to more passengers and higher mobility requirements, more flexible and efficient traction systems are always needed. In Europe, AC traction power systems are mainly classified according to the voltage and frequency parameters (15 kV, 16.7 Hz) or (1×25 kV or 2×25 kV, 50 Hz). In all cases, railway operators have an absolute interest to run the electrified trains with the lowest possible operation and maintenance costs. In this context, power quality improvement at the three-phase power grid, associated with the AC electrified railway has drawn more attention in the last decades, especially after the evolution in the Power Electronics field. Subsequently, various solutions based on Power Electronics converters have been proposed to improve power quality in the electrified railway, e.g., the flexible AC transmission systems (FACTS). Abstract Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho v The rail power conditioner (RPC) is one of the FACTS devices that can be used to improve power quality by compensating harmonic contents, reactive power and negative sequence components of currents generated by the railway system. Among the other possible multilevel power converters, the modular multilevel converter (MMC) is an attractive solution for medium-voltage applications due to harmonics reduction, lower switching losses, and higher flexibility, scalability and reliability. Therefore, the MMC has been enhanced to be combined with the FACTS family. Taking into consideration the existing opportunities in the railway industry, not only in the development of the electric train itself, but also on the power quality improvement in the electrified railway, there is a strong investment in technological development for electrified railway systems. Therefore, this work presents a new topology of Power Electronics converter (RPC based on MMC) that compensates power quality problems associated with traction power systems, thus, reducing the operating costs of the electrified trains and increasing the power capacity of the electric traction grid. The main innovations of the RPC based on MMC are the integration of the MMC topology to operate as a railway power quality conditioner, benefiting from the advantages of the MMC in the traction power supply system. In this context, the research work proposed and developed in this Ph.D. thesis aimed to design, develop and validate a reduced-scale laboratory prototype of the RPC based on MMC, including all the necessary control algorithms and simulation models that are important to support the correct operation of the proposed system. Under simulation conditions, this work developed control algorithms for different RPC topologies, (full-bridge, half-bridge, three-wire, etc.) for demonstrating the general capabilities of the RPC system, and also for two different transformers connections (V/V and Scott). The most favorable RPC based on MMC topology (based on half-bridge MMC) was deeply and extensively simulated, namely employing predictive control approach. The experimental results obtained from a developed reduced-scale prototype confirm the validity of the presented control theory, as well as the power quality improvement capability of the proposed solution. Keywords: Electrified Railway Systems, Flexible AC Transmission Systems (FACTS), Harmonic Distortion, Modular Multilevel Converter (MMC), Negative Sequence Component (NSC), Power Electronics, Electric Power Quality, Rail Power Conditioner (RPC), Traction Power System (TPS). Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho vi Resumo Os sistemas ferrovias foram progressivamente desenvolvidos desde que James Watt apresentou uma técnica de conversão da energia a vapor para um movimento circular em 1763. Com a novidade dos motores a vapor e a sua implementação nas redes ferroviárias, a indústria ferroviária rapidamente se tornou um catalisador econômico em todo o mundo devido às vantagens no transporte de passageiros e mercadorias. Em 1879, a empresa Siemens & Halske introduziu o primeiro comboio elétrico do mundo na cidade de Berlim, consistindo numa locomotiva com três vagões, alimentado por um terceiro trilho isolado alimentado com corrente contínua em 150 V (CC). A partir desse momento, o mundo começou a reconhecer a importante transição da energia a vapor para a energia elétrica e o potencial na ferrovia eletrificada como um meio de transporte de massa. Devido à abundância de combustível fóssil no século passado, os comboios a Diesel não eram apenas comuns, mas também dominaram o setor ferroviário. Consequentemente, o desenvolvimento das infraestruturas dos comboios elétricos desacelerou, e o caminho para haver comboios totalmente eletrificados tornou-se bastante longo. Nesse contexto, os comboios elétricos começaram a impor-se progressivamente, inicialmente pela combinação do motor Diesel e do motor elétrico, resultando numa locomotiva híbrida. No entanto, com o aumento da demanda pelo transporte, e com o aumento do preço dos combustíveis nas últimas décadas, os comboios elétricos afirmaram-se por poderem oferecer custos operacionais mais baixos, assim como melhor desempenho ambiental. Atualmente, a maioria dos comboios elétricos de alta velocidade utilizam sistema de tração em corrente alternada (CA), que oferece melhor desempenho na transmissão de energia a longa distância do que sistema de tração em corrente contínua CC. No entanto, o aumento do transporte ferroviário requer a melhoria da eficiência energética devido a haver mais passageiros e maiores requisitos de mobilidade. Na Europa, os sistemas de tração elétrica são classificados principalmente de acordo com os parâmetros de tensão e frequência (15 kV, 16,7 Hz) ou (1×25 kV ou 2×25 kV, 50 Hz). Em ambos os casos, os operadores ferroviários têm interesse absoluto em otimizar os custos. Nesse contexto, a melhoria da qualidade de energia elétrica na ferrovia suscitou mais atenção nas últimas décadas, principalmente pela introdução da eletrônica de potência. Posteriormente, várias soluções baseadas em conversores de eletrônica de potência foram propostas para melhorar a qualidade de energia elétrica na ferrovia, como por exemplo, os sistemas flexíveis de transmissão CA (FACTS – Flexible AC Transmission Systems). List of Figures Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xiii Figure 4.24. HB-RPC simulation results: (a) Load section currents; (b) Currents at the secondary windings of the V/V power transformer after compensation (when both of the load sections are loaded). ............................. 91 Figure 4.25. HB-RPC simulation results: (a) DC-link voltage; (b) Compensation currents (when both of the load sections are loaded). ................................................................................................................................. 92 Figure 4.26. Co-HRPC with a single-phase power transformer. ..................................................................................... 93 Figure 4.27. Double-side feeding co-HRPC substations. ................................................................................................ 94 Figure 4.28. Control strategy of the co-HRPC. .............................................................................................................. 95 Figure 4.29. Co-HRPC simulation results: (a) Three-phase currents before compensation; (b) Three-phase currents after compensation. ................................................................................................................................... 96 Figure 4.30. Co-HRPC simulation results: (a) Load section current; (b) Compensation currents..................................... 97 Figure 4.31. Co-HRPC simulation results: (a) Passive filter capacitor voltage; (b) DC-link voltage. .................................. 97 Figure 4.32. RPC system with a Scott power transformer. ............................................................................................ 98 Figure 4.33. Phasors of the RPC system with a Scott power transformer: (a) Scott transformer connection points; (b) Phasors diagram of the primary windings; (c) Phasors diagram of the secondary windings. .................... 99 Figure 4.34. Phasors of the RPC system with a V/V power transformer: (a) V/V transformer connection points; (b) Phasors diagram of the primary windings; (c) Phasors diagram of the secondary windings. .................. 100 Figure 4.35. RPC output voltages: (a) Output equivalent circuit of the RPC; (b) Phasors of the RPC output voltages in V/V power transformer; (c) Phasors of the RPC output voltages in Scott power transformer. .................. 100 Figure 4.36. Control strategy of the RPC system with a Scott power transformer......................................................... 101 Figure 4.37. Load section voltages: (a) Using the V/V power transformer; (b) Using the Scott power transformer. ....... 101 Figure 4.38. Public grid currents, PSC and NSC when the load sections are equally loaded before compensation: (a), (b) Using the V/V power transformer; (c), (d) Using the Scott power transformer................................. 103 Figure 4.39. Public grid currents, PSC and NSC when load sections are unequally loaded before compensation: (a), (b) Using the V/V power transformer; (c), (d) Using the Scott Power transformer. ............................... 103 Figure 4.40. RPC based on a Scott transformer (RPC is turned on after 0.1 s): (a) Three-phase grid currents; (b) Currents at the secondary windings of the Scott transformer; (c) PSC and NSC of three-phase grid currents. ................................................................................................................................................. 104 Figure 4.41. RPC based on a Scott transformer (RPC is turned on after 0.1 s): (a) Load section currents; (b) Compensation currents synthesized by the RPC; (c) DC-link voltage..................................................... 105 Figure 4.42. MRPC system with a V/V power transformer. ......................................................................................... 106 Figure 4.43. Control strategy of the MRPC. ................................................................................................................ 107 Figure 4.44. MRPC simulation results: (a) Three-phase currents after compensation; (b) Load section currents; (c) Compensation currents synthesized by an RPC module; (d) DC-link voltage for one RPC module. ................................................................................................................................................... 108 Figure 4.45. FB-MMC4 RPC system with a V/V power transformer. ............................................................................ 111 Figure 4.46. AC equivalent circuit of the FB-MMC4 RPC system. ................................................................................ 111 Figure 4.47. FB-MMC4 RPC: (a) Output equivalent circuit; (b) Phasors of output voltages when using V/V transformer. ............................................................................................................................................ 112 Figure 4.48. Block diagram for establishing the compensation current references. ..................................................... 114 Figure 4.49. DC-link voltage control and the calculation of the voltage reference signals of the FB-MMC4 RPC. ........... 114 Figure 4.50. SM capacitors voltage control of the FB-MMC4 RPC: (a) MMC leg averaging voltage control; (b) MMC arm averaging voltage control; (c) MMC SM individual voltage control. ...................................................... 115 Figure 4.51. Voltage command generation of each SM applied to a phase-shifted PWM. ............................................. 116 Figure 4.52. FB-MMC4 RPC simulation results: (a) Three-phase currents before compensation; (b) Three-phase currents after compensation; (c) Currents at the secondary windings of the V/V power transformer after compensation. ......................................................................................................................................... 117 Figure 4.53. FB-MMC4 RPC simulation results: (a) Load section currents; (b) Compensation currents. ....................... 118 Figure 4.54. FB-MMC4 RPC simulation results: (a) Arm currents of the positive leg of the section x converter; (b) Arm currents of the positive leg of the section y converter. ....................................................................... 118 Figure 4.55. FB-MMC4 RPC frequency spectrum: (a) Compensation currents; (b) Upper and lower arm currents of converter x; (c) Upper and lower arm currents of converter y. .................................................................. 119 List of Figures Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xiv Figure 4.56. FB-MMC4 RPC SM voltages: (a) Positive leg of converter x; (b) Negative leg of converter x; (c) Positive leg of converter y; (d) Negative leg of converter y. .................................................................................... 119 Figure 4.57. TW-MMC3 RPC system with a V/V power transformer. ........................................................................... 120 Figure 4.58. AC equivalent circuit of the TW-MMC3 RPC system. ............................................................................... 121 Figure 4.59. TW-MMC3 RPC: (a) Output equivalent circuit; (b) Phasors of output voltages when using V/V power transformer. ............................................................................................................................................ 121 Figure 4.60. DC-link voltage control and the calculation of the voltage reference signals of the TW-MMC3 RPC. ........... 124 Figure 4.61. SM capacitors voltage control of the TW-MMC3 RPC: (a) MMC leg averaging voltage control; (b) MMC arm averaging voltage control; (c) MMC SM individual voltage control. ...................................................... 125 Figure 4.62. TW-MMC3 RPC simulation results: (a) Load section currents; (b) Compensation currents. ....................... 126 Figure 4.63. TW-MMC3 RPC simulation results: (a) Arm currents of phase x; (b) Arm currents of phase y; (c) Arm currents of phase z. ................................................................................................................................. 127 Figure 4.64. TW-MMC3 RPC frequency spectrum: (a) Compensation currents; (b) Arm currents of phase x; (c) Arm currents of phase y; (d) Arm currents of phase z. ..................................................................................... 127 Figure 4.65. TW-MMC3 RPC SM voltages: (a) Phase x; (b) Phase y; (c) Phase z; (d) Main DC-link voltage. .................. 128 Figure 4.66. HB-MMC2 RPC system with a V/V power transformer. ........................................................................... 129 Figure 4.67. Number of SMs connected to the output terminals: (a) FB-MMC4 RPC; (b) HB-MMC2 RPC. .................... 129 Figure 4.68. AC equivalent circuit of the HB-MMC2 RPC system. ............................................................................... 130 Figure 4.69. HB-MMC2 RPC: one phase equivalent circuit. ........................................................................................ 132 Figure 4.70. DC-link voltage control and calculation of the voltage reference signals of the HB-MMC2 RPC. ................ 135 Figure 4.71. SM capacitors voltage control of the HB-MMC2 RPC: (a) MMC leg averaging voltage control; (b) MMC arm averaging voltage control; (c) MMC SM individual voltage control. ...................................................... 136 Figure 4.72. HB-MMC2 RPC simulation results: (a) Three-phase grid currents before compensation; (b) Three-phase grid currents after compensation; (c) Catenary section (x and y) currents. ....................... 138 Figure 4.73. HB-MMC2 RPC currents when using the proposed predictive current controller (I) and when using the conventional PI controllers (II): (a) Compensation currents; (b) Upper and lower arm currents of section x converter; (c) Upper and lower arm currents of section y converter; (d) MMC circulating currents. ................................................................................................................................................. 138 Figure 4.74. HB-MMC2 RPC frequency spectrum of currents when using the proposed predictive current controller (I) and when using the conventional PI controllers (II): (a) Compensation current of section x converter; (b) Compensation current of section y converter; (c) Circulating current of section x converter; (d) Circulating current of section y converter. ........................................................................................... 139 Figure 4.75. HB-MMC2 RPC frequency spectrum of currents when using the proposed predictive current controller (I) and when using the conventional PI controllers (II): (a) Upper arm current of section x converter; (b) Lower arm current of section x converter; (c) Upper arm current of section y converter; (d) Lower arm current of section y converter. .......................................................................................................... 140 Figure 4.76. HB-MMC2 RPC DC-link voltages when using the proposed predictive current controller (I) and when using the conventional PI controllers (II): (a) Main DC-link voltages; (b) SM voltages of section x converter; (c) SM voltages of section y converter. ..................................................................................... 140 Figure 4.77. Comparison of RPC topologies based on MMC: (a) V/V power transformer; (b) Scott power transformer. ............................................................................................................................................ 146 Figure 5.1. HB-MMC2 RPC simulation model using the PSIM V.9.1. ........................................................................... 152 Figure 5.2. HB-MMC2 RPC control blocks using the PSIM V.9.1. ................................................................................ 153 Figure 5.3. HB-MMC2 RPC reduced-scale prototype simulation results: (a) Three-phase grid currents before compensation; (b) Three-phase grid currents after compensation; (c) Catenary section (x and y) currents. ................................................................................................................................................. 154 Figure 5.4. HB-MMC2 RPC reduced-scale prototype currents (a) Compensation currents; (b) Upper and lower arm currents of section x converter; (c) Upper and lower arm currents of section y converter; (d) MMC circulating currents. ................................................................................................................................. 155 Figure 5.5. HB-MMC2 RPC reduced-scale prototype DC-link voltages: (a) Main DC-link voltages; (b) SM voltages of section x converter; (c) SM voltages of section y converter. ...................................................................... 155 Figure 5.6. Supplementary power equipment diagram. .............................................................................................. 156 Figure 5.7. Supplementary power equipment setup. .................................................................................................. 156 List of Figures Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xv Figure 5.8. User interface of PADS PCB design tool: (a) PADS Logic ; (b) PADS Layout. ............................................... 158 Figure 5.9. Driver circuit PCB scheme: internal schematic of Si8244 and the connections with the IGBT terminals. .... 159 Figure 5.10. Driver circuit PCB layout design: (a) Top layout; (b) Bottom layout. ......................................................... 159 Figure 5.11. Driver circuit PCB (developed under the scope of this Ph.D. thesis). ....................................................... 160 Figure 5.12. Driver circuit PCB experimental results: (a) Unipolar PWM; (b) Bipolar PWM; (VOA , VOB: 5 V/div; VPWM: 2 V/div). .................................................................................................................................... 161 Figure 5.13. Driver circuit PCB experimental results: (a) Deadtime results; (b) Results when Si8244 is disabled; (VOA , VOB , DISABLE: 5 V/div; VPWM: 2 V/div)................................................................................ 161 Figure 5.14. MMC faults: (a) External origin faults; (b) (c) Internal origin faults. ........................................................... 162 Figure 5.15. Half-bridge SM with the proposed protection scheme. ............................................................................ 164 Figure 5.16. Flowchart diagrams of the proposed protection scheme: (a) Overcurrent protection flowchart; (b) Overvoltage protection flowchart. ............................................................................................................. 165 Figure 5.17. Protection circuit PCB layout design: (a) Top layout; (b) Bottom layout. ................................................... 165 Figure 5.18. Protection circuit PCB: (a) Top; (b) Bottom (developed under the scope of this Ph.D. thesis). .................. 166 Figure 5.19. Schematic of the circuit implemented for testing the overcurrent protection. ........................................... 166 Figure 5.20. Protection circuit PCB experimental results (overcurrent protection): Test current (itest: 20 A/div, 100 mV per 1 A); Overcurrent protection actuation signal (vpr: 5 V/div); Differential output voltage of the current sensor (vdif : 1 V/div); Differential output voltage of the current sensor after using a differential amplifier (vamp: 2 V/div). ......................................................................................................... 167 Figure 5.21. Schematic of the circuit implemented for testing the overvoltage protection. ........................................... 168 Figure 5.22. Protection circuit PCB experimental results (overvoltage protection): Test voltage (vtest: 50 V/div); Voltage protection actuation signal (vpr: 2 V/div); Differential output voltage of the voltage sensor after using a differential amplifier (vamp: 2 V/div); Reference voltage signal (vref : 2 V/div). ................................ 168 Figure 5.23. Half-bridge SM with the proposed power circuit PCB scheme.................................................................. 169 Figure 5.24. Power circuit PCB layout design: (a) Top layout; (b) Bottom layout. ......................................................... 170 Figure 5.25. Power circuit PCB: (a) Top; (b) Bottom (developed under the scope of this Ph.D. thesis). ........................ 170 Figure 5.26. Testing scheme of the half-bridge SM. .................................................................................................... 171 Figure 5.27. Power circuit PCB experimental results without using TVS diodes or snubber capacitors: Collector-emitter voltage of IGBT top (VCE1: 50 V/div); Collector-emitter voltage of IGBT bottom (VCE2: 50 V/div); DC voltage source (Vdc: 50 V/div). ................................................................................ 173 Figure 5.28. Power circuit PCB experimental results: Output current (io: 2 A/div); Collector-emitter voltage of IGBT top (VCE1: 50 V/div); Collector-emitter voltage of IGBT bottom (VCE2: 50 V/div); DC voltage source (Vdc: 50 V/div). ....................................................................................................................................... 173 Figure 5.29. Power circuit PCB experimental results during the deadtime: Collector-emitter voltage of IGBT top (VCE1: 50 V/div); Collector-emitter voltage of IGBT bottom (VCE2: 50 V/div); DC voltage source (Vdc: 50 V/div). ....................................................................................................................................... 174 Figure 5.30. Power circuit PCB experimental results when an overcurrent condition is detected by the protection circuit PCB: Collector-emitter voltage of IGBT top (VCE1: 50 V/div); Collector-emitter voltage of IGBT bottom (VCE2: 50 V/div); Overcurrent protection actuation signal (vpr: 5 V/div); Output current (io: 5 A/div). ............................................................................................................................................ 174 Figure 5.31. Final SM structure. ................................................................................................................................ 175 Figure 5.32. Single MMC leg/phase (four SMs). ......................................................................................................... 175 Figure 5.33. Filter inductor used in MMC. .................................................................................................................. 176 Figure 5.34. Filter inductor saturation test: Pulse voltage (Vpulse: 5 V/div); Coil current (iL: 10 A/div); Coil voltage (vL: 5 V/div). ........................................................................................................................................... 176 Figure 5.35. Main DC-link capacitor (developed under the scope of this Ph.D. thesis). ................................................ 177 Figure 5.36. Control system hardware fitted in a metallic box. .................................................................................... 178 Figure 5.37. Global communication structure of the HB-MMC2 RPC reduced-scale prototype. ..................................... 179 Figure 5.38. Interface board to support the TMDSCNCD28335 control card. .............................................................. 181 Figure 5.39. User interface of the programming tool Code Composer Studio v5.5.0 from Texas Instruments . .............. 181 Figure 5.40. PCB of the CYHVS025A voltage sensor. ................................................................................................. 183 Figure 5.41. PCB of the LA 100-P current sensor. ...................................................................................................... 183 List of Figures Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xvi Figure 5.42. Interface PCB between the DSC and the DAC. ........................................................................................ 184 Figure 5.43. Signal conditioning PCB for the external ADC. ........................................................................................ 185 Figure 5.44. Signal conditioning PCB for the internal ADC: (a) Top; (b) Bottom (developed under the scope of this Ph.D. thesis)............................................................................................................................................ 186 Figure 5.45. Voltage sense application by using Si8920 , including the signal conditioning. ......................................... 187 Figure 5.46. Operation principle of the command circuit PCB. ................................................................................... 187 Figure 5.47. Command circuit PCB (developed under the scope of this Ph.D. thesis): (a) Top; (b) Bottom. .................. 188 Figure 5.48. The overall delay at no load conditions: (a) When using normal optocouplers; (b) When using Si8710 isolator from Silicon labs ; (vpr, vdisable, PWM: 5 V/div). ............................................................................. 189 Figure 5.49. PWM PCB adapter (developed under the scope of this Ph.D. thesis). ...................................................... 189 Figure 6.1. MMC workbench developed in GEPE. ....................................................................................................... 192 Figure 6.2. Enhanced phase-locked loop (E-PLL) results: Phase x voltage (ux: 30 V/div); Phase y voltage (uy: 30 V/div); Phase x voltage angle (θx:10 ms ⇔ 180°); Phase y voltage angle (θy: 10 ms ⇔ 180°). ............................................................................................................................................................... 193 Figure 6.3. Results of the digital moving average LPF at M = 200 samples: Load section x current (iLx: 5 A/div); Load section y current (iLy: 5 A/div); Moving average LPF output signal (iLPF: 5 A/div). ............................ 194 Figure 6.4. Results of the digital moving average LPF at M = 600 samples: Load section x current (iLx: 5 A/div); Load section y current (iLy: 5 A/div); Moving average LPF output signal (iLPF: 5 A/div). ............................ 194 Figure 6.5. Results of the digital moving average LPF at M = 800 samples: Load section x current (iLx: 5 A/div); Load section y current (iLy: 5 A/div); Moving average LPF output signal (iLPF: 5 A/div). ............................ 194 Figure 6.6. Testing of one MMC leg/phase in an open-loop control............................................................................. 195 Figure 6.7. Testing of one MMC leg/phase in an open-loop control experimental results: Output voltage (vo: 10 V/div); Upper arm voltage (v1: 10 V/div); Lower arm voltage (v2: 10 V/div). ........................................ 196 Figure 6.8. Test of one MMC leg/phase using a closed-loop control. .......................................................................... 197 Figure 6.9. MMC SM voltages when testing of one MMC leg/phase without commutation: SM1 voltage (VSMx1: 20 V/div); SM2 voltage (VSMx2: 20 V/div); SM3 voltage (VSMx3: 20 V/div); SM4 voltage (VSMx4: 20 V/div). .................................................................................................................................... 198 Figure 6.10. MMC SM voltages when testing of one MMC leg/phase with commutation: SM1 voltage (VSMx1: 20 V/div); SM2 voltage (VSMx2: 20 V/div); SM3 voltage (VSMx3: 20 V/div); SM4 voltage (VSMx4: 20 V/div). .................................................................................................................................... 198 Figure 6.11. Testing of one MMC leg/phase in a closed-loop control: DC-link voltage of the upper capacitor (Vdca: 10 V/div); DC-link voltage of the lower capacitor (Vdcb: 10 V/div); Reference current waveform (iref: 2 A/div); Output current waveform (irx: 2 A/div). .............................................................................. 199 Figure 6.12. Testing of one MMC leg/phase in a closed-loop control: MMC leg/phase output current (irx: 2 A/div); Upper arm current (irxu: 2 A/div); Lower arm current (irxl: 2 A/div)........................................................... 199 Figure 6.13. Testing two MMC legs/phases using a closed-loop control. ..................................................................... 200 Figure 6.14. SM voltages of phase x under operation, when testing two MMC legs/phases: SM1 voltage (VSMx1: 20 V/div); SM2 voltage (VSMx2: 20 V/div); SM3 voltage (VSMx3: 20 V/div); SM4 voltage (VSMx4: 20 V/div). .................................................................................................................................... 201 Figure 6.15. SM voltages of phase y under operation, when testing two MMC legs/phases: SM1 voltage (VSMy1: 20 V/div); SM2 voltage (VSMy2: 20 V/div); SM3 voltage (VSMy3: 20 V/div); SM4 voltage (VSMy4: 20 V/div). .................................................................................................................................... 201 Figure 6.16. Testing two MMC legs/phases using a closed-loop control: Phase x output current (irx: 2 A/div); Phase y output current (iry: 2 A/div). ....................................................................................................... 202 Figure 6.17. Testing two MMC legs/phases using a closed-loop control: Phase x upper and lower arm currents (irxu, irxl: 2 A/div); Phase y upper and lower arm currents (iryu, iryl: 2 A/div). ........................................... 202 Figure 6.18. Schematic of the HB-MMC2 RPC experimental setup. ............................................................................ 203 Figure 6.19. HB-MMC2 RPC experimental results (when both load sections are loaded): Phase x current before compensation (ix: 10 A/div); Phase y current before compensation (iy: 10 A/div); Phase z current before compensation (iz: 10 A/div). ......................................................................................................... 204 List of Figures Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xvii Figure 6.20. HB-MMC2 RPC experimental results (when both load sections are loaded): Phase x current before compensation (ix: 5 A/div); Phase y current before compensation (iy: 5 A/div); Phase A voltage (uA: 100 V/div)............................................................................................................................................... 204 Figure 6.21. HB-MMC2 RPC experimental results (when both load sections are loaded): Phase x current after compensation (ix: 10 A/div); Phase y current after compensation (iy:10 A/div); Phase z current after compensation (iz: 10 A/div). .................................................................................................................... 205 Figure 6.22. HB-MMC2 RPC experimental results (when both load sections are loaded): Phase x current after compensation (ix: 5 A/div); Phase y current after compensation (iy: 5 A/div); Phase A voltage (uA: 100 V/div)............................................................................................................................................... 205 Figure 6.23. Frequency spectrum of the three-phase currents at the secondary windings of the V/V transformer (when both load sections are loaded): (a) Before compensation; (b) After compensation. .......................... 206 Figure 6.24. Harmonic contents value of the three-phase currents at the secondary windings of the V/V transformer (when both load sections are loaded): (a) Before compensation; (b) After compensation. .......................... 206 Figure 6.25. Unbalance ratio and phasors diagram of the three-phase currents at the secondary of the V/V transformer (when both load sections are loaded): (a) Before compensation; (b) After compensation. ....... 207 Figure 6.26. HB-MMC2 RPC experimental results (when both load sections are loaded): Phase x compensation current (irx: 5 A/div); Phase y compensation current (iry: 5 A/div). ........................................................... 207 Figure 6.27. HB-MMC2 RPC experimental results – SM voltages of section x converter (when both load sections are loaded): SM1 voltage (VSMx1: 20 V/div); SM2 voltage (VSMx2: 20 V/div); SM3 voltage (VSMx3: 20 V/div); SM4 voltage (VSMx4: 20 V/div). ................................................................................................ 208 Figure 6.28. HB-MMC2 RPC experimental results - SM voltages of section y converter (when both load sections are loaded): SM1 voltage (VSMy1: 20 V/div); SM2 voltage (VSMy2: 20 V/div); SM3 voltage (VSMy3: 20 V/div); SM4 voltage (VSMy4: 20 V/div). ................................................................................................................ 208 Figure 6.29. HB-MMC2 RPC experimental results (when load section y is loaded): Phase x current before compensation (ix: 5 A/div); Phase y current before compensation (iy: 5 A/div); Phase z current before compensation (iz: 5 A/div). ...................................................................................................................... 209 Figure 6.30. HB-MMC2 RPC experimental results (when load section y is loaded): Phase x current after compensation (ix: 5 A/div); Phase y current after compensation (iy: 5 A/div); Phase z current after compensation (iz: 5 A/div). ...................................................................................................................... 209 Figure 6.31. Frequency spectrum of the three-phase currents at the secondary of the V/V transformer (when load section y is loaded): (a) Before compensation; (b) After compensation. ..................................................... 210 Figure 6.32. Harmonic contents value of the three-phase currents at the secondary of the V/V transformer (when load section y is loaded): (a) Before compensation; (b) After compensation. ............................................. 210 Figure 6.33. Unbalance ratio and phasors diagram of the three-phase currents at the secondary of the V/V transformer (when load section y is loaded): (a) Before compensation; (b) After compensation. ................ 211 Figure 6.34. HB-MMC2 RPC experimental results (when load section y is loaded): Phase x compensation current (irx: 5 A/div); Phase y compensation current (iry: 5 A/div). ....................................................................... 211 Figure 6.35. HB-MMC2 RPC experimental results – SM voltages of section x converter (when load section y is loaded): SM1 voltage (VSMx1: 20 V/div); SM2 voltage (VSMx2: 20 V/div); SM3 voltage (VSMx3: 20 V/div); SM4 voltage (VSMx4: 20 V/div). ................................................................................................................ 212 Figure 6.36. HB-MMC2 RPC experimental results – SM voltages of section y converter (when load section y is loaded): SM1 voltage (VSMy1: 20 V/div); SM2 voltage (VSMy2: 20 V/div); SM3 voltage (VSMy3: 20 V/div); SM4 voltage (VSMy4: 20 V/div). ................................................................................................................ 212 Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xviii List of Tables Table 1.1. Portuguese railway locomotives in the last 5 decades (part I) [7]. Source: [ Comboios de Portugal ] . ............... 8 Table 1.2. Portuguese railway locomotives in the last 5 decades (part II) [7]. Source: [ Comboios de Portugal ] . .............. 9 Table 1.3. Portuguese railway locomotives in the last 5 decades (part III) [7]. Source: [ Comboios de Portugal ] . ........... 10 Table 1.4. Future trends on high-speed locomotives [40]. ............................................................................................ 16 Table 2.1. Voltage harmonics at the point of power delivery, expressed as a percentage of the nominal voltage amplitude Uc (1 kV ≤ MV ≤ 36 kV, 36 kV ≤ HV ≤ 150 kV, 150 kV ≤ EHV ≤ 400 kV). ..................................... 25 Table 2.2. Comparison between several types of power transformers used in railway applications [77]. ........................ 34 Table 3.1. Full-bridge SM switch operation. .................................................................................................................. 56 Table 3.2. Operating states of the half-bridge SM. ........................................................................................................ 58 Table 3.3. Traction power system, public power system and MMC parameters. ............................................................ 60 Table 3.4. Comparison of SFC based MMC topologies for the application of rail electrification [118]. ............................ 61 Table 3.5. Comparison between different compensator topologies in railway electrification [49]. ................................... 63 Table 3.6. Comparison between SFC and RPC based on an indirect MMC: different inherent benefits and the abilities for each system. .............................................................................................................................. 65 Table 4.1. Capabilities of the RPC operation mode. ...................................................................................................... 70 Table 4.2. Capabilities of the RPC operating in the SVC mode. ..................................................................................... 71 Table 4.3. Capabilities of the RPC operating in the catenary voltage regulator mode. .................................................... 72 Table 4.4. Capabilities of the RPC operating as an interface converter between two substations. ................................... 73 Table 4.5. Components quantities of the FB-RPC. ........................................................................................................ 75 Table 4.6. Load parameters of the FB-RPC converter. .................................................................................................. 79 Table 4.7. Parameters of the FB-RPC simulation model................................................................................................ 80 Table 4.8. Components quantities of the TW-RPC. ....................................................................................................... 84 Table 4.9. Parameters of the TW-RPC simulation model. .............................................................................................. 86 Table 4.10. Comparison between the HB-RPC and the FB-RPC. ................................................................................... 88 Table 4.11. Components quantities of the HB-RPC. ..................................................................................................... 88 Table 4.12. Parameters of the HB-RPC simulation model. ............................................................................................ 90 Table 4.13. Components quantities of the co-HRPC. .................................................................................................... 93 Table 4.14. Simulation parameters of the FB-RPC and the co-HRPC models. ................................................................ 96 Table 4.15. Technical comparison between Scott and V/V power transformers. .......................................................... 102 Table 4.16. Components quantities of the MRPC. ...................................................................................................... 106 Table 4.17. Simulation parameters for the FB-RPC model and the MRPC model. ........................................................ 108 Table 4.18. Components quantities of the FB-MMC4 RPC. ......................................................................................... 111 Table 4.19. Parameters of the FB-MMC4 RPC simulation model. ............................................................................... 117 Table 4.20. Components quantities of the TW-MMC3 RPC. ........................................................................................ 120 Table 4.21. Parameters of the TW-MMC3 RPC simulation model. ............................................................................... 126 Table 4.22. Components quantities of the HB-MMC2 RPC. ........................................................................................ 129 Table 4.23. Load parameters of the HB-MMC2 RPC converter. .................................................................................. 137 Table 4.24. Parameters of the HB-MMC2 RPC simulation model. ............................................................................... 137 Table 4.25. IGBT characteristics of the RPC topologies [150]. .................................................................................... 142 Table 4.26. Quantitative comparison of the RPC topologies [150]............................................................................... 143 Table 4.27. Characteristics of the RPC topologies in V/V and Scott transformer [150]. ............................................... 145 Table 5.1. Reduced-scale MMC parameters. .............................................................................................................. 151 Table 5.2. Power circuit PCB components. ................................................................................................................ 170 Table 5.3. Parameters of single half-bridge SM test. ................................................................................................... 172 List of Tables Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xix Table 6.1. Parameters of testing one MMC leg/phase in an open-loop control. ........................................................... 195 Table 6.2. Parameters of testing one MMC leg/phase using a closed-loop control. ..................................................... 197 Table 6.3. Experimental parameters of the HB-MMC2 RPC. ....................................................................................... 203 Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xx Acronyms and Abbreviations Acronym Significance AC Alternating Current ADC Analogue-to-Digital Converter AVE Alta Velocidade Española (in Spanish) CAN Controller Area Network CMOS Complementary Metal-Oxide-Semiconductor CO2 Carbon Dioxide Co-HRPC Hybrid co-phase Rail Power Conditioner CP Comboios de Portugal (in Portuguese) DAC Digital-to-Analogue Converter DC Direct Current DSC Digital Signal Controller EHV Extra High-Voltage E-PLL Enhanced Phase-Locked Loop ERA European Railway Agency ERTMS European Railway Traffic Management System EU European Union FACTS Flexible AC Transmission Systems FB-MMC4 RPC RPC Based on Full-Bridge Indirect Modular Multilevel Converter FB-RPC Full-Bridge Back-to-Back RPC FPGA Field-Programmable Gate Array GEPE Grupo de Eletrónica de Potência e Energia (in Portuguese) GPIO General-Purpose Input/Output GTO Gate Turn-Off Thyristor Acronyms and Abbreviations Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xxi HB-MMC2 RPC RPC Based on Half-bridge Indirect Modular Multilevel Converter HB-RPC Half-Bridge Back-to-Back RPC HF Harmonic Filter HP Horse-power HV High-Voltage ICE Inter-City Express Train IC Integrated Circuit IEGT Injection-Enhanced Gate Transistor IGBT Insulated-Gate Bipolar Transistor LPF Low-Pass Filter MDLC Multiple DC-Link Static Frequency Converter MMC Modular Multilevel Converter MRPC Modular Rail Power Conditioner MSC Mechanically Switched Capacitor MSR Mechanically Switched Reactor MV Medium-Voltage NPC Neutral Point Clamp NSC Negative Sequence Component PCB Printed Circuit Board PI Proportional-Integral PPS Public Power System PR Proportional-Resonant PSC Positive Sequence Component PWM Pulse Width Modulation RMS Root Mean Square RPC Rail Power Conditioner S2R Shift2Rail Acronyms and Abbreviations Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho xxii SDLC Single DC-Link Static Frequency Converter SFC Static Frequency Converter SiC Silicon Carbide SM Submodule SNCF Société Nationale des Chemins de Fer Français (in French) SPI Serial Peripheral Interface STATCOM Static Synchronous Compensator SVC Static VAr Compensator TCR Thyristor controlled reactor TGV Train à Grande Vitesse (in French) THD Total Harmonic Distortion TPS Traction Power System TSC Thyristor Switched Capacitor TVS Transient Voltage Suppressor TW-MMC3 RPC RPC Based on Two-phase Three-wire Indirect Modular Multilevel Converter TW-RPC Three Wire Back-to-Back RPC ZOH Zero-order Hold Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 3 addition, the EU has liberalized the market for the international rail passenger services since January 2010. Harmonizing the hardware and the software between the EU rail operators is a real challenge to accomplish the rail integration in the EU, especially after realizing the differences in the technical specifications of rail infrastructure. For instance, different gauge width is applied between some EU countries as shown in Figure 1.1(a). Portugal and Spain (Iberian gauges) gauge width is 1668 mm [7], higher than the standard gauge in the rest of Europe which is 1435 mm. The electrification standards are different in the EU countries as shown in Figure 1.1(b). Owing principally to historical reasons, Germany, Sweden, Norway, Switzerland and Austria are using the 15 kV, 16.7 Hz for rail electrification, while the north part of France, Portugal, Denmark and Finland are using the 25 kV, 50 Hz for rail electrification [8]. Italy, Belgium, Poland and Spain are mainly using the 3 kV DC system for rail electrification when the train speeds are lower than 250 km/h [8]. Netherland and south of France are electrified with a 1.5 kV DC system. The southern part of the United Kingdom is electrified with a 0.65/0.75/1.2 kV DC [9]. This problem of five different electrification systems can be fixed by using multisystem or hybrid locomotives equipped with Power Electronics devices, hence the locomotives manufacturers should have the foreknowledge of the multisystem concept. The safety conditions and the signaling systems are also different among some EU countries. All previous reasons complicate the efforts to run trains from different EU countries. Therefore, specific EU legislations exist to promote the rail specifications in a way to make them under a unified standard. As a conclusion of these challenges, the European Railway Agency (ERA) was established in 2004 to coordinate and to harmonize the technical standards between EU countries [1]. The name of this agency changed in 2016 to the European Union agency for railways and it is located in Valenciennes and Lille cities in France. (a) (b) Figure 1.1. Railway maps in EU: (a) The used gauge width; (b) The used voltage level for railway electrification [10]. Source: [ Siemens mobility ] Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 4 1.1.3 Portuguese Railway Network The first train journey in Portugal took place 164 years ago, precisely on 28 October 1856 between Lisbon and Carregado. The railway line between Lisbon and Carregado was initially built in a standard gauge of 1435 mm, then it was substituted by the Iberian gauge of 1668/1676 mm due to the noted developments in Spain [11]. The railway network was gradually expanded in the first-half of the 20th century (1900−1950) to reach north of Portugal besides some of the southern parts. The current Portuguese railway network is shown in Figure 1.2, where nowadays, more than half of the Portuguese railway lines are electrified, but the total length of the railway network has decreased during the last decades. The interior cities of the country were the most affected area of this decrement. Out of the 3.750 km of railway lines in service in the 1960, today Portugal has only 2.630 km of railways [12]. The second half of the 20th century has carried the cars industrial revolution in Europe, then from the 1980s and later, Portugal has observed a high demand for the highway roads as an easy way of transport, and at the same time, railway infrastructure suffered from aging and neglecting. As a result, there was a lower demand for railway freight and transport, leading to a significant reduction in railway investment sector. In such cases, hundreds of railway lines and stations have been closed. In this context, Figure 1.3 shows the active and the abandoned Portuguese railway during the last 50 years [13]. Figure 1.4 shows the reduction in Portuguese railways stations number during the last 15 years. According to Pordata the contemporary database for Portugal, the number of Portuguese train stations has decreased from 669 in 2001 to only 571 stations in 2015 [14]. Figure 1.5 shows the abandoned railway lines in the north of Portugal in the last five decades. The most affected areas from the abandoned railway lines were the interior districts of the country [15]. Progressively, railway electrification started in Portugal in 1975, then a plan to finally link all the district capitals by fully electrified trains was to be performed in 2010. The electrified trains nowadays (2016-2017) cover about 1634 km according to Infraestruturas de Portugal . In addition, a plan called Ferrovia 2020 is in the execution phase to promote the Portuguese rail freight transport. For instance, railway network will tend to be fully electrified with a voltage of 25 kV, 50 Hz. Moreover, the network will be equipped with control commands and signaling systems according to the European Railway Traffic Management System (ERTMS) standard. Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 5 Figure 1.2. Portuguese railway network map in 2015 [16].Source: [ Infraestruturas de Portugal ] . Figure 1.6 shows the developments achieved in Portuguese electrified railway lines during the last 15 years [17]. Although the total length of Portuguese railway lines has decreased in the last 50 years, the electrified Portuguese railway lines are composing nowadays around 62% of the total railway length in Portugal. The old electrified lines are operating under 1.5 kV or 3 kV DC, whereas the new high-speed electrified railway lines operate with overhead AC power systems of 25 kV, 50 Hz. Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 6 Figure 1.3. The length of active and abandoned Portuguese railway during the last 50 years. Source: [ Pordata ] . Figure 1.4. Number of Portuguese train stations during the last 15 years. Source: [ Pordata ] . Figure 1.5. Railway network in north of Portugal plus the already abandoned rail lines (in brown) [15]. Source: [ Infraestruturas de Portugal ] . 0 500 1000 1500 2000 2500 3000 3500 4000 1960 1970 1980 1990 2000 2010 2020 km Active Lines Abandoned Lines 0 100 200 300 400 500 600 700 800 2000 2002 2004 2006 2008 2010 2012 2014 2016 Number of train stations in Portugal Year Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 7 Figure 1.7 shows the different voltage levels of the electrified Portuguese railways (such as 25 kV AC, 3 kV DC and 1.5 kV DC), besides the lines that are still using the Diesel locomotives (mainly the internal lines). Cascais line was the first electrified Portuguese line with a voltage level around 1.5 kV DC. The data presented in Figure 1.7 correspond to the year of 2012. Figure 1.6. Electrified and non-electrified Portuguese railway during the last 15 years. Source: [ Pordata ] . Figure 1.7. Portuguese railway based on the running mode in 2012 [18]. Source: [ Thorsten Büker ] . 0 500 1000 1500 2000 2500 3000 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 km Electrified Railway Non-electrified Railway Total Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 8 1.1.4 Portuguese Railway Locomotives Locomotives are the single units responsible to provide the kinetic energy to the train. The source of this kinetic energy can be the fossil fuels (e.g., coal and Diesel fuel), the electrical energy or the Hydrogen (converting the chemical energy of the hydrogen to a kinetic energyGermany launched in September 2018 the world's first hydrogen-powered train). Despite the importance of the locomotives, these units have no capability to carry passengers for several reasons: • The easier maintenance of a single power vehicle (locomotive); • Keeping the power source far from the passengers in case of any dangerous situation; • Replacing the locomotive and not the whole train in case of failure; • No necessary to change all the train vehicles, even when the locomotives become old. Table 1.1, Table 1.2 and Table 1.3 present the used locomotives in Portugal in the last 5 decades. The use of electric locomotives has been increased since the electrification of the Portuguese railway network has taken place many decades ago. Table 1.1. Portuguese railway locomotives in the last 5 decades (part I) [7]. Source: [ Comboios de Portugal ] . Power Proprieties Locomotive Diesel Class:1151−1186 Entered Service: 1966−1967 Maximum Speed: 58 km/h Power: 187 kW Diesel-Electric Class: 1401−1467 Entered Service: 1967−1969 Maximum Speed: 105 km/h Power: 992 kW Diesel-Electric Class: 1551−1570 Entered Service: 1973 Maximum Speed: 120 km/h Out of service: 2012 Power: 1268 kW Diesel-Electric Class: 1901−1913 Entered Service: 1981 Maximum Speed: 100 km/h Out of service: 2002 Power: 1686 kW Diesel-Electric Class: 1931−1947 Entered Service: 1981 Maximum Speed: 120 km/h Out of service: 2002−2005 Power: 1640 kW Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 9 Table 1.2. Portuguese railway locomotives in the last 5 decades (part II) [7]. Source: [ Comboios de Portugal ] . Power Proprieties Locomotive Diesel-Electric Class: 1961−1973 Entered Service: 1979 Maximum Speed: 120 km/h Out of service: 2012 Power: 1655 kW Electric Class: 2501−2515 Entered Service: 1956−1957 Out of service: 2009 Maximum Speed: 120 km/h Power: 2080 kW Electric Class: 2551−2570 Entered Service: 1963−1964 Out of service: 2009 Maximum Speed: 120 km/h Power: 2080 kW Electric Class: 2602−2612 Entered Service: 1974−1975 Out of service: 2012 Maximum Speed: 160 km/h Power: 2870 kW Electric Class: 2621−2629 Entered Service: 1987 Maximum Speed: 160 km/h Power: 2870 kW Diesel Class: 9601−9637 Entered Service: 1976 Out of service: 2002 Maximum Speed: 90 km/h Power: 286 kW Diesel Class: 0451−0469 Entered Service: 1999 Maximum Speed: 120 km/h Power: 544 kW Diesel-Electric Class: 0351−0371 Entered Service: 2000 Maximum Speed: 100 km/h Power: 269 kW Electric Class: 2301−2342 Entered Service: 1992 Maximum Speed: 120 km/h Power: 3100 kW Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 10 Table 1.3. Portuguese railway locomotives in the last 5 decades (part III) [7]. Source: [ Comboios de Portugal ] . Power Proprieties Locomotive Electric Class: 5601−5630 Entered Service: 1993 Maximum Speed: 220 km/h Power: 5600 kW Electric Class: 2451−2664 Entered Service: 1997 Maximum Speed: 120 km/h Electric Class: 3151−3163, 3255, 3261−3263 Entered Service: 1998 Maximum Speed: 90 km/h Voltage: 1.5 kV DC Electric Class: 3519−3530, 3569−3580 Entered Service: 1999 Maximum Speed: 140 km/h Power: 3475 kW Electric Class: 4001−4010, 4051−4060 Entered Service: 1999 Maximum Speed: 220 km/h Power: 4000 kW Electric Class: 3401−3434, 3451−3484 Entered Service: 2002 Maximum Speed: 140 km/h Power: 1400 kW Electric Class: 2241−2297 Entered Service: 2004 Maximum Speed: 120 km/h Power: 1230 kW Electric Class: 4701−4725 Entered Service: 2009 Maximum Speed: 140 km/h Power: 4684 kW 1.1.5 Shift2Rail – Achieving a Notable Shift from Road to Rail This Ph.D. research project belongs to the Shift2Rail (S2R) main objectives, namely the innovation program under the smart power supply framework (IP3). Therefore, this item presents an overview Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 11 about S2R, which is the first European rail organization aimed to support rail product solutions, carry on the major innovation and creativities for the European railway industry, besides applying some policies in railway sector for a larger share of transport demand in the next few decades. Creating an unified European railway area is one of the European commission objectives to achieve a notable shift from road to rail in terms of freight and passenger transport. This will accomplish a competitive and an effective European transport system. However, rail’s share in the European freight and passenger transport is still unsatisfying. Therefore, one of the main S2R tasks is to push forward the European research and innovation projects in a way that helps the rail of playing a broader role in global transportation. S2R also contributes to reduce the life-cycle cost of railway transports by 50%, obtaining twice the existing railway capacity, reducing congestion and CO2 emissions, retaining Europe’s leadership in the global rail market and increasing the trustworthiness and the punctuality by 50% [19]. Figure 1.8 shows the S2R members map in Europe with overall 11 participants from Portugal shared between academic institutions and companies as follow [20]: • Universidade do Minho; • Universidade do Porto; • Instituto Superior Técnico de Lisboa; • Comboios de Portugal (CP); • Fertagus Travessia do Tejo Transportes SA; • Infraestruturas de Portugal SA; • EMEF SA - Empresa de Manutenção de Equipamento Ferroviário SA; • STRA LDA; • Thales Portugal SA; • IP Patrimonio - administracao e gestao imobiliaria SA; • Evoleo Technologies LDA. Figure 1.8. Shift2Rail members map in Europe (red points) [20]. Source: [ Shift2Rail ] . Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 12 The S2R joint undertaking is a new public-private corporation in the rail sector, providing a platform to support future innovations. The founding members of S2R joint undertaking are the EU plus eight agents of the rail industry like Alstom , Ansaldo STS , Bombardier , Construcciones y Auxiliar de Ferrocarriles , Siemens and Thales , as well as infrastructure managers like Network Rail and Trafikverket . The estimated budget of the S2R joint undertaking for the period between 2014-2020 will be at least €920 million. The EU contributes to the budget of €450 million that will come from the Horizon 2020 program. Other members from the rail industry sector should contribute with at least €470 million to proceed with the main objectives of the S2R joint undertaking [21]. This partnership will keep track of the research and innovation activities in a way to achieve a unified European railway area. These activities include five aspects as follow [22]: • Cost-efficient and reliable trains, including high-speed trains and high-capacity trains; • Advanced traffic management and control systems; • Cost-efficient and reliable high capacity infrastructure; • Information technology Solutions for Attractive Railway Services; • Technologies for Sustainable & Attractive European Freight. 1.1.6 Timeframe of the Fastest Trains in the World Nowadays, high-speed trains are not only considered as a fast transport mode but also as a representation of the companies/operators and the belonging countries. In other words, the actual development level in the railway sector in any country could prove the ongoing rail improvements and the alignment with new technology. Thirty years ago, the Germans trains were the fastest, but today Germans retreated from the competition in this race [23]. The high-speed trains are classified according to the maximum operating speed. The 1st generation of the high-speed trains could reach a maximum speed of 250 km/h. The 2nd generation of high-speed trains was designed at an operational speed between 300 km/h and 350 km/h. The last 3rd generation of the high-speed trains nowadays can reach a maximum speed of 500 km/h. In this context, Japan has spent around three decades in the 1st generation and 16 years in the 2nd generation. France has spent 20 years and 10 years, while Germany has spent 12 years and 8 years for the 1st and the 2nd generation of the high-speed trains respectively [23]. The German high-speed train InterCity-Express (ICE) as shown in Figure 1.9 was the fastest train in the world 30 years ago, reaching a speed of 406.9 km/h in 1988 and has a power higher than 11,000 hp / 8.2 MW. At present, there are many trains that have exceeded this speed [24]. Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 19 Finally, in Chapter 7, named Conclusion, the main conclusions are presented according to the performed work throughout this Ph.D. thesis, bearing in mind, the suggestions for future work. 1.5 List of Publications In this item, the publications that have resulted directly from this Ph.D. thesis are presented, namely, publications in international journals, book chapters and indexed international conferences. 1.5.1 Publications in International Journals 1. M. Tanta, G. Pinto, V. Monteiro, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Deadbeat Predictive Current Control for Circulating Currents Reduction in a Modular Multilevel Converter Based Rail Power Conditioner”, Appl. Sci. 2020, 10, 1849. DOI: 10.3390/app10051849. 2. M. Tanta, G. Pinto, V. Monteiro, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Topologies and Operation Modes of Rail Power Conditioners in AC Traction Grids: Review and Comprehensive Comparison”, Energies. 2020, 13, 2151. DOI: 10.3390/en13092151. 1.5.2 Publications in Book Chapters 1. M. Tanta, J. A. Afonso, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Comprehensive Study for a Rail Power Conditioner Based on a Single–Phase Full–Bridge Back–to–Back Indirect Modular Multilevel Converter,” in Transactions on Engineering Technologies , 2019, pp.263-279. 1.5.3 Publications in International Conferences 1. M. Tanta, J. A. Afonso, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Rail Power Conditioner Based on Indirect AC/DC/AC Modular Multilevel Converter Using a Three-phase V/V Power Transformer,” in Lecture Notes in Engineering and Computer Science: Proceeding of the World Congress on Engineering, 5-7 July 2017 , London, UK, pp.289–294. 2. M. Tanta, V. Monteiro, J. G. Pinto, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Efficiency and Cost Estimation for a Static Frequency Converter and a Rail Power Conditioner Based on an Indirect Modular Multilevel Converter in Railways Applications,” in ICEE International Conference on Energy and Environment: Bringing Together Engineering and Economics , Porto, Portugal, 2017, pp.313–319. 3. M. Tanta, V. Monteiro, B. Exposto, J. G. Pinto, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Simplified Rail Power Conditioner Based on a Half-Bridge Indirect AC/DC/AC Modular Multilevel Converter and a V/V Power Transformer,” in IECON 2017 - 43rd Annual Conference Chapter 1 – Introduction Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 20 of the IEEE Industrial Electronics Society , 2017, pp.6431–6436. DOI: 10.1109/IECON.2017.8217120. 4. J. G. Pinto, M. Tanta, V. Monteiro, L. A. M. Barros, and Joao. L. Afonso, “Active Power Conditioner Based on a Voltage Source Converter for Harmonics and Negative Sequence Components Compensation in Electrified Railway Systems,” presented at the Transport Research Arena TRA 2018, Vienna, Austria, 2018, pp.1–10. DOI: 10.5281/zenodo.1491309. 5. M. Tanta, G. Pinto, V. Monteiro, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “A Comprehensive Comparison of Rail Power Conditioners Based on Two-level Converters and a V/V Power Transformer in Railway Traction Power Systems,” presented at the Transport Research Arena 2018, Vienna, Austria, 2018. DOI: 10.5281/zenodo.1483284. 6. M. Tanta, V. Monteiro, T. J. C. Sousa, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Power quality Phenomena in Electrified Railways: Conventional and New Trends in Power Quality Improvement toward Public Power Systems,” in 2018 International Young Engineers Forum (YEF-ECE) , 2018, pp.25–30. DOI: 10.1109/YEF-ECE.2018.8368934. 7. M. Tanta, G. Pinto, V. Monteiro, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Cost Estimation of Rail Power Conditioner Topologies based on Indirect Modular Multilevel Converter in V/V and Scott Power Transformers,” presented at the 4th International Conference on Energy and Environment: bringing together Engineering and Economics, Guimarães, Portugal, 2019, pp.365–370. 8. M. Tanta, J. Cunha, V. Monteiro, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “A Novel Hardware Protection Scheme for a Modular Multilevel Converter Half-Bridge Submodule,” in IECON 2019 - 45th Annual Conference of the IEEE Industrial Electronics Society , 2019, vol. 1, pp.6043–6048. DOI: 10.1109/IECON.2019.8927361. 9. L. A. M. Barros, M. Tanta, A. P. Martins, Joao. L. Afonso, and J. G. Pinto, “STATCOM Evaluation in Electrified Railway Using V/V and Scott Power Transformers,” in Sustainable Energy for Smart Cities , Cham, 2020, pp. 18–32, DOI: 10.1007/978-3-030-45694-8_2. 10.L. A. M. Barros, M. Tanta, A. P. Martins, Joao. L. Afonso, and G. Pinto, “Opportunities and Challenges of Power Electronics Systems in Future Railway Electrification,” presented at the IEEE CPE - POWERENG 2020, Setúbal-Portugal, 8-10 July, pp. 1–6 (Submitted). 11.M. Tanta, L. A. M. Barros, G. Pinto, A. P. Martins, A. S. Carvalho, and Joao. L. Afonso, “Modular Multilevel Converter in Electrified Railway Systems: Applications of Rail Static Frequency Converters and Rail Power Conditioners” in 2020 International Young Engineers Forum (YEF-ECE) , 2020, Caparica-Portugal, 3 July, pp. 1–6 (Accepted for publication). Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 21 Chapter 2 Power Quality Phenomena in Electrified Railway Systems 2.1 Introduction Railway electrification has been set out in the early 20th century. It was undertaken in many countries because of its advantages, such as the reduced air pollution, the higher efficiency, and the decreased carbon dioxide (CO2) emissions [43]. Establishing newly electrified railway lines is not seen enthusiastically by the transmission lines operators as they introduce harmonics and negative sequence components (NSCs) of currents, affecting the power quality of the public power system (PPS). Power quality has become a very important concept of power delivery especially in the second-half of the 1990s. From the early use of railway electrification, the power quality improvement in alternating current (AC) railway electrification was a significant concern for researchers, and many research studies have been devoted to this purpose. Nowadays, most of the high-speed electric locomotives are using the AC power system for a long-distance electrification [44]. The AC traction power system (TPS) is normally a single-phase system connected through power transformers to the three-phase PPS. Electric trains, especially the old ones that use half-controlled rectifiers usually cause significant distortions in voltage and current waveforms on both three-phase PPS and single-phase TPS. However, the power quality improvement on the single-phase TPS was not under interest by researchers since the electrical traction load has a poor power quality by nature [45]. Consequently, most of the power quality improvement studies in AC railway electrification are focusing on the power quality improvement of the three-phase PPS. The typical power quality problems in high-speed electrified railway systems are; currents imbalance, voltage and current harmonics, reactive power and the low-frequency voltage fluctuations, etc. [46]. Furthermore, the non-linear and the dynamic nature of the high-speed electric locomotives make the improvement of such power quality problems a rather difficult work. In this context, this chapter presents an overview about the power quality phenomena in electrified railway systems, as well as the power quality improvement methods that can be classified based on traditional power supply systems to interface with the electrified railway systems or based on the flexible AC transmission systems (FACTS) [47]. Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 22 The traditional power supply-based methods have been used since the beginning of railway electrification for the purpose of improving the PPS power quality. However, the experimental results show that, this choice is only useful in the non-dynamic operation of the locomotives. This led the researchers to search for a solution that can improve the power quality under the transient or the dynamic operation of the electric locomotives. The main contribution of this chapter is to present an overview of the power quality phenomena regarding the AC railway electrification, besides the traditional methods used to overcome the power quality deterioration on the PPS side. This chapter also gives an idea about the hazards resulting from power quality deterioration and it ends with the main conclusion. 2.2 Power Quality in AC Railway Electrification Railway operators have an absolute interest to run the electrified trains with the lowest possible costs. In this context, power quality improvement is important to guarantee reliability and avoid costly downtime. The traction load is considered as a non-linear load and it is varying dynamically, then voltage arcs may occur because of the pantograph/catenary connection. Harmonics are produced due to the old converter equipment (half-controlled bridges) in the electric locomotives and they have an adverse effect on the three-phase PPS side. On the other side, and since the TPS is a single-phase system connected through power transformers to the three-phase PPS, currents imbalance may occur, resulting in NSC of currents that highly deteriorate the power quality of the three-phase PPS [47]–[49]. Hereafter, power quality phenomena in AC railway electrification are presented in detail. 2.2.1 System Imbalance The three-phase PPS is considered balanced as long as the three-phase voltage and current waveforms are sinusoidal and have an equal amplitude with a 120˚phase shift. However, if these conditions are not met, the system is called imbalanced. In AC railway electrification, the voltage imbalance resulting from a non-linear asymmetrical load is inversely proportional to the PPS short circuit power [50]. Normally, the three-phase PPS with nominal voltage values higher than 110 kV has a high short circuit power (it has a low busbar impedance) and great enough to directly connect the single-side feeding TPS to the PPS [43]. On the other hand, when the primary windings of the traction substation are connected to a voltage level higher than 110 kV, the voltage imbalance issue has less priority to be solved. Also because at extra-high-voltage (EHV) level (higher than 150 kV: e.g., 220 kV, 400 kV), the allowed voltage variation according to the European standard EN-50160:1999 (Voltage characteristics of electricity supplied by public distribution systems) is between ± 5%, whereas, at a high-voltage (HV) level (between 35 kV and 150 kV), the allowed voltage variation according to the same European Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 23 standard is between ± 10% [51], [52]. As a result, and in most of the cases, the system imbalance in AC railway electrification mainly refers to the NSC of currents injected into the PPS. The three-phase currents imbalance will definitely result in three-phase voltage imbalance. In other words, the NSC of currents will produce NSC of voltages as well [45]. The traction motors in the electric locomotives are normally designed to operate appropriately at a reduced voltage amplitude by 24% or at amplitudes 10% higher than the nominal voltage amplitude of the electric locomotives drives based on the IEC-60850 and the EN-50163 [45]. Hence, the power quality deterioration-based currents imbalance has more priority to be solved than the voltage problems in railway electrification. Moreover, using the V/V power transformer to feed a complete line being fed from the same PPS, the voltage imbalance can be reduced in total for all substations. Voltage imbalance is generally measured during 10-period intervals for 50 Hz AC systems (After mitigating the effect of harmonics) according to IEC 61000-4-30 electromagnetic compatibility, testing and measurement techniques, power quality measurement methods. Currents imbalance is the most serious problem that affects the power quality of the PPS. The imbalance is clearer as long as a huge single-phase load is connected to the PPS, which in electrified railway case, several single-phase electric locomotives consume a huge amount of power estimated by MW. A single-phase load is normally causing the NSC of currents, if they are not compensated, they may cause power perturbation, as well as increasing the operating costs of the electrified trains. An imbalance ratio is usually used to evaluate the potential risks that could appear in the imbalanced system. A higher imbalance ratio indicates a big difference between the three-phase PPS currents and higher NSC of currents injected into the PPS [43]. When the imbalance ratio is close to one, the NSC of currents will have the same magnitude as the positive sequence components (PSCs) but they rotate in a direction opposite to the PSCs. A related point to consider is the zero sequence components of currents have no value in the electrified railway systems as long as the traction power transformers do not have the fourth wire [45]. The European standard EN-50160 describes the main principal characteristics at the point of power delivery to the client under normal operating conditions. The standard refers to characteristics of the three-phase supplied voltage, which states the following regulation for the clients at HV and EHV levels. For a period of one week, 95% of the voltage NSCs effective values, that are obtained for 10 minutes, should not exceed 2% of the corresponding voltage PSCs. In certain regions, the allowed voltage imbalance ratio can reach up to 3% [51] (such as the isolated islands). On the other hand, the European standard EN-50163 specifies the main characteristics of the supply voltages of the TPS side. Under Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 24 normal operating conditions, for 25 kV nominal catenary voltage, the voltage value (at the catenary) should always be between 19 kV and 27.5 kV and can reach the minimum of 17.5 kV or the maximum of 29 kV during 2 minutes at most [53]. 2.2.2 Harmonic Distortion Electric locomotives normally use power converters to drive the traction motors. These converters, especially the half-controlled ones, may cause harmonic contents flowing into the three-phase PPS [54]. There will be a series of harmonic orders (load current mainly contains all odd harmonics) that have the double, third, fourth, etc., the fundamental frequency. Among the most critical are the 3rd order harmonic contents that have three-times the fundamental frequency value. The 3rd order harmonics are generated when non-linear loads are under operation [43] (e.g., electric locomotives). Consequently, in order to evaluate the harmonic contents in the PPS, equation (2.1) helps to calculate the total harmonic distortion (THD) in a waveform, and is defined as a percentage ratio between the amplitude of the non-fundamental frequency harmonic contents (h2, h3, h4, etc.,.) to the amplitude of the fundamental frequency harmonics (h1). THD(%) = 100 × √ h22 + h32 + h42 +…+ hn2 h12 (2.1) Harmonic contents are also remaining in the DC railway electrification, in which some of the urban train locomotives contain some Power Electronics devices, namely, the 12 pulse rectifier that generates a large amount of the 11th and 13th harmonic contents [45]. Harmonic contents are considered as one of the main origins for power quality deterioration in railway electrification. Harmonics suppression could be either by installing passive filters at the load side of each feeder station or by using FACTS devices that are more expensive and more effective for the purpose of harmonics cancellation [43]. On the other hand, the power system may have harmonic resonance frequency because of the interaction between the transmission lines inductance and the distributed capacitances along the power transmission lines. However, a related point to consider is the harmonics problem can be worsened when the harmonic resonance may occur once the harmonic contents injected by the TPS match one or more of the power system natural frequencies (the natural frequency signifies a frequency value at which the electric power system tends to oscillate) [43], [55]. In such situations, the THD value will increase to reach a high level. In other words, the waveforms are being forced to oscillate at the natural frequency and harmonic resonance will happen accompanied by large amplitude oscillations. This scenario must be avoided since it may cause the total collapse of the power devices, Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 25 especially on the TPS side, besides the other serious impacts on both PPS and TPS, e.g., power perturbation, power distortion, wrong actuation of the protection relays [55]. According to the standard EN-50160 and under normal operating conditions, for each period of one week, 95% of the obtained voltage values during 10 minutes for each harmonic content should not exceed the values given in the Table 2.1 [51]. As a consequence of the harmonic resonance, higher voltage harmonics may occur, but the THD of the PPS (including the harmonics up to the 40th order) must not exceed 8% at medium-voltage (MV) or HV levels, and 4% at the EHV level [51], [52]. Table 2.1. Voltage harmonics at the point of power delivery, expressed as a percentage of the nominal voltage amplitude Uc (1 kV ≤ MV ≤ 36 kV, 36 kV ≤ HV ≤ 150 kV, 150 kV ≤ EHV ≤ 400 kV). Odd Harmonics Even Harmonics Not multiplied by 3 Multiplied by 3 Harmonic order Related Voltage % Harmonic order Related Voltage % Harmonic order Related Voltage % MV HV EHV MV HV EHV MV HV EHV 5 6 5 3 3 5* 3* 1 2 2 1.9 1.5 7 5 4 2 9 1.5 1.3 1 4 1 1 1 11 3.5 3 1.5 15 0.5 0.5 0.3 6 0.5 0.5 0.5 13 3 2.5 1.5 21 0.5 0.5 0.2 8 0.5 0.5 0.4 17 2 −** 1 >21 − 0.2 10 0.5 0.5 0.4 19 1.5 −** 1 12 0.5 0.5 0.2 23 1.5 −** 0.7 >12 0.5 0.5 0.2 25 1.5 −** 0.7 * According to the grid conception, this value of 3rd order harmonic can be much lower. ** These harmonic orders should be in consideration according to the EN-50160 European norm. Note 1: Harmonic values higher than 25 orders are not indicated in the table due to their small amplitude. Note 2: THD in percentage, calculated in accordance with EN-50160, should not exceed 4% at the EHV level. Nowadays, the traction motors are using Power Electronics converters based on the insulated-gate bipolar transistor (IGBT) technology instead of Thyristors. The IGBT can operate at a higher switching frequency (faster commutation), in which a lower amount of harmonics can be injected. On the other hand, silicon carbide (SiC) switches have shown an increase over the last years due to some advantages, e.g., lower power losses, size reduction, improved reliability and switching frequency range. According to Infineon LDA , in the next few years, SiC solutions will expand into new applications such as industrial or traction drives. This will result in lower harmonic contents since the SiC semiconductors are prepared to operate at hundreds or thousands of kHz. Using the SiC semiconductors in a multilevel converter topology allows to reach a higher amount of power, then, this innovative solution can be the future but still expensive. Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 26 2.2.3 Reactive Power Reactive power is the part of the complex power that cannot be dissipated in a purely reactive or active loads. This power is alternatively absorbed from and returned to the source. The power factor is normally used to estimate the reactive power amount of the electrical system. A low power factor signifies a large amount of reactive power. However, this power cannot be consumed by the active power components of traction loads, and it should be reduced as possible for lower operating costs of the electrified trains [48]. Power Converters with power factor correction feature that imposing sinusoidal grid currents are nowadays substituting the diode and multi-pulse rectifiers [56], then, modern AC Power Electronics converters of traction motors use the pulse width modulation (PWM) technique, which generates zero reactive power with a unitary power factor since PQ control can be employed [45]. Regarding the TPS side, reactive power is required in order to compensate the NSC of currents, hence, a contradiction between NSCs compensation and the reactive power compensation may occur [57]. Therefore, the NSCs in the single-phase TPS must be in antiphase with the three-phase voltages/currents of the three-phase PPS to eliminate reactive power [45]. As a result, the reactive power compensation, as well as the NSCs compensation should be performed together. Regarding the compatibility of rolling stocks with the infrastructure, there are some requirements about the power factor of electrified trains. For instance, the power factor of trains should be higher than 0.95 for locomotives having instantaneous power at the pantograph higher than 2 MW. As for those with power below 2 MW, their power factor should be higher than 0.85 over a complete timetable journey. These values are calculated on the basis of the fundamental frequency component only and determined in the standard of EN-50388 [58]. FACTS can be used for the purpose of reactive power compensation. For instance, static VAr compensator (SVC) can be installed into two load sections of the traction substation to improve the power factor and to overcome the NSC of currents [59]. In that case, SVC may increase the total reactive power in the traction substation when it is designed to compensate the NSCs, because there is a trade-off between the power factor correction or the reactive power compensation and the NSCs compensation when using the SVC in traction system [57], [60]. Rail power conditioner (RPC) system consisting in two back-to-back single-phase converters, that can shift active power from one load section to other and send out (or absorb) reactive power to (from) the load sections and, at the same time, achieving NSC compensation. A static synchronous compensator (STATCOM) based a three-phase three-wire AC/DC converter is normally installed at the three-phase PPS side to compensate both of Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 27 reactive power and NSC of currents. The three-phase three-wire converter only contains six switching devices, so it saves two switching devices compared with the RPC topology containing eight switching devices [60]. 2.2.4 Low-Frequency Voltage Fluctuations The low-frequency voltage fluctuations indicate the presence of voltages with a low-frequency less than 10 Hz for the 50 Hz railway TPS [43] and less than 5 Hz for the 16.7 Hz railway TPS [61]. In some cases, these fluctuations could trigger the locomotives protection relays, causing an overhead line power-off. This phenomenon affects the power quality of the TPS, as well as the safe operation of the electric locomotives [62]. It is hard to know the direct reason of the low-frequency voltage fluctuations, where this topic still did not get enough attention by the researchers. However, the fluctuation, in fact, could appear as a complicated locomotive-grid interaction problem [62], [63]. These voltage fluctuations can be induced either by impedance variation or by multiple active locomotives in one feeding section [61]. The fluctuations have been worldwide noted in a variety of AC locomotives and at different operating conditions. According to the published literature, this phenomenon, was first noted in a Norwegian railway, where the rotary converters were used in the TPS substations. The fluctuations have been noticed later in Germany, Switzerland, France, China and the USA. Among the aforementioned countries, the problem in France and China occurred for the same reasons, where many electric locomotives in railway stations were under a standstill state with only the auxiliary load powered by the DC-link of AC/DC/AC converters [63]. Another study has been performed on a traction substation in China that suffered from extremely low-frequency voltage fluctuations [62]. Researchers concluded that the fluctuations are caused by the current collectors of the electric Chinese HXD2B locomotives. 2.2.5 Voltage Arcing Voltage arcing is an unavoidable problem in railway electrification and it is more predominant in high-speed trains [64]. This phenomenon defines electromagnetic interference with a wide frequency spectrum up to several GHz, so it can be induced at wide range of frequencies (kHz to GHz). It occurs as a result of the interaction between the pantograph and the overhead catenary line or between brushes and the third or the fourth rail. Consequently, these arcs have a paramount importance in the high-speed railway lines where the trains run at a speed of over 300 km/h. Then, they may cause power perturbation and power quality deterioration during specific transient periods [64]. Because of the voltage arcs, a transient current component in the TPS is induced, which may trigger one or more of the protection relays, then power-off the main feeder lines of the electric locomotive. In addition, a Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 28 transient magnetic field resulting from the produced current component possibly will perturb the locomotives signaling system. The main reason for the voltage arcing in railway electrification is the varied airgap between the feeder line (overhead line/ third rail) and the current collector (pantograph or brush) due to the train mechanical oscillation [45]. The arcs increase at a subzero temperature, where a skinny ice layer prevents the direct contact between the pantograph and the catenary line. Since the voltage level in the DC railway electrification is lower than the one in AC railway, the produced current is always higher, the arcs are stronger and the electromagnetic radiation is more intense in the DC railway electrification [65]. Therefore, visible and bright arcs moving along the pantograph or along the brushes are well noticed in the DC rails underground metros, especially at acceleration conditions or heavy freight trains, in which the train draws heavy current. In addition, recent studies have proved that arc’s average duration is longer at a bigger traction load and the arc’s duration could be longer at inductive non-linear loads (e.g. traction loads). On the other hand, since the arc’s energy is influenced by the arc’s duration, the arc’s average duration could be shorter and the arc’s energy could be lower as long as the locomotives power factor is close to one [66]. The load power factor correction is one of the effective methods in order to reduce the arcs negative impacts in the electrified railway systems. 2.2.6 Poor Utilization of Supply Network Normally, there is no solution regarding the poor utilization of the TPS, especially in terms of the long-distance between traction substations. However, the irregular utilization of electric locomotives can be as a result of train accelerating/deceleration. For instance, a train going uphill will consume more power, while a train with a constant speed or going downhill will draw a minimal power [46]. Consequently, electric trains have a low electrical load factor or utilization rate by normal (electrical load factor is defined as the average load divided by the peak load in a specified time period). This highly indicates a varied power consumption along a given time (day, month, year). The recent electric locomotives are equipped with regenerative braking. However, if there are no electric trains ready to consume this amount of power, the regenerated power gets back into the supply PPS and will not be consumed by the electric trains [46]. Using the regenerative braking could help to reduce the operating costs of the electrified trains. However, the old locomotives only have unidirectional energy meters and the energy resulting from the regenerative braking cannot be measured. According to the 2017 European railway performance index, railway intensity of use in Portugal was lower than what it was in other European Union countries in both freight and passenger transport, where the prevailing culture Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 35 fixed capacitor value in each passive filter cannot totally compensate the reactive power since these devices are not able to follow the load dynamic changes [43]. Therefore and in order to implement a dynamic compensation, FACTS devices put in service since the 1970s [79], especially after the fast evolution in Power Electronics field. Figure 2.5. Various types of passive filters used in railway electrification. 2.4.5 Steinmetz Compensation Circuit The Steinmetz compensation circuit indicates one of the power quality solutions when a huge single-phase load (e.g., electric locomotives) is connected to the PPS. This solution is applied to balance the three-phase currents and to compensate reactive power. Figure 2.6 shows the main configuration of the Steinmetz compensation circuit. It is easy to be implemented in practice due to its independent structure and control. This power compensator may contain controlled impedance, both capacitive and inductive, as required. These impedances, connected across the phase voltages of the TPS, draw currents with a NSC that compensates the imbalance of the three-phase currents and the voltage imbalance produced by the locomotive load [81]. Steinmetz compensation circuit is a good solution to balance fixed single-phase loads. However, bearing in mind, if the single-phase load varies over a large range (as the case of electric locomotives), then Steinmetz compensation circuit cannot be an effective solution [82]. In such situations, using Steinmetz compensation circuit equipped with changeable reactive elements (capacitors and inductors) controlled by switching devices sounds a good technique. This solution is known as an active Steinmetz compensator or an SVC, which is presented in chapter 3. On the other hand and since the load in high-speed railway is heavy, the capacitors of the Steinmetz compensation circuit may be too large to achieve full compensation [43]. Figure 2.6. The configuration of Steinmetz compensation circuit. PPS AB V/V Transformer ix iy ux uy N2 N1 Double-tuned Passive Filter Single-tuned Passive Filter Damping Passive Filter L C R RL CL C C L TPS C N2 N1 Return Rail Return Rail Series tuned Parallel tuned ix iy ux uy yx z L C1C2 Steinmetz circuit Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 36 2.5 AC Feeding Methods of Traction Power System Since the early use of railway electrification, different methods have been adopted to connect the feeding transformers of the overhead catenary lines. Choosing a method is dependent on some fundamental aspects, such as the power transmission capacity as a relation to the line length, besides the generated electromagnetic disturbances near the train's surrounding area. In this context, the next items present the common AC feeding methods of the TPS. The catenaries (overhead lines) are supposed to be used in AC electrification instead of electrifying the rails. 2.5.1 Simple Feeding with a Return Rail This simple power system transmits the train power through the catenary and the rails. The main drawback of this feeding system is the significant rail-to-earth leakage current due to the unavoidable rail-to-earth impedance [83]. This leakage current is resulting from the difference between the catenary supply line current and the return current through the rails, causing electromagnetic interference with the telecommunication equipment [84]. In addition, the catenary current can induce electromagnetic fields in nearby signal causing interference as well. The leakage current is higher near the locomotive as well as the rail-to-earth voltage value. Reducing the rail-to-earth leakage current is possible by using a return wire connected to the rails at constant intervals as shown in Figure 2.7. This technique helps somewhat to reduce the leakage current [83]. The coupling transformers along the catenary can be connected in single-side feeding or in double-side feeding. The single-side feeding signifies that the neighboring coupling transformers are connected to a different line-to-line voltage, and the electric train draws power from only one traction substation. In this case, a neutral section should be inserted between substations. However, the double-side feeding implies to connect both neighboring coupling transformers to the same line-to-line voltage. In this case, the train draws power from two traction substations, leading to, reducing the required capacities of each individual substation and the voltage drop across the catenary line. However, the double-side feeding is not that common due to the constant current transit between substations [85], [86]. Figure 2.7. Simple direct feeding with a return rail and return wire configuration. Coupling Transformer Power Source (Grid/Converter) i Catenary Rail Return Wire Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 37 2.5.2 Boost Transformers with a Return Rail This system contains boost transformers along the catenary line (every 4 km-6 km [87]) to minimize the leakage current to the ground as presented in Figure 2.8. The effects of induction and noise are minimized in this configuration. Boost transformers are used in AC electrified railway systems with overhead catenary lines to collect the return current from the rails to the return wire or to collect the return current from the rail and the earth to the return wire (as in the item 2.5.3.). However, the boost transformers connected in series with the overhead catenary line increase the voltage drop in comparison to the simple direct feeding system with a return rail. Figure 2.8. Boost transformers with a return rail configuration. 2.5.3 Boost Transformers with a Return Wire The boost transformers feeding with a return wire provide a way to enforce the return current to flow in the return wire rather than the rails. However, this method has poor voltage regulation as a result of additional boost transformers impedances [83]. In some countries, these types of configuration were necessary to be used due to the high ground resistance of different soils. Therefore, Swedish and Norwegian catenary systems have introduced the Boost transformers in railway electrification [87]. In this scenario as in Figure 2.9, the return current flows to the rail only in partial sections. Figure 2.9. Boost transformers with a return wire configuration. Since the catenary and the return wire are much closer to each other, the induction effects are quite smaller than what they are in the previous cases. In other configurations, series capacitors can be TPS Coupling Transformer Power Source (Grid/Converter) Isolated Rail JointsIsolated Rail Joints Boost TransformerBoost Transformer Catenary Line i Coupling Transformer Power Source (Grid/Converter) Catenary Boost Transformer Boost TransformerBoost Transformer Return Wire Rail i Isolated Rail Joints Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 38 added to the return wire to reduce the voltage drop along the line and to decrease the arcs generated when the locomotive passes the boost transformer section [9]. 2.5.4 Auto-Transformers System Auto-transformers are designed to obtain voltage balancing, while the boost transformers operate by balancing the currents on the primary and the secondary windings [83]. The interference suppression, besides the improved system efficiency, are the main advantages of using the auto-transformer system. This system has a feeder called a negative feeder so that from the substation connected between the feeder and the catenary line there is a voltage twice the voltage between catenary and rail. The doubling of the voltage is created by a 180 degrees phase shift. This configuration allows transmitting the power at the voltage twice of the operating voltage (25 kV or 15 kV). Auto-transformers maintain better voltage regulation and a longer interval allowance between transformers around 10−12 km [9], [83]. As known, at a constant power value, a higher voltage leads to smaller currents, then, reducing the power losses and electromagnetic interference [87]. This makes the auto-transformer configuration is preferable one among the other feeding methods. The locomotive is fed by both neighboring auto-transformers as shown in Figure 2.10. Figure 2.10. Auto-transformers configuration. The locomotive current distribution varies according to the distance between the locomotive and the respective auto-transformers. The current magnitudes in Figure 2.10 are presented assuming the distance between the locomotive and the respective auto-transformers is the same on the left and the right sides. In some scenarios, series capacitors with the negative feeder line are added to decrease the voltage drop and to stabilize the catenary voltage. A study presented in [84] shows the voltage drop of an auto-transformers system is less than the voltage drop when using a boost transformers system by 40%, at a traction power of 8 MVA, 10 km spacing between auto-transformers and 5 km spacing between boost transformers. On the other hand, rail-to-earth voltage is lower in the auto-transformer system than in systems where the return current flows through the rails. Nowadays, this system is the Catenary Line Rail Feeder Traction Substation Auto-transformer Protection Wire 2 × 25 kV i/2 i/2 i i/2 i/2 i/4 i/4 i/43i/4 i/4 i/4 i/4 Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 39 most used for AC railway electrification, even some boost transformer systems have been modified into auto-transformer as in Norway and Sweden [87]. 2.5.5 Coaxial Power Cable As shown in Figure 2.11, the coaxial cable feeding system consists of a coaxial cable placed along the train path. Every several kilometers, the inner conductor of the cable is connected to the overhead catenary line and the outer conductor is connected to the rail through parallel connectors. This configuration is expensive compared to the previous feeding ways. However, it can be useful when space is limited. Figure 2.11. Coaxial power cable feeding system. The Shinkansen high-speed train in Japan is using in some sections this way of electrification. The Eurotunnel train is also using this method of electrification. In some situations, the cable small loop impedance makes this solution attractive for some applications as it reduces the electromagnetic interference with other communication devices. In general, the load current is boosted in the cable and the rail current distribution is similar to that of the auto-transformer system [9]. 2.6 Conclusion Power quality phenomena have been discussed in this chapter to demonstrate the effects caused by power quality deterioration in electrified railway systems. In this context, the power quality deterioration has directly adverse impacts either on the electric power system, including the electric locomotive itself, or on the signaling and communication system between trains. Power quality deterioration will result in higher operating costs of electrified trains, as well as in polluting the power system with high total harmonic distortion (THD) levels and negative sequence components (NSCs). Balanced three-phase power transformers can partially solve the problem of system imbalance in the electrified railway systems. However, the effectiveness of this solution is highly relying on the trains loading profile parameters. These transformers provide a better power quality improvement when both load sections are equally loaded. On the other hand, power compensators based on passive components have been presented in this chapter to explain the technology used in the last decades, and even in these days, Coupling Transformer Power Source (Grid/Converter) Catenary Line Rail Negative Feeder Cable ii Chapter 2 – Power Quality Phenomena in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 40 to overcome the power quality deterioration. These passive compensators are not able to follow the fast-dynamic changes of non-linear loads, which is the case of the traction load (electric locomotive). This reason has established a high demand for dynamic power compensators that can follow the dynamic changes of non-linear loads. From here, next chapter of this Ph.D. thesis serves that purpose. Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 41 Chapter 3 Active Power Compensators in Electrified Railway Systems 3.1 Introduction Chapter 2 presented an overview of power quality phenomena, besides the traditional-based methods of power quality improvements in electrified railway systems, such as balanced power transformers and passive power compensators. However, these techniques are not able to follow the non-linear load's dynamic changes and they cannot totally compensate harmonics and negative sequence components (NSCs) of currents [88]. Power Electronics converters are increasingly responsible for ensuring operation with high efficiency and for meeting power quality requirements [89]. Therefore, some of the flexible AC transmission systems (FACTS) devices are used for the purpose of power quality improvement of the three-phase public power system (PPS). In this case, these devices operate as active power compensators to increase the efficiency of the long-distance power transmission. Then, providing extra network capacity more economically and much faster than the option of building new transmission lines. They also contribute to enhance power quality, improve grid stability, besides increasing the reliability of AC power grids. Due to the FACTS advantages, nowadays, electrified trains are one of the biggest users of FACTS devices in Japan [90]. In this context, the static VAr compensator (SVC) is used for reactive power compensation and to mitigate active power oscillations. Like the SVC, but with a faster transient response, the static synchronous compensator (STATCOM) is considered as an evolution of the SVC technology. Both solutions are well-known devices and they are broadly used for power quality improvement applications. On the other hand, the static frequency converter (SFC) is used in certain countries to interface the single-phase traction power system (TPS) and the three-phase PPS. The SFC based on the modular multilevel converter (MMC) has been included in this chapter for a better understanding of MMC topology. The SFC solution prevents the harmonics and the NSC of currents passing from the TPS to the PPS side. However, the SFC has some drawbacks regarding the converter power ratings and the need for additional passive filters on the PPS side [91]. The rail power conditioner (RPC) is a recent Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 42 technology used in AC electrified railway systems. It has the ability to compensate the NSCs and the current harmonics produced by the locomotives [57]. Both types of power compensators (active and passive) are intended to operate together for the purpose of power quality improvement. This type of combination helps to reduce the power ratings of the active power compensators. In such cases, a considerable part of the reactive power compensation is handled by passive compensators. In this context, and since the previous chapter gave an idea about the passive power compensators, this chapter presents the active power compensators, or the FACTS devices used in electrified railway systems. At the end of this chapter, several case studies are presented to establish a comparative analysis according to the published literature. 3.2 Static VAr Compensator (SVC) At the end of seventies, with the emergence of new semiconductor materials, a switching devicesbased solution was introduced with the objective of improving power quality. At that epoch, Thyristor switching devices have drawn more interest and they were mainly used for dynamic compensation purposes. Since the SVC has the same operation principles of the Steinmetz circuit (presented in the item 2.4.5), this device can be called as an active Steinmetz compensator. SVC is typically modeled as variable three-phase reactive elements (capacitors and inductors), connected in delta or wye scheme [45]. Consequently, the SVC structure involves several reactive elements, in which they could be fixed or controlled by Thyristor switching devices. SVC systems are used to improve the balance of the PPS by adjusting the three-phase impedance connected to the grid. However, using these devices at the high-voltage (HV) or the extra-high-voltage (EHV) level requires a high number of series connected switches to withstand the high-voltage values. Otherwise, the connections can be accomplished through a step-down power transformer (coupling transformer) as shown in Figure 3.1. Another solution is by installing the SVC devices at the secondary windings of V/V power transformer or at the two feeder sections of the traction substation. SVC devices are well known as an economical solution for power quality improvement. This category of FACTS compensates both of NSCs and reactive power, leading to three-phase PPS power factor correction and balanced three-phase currents. However, the main drawback of such devices is the huge amount of harmonic contents injected into the PPS due to the slow Thyristors commutation [46]. For this reason, harmonics compensation using the SVC is a weak indicator, as the SVC performance relies entirely on the harmonic impedance of the grid. Moreover, SVC in traction system may reduce the power factor of the traction system when it is designed for NSCs compensation because there is a Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 43 trade-off between power factor correction and NSCs compensation when using the SVC in traction system for the purpose of power quality improvement [57]. In other words, the conflict is between NSCs and reactive power compensation. If the three-phase power system carries a high amount of reactive power, the SVC cannot be a helpful solution. Anyway, and regardless of this fact, nowadays, most of the new electric locomotives use pulse width modulation (PWM) technique in the traction drivers. This technology can achieve an almost unitary power factor in the traction system, and the SVC in this case compensates the NSC. Figure 3.1. SVC installation via a step-down power transformer in railway electrification. At the present time, SVC is branded as a low cost dynamic compensator and is used in more flexible power systems (e.g., near the giant power plants) which can be technically sufficient and economically effective [45]. Since there are various types of SVC in the market, manufacturers are customizing the SVC to fit each customer with their specific needs. The SVC contains several fixed or switched branches, of which, at least one branch includes thyristors, and the combination of branches can be diversified depending on the customer requirements. Typically, the SVC includes a combination of at least two of the following branches: Thyristor controlled reactor (TCR); Thyristor switched capacitor (TSC); Harmonic filter (HF); Mechanically switched capacitor bank (MSC) and Mechanically switched reactor bank (MSR) [92]. The most common topologies are the ones include the combination of TCR/HF or TCR/TSC/HF. Using the TSC topology is useful to reduce the total SVC losses, where the TSC results in reducing the overall SVC size. On the other hand, the MSR or the MSC topologies are the ones used at the high-voltage and the medium/low-voltage side of the SVC coupling transformer to have extra reactive power support outside the dynamic range [92]. As a result, SVC dynamic performance is poor, and the system occupies a large physical area compared to other active power compensators [93]. 3.3 Static Synchronous Compensator (STATCOM) During the last two decades, the insulated-gate bipolar transistors (IGBTs) have begun replacing the Thyristor based SVC (new SVC IGBT based). This was due to the higher IGBTs switching frequency V/V Transformer PPS 220 kV TPS SVC Coupling Transformer Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 44 compared to the Thyristor switching devices. Consequently, STATCOM devices have been considered as the next-generation of SVC power compensators but with improved performance. Normally, STATCOM devices consist of a voltage-source converter, coupling power transformer and filter inductors installed on the three-phase AC grid. These devices allow reactive power exchange without the need for other passive filters [94]. In some cases and if needed, switched, or fixed air core coils and capacitors can be used with the voltage-source converter as supplementary reactive power components to reach any desired range. In Japan, five sets of 34 MVA to 60 MVA STATCOM devices are available in Tokaido Shinkansen [90]. STATCOM devices can be used in medium-voltage levels at a frequency of 50 or 60 Hz. Consequently, they are normally manufactured with a larger volume in comparison to the SVC devices. However, and in some applications (e.g., an extension of the dynamic compensation range and for better overloading performance), combining the best of SVC and STATCOM technologies to create a new solution called hybrid STATCOM (manufactured by ABB Ltd.) in which the TCR and TSC branches are connected in parallel with STATCOM device. On the other hand, the STATCOM based on cascade connected modular multilevel voltage-source converter has low harmonic generation. Therefore, and since the TSC does not add any harmonic contents, there is no need for low-order harmonic filters in the substation. The STATCOM is usually used for power factor correction, current compensation and voltage regulation at the point of power delivery, contributing in electrical PPS stability. However, voltage regulation by the STATCOM is only possible due to the inductive part of the upstream grid line impedance at the point of interconnection between the STATCOM and the PPS [42]. Figure 3.2 depicts the STATCOM device connected in parallel with the PPS. This parallel connection signifies that STATCOM can be connected or disconnected without disturbing the operation of the traction substation. Consequently, the robustness and readiness of the railway substation will not be affected by the STATCOM operation [95]. Moreover, this device does not require an internal power supply, meaning that, the STATCOM has a neutral contribution in terms of active power. By another meaning and after neglecting the STATCOM power losses, STATCOM will not consume or provide active power to the PPS, and the main objective is to exchange only reactive power with the PPS in order to improve the power quality. This is possible by exchanging the instantaneous reactive power among the phases of the PPS since the STATCOM can act as either a source or as a sink of reactive power [96]. As a fully controllable compensator, the STATCOM can dynamically provide reactive power to compensate NSC of currents. Considering another aspect, and since the necessary load active power flows directly to the catenary through the V/V power transformer or any of the balanced transformers Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 51 • SFC based on MMC is a transformer-less topology. In other words, power transformers are dispensable devices at the TPS side. • The power ratings will be divided equally among several SMs. Therefore, there is no need to use switching devices with high power ratings. • MMC equivalent switching frequency is equal to the SM switching frequency multiplied by the total number of SMs in each arm (each MMC leg contains two arms, the upper and the lower arms). Accordingly, MMC output waveforms contain lower harmonics content and a smaller size of passive filters (higher output voltage level results in minimal use of three-phase grid filters). • The possibility to increase the total MMC power with ease by inserting extra SMs. • High reliability due to the MMC flexibility of redundancy. MMC SMs can be half-bridge or full-bridge according to the application and they sequentially switch during one complete cycle. MMC equipped with full-bridge SMs is an interesting solution in 15 kV, 16.7 Hz traction substations. This technology is called a direct AC/AC MMC. On the other aspect, the other MMC type called an indirect AC/DC/AC MMC consists of several half-bridge SMs and it is the suitable choice for 25 kV, 50 Hz traction substations. The main reasons for such a selection are explained in the further items of this chapter. 3.5.1 Modular Multilevel Converter Operation Principle Figure 3.8 depicts operation approach of the 5-level MMC, besides a simplified representation of the MMC [105]. The AC output voltage uo is acquired by inserting or bypassing a specific number of SMs in each MMC leg. The DC-link voltage capacitor of each SM can be inserted (as the case of SM2, SM3, SM4 and SM1n) or bypassed (as the case of SM1, SM2n, SM3n and SM4n) as shown in Figure 3.8(a). A higher number of DC-link capacitors signifies a higher output voltage level, lower harmonic distortion, and a lower noise to the neighboring electrical equipment [106]. The simplified representation of MMC can be equivalent to a switch that changes its position and connection points with charged floating capacitors during a time cycle as shown in Figure 3.8(b). In that case, a multilevel output voltage uo is generated [107]. A review of MMC SM topologies and PWM techniques is presented in [108]. Three-phase MMC contains three legs that correspond to the three-phase AC grid. Each phase leg consists of two arms, namely an upper arm and a lower arm. Each arm contains a few tens of cascade-connected SMs. Each SM is an independent voltage-source converter with at least two switching devices and one floating DC-link capacitor. The total DC-link voltages of SMs in one MMC arm should be able to withstand the entire converter DC-link voltage Vdc. In other words, each leg must have a total DC-link voltage with a value double the entire MMC DC-link voltage. By adopting an Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 52 appropriate switching strategy, each MMC arm voltage (including the filter inductance voltage in the MMC arm) has a sinusoidal output waveform with a DC offset equal to half the entire DC-link voltage. Figure 3.8. Operation approach of the 5-level MMC: (a) Representation of the MMC; (b) Simplified representation of the MMC. MMC leg output AC voltage is obtained collaboratively through the upper and the lower MMC arms, after considering mid-neutral point capacitors at the DC-link, Vdc, to create a neutral point. [109]. States of the 5-level MMC are presented in Figure 3.9, Figure 3.10, Figure 3.11 and Figure 3.12. MMC is structurally scalable and can hypothetically meet any voltage level requirement. The voltage level at the midpoint of the phase leg uo is defined by the number of SMs that are connected in the upper and lower MMC arms [107]. By considering 5-level MMC, state 1 happens when all the upper arm SMs are inserted and the lower arm SMs are bypassed. In that case, the output voltage uo has the highest negative value. On the other hand, state 2 is when three SMs in the upper arm and one SM in the lower arm are inserted. State 3 is when two SMs in the upper arm and two SMs in the lower arm are inserted and the output voltage uo is close to zero. State 4 is when one SM in the upper arm and three SMs in the lower arm are inserted. State 5 is when all the lower arm SMs are inserted and the upper arm SMs are bypassed. In that case, the output voltage uo has the highest positive value [110]. Vdc/2 Vdc/2 uo Vdc uo SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n (a) (b) + _ Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 53 Figure 3.9. States of the 5-level MMC (state 1 to state 2 example). Figure 3.10. States of the 5-level MMC (state 2 to state 3 example). Vdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uoVdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uo State 1 State 2 State 1 State 2 uo t Vdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uoVdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uo State 2 State 3 State 3 State 2 uo t Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 54 Figure 3.11. States of the 5-level MMC (state 3 to state 4 example). As a conclusion, the MMC voltage level (N) is always equal to the number of SMs in one MMC arm plus one, then, the number of SMs in one arm is: (N−1). These nomenclatures are the ones used in this Ph.D. thesis. Figure 3.12. States of the 5-level MMC (state 4 to state 5 example). Vdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uoVdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uo State 3 State 4 State 3 State 4 uo t Vdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uoVdc SM1 SM2 SM3 SM4 SM1n SM2n SM3n SM4n + _ uo State 4 State 5 State 4 State 5 uo t Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 55 3.5.2 Direct Modular Multilevel Converter This MMC type fulfills a direct three-phase to single-phase AC/AC conversion without DC-link connection. The main structure of direct MMC is composed of identical series-connected bipolar full-bridge SMs [111]. Each SM consists of four power electronic modules (IGBTs + freewheeling diodes) and a DC-link capacitor as shown in Figure 3.13. The direct MMC full-bridge SM allows the SM capacitor to be inserted into the circuit in any polarity as presented in Table 3.1 [112]. The MMC leg contains two arms and two reactors to smooth the current waveforms and to limit the circulating currents between the MMC arms. The main MMC advantage is to have a low actual switching frequency of switching devices [113]. Consequently, the equivalent switching frequency in this case is equal to the actual switching frequency for each SM multiplied by the number of SMs connected in series in each arm (each leg consists of two arms, as shown in Figure 3.13). This leads to lower switching losses and a higher efficiency. Moreover, and compared to the SDLC and the MDLC, there is no need for the step-up power transformer at the TPS side, since the MMC can withstand the catenary medium-voltage of 25 kV or 15 kV. This reduces the total costs, size, and the cooling equipment of the overall system [44]. The number of series-connected SMs in each arm can be defined according to the desired power level. Normally, in SFC railway substation, the number of SMs in each arm is between 10 and 30 with a power level between 20 MVA and 60 MVA. For example and for a catenary voltage of 15 kV, the peak voltage value is around 21 kV could be obtained by seven SMs per arm (14 SMs per MMC leg) with DC-link capacitors each of 3.25 kV [114], [44]. The converter design should guarantee the MMC ability to deal with the voltage oscillations according to the grid norm. Figure 3.13. Direct AC/AC MMC topology. PPS 50 Hz Step-down power transformer 16.7 Hz Single submodule Lau Lal Lbu Lbl Lcu Lcl T1 T2 T3 T4 MMC leg MMC arm Lo io uo uin Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 56 Table 3.1. Full-bridge SM switch operation. Switching devices (IGBTs) Output voltage T1 + T4 + Vout T2 + T3 − Vout T1 + T3 0 T2 + T4 0 Under normal operation, the direct AC/AC MMC converter circulates power between the three legs to compensate pulsating power on the catenary line. This circulated power has two frequencies, the sum and the difference between input and output power frequencies. If the input and the output frequencies are the same, so there is no circulating power between legs. This converter is convenient to be used for 16.7 Hz output frequency in railway substations because the circulating power difference frequency between legs is higher than the output frequency, thus this decreases the importance for filters requirement. Nevertheless, using direct MMC topology in practical applications is still limited and has some difficulties as it requires a frequency separation accompanied by a complex control as well. Otherwise, the implementation will result in a harmonic interaction between the power grids (PPS and TPS), especially at low numbers of SMs and low switching frequency values [111]. Arm currents of the direct AC/AC MMC have mainly three different frequency components, the fundamental frequency current component of 50 Hz that has the main responsibility for power exchange between the direct MMC and the PPS, the 16.7 Hz (50/3 Hz) current component that manages the power exchange between the direct MMC and the TPS, and the two-third of (100/3 Hz) fundamental frequency current component (the difference of input and output current frequencies) that controls the power exchange between the direct MMC arms (circulating current between arms). The last component refers to the circulating current flows between the MMC arms, which should be well controlled to avoid high power losses in the converter. The circulating current between arms can also be created from an additional current component that has the frequency of (50+16.7 Hz) (the sum of input and output current frequencies). However, the last indicated component could be ignored since the main part of the circulating current has the two-third of the fundamental frequency. Finally, current waveforms may also contain some additional harmonic contents. The main challenge in direct MMC control system is the simultaneous currents control for the MMC legs in a way to generate AC current reference and also to maintain equal DC-link voltages among the MMC SMs [44]. Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 57 3.5.3 Indirect Modular Multilevel Converter This MMC topology comprises series-connected half-bridge SMs as shown in Figure 3.14. Each half-bridge SM contains two power electronic modules (IGBTs + freewheeling diodes) and a DC-link capacitor. Besides the SM DC-links, there is a main DC-link connection between the three-phase AC/DC converter and the single-phase DC/AC converter. Although the indirect MMC can accomplish direct frequency conversion. However, this topology is used when the input frequency and the output frequency are the same since there is no limitation on frequency separation [115]. Concerning the railway applications, indirect MMC is the best choice for 50 Hz catenary supply. On the other side and for 16.7 Hz catenary supply, indirect MMC should be used with larger SM capacitors and higher current rating devices on its single-phase DC/AC converter compared to the direct MMC topology (see the item 0 case study). Figure 3.14. Indirect AC/DC/AC MMC topology. Each SM acts as an independent two-level converter generating a voltage of either 0 V or Vdc (SM capacitor voltage). Using an enough number of series-connected SMs increases the converter power capacity. In addition, this makes the power electronic modules synthesize a stepped voltage that varies near a Sine waveform with very low harmonic contents. The main difference between MMC and other converter types is the MMC arm currents that flow continuously in all arm SMs of the converter over the frequency cycle. As a result, concepts such as on-state and off-state in MMC have no physical meaning. This converter has two principal disadvantages. First, the control is more complex than other converter types, then, balancing the SM voltages is a profound challenge that requires a significant computing power and high-speed communication between central control unit and actuators. Figure 3.15 shows the switching possibilities of indirect AC/DC/AC MMC SM, where the output voltage is either equal to the capacitor voltage when the capacitor is inserted or equal to zero when the capacitor is bypassed. Single submodule PPS 50 Hz Step-down power transformer 50 Hz DC-link Lau Lal Lbu Lbl Lcu Lcl Lxu Lxl Lyu Lyl uin Lo io uo T1 T2 Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 58 Among the SM topologies of MMC, half-bridge SM is the most common because of its simple configuration and due to the low number of power electronic modules (switching devices) as shown in Figure 3.15. In addition to that, using the half-bridge SM can reduce the MMC control complexity, hereafter the positions of half-bridge SM switching devices are presented in Figure 3.15 [116]. Switching devices T1/D1 represent the upper IGBT/diode of the half-bridge SM and the switching devices T2/D2 signify the lower IGBT/diode of the half-bridge SM. At normal operating condition, SM capacitor is either inserted or bypassed and the output voltage is either equal to Vdc or 0. Depending on the switches positions of T1/D1, T2/D2 and the current direction, there are six switching states, as shown in Figure 3.15 and Table 3.2 [109]. Figure 3.15. Positions of half-bridge SM switching devices: (a) SM switched on; (b) SM switched off; (c) SM blocked during the deadtime. Table 3.2. Operating states of the half-bridge SM. States SM switches positions T1 state T2 state Conducting switch Direction of current Capacitor state 1 Switch on On Off T1 Inserted 2 Switch on Off Off D1 Inserted 3 Switch off Off Off D2 Bypassed 4 Switch off Off On T2 Bypassed 5 Block (during deadtime) Off Off D2 Bypassed 6 Block (during deadtime) Off Off D1 Inserted When the SM is required to be switched into MMC arm circuit, T1/D1 will switch on and T2/D2 will switch off. In this case, SM capacitor is charging or discharging according to the current direction iSM. However, the opposite scenario is when the SM outputs a zero voltage into the MMC arm circuit. In this case, T2/D2 will switch on and T1/D1 will switch off. As a result, the SM capacitor will be always bypassed regardless the current direction iSM. A deadtime band is required between both actions to avoid a short circuit fault during the switch transient state. Therefore and in such situations, when the system is in the pre-energization state or in deadtime band between switches, SM capacitor can be (a) (b) (c) T1D1 T2D2 T1D1 T2D2 T1D1 T2D2 T1D1 T2D2 T1D1 T2D2 T1D1 T2D2 Vdc Vdc Vdc Vdc Vdc Vdc Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 59 either inserted or bypassed according to the current direction iSM, and the current in this case flows only through the diodes D1 or D2 as explained in Figure 3.15(c) [97]. 3.5.4 Case Study: Using Indirect MMC / Direct MMC as SFC for the 15 kV, 16.7 Hz Electrified Railway The conventional AC/DC/AC back-to-back two-level converter normally requires a step-up power transformer at the side of 16.7 Hz. In addition, AC filters are required on both AC sides to meet the standards of 50 Hz PPS and 16.7 Hz of TPS. A single-line diagram of such system is presented in Figure 3.16 with two power transformers and AC filters. The converter efficiency is normally high, but the total system efficiency is low because of the filters and the power transformers losses. The overall system is bulky and entails some drawbacks, such as filtering requirement and the low efficiency. Therefore and as a result of the multilevel converter features, these transformer-less power converters are nowadays under interest [44]. SFC devices based on MMC have recently been proposed to interface the TPS (50 Hz or 16.7 Hz) and the PPS (50 Hz or 60 Hz). Normally, the SFC based on an indirect MMC is intended for fixed frequency applications, whereas, the SFC based on a direct MMC is proposed to obtain a lower value of output frequency (16.7 Hz or 25 Hz). In this framework, this item presents a detailed explanation to clarify the main reasons for this selection. Serving that purpose, direct and indirect MMC for 15 kV, 16.7 Hz are compared in terms of SMs energy storage, power losses, and complexity with regard to the number of filters and semiconductors [117]. As mentioned earlier, indirect MMC is equipped with half-bridge SMs and consists of a three-phase AC/DC converter and a single-phase DC/AC converter linked to a DC-link as shown in Figure 3.14. Filter inductors between MMC arms play an important role to limit the circulating currents between MMC arms. In addition, there is no need for using AC filters in both AC sides since the MMC passive components operate as inner filters. The multilevel output voltage waveform contains low harmonic contents [117]. On the other side, direct AC/AC MMC is equipped with full-bridge SMs and consists of a three-phase AC/AC converter without the need for a DC-link as shown in Figure 3.13. As already stated, the filter inductors between MMC arms can effectively reduce the power fluctuations, hence there is no need for using power filters. A research team from ABB Ltd has accomplished a study about the SFC based on direct and indirect MMC for railway applications. The results have been published in [117] after considering the same rated power and the same parameters for PPS and TPS as presented in Table 3.3. In other words, this Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 60 study is performed under the same loading parameters when using the SFC based on direct or indirect MMC. Figure 3.16. Conventional AC/DC/AC back-to-back two-level converter in 15 kV, 16.7 Hz traction power system. Both converters were designed in ABB Ltd after considering the customer requirements in terms of overvoltage and other operation points. Maximum overvoltage was set to be around 18 kV (catenary voltage). Therefore, the maximum peak output voltage for each converter was around 25.5 kV. Each converter included eleven SMs with a DC-link voltage around 2.6 kV for each SM after assuming a voltage fluctuation around ± 10%. Both indirect and direct MMC with eleven SMs in each MMC arm create 12-level catenary line voltage. The final results waveforms for both solutions are presented in [117]. The capacitance of the SM capacitor is normally defined by the maximum SM voltage fluctuations, besides the energy fluctuations in MMC arms. The final selected values of the SM capacitors by ABB Ltd are presented in Table 3.3. Table 3.3. Traction power system, public power system and MMC parameters. Parameters Symbols Values TPS Rated power Sn 15 MVA Rated phase voltage Un 15 kV Rated current In 1000 A Rated frequency fn 16.7 Hz PPS Rated line-to-line voltage ULL 15 kV Rated frequency fLn 50 Hz Rated current ILn 575 A Indirect and Direct MMC parameters design Number of SMs in one arm NSM 11 SM switching frequency fisw 250 Hz Equivalent switching frequency fsw 2.75 kHz Capacitance of the SM Capacitor Direct MMC CDSM 2.6 mF Indirect MMC (AC/DC side) CI3SM 1.3 mF Indirect MMC (DC/AC side) CI2SM 10.6 mF AC filter AC filter 33 Hz filter AC/DC converter DC/AC converter 15 kV, 16.7 Hz Single phase 110 kV, 50 Hz Three phase Power transformerPower transformer Chapter 3 – Active Power Compensators in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 67 and negative sequence components (NSCs) of currents. The static VAr compensator (SVC), the static synchronous compensator (STATCOM), the static frequency converter (SFC) and the rail power conditioner (RPC) were the main FACTS introduced along the chapter. The focus has been more on the modular multilevel converter (MMC) topology with comprehensive introductory information, since it is one of the main topics of this Ph.D. thesis. In this term, comparative case studies between the SFC and the RPC based on MMC have been under the scope of this chapter. The direct AC/AC MMC and the indirect AC/DC/AC MMC, besides the half-bridge and the full-bridge MMC submodules, have been described to understand the MMC operation principle, which is significant to perceive the RPC based on MMC system that will be explained in the next chapter. Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 68 Chapter 4 Rail Power Conditioners in Electrified Railway Systems 4.1 Introduction Electric locomotives in the AC traction power system (TPS) represent a huge single-phase non-linear load and detrimentally affect the power quality and the efficiency of the three-phase public power system (PPS) [46]. Some drawbacks are related to the continuous variations of the traction load, the harmonics distortion produced by the electric locomotives and the negative sequence components (NSCs) created by the three-phase currents imbalance [126]. Many power quality conditioners and compensation strategies have been developed and investigated in the last decades to overcome such phenomena and to guarantee an advantageous interface with the three-phase PPS. However, and within the appearance of high-speed electrified railway systems, those compensation strategies and power compensators have been recently enhanced to follow the latest technology and developments in the high-speed electrified railway systems that use the 25 kV, 50 Hz supply voltage [46]. Power quality improvement determines to use flexible AC transmission systems (FACTS) installed either at the PPS or at the TPS. For instance, static VAr compensators (SVC) are normally used to compensate the NSCs, but they have a limited tracking ability and a poor power factor correction. In addition, the SVC systems generate some harmonic contents [127]. The static synchronous compensator (STATCOM) devices are normally used to be installed at the three-phase PPS. Therefore, the cost is usually high compared with their compensation capacity. In this framework, the rail power conditioner (RPC) is basically used to overcome the power quality deterioration, then maintaining balanced the three-phase power grid currents, without NSCs and with lower harmonic distortion. Usually, the RPC system is installed near the catenary neutral section of the traction substation (close to the secondary windings of the traction power transformer), which has two load sections with the same frequency and RMS voltage magnitude [44], [128]. Over the last decades, some railway substations, especially the ones in East Asia countries (Japan and China), have been equipped with RPC systems for their beneficial utilization with the balanced power Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 69 transformers (Scott, Woodbridge, LeBlanc and impedance-matching transformers) that are commonly used in that region. In that case, the RPC nominal power ratings will be lower since the RPC mainly shifts active power between load sections, and the reactive power compensation requirement is lower. By considering the V/V transformer features presented in Table 2.2, the RPC with the V/V unbalanced power transformer can be an interesting solution, and therefore, this chapter presents a comprehensive analysis of the RPC systems with V/V power transformer at 25 kV, 50 Hz. The RPC topologies based on two-level converters can use single-phase full-bridge back-to-back converters, or two-phase three-wire converters, or even half-bridge back-to-back converters, sharing a DC-link in between [54]. This chapter also explains the RPC topologies based on the indirect AC/DC/AC modular multilevel converters (MMC), which is one of the main interests of this Ph.D. thesis. In addition to that, and along the chapter, it is explained the RPC operation principle and the associated control algorithms. In the final analysis, a comparative study between the RPC topologies under interest is executed in terms of the number of power component, control complexity, costs, volume, reliability and overall performance. 4.2 Rail Power Conditioner (RPC) Operation Modes High-speed trains are single-phase non-linear loads that usually cause NSCs, current harmonics, imbalance of the three-phase currents, and low power factor in the three-phase PPS [129]. These power quality phenomena occur regardless of the used power transformers (unbalanced transformers or balanced transformers) that interface the PPS and the TPS. However, the effect of power quality distortion is higher when using unbalanced transformers. For instance, and to solve the problem of poor power factor, reactive power exchange should not be between the traction loads and the three-phase PPS, then, traction loads have to exchange reactive power with a third party (e.g., power compensator) [49]. The key feature of the RPC is the potential of different operation modes, in which each operation mode can be implemented by using the same converter hardware, but the converter control is quite different. The operation modes can be either implemented to increase the power capacity of the traction substation or to solve one or more of the power quality problems. When the two terminals of the RPC are connected to a single-phase traction power grid, the RPC can operate as an SVC. On the other hand, when the two terminals of the RPC are connected to two-phase traction power grid, the RPC can transfer active power between the two-phases [121]. 4.2.1 Rail Power Conditioner Traditional Operation Mode RPC devices are very effective in compensating the NSC of currents and the harmonic contents in electrified railway systems. The presented RPC schemes, in this chapter, are based on the split of the Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 70 TPS into two load sections separated by isolators, known as catenary neutral sections. The pantograph may produce arcs when it crosses over the neutral sections due to the electric current interruption. To avoid such circumstances, the speed limit is usually applied when the locomotive moves from a load section to another [129]. RPC devices are designed to tolerate the overloading circumstances that could happen when several locomotives operate at the same load section. The RPC system should deal with harmonics compensation, presenting a balanced load seen by the PPS and correcting the three-phase grid power factor to unitary. Figure 4.1 shows the RPC system basic operation principles when both load sections (x and y) are unequally loaded (Px > Py) and when using V/V power transformer. In this case, the RPC system shifts half of the active power difference from the highly loaded section (section x) to the lightly loaded one (section y). Table 4.1 presents the capabilities of the RPC operation mode. The shifted active power by the RPC, PC, is presented as in (4.1). The power factor of PPS is close to one since the required reactive power for both load sections is always provided by the RPC compensator. The reactive power exchange between the TPS and the RPC is bidirectional according to the loading conditions and the used power transformer to interface the three-phase PPS and the single-phase TPS. It is noteworthy to mention, the power consumption losses of the TPS equipped with RPC are higher due to the additional losses in the RPC components [130]. PC = 0.5 (Px – Py) (4.1) Table 4.1. Capabilities of the RPC operation mode. Active power shifting Reactive power compensation Harmonics cancelation Catenary voltage stability ✓ ✓ ✓ × Figure 4.1. Principles of the RPC system. V/V Transformer Converter (y) Converter (x) Catenary Line Section (y) Catenary Line Section (x) Px Qx Py Qy Qx Qy Px −PC PC PC PC Py +PCPx > Py Lx Ly Px + Py+Plosses Q = 0Three-phase Public Grid Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 71 4.2.2 Static VAr Compensator Operation Mode Normally, the traction power transformers, such as the V/V and the Scott, are constituted by a three-phase input and two-phase output. When one of the traction transformer output phases is disconnected due to the periodic maintenance, in this case, the SVC mode based on a single-phase power transformer is implemented, as shown in Figure 4.2. Accordingly, the active power cannot be shifted between the load sections, and the RPC operates as an SVC system only to compensate reactive power and harmonic distortion as presented in Table 4.2. The RPC based on the SVC mode calculates the reactive power of the substation according to the load section currents and voltage deviation, then, the converter controls the reactive power that has a reverse polarity of the calculated reactive power to accomplish reactive power compensation. The converter x and converter y are connected in parallel, and therefore, each converter injects half of the reactive power amount [120], [131]. Table 4.2. Capabilities of the RPC operating in the SVC mode. Active power shifting Reactive power compensation Harmonics cancelation Catenary voltage stability × ✓ ✓ × Figure 4.2. RPC operating in the SVC mode (RPC based SVC). 4.2.3 Catenary Voltage Regulator Operation Mode Additional operation mode is when the RPC is not directly connected to the traction substation, but it is connected to a certain point along the overhead catenary line, as shown in Figure 4.3. In that case, no active power shifting, or reactive power compensation is applied in this scenario. However, the converter, in this case, can produce reactive power to compensate the feeding voltage drop caused by the loads of the catenary load sections, as presented in Table 4.3. The voltage regulation in the overhead catenary lines occurs due to the internal impedance of the line. On the other hand, this Single-phase power transformer Converter (y) Converter (x) Catenary Line Section (y) Catenary Line Section (x) Px Qx Py Qy 0.5(Qx + Qy) Px > Py Lx Ly Px + Py+Plosses Q = 0 0.5(Qx + Qy) Three-phase Public Grid Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 72 operation mode overcomes the effect of the three-phase power grid voltage oscillations to maintain constant overhead catenary voltages. The catenary voltage adjustment close to the connection point is achieved by controlling the reactive power output, Qx, Qy [119], [131]. It is worthy to mention that reactive power compensation can be made instead of catenary voltage stability. On the other side, this scheme is hardly possible. The open-loop voltage in each traction substation is different and high circulating currents can happen. Table 4.3. Capabilities of the RPC operating in the catenary voltage regulator mode. Active power shifting Reactive power compensation Harmonics cancelation Catenary voltage stability × × × ✓ Figure 4.3. RPC operating in the catenary voltage regulator mode. 4.2.4 Interface Converter Between Two Collateral Substations Additional operation mode is when the RPC operates as an interface converter between two collateral substations, as shown in Figure 4.4. The interface converter, in this case, is used to interface between two phases (with different out-of-phase angles). The control of the interface converter is quite different from the one in the conventional RPC application. The RPC operating as an interface converter, is mainly used for the purpose of power transfer by controlling the converter as a voltage source or current source to extract power from two substations instead of only one substation. In other words, it is possible to shift the active power from one substation to another, but this solution has no ability to compensate the NSC of currents, then, the three-phase currents may suffer from imbalance. Table 4.4 presents the main capabilities of this solution. In [132], an RPC interface converter located between two traction substations is presented and commercialized by Hitachi , Japan. The system commenced operation in 2015 and is designed to shift V/V Transformer Qx Lx Ly ZyZx Qy V/V Transformer Converter (y) Converter (x) Three-phase public grid Three-phase public grid ZL Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 73 active power from one substation to another, including the energy resulting from the regenerative braking of the trains. The RPC converter was manufactured for the 22 kV AC catenary line and installed at the Ushiku neutral section between Tsuchiura and Fujishiro traction substations in Japan. Both traction substations are equipped with arithmetic units that compute the amount of electric power consumed and regenerated. Each of these units transmits the power data to a central control unit of the RPC installed at the neutral section between the traction substations. Digital subscriber line communication technology is used to a range of 10 km-20 km. The RPC was designed with a total capacity around 2.6 MW. However, and since regenerative electric power is frequently generated in large amount over short time duration, the power transformer and cooling system have been adapted by giving the system a one-minute overload capacity of 5.3 MW. A single-phase full-bridge power converter using the insulated-gate bipolar transistor (IGBT) switching devices is used, with a total DC-link of 6 kV. Besides the active power shifting capability, it is worthy to mention that reactive power compensation capability or catenary voltage stability can be implemented (not both simultaneously). Table 4.4. Capabilities of the RPC operating as an interface converter between two substations. Active power shifting Reactive power compensation Harmonics cancelation Catenary voltage stability ✓ × × × Figure 4.4. RPC operating as an interface converter between two substations. 4.3 RPC Based on Full-Bridge Back-to-Back Two-Level Converter and V/V Power Transformer In order to eliminate the effects of NSC and harmonic distortion in the three-phase PPS, when a V/V unbalanced power transformer is used to interface the PPS and the TPS, RPC based on full-bridge back-to-back two-level converters (FB-RPC), sharing the same DC-link, can provide power quality improvement. The amount of NSC depends on the TPS topology, particularly, the type of used power Three-phase public grid V/V Transformer Qx Lx Ly ZyZx Qy V/V Transformer Converter (y) Converter (x) Px Py Three-phase public grid Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 74 transformer. For instance, V/V transformers are widely used in high-speed railways due to their simple structure, low cost and high overload capacity compared to other power transformers [54]. It is important to declare that when using a balanced power transformer (presented in item 2.4.3), no NSCs of currents are injected into the PPS when both load sections consume the same power. However, when an unbalanced V/V power transformer is in use, the NSCs of currents injected into the PPS are half of the fundamental positive sequence components (PSCs) when both load sections are equally loaded [54]. RPC system is equipped with an ability of energy storage system is presented in [133]. This system uses a supercapacitor that is connected onto the DC-link of the RPC via a bidirectional DC-DC converter. The energy storage system is useful for reducing the peak power demand and the operating costs in a railway substation. In AC railway electrification, the catenary line system can be modeled in sections with lumped impedances along it. However, in this Ph.D. thesis, these lumped impedances were not added to the figures of RPC leading to some assumptions in the RPC configurations. Nevertheless, different assumed values of lumped impedance were considered in the simulation of RPC systems. According to the European standard EN-50641, the lumped impedance for one tracking line has the value of ZL = 0.1 + j 0.1 Ω/km in the 15 kV, 16.7 Hz system. On the other hand, the lumped impedance for one tracking line has the value of ZL = 0.15 + j 0.45 Ω/km in the 25 kV, 50 Hz system [134]. The lumped parameters utilize the network equivalent impedance and they add more complexity to the RPC models. Therefore, the lumped impedance values in AC railway systems are only informative. The presented control algorithm and studies in this chapter are for the conventional application of RPC (NSC and harmonics compensation). Photos of the RPC devices in Shin-Numakunai substation in Japan, besides some technical aspects, are presented in [135]. Figure 4.5 depicts the RPC system based on full-bridge converters and a V/V power transformer. The line-to-line three-phase voltage of PPS is stepped-down through the V/V power transformer to 25 kV catenary voltage. Since two of the RPC legs are connected to the same grounding point (the rail), step-down coupling transformers are necessary to avoid short circuits in the FB-RPC. In addition, and by considering the maximum voltage blocking of 6.5 kV of the commercially available IGBTs [112], the coupling transformers are important to step-down the catenary medium-voltage value to a lower value. Table 4.5 presents the components quantities of the FB-RPC. Two filter inductors, Lx, Ly, are added to operate as an RPC filter. Figure 4.5 assumes that the right load section is called phase x and the left one is called phase y. The corresponded phases for U 󰇗x and U 󰇗y, are respectively U 󰇗AC and U 󰇗BC. Consequently, phasors diagram of the PPS before compensation Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 75 is shown in Figure 4.6(a) after assuming both load sections have a unitary power factor. The turns ratio of the three-phase V/V power transformer and the single-phase step-down coupling transformers are KV and KD, respectively. Normally, the three-phase currents of PPS are suffering from current imbalance and harmonic contents, then the three-phase currents are presented in (4.2) [136]. By assuming negligible power losses in the RPC, there will be an active power transfer with an amplitude of half the active power difference between load sections as in (4.3). This active power flows from the highly loaded section to the lightly loaded one. In this context, Figure 4.6(b) presents the phasors diagram when RPC shifts only active power from phase x to phase y [54], [136]. Phase A and phase B currents after active power shifting are as presented in (4.4) and (4.5). Table 4.5. Components quantities of the FB-RPC. Number of IGBTs Number of capacitors Number of inductors Isolation/coupling transformer 8 1 2 2 Figure 4.5. FB-RPC system with a V/V power transformer. 𝐼󰇗A = ILx KV e-j30°, 𝐼󰇗B = ILy KV e-j90°, 𝐼󰇗C = – ILx KV e-j30–ILy KV e-j90° (4.2) ∆I = 1 2(ILx – ILy) (4.3) 𝐼󰇗A1 = 𝐼󰇗A – ∆I KV e-j30° = 1 2KV (ILx + ILy) e-j30° (4.4) 𝐼󰇗B1 = 𝐼󰇗B + ∆I KV e-j90° = 1 2KV (ILx+ILy) e-j90° (4.5) On the TPS side (x, y, z), equation (4.6), Figure 4.6(b) and Figure 4.7 confirm that the RPC equalizes in magnitude between phase x and phase y currents after shifting the active power difference between Public Grid 220 kV iA DC-link iC iB A B C V/V Transformer x yz Step-down Transformer Step-down Transformer Catenary Line Section (y) Catenary Line Section (x) ixiLx irx iry iyiLy 25 kV ux 25 kV uy LxLyirx1iry1 Two-Level Converter (y) Two-Level Converter (x) Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 76 load sections. However, the three-phase currents, in that case, do not have equal magnitude as presented in (4.6). Ix1 = ILx – ∆I , Iy1 = ILy + ∆I , Iz1 = 3 Iy1 = 3 Ix1 (4.6) Phase A and phase B currents have now the same root mean square (RMS) value, but phase C has a different magnitude since the three-phase currents are still imbalanced (different magnitudes and different out-of-phase angles), as presented in Figure 4.6(b). However, phase C current is in phase now with its phase voltage, while the other two phases (phase A and phase B currents) have 30°out-of-phase with the corresponded phase voltages because the reactive power is not yet compensated. In order to balance the three-phase currents, it is important to add a certain reactive current component to phase x and phase y as shown in Figure 4.6(c) [54]. Phase x generates reactive power because the reactive current component, I󰇗rxr, that is synthesized by the converter x leads the line-to-line voltage U 󰇗AC, and its corresponded phase voltage on the secondary windings of the V/V power transformer, U 󰇗x. Phase y receives reactive power because the reactive current component, I󰇗ryr, that is received by the converter y lags the line-to-line voltage U 󰇗BC, and its corresponded phase voltage on the secondary windings of the V/V power transformer U 󰇗y, as shown in Figure 4.7. The following equation (4.7) confirms equal RMS reactive components of compensation currents. Equation (4.8) presents the RMS then the instantaneous value of the section x current after compensation. Irxr = Ix1 tan π 6 = 1 2 (ILx + ILy) tan π 6 Iryr = Iy1 tan π 6 = 1 2 (ILx + ILy) tan π 6 (4.7) Figure 4.6. PPS phasors diagram: (a) Without compensation; (b) After shifting the active power difference; (c) After shifting the active power difference and compensate reactive power [57]. (c) (a) 30º UC UA UB UAC UBC IA IB IC (b) 30º UC UA UB UAC UBC IC1 IA1 IB1 UC UA UB UAC UBC IB2 IA2 IC2 30º Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 83 Figure 4.14. FB-RPC simulation results: (a) Load section current; (b) Currents at the secondary windings of the V/V power transformer after compensation (when the load section x is loaded). On the other hand and although the three-phase PPS currents after compensation have a smaller RMS value than the case when both load sections were loaded, the compensation currents synthesized by the RPC in this case, as shown in Figure 4.15(b), are higher than the case when both load sections were loaded, as shown in Figure 4.12(b). This signifies higher operating power demand for the RPC system, including capacitors, inductors and power switching devices. As a conclusion, the design of the RPC system should always consider the worst-case scenario when only one load section is loaded. Figure 4.15. FB-RPC simulation results: (a) DC-link voltage; (b) Compensation currents (when the load section x is loaded). 4.4 RPC Based on Two-Phase Three-Wire Converter and V/V Power Transformer This RPC system consists of two-phase three-wire converter (TW-RPC), which is fed by single-phase step-down coupling transformers connected to the TPS. There are six bidirectional switching devices, where one of the TW-RPC wires is connected to the grounded rail of the TPS. Hence, a leg with two switching devices are saved compared to the FB-RPC as shown in Figure 4.16. In that regard, authors Time (s) 0.25 0.3 (a) i (A) -200 (b) 200 0.2 0.35 0.4 0 ixiy 400 iLx -200 200 0 0.25 0.3 (a) Time (s) i (kA) (b) 0.2 0.35 0.4 c 1 irx1iry1 0 4 -4 v(kV) 2 3 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 84 in [138] propose a three-wire quasi-Z-source RPC and authors in [139] propose a three-wire Z-source RPC to decrease the ratings of the RPC converter. It is worthwhile to mention that using a lower number of switching devices does not affect the TW-RPC power ratings, which is similar to the power ratings of the FB-RPC [60]. In other words, the TW-RPC topology has the same characteristics of the FB-RPC. Therefore, it can be considered as two single-phase converters merged together and sharing the same DC-link voltage [140]. Since each leg of the TW-RPC is connected to different phase or wire, it is not essential to have isolation transformers between the converter and the traction feeders. However, step-down coupling transformers are indispensable in this topology [48]. Also, the outer legs generate the compensation currents for phase x and phase y, respectively. In turn, the inner leg generates the compensation current of the common ground phase. Then, all the IGBTs in the TW-RPC topology have homogeneous stress. Table 4.8 presents the components quantities of the TW-RPC. Table 4.8. Components quantities of the TW-RPC. Number of IGBTs Number of capacitors Number of inductors Isolation/coupling transformer 6 1 2 2 Figure 4.16. TW-RPC system with a V/V power transformer. In some applications, the TW-RPC converter can be connected through three-phase step-down coupling transformer to the PPS instead of the single-phase TPS [95]. In that case, the solution is called shunt active power conditioner, which does not offer the operation modes flexibility that the RPC presents. Due to the use of the three-phase coupling power transformer, the shunt active power conditioner can be bulkier and more expensive converter than the TW-RPC converter. The phasors diagram presented in Figure 4.6 and Figure 4.7 can be applied in the case of TW-RPC [60]. This results in obtaining the compensation current equations in (4.10), (4.14) and (4.15), where (4.14) shows the compensation currents in the phases after considering the turns Public Grid 220 kV iA DC-link iC iB A B C V/V Transformer xy z Step-down Transformer Step-down Transformer Catenary Line Section (y) Catenary Line Section (x) irz1 ixiLx irx iry iyiLy 25 kV ux 25 kV uy LxLyirx1iry1 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 85 ratio of the step-down single-phase coupling transformer KD. However, summing the vectors of 𝐼󰇗rx and 𝐼󰇗ry, in Figure 4.7, gives the compensation current of phase z, 𝐼󰇗rz, as presented in (4.15). The compensation current of phase z has an amplitude very close to the phase x and phase y compensation current amplitudes. Consequently, current ratings of the two switching devices connected to the grounded leg or phase are not increased. This is one of the main advantages of the TW-RPC topology [60], [140]. Irx1 = KD Irx , Iry1 = KD Iry , Irz1 = KD Irz (4.14) irz1*= – (irx1*+ iry1*) (4.15) 4.4.1 Control Algorithm The control strategy of the TW-RPC has the details presented in the item 4.3.1 but with adding the leg z control, as shown in Figure 4.17. The load section currents of iLx and iLy, are the main keys to calculate the compensation current references of irx* and iry*. Two single-phase E-PLL are used to create a signal whose phase angle is adaptively tracking the variations in the voltage x and the voltage y waveforms. In addition, 2nd order (LPF) is used to extract the peak value of iLx sin (ωt – π/6)+ iLy sin (ωt – π/2). Subtracting the waveforms of ix2 and iy2 (secondary windings of the V/V power transformer currents after compensation) from the waveforms of iLx and iLy, gives the required compensation currents synthesized by the TW-RPC as presented in (4.10) [60]. Figure 4.17. Control strategy of the TW-RPC converter. The DC-link voltage control is similar to the one presented in Figure 4.8. In addition, three hysteresis controllers are used to correct the error resulting from the difference between the reference and the measured compensation currents. The final three-phase compensation current references have the values as in (4.10) and (4.15). It is worth noting that, using the TW-RPC instead of the FB-RPC does not increase the number of sensors required but additional current tracking controller is mandatory to drive the leg z switching devices [60]. × - iLx KD KD irx iry * * irx1 * iry1 * × - iLy × × × × × - × - irx1 iry1 × × Vdc × -irz1 * - irz1 PWM PI Vdc * sin(ωt-π/6) sin(ωt-π/2) - - × × iLx × 2 / 3 2nd-LPF × iLy sin(ωt-π/2) ux sin(ωt-π/6) uy E-PLL sin(ωt-2π/3) -1 cos(ωt-π/2) = sin(ωt) cos(ωt-π/6) E-PLL × × ix2 iy2 Hys Con Hys Con Hys Con Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 86 4.4.2 Simulation Results The overhead catenary lines of the load section x and the load section y are loaded with electric locomotives, which can be modeled using the equivalent circuit of the electric locomotive, as in Figure 4.9. The simulation model assumes that both load sections are unequally loaded. In this study, the overhead catenary line of phase x is loaded with a higher load, which has a value of 4.8 MW. On the other side, the overhead catenary line of phase y is loaded with a value of 2.4 MW (half of the loading value of phase x), as presented in Table 4.6. The parameters of the TW-RPC simulation model are presented in Table 4.9. Table 4.9. Parameters of the TW-RPC simulation model. Description Symbols Values Line-to-line PPS voltage UAB, UBC, UCA 220 kV Catenary voltage Ux, Uy 25 kV RPC filter inductance Lx, Ly 2 mH Catenary lumped impedance ZL 0.03 + j 0.12 Ω Capacitance of the DC-link capacitor Cdc 80 mF Turns ratio of the V/V power transformer KV 220/25 Turns ratio of the step-down coupling transformer KD 25/1 Figure 4.18(a) presents the three-phase PPS currents before compensation. These currents are imbalanced and contain harmonics and NSCs. There is 60° out-of-phase between the phase A and phase B currents. Under other conditions, Figure 4.18(b) presents the three-phase PPS currents after compensation when the TW-RPC balances the active power between the load sections, and compensates the reactive power. In that case, the three-phase PPS has a unitary power factor. Figure 4.18. TW-RPC simulation results: (a) Three-phase currents before compensation; (b) Three-phase currents after compensation (when both of the load sections are loaded). 0.25 0.3 (a) Time (s) i (A) 0 (b) 40 -40 0.2 0.35 0.4 iAiBiC 20 -20 iAiBiC 0 40 -40 20 -20 60 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 87 Figure 4.19(a) shows the same load section currents of iLx and iLy, that have been presented in Figure 4.11(a), where the load section x has double the value of the load section y. Figure 4.19(b) presents the currents at the secondary windings of the V/V power transformer after compensation. These currents have 120° out-of-phase (after compensation) instead of 60° (before compensation). In general, the simulation results presented in Figure 4.18 and Figure 4.19 confirm that the TW-RPC has a similar performance to the FB-RPC. Figure 4.19. TW-RPC simulation results: (a) Load section currents; (b) Currents at the secondary windings of the V/V power transformer after compensation (when both of the load sections are loaded). The DC-link voltage waveform of the TW-RPC is presented in Figure 4.20(a), where the waveform has voltage ripples with a frequency equals to 100 Hz. The DC-link voltage, in this case, follows its reference value of 2 kV, as was the case in the FB-RPC. The compensation currents synthesized by the TW-RPC to compensate NSC and harmonic contents are presented in Figure 4.20(b). Figure 4.20. TW-RPC simulation results: (a) DC-link voltage; (b) Compensation currents (when both of the load sections are loaded). The voltage stress across each of the power switching devices is similar to the one when using the FB-RPC as both of the topologies have the same DC-link voltage value. As a conclusion, the 0.25 0.3 (a) Time (s) i (A) -200 (b) 200 0.2 0.35 0.4 0 ixiy 400 iLx iL -200 200 0 0.25 0.3 (a) Time (s) i (kA) (b) 0.2 0.35 0.4 c 1 irx1iry1irz1 0 4 -4 v (kV) 2 3 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 88 compensation capacity of the TW-RPC is similar to the FB-RPC topology but with a lower number of power components for the TW-RPC [60]. 4.5 Simplified RPC Based on Half-Bridge Two-Level Converter and V/V Power Transformer This system consists of two half-bridge back-to-back converters connected by two capacitors with a mid-neutral point (HB-RPC), as presented in Figure 4.21. This RPC can reduce half of the required switching devices compared to the FB-RPC [130], which can decrease the power losses, complexity and the total costs of the HB-RPC system. However, the voltage stress of each switching device in the HB-RPC is double compared with the FB-RPC and with the TW-RPC as presented in Table 4.10. This determines to use switching devices with a higher voltage stress capability [140]. Table 4.11 presents the components quantities of the HB-RPC. Table 4.10. Comparison between the HB-RPC and the FB-RPC. Characteristics FB-RPC HB-RPC IGBTs number 8 4 Voltage stress 1 p.u. 2 p.u. Current stress 1 p.u. 1 p.u. Switching frequency of IGBTs fsw 2 fsw Table 4.11. Components quantities of the HB-RPC. Number of IGBTs Number of capacitors Number of inductors Isolation/coupling transformer 4 2 2 2 Figure 4.21. HB-RPC system with a V/V power transformer. To have a similar performance between the RPC topologies under interest (FB-RPC, TW-RPC and HB-RPC), and by considering identical filter inductors in these topologies, the switching stress for each Public Grid 220 kV iAiC iB A B C V/V Transformer xy z Step-down Transformer Step-down Transformer Catenary Line Section (y) Catenary Line Section (x) ixiLx irx iry iyiLy 25 kV ux 25 kV uy LxLyirx1iry1 DC-link C1 C2 irz1 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 89 switching device in the HB-RPC should be twice the value of the applied switching frequency in the FB-RPC and the TW-RPC topologies. Otherwise, a higher value of filter inductance is required for the HB-RPC because of the high current ripple as a result of the half-bridge schematic. On the other hand, using high switching frequency increases the switching losses of the switching devices. Simulation results, presented in [130], confirm that HB-RPC has a good performance regarding the power quality improvement in electrified railway systems, including NSC and harmonic contents compensation. Although the HB-RPC has some drawbacks as previously indicated but it provides one of the attempts to accomplish power quality improvement in the high-speed electrified railway systems [130]. 4.5.1 Control Algorithm The control algorithm of the HB-RPC is very similar to the ones of the FB-RPC and the TW-RPC topologies with some differences in the DC-link voltage control due to the mid-neutral point configuration [130]. This control algorithm is presented in Figure 4.22 after considering the equations introduced in the item 4.3. As was the case of the FB-RPC, there are only two hysteresis current tracking controllers. If the voltages of the DC-link capacitors are imbalanced, this will cause current distortion and waveforms fluctuation. Therefore, two PI controllers are important to achieve the voltage balancing between the capacitors. The first PI controller is used to compare the actual value of the DC-link voltage with its reference value (Vdc* = 4 kV). The output of this PI controller is synchronized by multiplying the output signal with the correspondent waveforms. The second PI controller is mainly used to achieve the balance between DC-link voltages. One of the capacitor DC-link voltage is considered as a reference for the second capacitor voltage. Then, the input signal of the second PI controller is ∆Vout = Vdc2 − Vdc1, while the output signal is subtracted or added to the compensation current references irx1 * and iry1 * [130] . When the control system is at the positive half-cycle of ir𝜎1 *, 𝜎 ∈ {x , y}, and in order to get an increase in the reference current value to |ir𝜎1 *+∆Vout |, capacitor C2 should discharge with more time in the control cycle to make the current increase, so the voltage of the capacitor C2 will decrease. In a similar procedure, when the control system is at the negative half-cycle of ir𝜎1 *, and in order to get a reduction in the reference current value to |ir𝜎1 *+∆Vout |, the capacitor C2 should charge with more time in the control cycle to make the current decrease, so the voltage of the capacitor C2 will increase, making ∆Vout reduce to zero [48]. This control attempts to overcome the high fluctuations in the DC voltage waveforms of the DC-link capacitors [130]. Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 90 Figure 4.22. Control strategy of the HB-RPC. 4.5.2 Simulation Results Parameters of the HB-RPC simulation model are presented in Table 4.12. The same load parameters presented in Table 4.6 are used in this simulation after considering a unitary load power factor for both load sections. It is worthwhile to mention how the HB-RPC requires a higher value of filter inductance, Lx and Ly, at the same switching frequency applied in the FB-RPC and the TW-RPC (using the same width of the hysteresis). However, it is possible to reduce the filter inductance value of the HB-RPC if the switching frequency is higher (by reducing the width of hysteresis). Higher switching frequency can increase the switching losses and the temperature of the power switching devices. Otherwise, the current waveforms of the three-phase PPS may contain higher harmonic contents. This is the main disadvantage of using the HB-RPC, besides the higher voltage stress across the power switching devices. Therefore, parameters of the HB-RPC should be carefully selected according to the aforementioned trade-off. Table 4.12. Parameters of the HB-RPC simulation model. Description Symbols Values Line-to-line PPS voltage UAB, UBC, UCA 220 kV Catenary voltage Ux, Uy 25 kV RPC filter inductance Lx, Ly 3 mH Catenary lumped impedance ZL 0.03 + j 0.12 Ω Capacitance of the DC-link capacitors C1=C2 50 mF Turns ratio of the V/V power transformer KV 220/25 Turns ratio of the step-down coupling transformer KD 25/1 Figure 4.23(a) presents the waveforms of the three-phase currents before compensation. These currents are imbalanced and contain harmonic contents and NSCs of currents. Figure 4.23(b) shows the three-phase currents after compensating the reactive power and shifting half of the active power × × × - × - iLx iLy KD KD ix2 iy2 irx iry * * × × × × × - × - irx1 * iry1 * irx1 iry1 × × Vdc Vdc1+Vdc2 × × - Vdc2 Vdc1 - × - PWM PI PI * sin(ωt-π/6) sin(ωt-π/2) × iLx × 2 / 3 2nd-LPF × iLy sin(ωt-π/2) ux sin(ωt-π/6) uy E-PLL sin(ωt-2π/3) -1 cos(ωt-π/2) = sin(ωt) cos(ωt-π/6) E-PLL Hys Con Hys Con ∆Vout Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 91 difference from the highly to the lightly loaded section (from section x to section y). In that case, the imbalance ratio is lower since the three-phase currents are balanced and have sinusoidal waveforms. Figure 4.23. HB-RPC simulation results: (a) Three-phase currents before compensation; (b) Three-phase currents after compensation (when both of the load sections are loaded). Figure 4.24(a) shows the load section currents of iLx and iLy, where the load section x has double the value of the load section y. The currents at the secondary windings of the V/V power transformer, ix and iy, are presented in Figure 4.24(b). After compensation, there is 120° out-of-phase between the currents of ix and iy. However, this phase difference was 60° before applying the compensation strategy. The results introduced in the Figure 4.24 are very similar to the ones presented in the Figure 4.11 (the case of the FB-RPC) and the Figure 4.19 (the case of the TW-RPC). Figure 4.24. HB-RPC simulation results: (a) Load section currents; (b) Currents at the secondary windings of the V/V power transformer after compensation (when both of the load sections are loaded). Figure 4.25(a) shows the total DC-link voltage of the HB-RPC. The PI controllers maintain the DC-link voltage around its reference value of Vdc* = 4 kV. The higher the value of the stored energy in the HB-RPC DC-link capacitors, the better the dynamic response of the HB-RPC compensator. This is important to match the same dynamic performance of the FB-RPC and the TW-RPC compensators 0.25 0.3 (a) Time (s) i (A) 0 (b) 40 -40 0.2 0.35 0.4 iAiBiC 20 -20 iAiBiC 0 40 -40 20 -20 60 Time (s) 0.25 0.3 (a) i (A) -200 (b) 200 0.2 0.35 0.4 0 ixiy 400 iLx iL -200 200 0 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 92 [140]. The voltage waveforms of vdc1 and vdc2, show that there is always one DC-link capacitor is charging while the other one is discharging, as has been previously explained in the item 4.5.1. The frequency of the total DC-link voltage ripples has a value close to 100 Hz. However, the voltage ripples across each capacitor have the frequency value close to 50 Hz, which implies using capacitors with high capacitance value, making the solution of the HB-RPC bulky and expensive. Figure 4.25(b) presents the compensation currents synthesized by the HB-RPC to compensate the NSCs and the current harmonics. These currents are calculated by using the control strategy presented in Figure 4.22. The results are very similar to the ones presented in Figure 4.12(b) and Figure 4.20(b), but the current ripples are higher which demands to use a higher switching frequency or a higher value of filter inductors. Figure 4.25. HB-RPC simulation results: (a) DC-link voltage; (b) Compensation currents (when both of the load sections are loaded). 4.6 Hybrid Co-Phase RPC Based on Full-Bridge Back-to-Back Two-Level Converter and Single-Phase Power Transformer The aforementioned RPC topologies are connected to the TPS with an inductive coupling structure, bearing in mind that most of the traction loads are inductive in nature, this will increase the voltage drop across the coupling structure, leading to a higher RPC DC-link voltage. However, this voltage can be decreased by using a capacitive-inductive coupled structure such as a series LC branch [88]. The LC passive filter is used to compensate harmonics, besides its main role in minimizing the DC-link voltage. In that regard, a developed system, called hybrid co-phase RPC (co-HRPC) eliminates the needs for a neutral section at the TPS side when using a single-phase traction substation as shown in the Figure 4.26. Table 4.13 presents the components quantities of the co-HRPC. 0.25 0.3 (a) Time (s) i (kA) (b) 3 0.2 0.35 0.4 c1 c2 c 1 irx1iry1irz1 0 4 -4 5 v(kV) 2 4 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 99 Figure 4.33. Phasors of the RPC system with a Scott power transformer: (a) Scott transformer connection points; (b) Phasors diagram of the primary windings; (c) Phasors diagram of the secondary windings. U 󰇗BD = 1 2UBC e-j90°; U 󰇗AB = UAB ej30°; |UAB| = |UBC| (4.16) U 󰇗AD = U 󰇗AB + U 󰇗BD = 3 2UAB (4.17) The compensation currents synthesized by the RPC based on the Scott power transformer, in that case, have no reactive components to compensate the NSC of currents, then, there is no need to inject reactive power for NSC compensation. However, a small value of reactive power can be injected by the RPC converter to compensate for the harmonics produced by the electric locomotives. As a result, the Scott power transformer helps to reduce the RPC power ratings for some RPC topologies (see item 4.10 for more information). This is due to the fact that in the RPC based on the Scott power transformer the reactive power compensation is not a vital factor and the compensator mainly shifts the active power difference from the highly loaded section to the lightly loaded one [48], [146]. Figure 4.34 shows the V/V power transformer phasors diagram. In this case, there is a phase shift of 60° between the load section currents of I󰇗Lx and I󰇗Ly . The two-phase secondary windings voltages, U 󰇗x and U 󰇗y are in phase with the primary windings voltages of U 󰇗AC and U 󰇗BC, respectively. The secondary windings voltages of the V/V power transformer are expressed in (4.18). As a result, in the V/V power transformer, the line-to-line or the voltage between the overhead catenary lines has an RMS voltage equals to the phase-to-neutral voltage. However, in the Scott power transformer, the line-to-line voltage between the overhead catenary lines has a higher RMS value than the traction feeder voltages, which results in a higher DC-link voltage of the RPC. This is only correct in the RPC topologies that are designed with respect to the line-to-line voltage (e.g., TW-RPC, HB-RPC) [150]. The compensation currents synthesized by the RPC based on the V/V power transformer, I󰇗rx and I󰇗ry have both of the active and the reactive current components for the compensation of current harmonics and the NSC. It is worth mentioning the RPC rated power, in this case, is higher since the converter mainly shifts the active power and compensates the reactive power [150]. 30º UC UA UB UBC UAB D UDC UBD UBD UAD (c) (b) UAD UBC Ux Uy ILx ILy Irx Iy2 IB2 Ix2 IC2 Iry IA2 Iz2 (a) Ux Uy y A B C UAD UBC D x UA Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 100 Figure 4.34. Phasors of the RPC system with a V/V power transformer: (a) V/V transformer connection points; (b) Phasors diagram of the primary windings; (c) Phasors diagram of the secondary windings. U 󰇗x = U 󰇗AC KV e-j30° , U 󰇗y = U 󰇗BC KV e-j90° (4.18) The DC-link voltage value when using the Scott power transformer is normally lower in the RPC topologies that are designed with respect to the phase-to-neutral voltage (e.g., FB-RPC). This is an extra advantage of using the RPC based on the Scott power transformer. The justification is presented in Figure 4.35, where the phasors diagram of the RPC output voltages are presented in both of the cases when using the V/V and the Scott power transformers. Figure 4.35(a) presents the equivalent circuit of the single-phase RPC load section converter [150]. The voltage across the filter inductors, Lx and Ly, leads the compensation current synthesized by the RPC, then, when using the Scott power transformer, the RPC phase-to-neutral output voltages, (U 󰇗xo)Scott and (U 󰇗yo)Scott, have almost a similar magnitude since the reactive power compensation is not a critical factor, as shown in Figure 4.35(c). On the other hand, as long as the reactive power compensation is a vital factor when using the V/V power transformer, the phase x output voltage, (U 󰇗xo)V/V, is higher than the phase y output voltage, (U 󰇗yo)V/V, and slightly higher than the phase-to-neutral output voltages of the RPC based on the Scott power transformer (U 󰇗xo)Scott and (U 󰇗yo)Scott. In the FB-RPC, the DC-link voltage should at least have the peak value of the phase voltages, uxo and uyo. Figure 4.35. RPC output voltages: (a) Output equivalent circuit of the RPC; (b) Phasors of the RPC output voltages in V/V power transformer; (c) Phasors of the RPC output voltages in Scott power transformer. UAC UAC UC UA UB UBC 30º ILx Irx Iry ILy Iy2 Ix2 Iz2 30º (c) UBC Ux Uy IB2 IC2IA2 (b) (a) Ux Uy y A C B UAC UBC x UA Uy1 ULx Ux1 (c) Uxo Irx1 Iry1 ULy Uyo Uy1 Ux1 (b) Uxo Irx1 Iry1 ULy Uyo ULx (a) uy uyo uy1 Ly iry1 ux1 uxo Lx irx1 ux Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 101 As a result, the DC-link voltage of the FB-RPC based on a V/V transformer is normally higher than the DC-link voltage when using the Scott power transformer [150]. The previous conclusion is not correct in the RPC topologies that are designed with respect to the line-to-line voltage (e.g., TW-RPC, HB-RPC). In that case, the DC-link voltage value when using the Scott power transformer will be higher since the line-to-line voltage in the Scott power transformer is higher than the line-to-line voltage in the V/V power transformer (The line-to-line voltage is equal to the phase-to-neutral voltage in the V/V power transformer, but it is higher than the phase-to-neutral voltage in the Scott power transformer). 4.7.1 Control Algorithm The control algorithm of the RPC based on a Scott power transformer is presented in Figure 4.36. Compared to the RPC based on a V/V power transformer control strategy, the main difference is in the phase angles of the correspondent Sine waves. As shown in Figure 4.37, there are θx = −30º and θy = −90º in the RPC based on the V/V power transformer, so there is θ = 60º out-of-phase between the load section voltages. However, there are θx = 0º and θy = −90º in the RPC based on the Scott power transformer, so there is θ = 90º out-of-phase between the load section voltages. Figure 4.36. Control strategy of the RPC system with a Scott power transformer. Figure 4.37. Load section voltages: (a) Using the V/V power transformer; (b) Using the Scott power transformer. × × × iLx × × × × × × × × sin(ωt) × sin(ωt-π/2) KD KD PI Vdc Vdc ix2irx iry * * irx1 * iry1 * irx1 iry1 PWM * × iLy iy2 - -- - - IP × iLx × 2 / 3 2nd-LPF × iLy uxsin(ωt) E-PLL uyE-PLL sin(ωt) sin(ωt-π/2) sin(ωt-π/2) Hys Con Hys Con 0.25 0.3 (a) Time (s) u (kV) 0 (b) 40 -40 0.2 0.35 0.4 ux 20 -20 uy uxuy θ 60 θ 90 0 40 -40 20 -20 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 102 Table 4.15 shows a technical comparison between the Scott and the V/V power transformers [151]. The Scott power transformer has more complex structure than the V/V. Moreover, V/V power transformer has a higher material utilization factor, which signifies a smaller transformer volume at the same nominal power. Consequently, the manufacturing costs of the Scott power transformer are higher. Nevertheless, and since the traction power system based on the Scott power transformer exchanges a lower amount of reactive power with the RPC, the Scott power transformer can effectively reduce the costs of the RPC or the power compensator. Table 4.15. Technical comparison between Scott and V/V power transformers. Compared Item V/V Transformer Scott Transformer Structure Simple Complex Material utilization factor High (94%) Low (81.6%) Requirement for reactive power compensation Higher Lower Manufacturing costs Lower Higher 4.7.2 Simulation Results A comparative study between the RPC based on a V/V power transformer and the RPC based on a Scott power transformer has been established using the PSIM V.9.1 software. PSIM advantages for Power Electronics simulation are presented in [152]. Figure 4.38 shows the performance of each traction power transformer (V/V and Scott power transformer) when both of the load sections, x and y, are equally loaded (each load section is loaded with 4.8 MW and the PPS line-to-line voltage is 220 kV). During the simulation, the RPC system is turned-off and there are no compensation currents injected into the PPS. Mathematically, PSC and NSC in a system with ABC rotation are defined as in (4.19) and (4.20), where these equations are implemented in the simulation model by using C code. PSC 󰇗 = IA 󰇗 + a IB 󰇗 + a2 IC 󰇗 3; a = 1∠120° (4.19) NSC 󰇗 = IA 󰇗 + a2 IB 󰇗 + a IC 󰇗 3; a = 1∠120° (4.20) Figure 4.38(a) presents the PPS currents without compensation when using the V/V power transformer. The PSC and the NSC of currents are calculated at the fundamental frequency of 50 Hz, as shown in Figure 4.38(b). In this case, the NSC of currents is almost 50% of the PSC. On the other hand, the simulation results when using the Scott power transformer show a good capability to present balanced PPS currents without compensation, as shown in Figure 4.38(c). The NSC of currents, on these terms, has a value close to zero, as shown in Figure 4.38(d). In addition, the PPS currents only contain harmonic contents without NSC. Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 103 Figure 4.38. Public grid currents, PSC and NSC when the load sections are equally loaded before compensation: (a), (b) Using the V/V power transformer; (c), (d) Using the Scott power transformer. Consequently, Scott power transformer shows a power quality improvement capability better than the V/V power transformer, even when both of the load sections are unequally loaded (section x is loaded with 4.8 MW and section y is loaded with 2.4 MW), as shown in the Figure 4.39(c) and Figure 4.39(d). In that regard, the NSC of currents at the fundamental frequency of 50 Hz when using the Scott power transformer is lower than the NSC when using the V/V power transformer. The value of NSC of currents has a value more than 50% of the PSC when using the V/V power transformer, as shown in Figure 4.39(b), while its value is lower than 40% of the PSC when using the Scott power transformer, as shown in Figure 4.39(d). As a result, and at similar loading conditions, any of the active power compensators (e.g., RPC and SVC) used to compensate the currents imbalance should have higher power ratings in the V/V traction system [150]. Figure 4.39. Public grid currents, PSC and NSC when load sections are unequally loaded before compensation: (a), (b) Using the V/V power transformer; (c), (d) Using the Scott Power transformer. Simulation results of the RPC based on a Scott power transformer, after compensation, are shown in Figure 4.40. The same load parameters presented in Table 4.6 and Table 4.7 are used in this 0.25 0.3 (a) Time (s) i (A) 0 (c) -50 0 0.2 0.35 0.4 iAiBiC 20 iAiBiC 50 (b) (d) PSC NSC PSC NSC 0 -50 50 0 20 0.25 0.3 Time (s) i (A) 0 -50 0 0.2 0.35 0.4 20 50 (a) (c) iAiBiC iAiBiC (b) (d) PSC NSC PSC 0 -50 50 0 20 NSC Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 104 simulation after considering a unitary load power factor for both load sections. The RPC is turned on after 0.1 s to improve the public grid power quality when both load sections are unequally loaded (section x active power is 4.8 MW; section y active power is 2.4 MW). Before the indicated time (0.1 s), the public grid currents are imbalanced and the NSC of currents has a high value (almost 40% of the PSC). After 0.1 s, the RPC is turned on to compensate for both of harmonics and NSC of currents. Consequently, and as shown in Figure 4.40(c), the NSC of currents at the fundamental frequency 50 Hz is reduced to be almost zero, and the PSC of currents is increased. Figure 4.40(b) shows the secondary windings currents of the Scott power transformer. These currents are imbalanced since the primary and the secondary end windings points of the Scott power transformer are different. This imbalance does not appear when using the V/V power transformer since it has common end windings points on the primary and the secondary windings [150]. Figure 4.40. RPC based on a Scott transformer (RPC is turned on after 0.1 s): (a) Three-phase grid currents; (b) Currents at the secondary windings of the Scott transformer; (c) PSC and NSC of three-phase grid currents. Figure 4.41(a) shows the load section currents that are considered as harmonic sources in the power system. During the simulation, the load currents have a constant RMS value. Figure 4.41(b) presents the compensation currents synthesized by the RPC converter. Since the load sections are unequally loaded (load section x has double the value of load section y), section x converter injects an active power component, which is opposite in direction to load section x active power. On the other hand, section y converter injects an active power component which is similar in direction to the load section y active power. The compensation current of the section x converter, irx1, has 180º out-of-phase with the load section x current, iLx. In addition, the compensation current of the section y converter, iry1, is in phase with the load section y current, iLy. This is also confirmed in the phasors diagram presented in the Figure 4.33(c) [150]. 0.1 0.15 Time (s) i (A) 0 -40 0.05 0.25 0.3 400 40 -400 0 10 20 0 (a) iAiBiC (b) (c) ixiy PSC iz NSC 0.2 30 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 105 Figure 4.41. RPC based on a Scott transformer (RPC is turned on after 0.1 s): (a) Load section currents; (b) Compensation currents synthesized by the RPC; (c) DC-link voltage. As has mentioned before, using the Scott power transformer helps to decrease the DC-link voltage in the RPC topologies that are designed with regard to phase-to-neutral voltage (e.g., FB-RPC). Consequently, and as shown in Figure 4.41(c), the initial value of the DC-link voltage is 1.5 kV before compensation. In the steady-state and after introducing the RPC compensation currents, the DC-link voltage reaches a value close to 1.8 kV. The DC-link voltage in the RPC based on the V/V power transformer, at the same loading conditions, reached a value close to 2 kV. In both of the cases, the DC-link voltage is supposed to be controlled in closed-loop with a constant reference. This reference has a lower value in the RPC topologies based on the Scott power transformer that are designed with regard to phase-to-neutral voltage (e.g., FB-RPC). On the other hand, the value of this reference is higher in the RPC topologies based on the Scott power transformer that are designed with regard to line-to-line voltage (e.g., TW-RPC, HB-RPC). 4.8 Modular RPC Based on Full-Bridge Back-to-Back Converters and V/V Power Transformer Modular rail power conditioner (MRPC), as shown in Figure 4.42, is mainly used to decrease the current stress of the switching devices, in which each RPC module can be considered as an independent single-phase back-to-back full-bridge converter. Using the MRPC topology is more common when a single RPC cannot withstand the high ratings of the compensation currents [146], [153]. The MRPC improves the power grid reliability as one or more of the RPC modules is out of service, the MRPC system can present power quality improvement, allowing full utilization of the installing capacity of the traction power transformer. On the other hand, the MRPC solution and its control strategy with renewable energy access is presented in [154]. The proposed solution in [154] has a better integration of the distributed renewable 0.1 0.15 Time (s) i (kA) 0 -200 0.05 0.25 0.3 4 200 -4 0 1 (a) iLx iLy (b) (c) irx1iry1 0.2 2 c 1.5 i (A) v(kV) Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 106 energy sources along the railway, besides the power quality control of the railway system. In addition, authors in [155] present MRPC topology in a co-phase traction power supply system, in which separated DC-links of the back-to-back power converters are used instead of a single DC-link in between, and therefore, the two converters of the RPC can operate independently with a relatively simple controlling method. However and due to the separated DC-links, the main drawback is that the active power cannot be exchanged between the RPC power converters, so the compensation performance is just satisfactory for traction loads at nearly unitary power factor [155]. By considering k is the number of the RPC modules, Table 4.16 presents the components quantities of the MRPC. The coupling step-down transformer for each load section converter is assumed as an ideal transformer with multiple secondary windings. The turns ratio of the coupling step-down transformer, KD, is given in the (4.21), where Ux and Uy, are the primary windings voltages. The voltages, Uxn and Uyn, are the phase n voltages of the secondary windings of the step-down coupling transformer. KD = Ux / Uxn = Uy / Uyn (4.21) Table 4.16. Components quantities of the MRPC. Number of IGBTs Number of capacitors Number of inductors Isolation/coupling transformer 8*k k 2*k 2 Figure 4.42. MRPC system with a V/V power transformer. 4.8.1 Control Algorithm Each RPC module can be considered as an independent FB-RPC unit. Consequently, the RPC module compensates a part of the reactive power and shifts a part of the active power difference between the load sections. The RPC module has its independent control in terms of producing the compensation current references and the DC-link voltage control. Therefore, it is not appropriate to use a common Public Grid 220 kV iAiC iB A B C V/V Transformer xy z Catenary Line Section (y) Catenary Line Section (x) ixiLx irx iry iyiLy 25 kV ux 25 kV uy Step-down Transformer DC-link Lxa Lxn Lya Lyn irxa irxn irya iryn Step-down Transformer Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 107 single DC-link for all the RPC modules. Otherwise, this will result in a bulky and expensive DC-link capacitor, besides the necessary coordinative control between the RPC modules in this case. Control algorithm of MRPC system is very similar to the FB-RPC control algorithm presented in Figure 4.8. However, the main difference is that each RPC module should contribute with a part of the total compensation currents, irx and iry. In addition, each RPC module control should guarantee a constant DC-link capacitor voltage. In this context, an averaging control is important to ensure equal DC-link voltage values and similar loading conditions for the RPC modules. In other words, this control improves the MRPC performance to validate the same compensation power contribution among the RPC modules. For that purpose, additional PI controller is used in this topology to perform the averaging control, then equalizing between all the DC-link voltages of the RPC modules. The output signals of the averaging controller, Ax and Ay, are multiplied with the correspondent Sine waves. Finally, these signals are added to the compensation current references for each RPC module. Figure 4.43 presents the control strategy of the MRPC system. Hysteresis controllers are used to track the compensation current references. Figure 4.43. Control strategy of the MRPC. 4.8.2 Simulation Results Simulation parameters of the MRPC model and the FB-RPC are presented in Table 4.17 to show the difference in parameters selection between the MRPC and the FB-RPC. Since the power provided by each RPC module is always lower than the total power provided by the FB-RPC system, the required DC-link capacitor for an RPC module is lower than the one for the FB-RPC. In addition, the IGBTs current stress is reduced by 1/k. For this case study, it has been considered four RPC modules (k = 4) in the simulation model of the MRPC. Consequently, the currents flowing through the RPC modules or the compensation currents synthesized by the RPC modules are given in (4.22). Irxn = Irx (KD / k), Iryn = Iry (KD / k) ; n = a → d ; k = 4 (4.22) × * Vdc Vdcn / k ∑× sin(ωt-π/6) sin(ωt-π/2) × PI Ax Ay - × iLx × 2 / 3 × × × iLx × × × × × - × - KD / k 2nd-LPF ix2irx iry * * irxn * iryn * irxn iryn PWM × iLy sin(ωt-π/2) × iLy iy2KD / k n = a → ux × × × × Ax Ay - - - - sin(ωt-π/6) uy E-PLL sin(ωt-2π/3) -1 cos(ωt-π/2) = sin(ωt) cos(ωt-π/6) E-PLL × × * Vdc × sin(ωt-π/6) sin(ωt-π/2) PI - Vdcn × Hys Con Hys Con Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 108 Table 4.17. Simulation parameters for the FB-RPC model and the MRPC model. Description Symbols FB-RPC MRPC Line-to-line PPS voltage UAB, UBC, UCA 220 kV 220 kV Catenary voltage Ux, Uy 25 kV 25 kV RPC filter inductance Lx, Ly 2.2 mH 2 mH Catenary lumped impedance ZL 0.05 + j 0.15 Ω 0.05 + j 0.15 Ω Section x load power Px 4.8 MW 4.8 MW Section y load power Py 2.4 MW 2.4 MW Capacitance of the DC-link capacitor Cdc 80 mF 20 mF DC-link Voltage Vdc 2 kV 2 kV IGBT Current Stress − 1 p.u. 1/k p.u. IGBT Voltage Stress − 1 p.u. 1 p.u. Simulation results of the MRPC are presented in Figure 4.44. The three-phase currents after compensation are sinusoidal, and similar to the previous case studies when both of the load sections were unequally loaded, as shown in the Figure 4.44(a). The three-phase currents are balanced without NSCs and with lower harmonic contents. Figure 4.44(b) presents the load section currents at the loading values presented in Table 4.17. These currents cause harmonic contents and imbalance in the currents of the three-phase power grid. Figure 4.44(c) shows the compensation currents synthesized by one RPC module. In comparison to the compensation currents presented in Figure 4.12(b), Figure 4.20(b) and Figure 4.25(b) for the FB-RPC, TW-RPC and HB-RPC, respectively, each RPC module in the MRPC system only carries a portion of the total compensation current. Figure 4.44(d) shows the DC-link voltage for each RPC module. The voltage stress value is similar to the one in the FB-RPC, and the TW-RPC topologies, but the current stress is quite lower. Figure 4.44. MRPC simulation results: (a) Three-phase currents after compensation; (b) Load section currents; (c) Compensation currents synthesized by an RPC module; (d) DC-link voltage for one RPC module. 0.25 0.3 Time (s) i (A) 0 -50 0.2 0.35 0.4 50 0 -1 1 1.6 2 2.4 iLx iLy iAiBiC irxn iryn cn 200 -200 0 i (A) i (kA) v (kV) (a) (c) (b) (d) Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 115 summing the arm currents, irσu and irσl, where σ ∈ {x, y}, in the upper and the lower arms that belong to the same MMC leg. The output voltage command of the MMC leg averaging voltage control, nominated as Aσ, will be added to a voltage command generation for each SM, as presented in Figure 4.51 [100]. Circulating current, icir, presented in (4.32), is considered as a part of the MMC arm current. However, it consists of a DC current component, AC current component that has double of the fundamental frequency (second harmonic order) and other harmonic orders. This current circulates in the MMC loops and it does not contribute to the compensation currents synthesized by the FB-MMC4 RPC, irx and iry. Therefore, it should be controlled as it causes extra losses and may increase the ratings of the power components. The DC current component is responsible for the active power flow between the DC-link and the AC side. The second harmonic content, that has double of the fundamental frequency, is mainly responsible for the power exchange between the MMC legs [160]. irσl = irσ + irσu ⟹ icir = (irσu + irσl) / 2 ; σ ∈ {x, y} (4.32) The second control block, presented in Figure 4.50(b), indicates the MMC arm averaging voltage control [161]. A proportional controller, KP1, is responsible to adjust the SM voltages in one arm to the reference voltage value, Varm* . The third control block in Figure 4.50(c) represents the MMC SM individual voltage control. A proportional controller, KP2, is responsible to adjust each SM voltage to its reference voltage value VSM* . The proportional controllers, KP1 and KP2, act only dynamically in the balancing process in every switching period. The output signal of KP1 and KP2, is multiplied by +1 when the current direction of irσu and irσl, where σ ∈ {x, y}, is to charge the SM capacitor, or it can be multiplied by −1 when the current direction is to discharge the SM capacitor. Figure 4.50. SM capacitors voltage control of the FB-MMC4 RPC: (a) MMC leg averaging voltage control; (b) MMC arm averaging voltage control; (c) MMC SM individual voltage control. The final signals of MMC leg averaging control, Aσ, MMC arm averaging control, Vrσu and Vrσl, MMC SM individual voltage control, Viσu and Viσl, where σ ∈ {x, y}, are added to the voltage reference signals, urσ*, to generate a voltage command (modulating signal) for each SM, which is to be compared × * Vsm Vsm PI × + × irσl, irσnl 1/2 PI Aσ irσu, irσnu σ ∈ x, y (a) × * Vsm Vsmiu KP2 ±1 +1: irσu, irσnu > 0 -1: irσu, irσnu < 0 × * Vsm Vsmil KP2 ±1 +1: irσl, irσnl > 0 -1: irσl, irσnl < 0 Viσu Viσl (c) × * Varm Varmu KP1 ±1 +1: irσu, irσnu > 0 -1: irσu, irσnu < 0 × * Varm Varml KP1 ±1 +1: irσl, irσnl > 0 -1: irσl, irσnl < 0 Vrσu Vrσl (b) --- - - - Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 116 with the phase-shifted PWM triangular carriers, as shown in Figure 4.51. The Phase-shifted PWM can be easily applied after shifting the carriers phase angle in the same MMC arm, consecutively, and with a fixed angle of 360/(N−1). It should be declared that, all of the above-mentioned explanation is applicable when using the FB-MMC4 RPC with a V/V power transformer. When using the Scott power transformer, several aspects should be considered in the control blocks, namely in the correspondent Sine waves that have been used to correct the control signals (more details about this case are presented in the item 4.7 and the item 4.10). Figure 4.51. Voltage command generation of each SM applied to a phase-shifted PWM. 4.9.1.2. Simulation Results Simulation model is built by using the PSIM V.9.1 software for Power Electronics simulation to validate the system of FB-MMC4 RPC and the associated control algorithm. The simulation model consists of 7-levels (N = 7) MMC with a total number of 48 SMs. The main parameters of the simulation model are shown in the Table 4.19. In this framework, the same load parameters presented in Table 4.6 are used in this simulation after considering a unitary load power factor for both load sections. Therefore, the locomotives can be modeled as a resistive load connected in parallel with an uncontrolled full-bridge rectifier on the secondary windings of the locomotive transformer [57]. This full-bridge rectifier is considered as a harmonic source, where the full locomotive model is shown in Figure 4.9. The simulation considers that both of the load sections are unequally loaded. Then, in this study, load section x is loaded with a value of 4.8 MW and load section y is loaded with a value of 2.4 MW, as presented in Table 4.6. The three-phase currents before applying the compensation strategy are presented in Figure 4.52(a) and they are imbalanced currents with high harmonic contents. Figure 4.52(b) shows the same currents after compensation. The three-phase currents are now balanced without NSC and with lower harmonic contents, as shown in the phasors diagram presented in Figure 4.6(c). Figure 4.52(c) shows the currents on the secondary windings of the V/V power transformer after compensation. These currents have 120° out-of-phase and sinusoidal waveforms (after compensation) instead of 60° out-of-phase (before compensation). It is noteworthy to mention, the used power transformers in the simulation model are considered as ideal ones without considering leakage flux, copper losses, and core losses. In other words, the ideal transformer gives an output power exactly equal to its input power. Phase Shifted PWM + × Aσ σ ∈ x, y + + Viσ (l,u) urσ (l,u) * 1 N-1 + + Vrσ (l,u) Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 117 Table 4.19. Parameters of the FB-MMC4 RPC simulation model. Description Symbols Values Line-to-line PPS voltage UAB, UBC, UCA 220 kV Catenary voltage Ux, Uy 25 kV RPC filter inductance Lσu, Lσnu, Lσl, Lσnl, 3 mH Catenary lumped impedance ZL 0.05 + j 0.15 Ω SM capacitor CSM 600 µF SM Switching frequency fisw 3 kHz Equivalent switching frequency fsw 18 kHz SM voltage VSM 8.3 kV DC-link voltage Vdc 50 kV Number of SMs in each arm N−1 6 Figure 4.52. FB-MMC4 RPC simulation results: (a) Three-phase currents before compensation; (b) Three-phase currents after compensation; (c) Currents at the secondary windings of the V/V power transformer after compensation. The load section currents are presented in Figure 4.53(a), where, iLx has double the value of iLy. Both currents are considered as harmonic sources because of using uncontrolled rectifiers in the locomotive model. The compensation currents synthesized by the RPC are demonstrated in Figure 4.53(b). These currents are calculated by using the control strategy presented in Figure 4.48. The results are very similar to the ones presented in Figure 4.12(b), but the RMS value of currents, in this case, are lower due to the abandon of the step-down coupling transformers, but isolation transformers with turns ratio 1/1 are used to interface the converter with the single-phase TPS. The compensation currents, irx and iry are shared among the upper and the lower arms of the MMC, where, irxu is the upper arm current of the positive leg of the section x converter, irxl is the lower arm current of the positive leg of the section x converter, iryu is the upper arm current of the positive leg of the section y converter, iryl is the lower arm current of the positive leg of the section y converter. (c) (b) (a) Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 118 In addition, the arm currents presented in the Figure 4.54 contain three current components, as shown the frequency spectrum presented in Figure 4.55. These three components are: the fundamental frequency current component of 50 Hz that contributes to create the compensation currents of irx and iry, the double fundamental frequency current component of 100 Hz (second harmonic current) that circulates between the MMC legs, the DC current component that has the responsibility of charging / discharging of the SM capacitors. The DC current component is also responsible for the DC/AC power transfer. This component has a value close to 18 A, as shown in the Figure 4.55. Figure 4.53. FB-MMC4 RPC simulation results: (a) Load section currents; (b) Compensation currents. Figure 4.54. FB-MMC4 RPC simulation results: (a) Arm currents of the positive leg of the section x converter; (b) Arm currents of the positive leg of the section y converter. It should be highlighted that, the circulating currents normally do not affect the AC output of the MMC, since these currents only circulate between the MMC phases. In addition, circulating currents are normally generated because of the inner voltage differences between MMC arms, and they increase the total RMS values of the MMC arm currents, resulting in higher power losses. One of the main applied techniques to suppress the circulating currents in the MMC is by using an inner filter (inductor) between the MMC arms. The effect of the circulating currents can be reduced by adjusting the Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 119 circulating current controller (the second PI controller presented in the Figure 4.50(a)). Figure 4.56 shows the simulation results of the FB-MMC4 RPC SM voltages. The main aim of the FB-MMC4 RPC control is to maintain the SMs voltages at their reference value, which is in this case a value close to 8.3 kV, as presented in Figure 4.56. These results confirm the effectiveness of the SM capacitors voltage balancing control presented in Figure 4.50. The SM voltage value of 8.3 kV is not reasonable for today’s IGBTs, where the maximum voltage blocking of the commercially available IGBTs is close to 6.5 kV [112], consequently, the MMC should be designed with a higher level to reduce the SM voltage or two IGBTs connected in series have to be used. Figure 4.55. FB-MMC4 RPC frequency spectrum: (a) Compensation currents; (b) Upper and lower arm currents of converter x; (c) Upper and lower arm currents of converter y. Figure 4.56. FB-MMC4 RPC SM voltages: (a) Positive leg of converter x; (b) Negative leg of converter x; (c) Positive leg of converter y; (d) Negative leg of converter y. 4.9.2 RPC Based on Two-Phase Three-Wire Indirect Modular Multilevel Converter This RPC topology is quite different from the FB-MMC4 RPC, and it can be considered as a three-phase MMC operating to compensate reactive power, NSC of currents, harmonic contents and balance the 0 50 100 150 200 0 25 50 75 100 0 25 50 75 100 050 100 150 200 250 300 𝑥 f (Hz) (A) (a) (b) (c) ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 𝑥 𝑥 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 ( ) i (A) 0 ( ) 50 iAiBiC -50 iB 0 -100 100 0 50 -50 100 100 ( ) iC iA i i 𝑥 0.22 0.24 Time (s) 0.2 0.26 0.28 0.3 2 Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 120 active power. Since the RPC based on two-phase three-wire indirect modular multilevel converter (TW-MMC3 RPC) does not have the potential of an inherent DC-link between the converters as was the case of the FB-MMC4 RPC, it is mandatory to add a DC-link capacitor to ensure a good performance, and to reduce the size of the SM capacitors. On the other hand, the TW-MMC3 RPC solution does not require to install step-down coupling transformers or isolation transformers to interface the converter with the single-phase TPS. The TW-MMC3 RPC based on a V/V power transformer is shown in Figure 4.57. Each half-bridge SM contains two switching devices and one capacitor, which can be inserted or bypassed. The leg inductors are considered as the MMC inner filters, which are important to adjust the circulating current flow between the MMC phases [150]. By using the TW-MMC3 RPC instead of the FB-MMC4 RPC, it is possible to reach the same output voltage level using only 75% of the FB-MMC4 RPC hardware. In other words, the TW-MMC3 RPC topology saves 25% of the SMs required in the FB-MMC4 RPC topology, and at the same RPC power ratings. In addition, Table 4.20 presents the components quantities of the TW-MMC3 RPC [150]. Table 4.20. Components quantities of the TW-MMC3 RPC. Number of IGBTs Number of capacitors Number of inductors Isolation/coupling transformer 12(N−1) 6(N−1)+1 6 0 Figure 4.57. TW-MMC3 RPC system with a V/V power transformer. The load section instantaneous voltages, ux and uy, are given in (4.23). In the V/V power transformer, there is 60° out-of-phase between the phasors of the load section voltages. However, in the Scott power transformer, this phase shift is equal to 90°, as demonstrated in the phasors diagram presented Public Power Grid Section (x) Section (y) A B C V/V Power Transformer Single Submodule SM ixiLx iy iLy izux uy iry irx SM SM SM SM SM SM SM SM SM SM SM SM Lxu Lxl Lyu Lyl Lzu Lzl irzu irxl irzl irxu iryu Vdc iryl irz Leg (z) Converter Leg (y) Converter Leg (x) Converter iA iB iC Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 121 in Figure 4.35(b) and Figure 4.35(c). Figure 4.58 shows the AC equivalent circuit of the TW-MMC3 RPC, where the zero-sequence voltage is given by (4.33), and the voltages of uxn, uyn and uzn are the phase-to-neutral voltages [146]. uno = (ux + uy) / 3 (4.33) Figure 4.58. AC equivalent circuit of the TW-MMC3 RPC system. On the other hand, Figure 4.59(a) and Figure 4.59(b) show the output equivalent circuit of single-phase TW-MMC3 RPC converter, and the associated phasors when using the V/V power transformer. Consequently, the load sections instantaneous voltages are calculated as in (4.34), where, uno is the zero-sequence voltage [146]. Figure 4.59. TW-MMC3 RPC: (a) Output equivalent circuit; (b) Phasors of output voltages when using V/V power transformer. ux = uxo + uLx = uxn + uno uy = uyo + uLy = uyn + uno 0 = uzo + uLz = uzn + uno (4.34) The instantaneous compensation currents synthesized by the TW-MMC3 RPC are given in (4.25) and (4.35). The active and the reactive compensation currents components of the phase x (RMS value) are Irxa and Irxr, respectively. In a similar way, the active and the reactive compensation currents components of the phase y (RMS value) are Irya and Iryr, respectively. The active and the reactive Leg (z) Converter Leg (y) Converter Leg (x) Converter Lxu Lxl Lyu Lyl Lzu Lzl irzu irxl irzl irxu Vdc iryu iryl uzn uxu uxl uzu uyu uzl uyl iry irx uxn uyn uno uno uno o irz (a) (uxl-uxu) Lx irx ux Ly iry uy /2 /2 (uyl-uyu) /2 /2 (uzl-uzu) Lz irz /2 /2 Uy Ux (b) Uxo Irx Iry ULy Uyo ULx Irz Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 122 compensation currents components are presented in (4.3) and (4.7), respectively. From (4.25), the equation that gives the instantaneous values of the compensation currents, it is possible to conclude the voltage drop across the filtering inductance of leg x converter, which can be expressed as in (4.26) after supposing the filtering inductance of the MMC inner filter is 0.1 p.u, namely, Lx = 0.1 Ux / (Irx*ω), where Lx, is the equivalent inductance of the leg x converter, and ω is the angular frequency [146]. irz = – (irx + iry) (4.35) Substituting (4.26) into (4.34), phase x output voltage of the TW-MMC3 RPC can be obtained as in (4.27). The same analysis is applicable for the leg y converter, as presented in (4.28) and (4.29) [146]. The voltage drop across the filtering inductance of leg z converter can be expressed as in (4.36), after supposing that the inductance of the MMC inner filter is 0.1 p.u, namely, Lz = 0.1 Uσ / (Irz*ω), where Lz, is the equivalent inductance of the leg z converter, ω is the angular frequency, and Uσ is the RMS value of the feeder voltages Ux or Uy [146]. uLz = 𝐿𝑧 irz t = 2 U σ 10 √Irza2 + Irzr2 [Irxa cos (ωt + θx) + Irxr sin (ωt + θx) – Irya cos (ωt +θy) – Iryr sin (ωt +θy)] (4.36) Substituting (4.36) into (4.34), the voltage of phase z can be obtained as in (4.37) [146]. uzo= – 2 U σ 10 √Irza2 + Irzr2 [Irxa cos(ωt + θx) + Irxr sin (ωt + θx) – Irya cos (ωt +θy) – Iryr sin (ωt +θy)] (4.37) The peak voltage values of uxo, uyo and uzo can be obtained from (4.27), (4.29) and (4.37), respectively. These are the output voltages of the TW-MMC3 RPC after taking into consideration the voltage drop across the filter inductors. With regard to the three-phase structure of the TW-MMC3 RPC, the DC-link voltage should not be less than the magnitude of the line-to-line voltage. Since the magnitude of the line-to-line voltage in the V/V power transformer is equal to the amplitude of the phase-to-neutral voltage (this is due to the 60° out-of-phase between the phase voltages, ux and uy, as shown in Figure 4.34), then the condition presented in (4.38) is only applicable when using the TW-MMC3 RPC in the V/V power transformer [146], [150]. |Uxo|, |Uyo| > |Uzo| V c ≥ 2 Uσo ; σ ∈ {x, y} for the V/V power transformer (4.38) However, the condition in (4.38) is not applicable when using the Scott power transformer since the magnitude of the line-to-line voltage in the Scott power transformer is higher than the amplitude of the Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 123 phase-to-neutral voltage (this is due to the 90° out-of-phase between the phase voltages, ux and uy, as shown in Figure 4.33), then the condition presented in (4.39) is applicable when using the TW-MMC3 RPC in the Scott power transformer. Therefore, the TW-MMC3 RPC is the best solution to be used with the V/V power transformer since it demands lower number of hardware power components compared to the FB-MMC4 RPC. The number of half-bridge SMs in one MMC arm should be equal to Vdc/Vsm. Further characteristics of the TW-MMC3 RPC are presented in the item 4.10, Table 4.25, Table 4.26 and Table 4.27 [146]. The TW-MMC3 RPC has the same parameters design of the FB-MMC4 RPC when using the V/V power transformer. On the other hand, in the Scott power transformer, the TW-MMC3 RPC should have higher SM voltages and a higher main DC-link voltage not less than the magnitude of the line-to-line voltage. The previous conditions obligate to use power switching devices with a higher blocking voltage, which makes this solution expensive and bulky. Therefore, it is not recommended to use the TW-MMC3 RPC topology when using the Scott power transformer instead of the V/V power transformer [146], [150]. |Uxo| , |Uyo| > |Uzo| V c ≥ 2 Uσo ; σ ∈ {x, y} for the Scott power transformer (4.39) 4.9.2.1. Control Algorithm Control algorithm of the TW-MMC3 RPC should guarantee the correct calculations of the compensation currents to achieve power quality improvement, maintain a constant MMC DC-link voltage, and maintain constant SM voltages. The malfunctioning of one or more of the aforementioned conditions may cause severe impacts on the safety and the efficient operation of MMC. In that regard, control algorithm is proposed in this item to obtain a robust performance of the TW-MMC3 RPC. Calculations of the reference currents, irx* and iry*, can be achieved as presented in the Figure 4.48. However, an additional reference voltage signal for the third MMC phase (leg z) is required to assure the correct operation of the TW-MMC3 RPC. The final reference signals, irx*, iry* and irz* are compared with the actual compensation currents synthesized by the TW-MMC3 RPC, irx, iry and irz, as shown in the control diagram presented in Figure 4.60. In that respect, PR controllers are used to correct the error resulting from the difference between the reference and the actual values. Output of the PR controllers gives the voltage reference signals, urx*, ury* and urz*. Although the additional requirement to generate the reference voltage signal, urz*, however, using the TW-MMC3 RPC instead of the FB-MMC4 RPC results in lower number of hardware components. Consequently, it leads to simplify the control of the RPC system. Chapter 4 – Rail Power Conditioners in Electrified Railway Systems Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 124 Figure 4.60. DC-link voltage control and the calculation of the voltage reference signals of the TW-MMC3 RPC. Figure 4.61 shows three control blocks that are responsible to balance the voltage across each of the SM capacitors. A robust SM voltage control leads to better MMC performance and then, this control is achieved through three steps: • MMC leg averaging voltage control, as shown in Figure 4.61(a); • MMC arm averaging voltage control, as shown in Figure 4.61(b); • MMC SM individual voltage control, as shown in Figure 4.61(c). Figure 4.61 represents the SM capacitors voltage control of the TW-MMC3 RPC after considering that σ ∈ {x, y, z}. The overall required control blocks to achieve that purpose in the TW-MMC3 RPC are six (two control blocks for each MMC leg), whereas, the required control blocks in the FB-MMC4 RPC are eight control blocks. The first control block, presented in Figure 4.61(a), shows the MMC leg averaging voltage control and the circulating current control, where two PI controllers are used for each MMC leg. The averaging control is implemented by summing the actual voltage values for all SM capacitors, then it compares the average of this actual value with a value provided as a reference [122]. The output of the first PI controller is considered as a reference value for a circulating current controller, which is implemented after summing the arm currents irσu, irσl, where σ ∈ {x, y, z}, in the upper and the lower arms that belong to the same MMC leg. Circulating current, icir, presented in (4.32) is considered as a part of the MMC arm current. However, it consists of a DC current component, AC current component that has double of the fundamental frequency (second harmonic current) and other harmonic currents. This current should be controlled as it causes extra losses and may increase the ratings of the power components. The second control block, presented in Figure 4.61(b), represents the MMC arm averaging voltage control [161]. The same explanation of Figure 4.50(b) and Figure 4.50(c) can be applied for the Figure 4.61(b) and Figure 4.61(c), respectively, where σ ∈ {x, y, z} in the TW-MMC3 RPC. The final signals of MMC leg averaging control, Aσ, MMC arm averaging control, Vrσu and Vrσl, MMC SM individual voltage control, Viσu and Viσl, where σ ∈ {x, y, z}, are added to the voltage reference signals, urσ*, to generate a voltage command (modulating signal) for each SM, which is to be compared × × urx ury * * × × × × Vdc * Vdc × PI × sin(ωt-π/6) sin(ωt-π/2) × PR PR irx iry * * irx iry irz irz *urz * PR × -- - - - - List of References Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks Mohamed Tanta – University of Minho 227 [68] Y. Hu, J. Huang, Z. Wang, and Y. Tao, “Research on the Impact of Electrified Railway on Power Grid,” in CICED 2010 Proceedings , Nanjing, 2010, pp. 1–9. [69] P. M. Kalla-Bishop, Italian Railways (Railway Histories of the World) . 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