Study on Dynamic Characteristics of UHVDC System under Hierarchical Infeed Mode with STATCOM

POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH Study on Dynamic Characteristics of UHVDC System under Hierarchical Infeed Mode with STATCOM Atiq Ur REHMAN 1, Zmarrak Wali KHAN 2, Bilawal REHMAN 2, Sheeraz IQBAL3, Mehr GUL1 1Department of Electrical Engineering, Balochistan University of Information Technology, Engineering and Management Sciences, 87300 Quetta, Pakistan 2State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beinong Road 2, 102206 Beijing, China 3Department of Electrical Engineering, University of Azad Jammu and Kashmir, Chela Campus, 13100 Muzaffarabad, Pakistan [email protected], [email protected], [email protected], [email protected], [email protected] DOI: 10.15598/aeee.v19i1.3937 Article history: Received Sep 03, 2020; Revised Nov 30, 2020; Accepted Jan 01, 2021; Published Mar 31, 2021. This is an open access article under the BY-CC license. Abstract. This paper focuses on the investigation of the dynamic characteristics of Ultra High Voltage Direct Current (UHVDC) system Under Hierarchical Infeed Mode (UHVDC-HIM) with Static Synchronous Compensator (STATCOM). The STATCOM at the inverter side of UHVDC-HIM is developed in PSCAD/EMTDC, and their power flow equations are derived. Based on these equations, the control schemes for rectifier and inverter side of UHVDC-HIM and STATCOM are properly designed. Several dynamic characteristics indices such as Commutation Failure Immunity Index (CFII), Commutation Failure Probability Index (CFPI), fault recovery time and Temporary Overvoltage (TOV) are evaluated in detail. The impact of different sizes of STATCOM and Short Circuit Ratio (SCR) of AC sources on dynamic characteristics of UHVDC-HIM system during single-phase and three-phase faults are examined. The analysis shows that higher SCR values and greater capacity of STATCOM can make the UHVDC-HIM system less vulnerable to Commutation Failure (CF), considerably enhance the fault recovery time and effectively reduce the TOV. Keywords Commutation Failure, fault recovery time, Hierarchical Infeed Mode (HIM), Line Commutated Converter, STATCOM, Temporary Overvoltage, UHVDC system. 1. Introduction LINE Commutated Converter (LCC) based High Voltage Direct Current (HVDC) system is globally employed for asynchronous grid interconnection and transmission of bulk power from distant areas to local grids [1]. Due to greater demand for LCC-HVDC transmission, multiple converter stations have been built in close proximity. An HVDC system where multiple converters from different power transmissions are linked to the same AC bus is termed as a multi-infeed HVDC transmission system. However, the multi-infeed LCC-HVDC system faces few operational problems, such as inadequate power transfer and excessive absorption of reactive power at the inverter station, which eventually outcomes in certain stability concerns [2]. To resolve the mentioned issues, a novel UHVDC scheme referred to as Hierarchical Infeed Mode (HIM) is presented [3]. The HIM implies diverse AC voltage levels (i.e. 500 kV and 1000 kV) at the inverter ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 16 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH Bus 1 Bus 2 U1∠ δ1 Z1∠ θ1E1∠ ξ1 U2∠ δ2 Z2∠ θ2E2∠ ξ2 Pd1+ jQd1 Pd1+ jQd1 Pac1+ jQac1 Pac2+ jQac2 Pd2+ jQd2 Pd2+ jQd2 Ud STATCOM k1:1 k1:1 k2:1 k2:1 HEC HEC LEC LEC Id Id Qc2 Ust 1:kst Qc1 RdLd RdLd Qc3 Bus 3 U3∠ δ3 E3∠ ξ3Z3∠ θ3 1:k3 1:k3 1:k3 1:k3 Inverter’s EndRectifier’s End Fig. 1: Schematic diagram of UHVDC-HIM with STATCOM. side [3]. Currently, the state grid of China employed the HIM scheme in under-construction UHVDC transmission links, such as Xilingol League to Taizhou city and Hulunbuir to Wannan [4]. The UHVDC-HIM system has many advantages, including voltage support capability, high power transmission capacity over long distances, and lesser capital cost [5]. Besides several benefits, it has some operating challenges, such as frequency deviation, Commutation Failure (CF), longer fault recovery time and high Temporary Overvoltage (TOV), specifically when the inverter station is allied to a weak AC network. Unfortunately, not much literature exists regarding dynamic performance improvement of the UHVDCHIM system. The power flow modeling of the UHVDCHIM system is analyzed in [6]. Reference [4] proposed voltage stability factor for UHVDC system hierarchically embedded into grid. The stability of UHVDCHIM is discussed in [5]. Reference [7] derived a new index as the multi-infeed short circuit ratio for UHVDC transmission hierarchically attached to AC system. Reference [8] presented a mathematical model to assess Multi-Infeed Interaction Factor (MIIF) of UHVDCHIM. The mentioned literature has not focused on the operational challenges and dynamic characteristics assessment of UHVDC-HIM. To overwhelmed the operating issues associated with LCC-HVDC transmission, different reactive power compensation devices like Fixed Capacitor (FC), Static Var Compensator (SVC), Synchronous Compensator (SC), and Static Synchronous Compensator (STATCOM) are used [9], [10], [11] and [12]. Reference [9] compared various compensator schemes (FC, SC, SVC, combination of SC and SVC) for LCC-HVDC transmission allied to a weak AC system. It is determined that the combination of SVC and SC gives the best performance as compared to other reactive power compensation schemes. Since STATCOM has the capability to regulate AC bus voltages and provide continuous current even under the worst system’s voltage [12], it could be utilized to improve operating characteristics of LCC-HVDC transmission. Reference [12] investigates the influence of reactive power compensation devices on dynamic characteristics of LCC-HVDC transmission under lower Short Circuit Ratio (SCR) conditions. The operating issues related to LCC-HVDC transmission can also be resolved by interconnecting it with a Voltage Source Converter (VSC) based HVDC system [13]. The detail about the VSC-HVDC system can be referred to [14], [15] and [16]. As the UHVDCHIM system is a new scheme, so it is necessary to do research that mainly focuses on dynamic characteristics improvement of the system. Considering the capability of STATCOM to flexibly adjust AC bus voltage, this work examines the influence of STATCOM on dynamics characteristics of the UHVDC-HIM, by evaluating fault recovery time, commutation failure immunity, commutation failure probability, and TOV. The impact of different sizes of STATCOM and AC system’s SCR values on operational characteristics of UHVDC-HIM system is also comprehensively elaborated. The results depict that STATCOM with larger capacity and higher SCR values can enhance dynamic characteristics of UHVDC-HIM. Moreover, it is also examined that when AC system (500 kV) is weak i.e. lower SCR values, then the impact of STATCOM on dynamic characteristics of the system is more prominent. 2. UHVDC-HIM System 2.1. Study System To investigate the effect of STATCOM on dynamic characteristics of UHVDC-HIM as presented in Fig. 1, the system is modeled in PSCAD/EMTDC. At the rectifier’s end, line commutated converters are linked to the same AC bus, whereas at the inverter’s end, the Higher End Converters (HEC) are linked to bus 1 (500 kV), while Lower End Converters (LEC) are attached to bus 2 (1000 kV). Single STATCOM ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 17 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH (300 Mvar) and dual STATCOM (2·300 Mvar) are embedded into bus 1 to assess their influence on different operational phenomena, such as fault recovery time, commutation failure immunity, commutation failure probability and TOV of UHVDC-HIM system. In Fig. 1, U1∠δ1,U2∠δ2and U3∠δ3are line to line voltages and their respective angles; E1∠ξ1,E2∠ξ2 and E3∠ξ3are electromotive forces and their respective angles; Z1∠θ1,E2∠θ2and E3∠θ3are equivalent impedances and their respective angels; k1,k2,k3and kst are turn ratio of transformer; Pd1,Pd2and Qd1,Qd2 are active and reactive powers of converter; Pac1,Pac2 and Qac1,Qac2are active and reactive powers of AC sources; Qc1,Qc2,Qc3indicates capacity of reactive power of AC filters and shunt capacitors; Udand Id represents DC voltage and current; Ld2,Rd2depicts inductance and resistance of DC link. 3. Mathematical Modelling 3.1. Modelling of UHVDC-HIM System Considering the schematic diagram depicted in Fig. 1, the following power equations are derived. The DC voltage of UHVDC-HIM can be computed as Eq. (1). Udi = 2 · 3√2Ui πki cos γi−3 πXiIdi!.(1) The DC current can be evaluated as Eq. (2). Idi =Ui[cos γi−cos (γi+µi)] √2kiXi ,(2) where i= 1 and 2 for bus 1 and bus 2, respectively. Xiindicates leakage reactance of transformer, γiand µiare converter extinction angles and commutation overlap angles, respectively. The commutation overlap angle is given in Eq. (3). µi= arccos cos γi−2XiIdi √2Udi −γi.(3) The analytical expression for DC transmitted power of UHVDC-HIM is established as in Eq. (4). Pdi =UdiIdi.(4) The reactive power absorption by HEC and LEC can be obtained by using Eq. (5). Qdi =Pdi tan ψi,(5) where: cos ψi=−cos γi+ cos (γi+µi) 2.(6) The expressions for real and reactive power of AC system are developed as in Eq. (7) and Eq. (8), respectively. Paci =U2 icos θi−EiUicos (δi+θi) |Zi|.(7) Qaci =U2 isin θi−EiUisin (δi+θi) |Zi|.(8) The reactive power supplied by combined AC filters and shunt capacitors is calculated as in Eq. (9). Qci =BciU2 i,(9) where Bci indicates the equivalent susceptance of AC filters and shunt capacitors. The real power of converters (HEC and LEC) and AC systems are related as in Eq. (10). 2Pdi −Paci = 0.(10) The relationship between reactive power of converters (HEC and LEC), AC systems and filters are given in Eq. (11). 2Qdi −Qaci +Qci = 0.(11) When STATCOM is attached to bus 1, then Eq. (11) is modified as Eq. (12). 2Qdi −Qaci +Qci +Qs= 0,(12) where Qsis reactive power supplied into UHVDC-HIM by STATCOM during transient state. 3.2. Modelling of STATCOM The simplified single line diagram of STATCOM attached to 500 kV bus is presented in Fig. 2, in which I1represents three-phase AC current, Usis three-phase AC voltage, Ucis the voltage across capacitor, R,Lindicate equivalent resistances and inductances of transformer linking STATCOM to 500 kV bus, R1and L1represent equivalent resistances and inductances of converter transformer. L R AC Filters Us∠0 I1 L1 R1 Ud Id HEC UcC STATCOM E1∠ξ1 U1∠δ1 Z1∠θ1 Fig. 2: Single line diagram of STATCOM linked to 500 kV bus at inverter’s end. The three-phase current (I1) inserted to STATCOM is depicted in Eq. (13). Ld dt   I1a I1b I1c =  U1a U1b U1c −  Usa Usb Usc  −R  I1a I1b I1c . (13) ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 18 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH The three-phase AC voltages of bus 1 and STATCOM are related in Eq. (14). Usabc =mUc 2  sin (ωt +δ1) sin (ωt +δ1−120◦) sin (ωt +δ1+ 120◦) ,(14) where δ1is the angle that Uslags to U1and mis the modulation index. The Park’s transformation matrix in Eq. (15) is utilized in order to transfer threephase current and voltage variables into dq frame: Tm=2 3   cos (ωt) cos (ωt −120◦) cos (ωt + 120◦) sin (ωt) sin (ωt −120◦) sin (ωt + 120◦) 1 2 1 2 1 2   . (15) Applying Eq. (15) to Eq. (13), the matrix deduced as Eq. (16).        d dtI1d=U1d−Usd L−R LI1d+ωI1q d dtI1q=U1q−Usq L−R LI1q−ωI1d ,(16) where: Usd Usq =mUc 2sin δ1 cos δ1,(17) m= 2qU2 sd +U2 sq Uc ,(18) δ1= arctan Usd Usq .(19) From Eq. (16), the following expression can be obtained.        Usd =U1d−RI1d−Ld dtI1d+ωLI1q Usq =U1q−RI1q−Ld dtI1q−ωLI1d .(20) The control strategy of UHVDC-HIM with STATCOM based on their mathematical modeling is designed and dynamic characteristics of the developed model are analyzed under various fault conditions. 4. Control Scheme of Study System 4.1. Control Scheme of UHVDC-HIM System The control scheme of UHVDC-HIM system is given in Fig. 3. The rectifier’s end is equipped with constant current control. At the inverter’s end, based on Eq. (1), Eq. (2), Eq. (3) and Eq. (4), constant extinction angle control and constant current control are adopted as primary and secondary control mechanisms. The main purpose of using constant current control at the inverter’s end is to maintain power flow in the DC transmission link during various disturbances at the rectifier’s end. Moreover, Voltage Dependent Current Order Limiter (VDCOL) is employed at the inverter’s end in order to limit DC current under the worst DC voltage. PI Filter Idcm αmax To Switches Firing Control System αmin αord Idc-ref (a) Constant current control at rectifier’s end. Constant DC Current Control Constant Extinction Angle Control γref γm β1 VDCOL Udi Ilim min Im ∆Idmin PI Filter To Switches Firing Control System PI β2 Iord Idi βord (b) Constant extinction angle and constant current control at inverter’s end. Fig. 3: Control scheme of UHVDC-HIM. 4.2. Control Scheme of STATCOM Uc-ref U1-ref ωL I1d-ref I1q-ref Usd Uc I1d I1q U1d U1 PI PI PI PI max max max max min min min min Usdo Usqo dq abc Usa φPLL VSC PWM PLL U1(abc) Usb Usc Usq U1q ωL Fig. 4: Complete control scheme of STATCOM. The overall control approach of STATCOM [17] relying on Eq. (13), Eq. (14), Eq. (15), Eq. (16), Eq. (17), Eq. (18), Eq. (19) and Eq. (20) is illustrated in Fig. 4. ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 19 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH It comprises of outer loop and inner loop controllers. The outer loop controllers regulate DC and AC voltages and generate d-axis and q-axis reference current (I1d−ref and I1q−ref ). The inner loop controllers are used to adjust d-axis and q-axis currents to set with reference currents. The voltage orders (Usd and Usq) are produced by combining output voltages (Usdo and Usqo) of inner loop PI controllers, measured voltages (U1dand U1q) and decoupling terms (ωLI1qand ωLI1d). These voltage orders in dq frame are transformed into abc frame employing inverse Park’s transformation, which are then utilized to produce switching signals for converters. 5. System Parameters The real world UHVDC (±800 kV) transmission system with a rated power capacity of 10,000 MW is considered in this paper. The DC current of the system is fixed at 6.25 kA. The system AC source frequency is 50 Hz. The inverter extinction angle is set at 17◦. The SCR values of 500 kV and 1000 kV AC sources at the inverter side are changed from 3 to 8. The study system for different SCR values is modified by using power flow Eq. (1), Eq. (2), Eq. (3), Eq. (4), Eq. (5), Eq. (6), Eq. (7), Eq. (8), Eq. (9), Eq. (10) and Eq. (11) of UHDC-HIM. The single STATCOM and dual STATCOM are embedded into bus 1 (500 kV) in order to assess their impact on dynamic characteristics of the UHVDC-HIM system. All other parameters of UHVDC-HIM and STATCOM are given in Tab. 1 and Tab. 2, respectively. Tab. 1: Parameters specification of UHVDC-HIM. Parameters Rectifier’s End Inverter’s End AC System’s 530 kV E1= 525 kV Voltage E2= 1050 kV AC Source θr= 84◦θ1= 85◦ Impedance Angle θ2= 75◦ Transformer 1527.9MVA 1466.1MVA Maximum Capacity Transformer’s 530/172.8kV 525/165.8 Turn Ratio 1050/165.8 Transformer’s 0.20 p.u. 0.20 p.u. leakage Reactance Tab. 2: Parameters Specification of STATCOM. System Parameters Value Maximum Capacity 300 Mvar Transformer’s Turn Ratio 525/13.8 Transformer’s Leakage Reactance 0.18 p.u. Capacitance 5000.0µF PI Controller (DC Voltage) kp= 10,ki= 0.01 PI Controller (AC Voltage) kp= 10,ki= 0.01 Inner I1dController kp= 15,ki= 0.001 Inner I1qController kp= 15,ki= 0.001 6. Simulation Results 6.1. Evaluation of Commutation Failure Immunity of UHVDC-HIM System with and without STATCOM CF is the worst dynamic event during inverter operation. It arises when converters’ off-going valves endure conducting without shifting currents to ongoing valves [13]. It is obvious from Eq. (1), Eq. (2) and Eq. (3) that when AC bus voltage drops at the inverter’s end of UHVDC-HIM, it will drop DC voltage and conversely increase current. The extinction angle γiof converters will decrease as a result of increase in commutation angle µiand the net effect is the occurrence of CF in the inverter station. It results in interruption of power flow to AC system. In order to mitigate CF, there is a need for dynamic reactive power compensators that can quickly bring the AC bus voltage to nominal value after clearance of fault. For this purpose, STATCOM is used to regulate AC bus voltage by supplying surplus reactive power to converter stations under various transient conditions. Commutation Failure Immunity Index (CFII) is utilized to assess the immunity of line commutated converters to CF. CFIIiis the ratio of critical fault MVA to DC transmitted power (Pdi) of UHVDC-HIM [18], as established in Eq. (21). CFIIi(%) = Worst Critical Fault MVA Pdi ·100 = =U2 i ωLminPdi ·100. (21) where i= 1, 2 for 500 kV bus and 1000 kV bus respectively, U2 iindicates AC system voltage at inverter’s end and Lmin is lower inductance that does not cause CF. It is obvious from Eq. (21) that greater the fault MVA, higher will be the CFIIivalue and less vulnerable is the LCC to CF. CFIIiconsiders HVDC controls and AC system strength and their effect on commutation process. To evaluate CFIIi, multiple run technique is developed in PSCAD/EMTDC. The three-phase inductance fault with increasing severity is employed at the inverter’s end, while continuously varying fault points on the wave. Repeated simulations are conducted for different points on wave and minimum inductance value with worst fault conditions at which CF does not occur is then utilized to evaluate CFIIi. The three-phase inductance fault is selected as it is the most severe fault as compared to resistive and capacitive that can cause CF in UHVDC-HIM. Two cases are considered for assessing commutation failure immunity of UHVDC-HIM. When CFII1 ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 20 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH (CFII2) is calculated, three-phase inductance fault is applied into bus 1 (bus 2). In both cases, STATCOM is linked to bus 1. The feature of the two cases are: Case 1: CFII1and CFII2values are evaluated relying on the situation that SCR2is set at 3 and SCR1 value is increased from 3 to 8. Case 2: CFII1and CFII2are assessed relying on the situation that SCR1is set at 3 and SCR2is increased from 3 to 8. The analytical results for mentioned cases are shown in Fig. 5 and their corresponding values are given in Appendix (Tab. 4 and Tab. 5). It is clear from Fig. 5(a), that CFII1value without STATCOM is 16.97 %, with single STATCOM is 18 % and with dual STATCOM is 20.74 %. This shows that STATCOM can help UHVDC-HIM in mitigating CF. Moreover, it is also examined that for increasing SCR1value from 3 to 8, the CFII1value without STATCOM ranges in between 17.56 % and 61.48 %, CFII1value with single STATCOM varies from 18 % to 63.76 %, and CFII1with dual STATCOM lies in between 20.74 % and 68.85 %. This depicts that higher SCR values can enhance CFII1value. The CFII2value with and without STATCOM is 16.95 %, as shown in Fig. 5(b). This clearly indicates that STATCOM has no influence on CFII2, as there is no interconnection between two AC sources. Similarly, it can be seen from Fig. 5(c) that increasing SCR2value has no impact on CFII2. Fig. 5(d) indicates that increasing SCR2value results in improving CFII2value in range of 16.95 % to 58.48 %. In this case, STATCOM has no effect on CFII2since STATCOM is connected to bus 1. 6.2. Evaluation of Commutation Failure Probability of UHVDC-HIM with and without STATCOM To further elaborate commutation failure phenomena of UHVDC-HIM, the Commutation Failure Probability Index (CFPI) is calculated utilizing statistical program data obtained from multiple run technique of PSCAD/EMTDC simulations. CFPI is referred to as the ratio of summation of fault points resulting in CF to the total number of various fault points undertaken in a cycle [18]. The CFPI of HEC and LEC is denoted as CFPI1and CFPI2, respectively. The SCR value of both AC sources is fixed at 3. As the voltage level of both buses is different, fault MVA is considered in perunit in order to have the same fault conditions for both buses at the inverter’s end. The per-unit fault level is the ratio of fault MVA to the DC transmitted power of UHVDC-HIM. The per-unit fault level for CFPI1ranges from 16 % to 25 %, and for CFPI2, it lies in between 16 % and 25 %. 0 10 20 30 40 50 60 70 80 3 4 5 6 7 8 SCR1 CFII1 (%) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) (a) 0 5 10 15 20 25 30 35 3 4 5 6 7 8 SCR1 CFII2 (%) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) (b) 5 10 15 20 25 3 4 5 6 7 8 SCR2 CFII1 (%) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) (c) 0 10 20 30 40 50 60 70 3 4 5 6 7 8 SCR2 CFII2 (%) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) (d) Fig. 5: CFII1and CFII2values under three-phase fault (a) and (b) Case 1: SCR2= 3, SCR1varies in range of 3 to 8. (c) and (d) Case 2: SCR1= 3, SCR2varies in range of 3 to 8. When CFPI1(CFPI2) is calculated, a three-phase inductance fault is applied into bus 1 (bus 2). For cal- ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 21 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH culation of CFPI1and CFPI2, a total of 100 equally distributed fault points are considered in an AC cycle of 20 ms duration. The results for CFPI1and CFPI2with and without STATCOM during threephase inductance fault with different fault levels are illustrated in Fig. 6 and Fig. 7, respectively. It is obvious from Fig. 6 that CFPI1without STATCOM is 0 for fault level less than 16.97 %. The chances of CF vary for fault level in between 16.97 and 19 %. At a higher fault level greater than 19 %, the probability of CF is 100 %. When a single STATCOM is connected to bus1, the CFPI1results are improved. The CFPI1with a single STATCOM is 0 for fault level less than 18 %, and it varies for fault level between 18 % and 19.5 %. At fault level above 19.5 %, the CFPI1with single STATCOM is 100 %. When two STATCOMs with an overall capacity of 600 Mvar are attached to bus 1, the CFPI1is reduced and there is no chance of CF up to fault level of 19.5 %. The CFPI1with dual STATCOM varies for fault level in between 19.5 % and 23 %. At fault level greater than 23 %, the CFPI1with dual STATCOM is 100 %. 0 20 40 60 80 100 16 17 18 19 20 21 22 23 24 25 Fault Level (%) CFPI1 (%) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Fig. 6: CFPI1with and without STATCOM under three-phase fault. Figure 7 indicates that CFPI2with and without STATCOM is 0 for fault level less than 16.95 %. The chances of CF increase with severe fault level and it reaches 100 % at 18.3 % fault level. It is observed that at higher fault level, the chances of CF in HEC are higher as compared to LEC. Moreover, it is also obvious from Fig. 7 that STATCOM has no contribution in improving CFPI2. The results of Fig. 6 are consistent with the results presented in Fig. 5(b) and Fig. 5(d). 0 20 40 60 80 100 16 17 18 19 20 Fault Level (%) CFPI2 (%) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Fig. 7: CFPI2with and without STATCOM under three-phase fault. Thus it is concluded from the above results that chances of CF in HEC and LEC are higher at sever fault conditions. At lower fault level, the CFPI1can be reduced by 1.03 % with single STATCOM and 2.53 % with dual STATCOM. At higher fault level, the CFPI1 can be improved by 0.5 % with single STATCOM and 4 % with dual STATCOM. 6.3. Evaluation of Fault Recovery Performance of UHVDC-HIM with and without STATCOM In this paper, DC power recovery time is utilized as fundamental metric to assess the fault recovery time of UHVDC-HIM quantitatively. Fault recovery time is termed as the time taken by DC power to reinstate to 90 % of pre-fault value after clearance of fault [19]. To investigate the fault recovery time of UHVDC-HIM with and without STATCOM, single-phase and threephase faults are applied into bus 1. 1) Single-Phase Fault at Bus 1 Initially, SCR1and SCR2of the developed system given in Fig. 1 are fixed at 3. A single-phase to ground fault with 0.15 H inductor is applied at 2.0 s and lasts for five cycles (0.1 s) duration. Here, 0.15 H is the critical inductance value at which the studied system cannot experience CF under single-phase fault. The fault recovery time with and without STATCOM is shown in Fig. 8. It is examined that fault recovery time without STATCOM is 194 ms, which is very high. The fault recovery time with single and dual STATCOM is 169 ms and 52 ms, respectively. This depicts that STATCOM can reduce the faulty recovery time of UHVDC-HIM, by supplying surplus reactive power into the system at the instant of fault. 1.9 2 2.1 2.2 2.3 2.4 2.5 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2.19 2.22 2.25 2.28 2.31 0.7 0.8 0.86 0.88 0.9 Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Time (s) Inverter DC Power (p.u.) Fig. 8: DC power of bus 1 under single-phase fault, SCR1= SCR2= 3. To evaluate the influence of AC system strength on fault recovery time, SCR2is kept constant at 3 and SCR1value is changed from 3 to 8. The results for fault recovery time of UHVDC-HIM with and without ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 22 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH STATCOM are shown in Fig. 9 and their corresponding values are summarized in Appendix (Tab. 6). 0 50 100 150 200 250 3 4 5 6 7 8 SCR1 Fault Recovery Time (ms) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Fig. 9: Fault recovery time under single-phase fault at bus 1 for SCR2= 3 and SCR1varies from 3 to 8. The fault recovery time without STATCOM decreases from 194 ms to 32 ms, when SCR1value increases from 3 to 8. With single STATCOM, the fault recovery time reduces from 169 ms to 29 ms. And with dual STATCOM, the fault recovery time decreases from 52 ms to 29 ms when the SCR1value varies from 3 to 8. This indicates that the system with low SCR1 values has larger fault recovery times and vice versa. Moreover, it is also observed that the impact of STATCOM is more prominent for the system having low SCR1values. As SCR1value increases from 3 to 8, the impact of STATCOM on improving fault recovery time decreases. 2) Three-Phase Fault at Bus 1 Initially, SCR1and SCR2of the developed system given in Fig. 1 are fixed at 3. The five cycles (0.1 s duration) three-phase to ground fault with 0.25 H inductor is applied at 2.0 s. Here, 0.25 H is the critical inductance value at which the studied system cannot experience commutation failure under a three-phase fault. The fault recovery time with and without STATCOM is presented in Fig. 10. It is observed from Fig. 10 that fault recovery time without STATCOM is 270 ms, with single STATCOM is 240 ms and with dual STATCOM is 230 ms. This indicates that the addition of STATCOM into the UHVDC-HIM system can help in improving the fault recovery time. To analyze the effect of system strength on fault recovery time under three-phase fault, the SCR1value is varied from 3 to 8 and SCR2is fixed at 3. The results are illustrated in Fig. 11 and their corresponding values are given in Appendix (Tab. 7). The fault recovery time without STATCOM decreases from 270 ms to 34 ms when SCR1value increases from 3 to 8. With single STATCOM, the fault recovery time reduces from 240 ms to 30 ms. And with dual STATCOM, the fault recovery time decreases from 230 ms to 30 ms when SCR1value varies from 3 to 8. It is examined that fault recovery time under three-phase fault is higher as compared to fault recovery time during single-phase fault. 1.9 2 2.1 2.2 2.3 2.4 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) 2.2 2.25 2.3 2.35 0.78 0.82 0.86 0.9 Time (s) Inverter DC Power (p.u.) 2.5 Fig. 10: DC power of bus 1 under three-phase fault, SCR1= SCR2= 3. 0 50 100 150 200 250 300 3 4 5 6 7 8 SCR1 Fault Recovery Time (ms) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Fig. 11: Fault recovery time under three-phase fault at bus 1 for SCR2= 3 and SCR1varies from 3 to 8. 6.4. Investigation of TOV under Converter Blocks of LCC-UHVDC-HIM with STATCOM LCC–HVDC station absorbs reactive power in range of 0.5 to 0.6 p.u. of rated transmitted DC power [16], which is usually supplied by filters and fixed capacitors connected to AC bus. The reactive power consumption is negligible when converters are blocked. The excessive reactive power supplied by filters and shunt capacitors results in temporary overvoltage. The case is most severe at the inverter side when AC system is weak. TOV greatly depends on AC system’s impedance, angle and reactive power compensating devices connected to AC bus. In this paper, TOV of bus 1 (TOV1) is examined while blocking both HEC of the system. Initially, SCR1and SCR2of the developed system are fixed at 3. It is evaluated that TOV1without STATCOM is 1.24 p.u., with single STATCOM is 1.21 p.u. and with dual STATCOM is 1.17 p.u. This indicates that STATCOM can support UHVDC-HIM in reducing TOV1. ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 23 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 19 |NUMBER: 1 |2021 |MARCH In order to assess the effect of system strength on TOV1, the SCR1value is increased from 3 to 8. The TOV1with and without STATCOM, evaluated for different SCR1values (SCR2= 3), are shown in Fig. 12 and their associated values are given in Tab. 3. The TOV1without STACOM reduces from 1.24 to 1.08 p.u. when SCR1value changes from 3 to 8. With single STATCOM, the TOV1decreases from 1.21 to 1.06 p.u. and with dual STATCOM, the TOV1reduces from 1.17 to 1.03 p.u. The improvement in TOV1with STATCOM is because of the fact that STATCOM absorbs surplus reactive power in case when both HEC are blocked. It is thus concluded that higher SCR1values and greater STATCOM capacity can result in a lower value of TOV1. 0.85 0.95 1.05 1.15 1.25 1.35 3 4 5 6 7 8 SCR1 TOV1 (p.u.) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Fig. 12: TOV of bus 1 (TOV1) with and without STATCOM for SCR2= 3 and SCR1value varies from 3 to 8. Tab. 3: TOV of bus 1 with and without STATCOM for SCR2= 3, SCR1varies from 3 to 8. SCR1 Temporary Overvoltage of Bus 1 (TOV1p.u.) Without STATCOM STATCOM STATCOM (300 Mvar) (2·300 Mvar) 3 1.24 1.21 1.17 4 1.16 1.13 1.11 5 1.12 1.10 1.09 6 1.10 1.08 1.07 7 1.09 1.07 1.05 8 1.08 1.06 1.03 The TOV of bus 2 (TOV2) is assessed while blocking both LEC. The results for TOV2with and without STATCOM for different SCR2values (SCR1= 3) are shown in Fig. 13. The TOV2with and without STATCOM varies from 1.12 to 1.04 p.u., when SCR1value ranges between 3 and 8. The TOV2is lower than TOV1 due to the reason that source 2 has a lower impedance angle as compared to source 1. The results also indicate that the greater the SCR2value, the lower will be TOV2and vice versa. However, STATCOM connected to bus 1 has no contribution in reducing the temporary overvoltage of bus 2. 1 1.03 1.06 1.09 1.12 1.15 3 4 5 6 7 8 SCR2 TOV2 (p.u.) Without STATCOM STATCOM (300 Mvar) STATCOM (2·300 Mvar) Fig. 13: TOV of bus 2 (TOV2) with and without STATCOM for SCR1= 3 and SCR2value varies from 3 to 8. 7. Conclusion In this paper, the dynamic characteristics of UHVDC transmission system under HIM with STATCOM are analyzed. Voltage source converter based STATCOM and LCC based UHVDC-HIM model is developed in PSCAD/EMTDC. The dynamic characteristics like commutation failure immunity, commutation failure probability, fault recovery time and temporary overvoltage of UHVDC-HIM are examined while considering the impact of system’s strength and capacity of STATCOM. The following conclusions can be drawn from simulation results: •STATCOM can make UHVDC-HIM less vulnerable to CF by improving CFII1. When SCR1and SCR2values of AC systems are increased, the improvements in CFII1and CFII2are observed. •The commutation failure probability index (CFPI1and CFPI2) is effectively decreased with STATCOM and by using larger SCR1and SCR2 values of AC sources. •The fault recovery time under single-phase and three-phase faults is improved when single and dual STATCOM is connected at bus 1 of UHVDCHIM. •The temporary overvoltage of bus 1 is considerably reduced with STATCOM when both high end converters are blocked. The TOV1and TOV2can also be reduced if higher SCR1and SCR2values are considered for the AC systems. •Moreover, it is also observed that the impact of STATCOM will be more apparent in case the AC system’s SCR value is low. •This paper can be used as a technical reference regarding dynamic characteristics analysis of UHVDC-HIM system with and without STATCOM. ©2021 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 24