AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems

MSc SELECT is a cooperation between KTH-Royal Institute of Technology, Sweden │ Aalto University, Finland │ Universitat Politècnica de Catalunya, Spain │ Eindhoven University of Technology, Netherlands │Politecnico di Torino, Italy │ AGH University of Science and Technology, Poland │ Instituto Superior Técnico, Portugal MSc Environomical Pathways for Sustainable Energy Systems – SELECT MSc Thesis AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Author: Josef Weizenbeck Principal supervisor: Oriol Gomis-Bellmunt, Universitat Politècnica de Catalunya Session: 2013 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems i Abstract This work presents a novel control approach for AC Hubs, interconnecting offshore wind farms and different mainland grids via HVDC links. There are several proposals for a European overlay grid by different associations. This is where AC Hubs come into play. In order to implement such a grid in the North Sea, wind farms and long distance HVDC transmission lines need to be interconnected. The two options considered for interconnection are multiterminal HVDC configurations or AC Hubs. This work concerns itself with a novel approach for the control of AC Hubs. A technical model based on common control strategies is established and verified by simulation in MATLAB/Simulink. The main objective is to guarantee flawless operation of the AC Hub in absence of fast and reliable communication. This is achieved by applying a droop control scheme, which is similar to control strategies that are typically applied to microgrids or synchronous generators in the conventional electrical grid. After introducing the control strategy, a system layout is chosen, which is loosely based on existing data of offshore wind farms and literature discussing the future scenario of an HVDC grid in the North Sea. Issues with power sharing of parallel power converters, power transmission between different nodes as well as operation during faults are addressed within this work. The results show that by applying the proposed droop-control scheme, the operation of the AC Hub is possible without fast and reliable communication systems. ii AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Table of Contents ABSTRACT ___________________________________________________ I TABLE OF CONTENTS _________________________________________ II GLOSSARY _________________________________________________ V 1. INTRODUCTION ___________________________________________ 1 2. BACKGROUND ___________________________________________ 3 2.1. Design of the European Offshore Transmission Grid in the North Sea ......... 3 2.2. HVDC Conversion Technology ...................................................................... 5 2.3. Control of VSCs ............................................................................................. 8 2.3.1. Park Transformation ........................................................................................ 10 2.3.2. Instantaneous Power in the Park Reference Frame......................................... 10 2.3.3. Idealised VSC Model ....................................................................................... 10 2.3.4. Voltage Equations in the qd0 Frame ................................................................ 11 2.3.5. Phase Locked Loop ......................................................................................... 12 2.3.6. Current Reference ........................................................................................... 13 2.3.7. Current Loop .................................................................................................... 13 2.3.8. Voltage Control ................................................................................................ 14 3. PROPOSED CONTROL APPROACH FOR AC HUB _____________ 16 3.1. Active and Reactive Power Sharing ............................................................. 17 3.2. Power Transmission via the AC Hub ........................................................... 18 3.3. VSC Loss or Master/Slave Operation .......................................................... 20 3.4. The Different VSC Control Schemes within the Model ................................ 20 4. DESIGN OF THE AC HUB __________________________________ 22 4.1. Power Rating and Voltage Level .................................................................. 23 4.2. Model of AC lines ......................................................................................... 24 4.3. Transformer and Converter Output Impedance Model ................................ 25 4.4. Wind Farm Model ......................................................................................... 25 5. SIMULATION IN MATLAB/SIMULINK _________________________ 26 5.1. Model with Central Platform and PCC ......................................................... 26 5.2. Model with Separate Platforms and Tie Cable............................................. 27 6. RESULTS _______________________________________________ 29 6.1. Standard Operation – Power Sharing .......................................................... 29 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems iii 6.1.1. AC Hub Model with PCC ................................................................................. 29 6.1.2. AC Hub Model with Tie Cable ......................................................................... 33 6.2. Power Transmission via the AC Hub ............................................................ 36 6.3. Converter Loss ............................................................................................. 38 6.4. Response of Voltages and Currents ............................................................ 41 6.5. Response to more Realistic Wind Data Input ............................................... 45 7. FUTURE WORK __________________________________________ 48 8. CONCLUSION ___________________________________________ 49 LIST OF FIGURES ____________________________________________ 51 LIST OF TABLES _____________________________________________ 54 ACKNOWLEDGEMENTS _______________________________________ 55 REFERENCES _______________________________________________ 57 APPENDIX __________________________________________________ 59 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems v Glossary AC Alternating Current AC Hub In this report, AC Hub refers to HVDC lines connected with a number of wind farms via an AC system by using VSCs. DC Direct Current HV High Voltage IGBT Insulated-Gate Bipolar Transistor LCC Line Commutated Converter MMC Modular Multilevel Converter MV Medium Voltage PI Proportional-Integral PLL Phase Locked Loop pu Per-Unit System PWM Pulse-Width Modulation VSC Voltage Source Converter WF Wind Farm XLPE cables Cross-Linked Polyethylene Cables AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 1 1. Introduction The number of offshore wind farms in Europe is growing continuously. There is an increasing interest to tap wind resources, which are located further from shore. In the beginning of the year 2012, plants with a capacity of more than 5 GW were under construction [1]. For distances of over 50 to 100 km from the shore, HVDC transmission usually has advantages over HVAC transmission for offshore wind farms in terms of efficiency and cost [2, 3]. Moreover, there are plans for building a trans-European grid in the North Sea, which is often referred to as the ’Supergrid’. Names and definition vary in literature; however, the purpose is to transmit power generated in areas with low or no demand for electricity at all (e.g. offshore wind power plants) to areas of huge demand (big metropolitan and industrial areas). In this report, mainly the vision advertised by the association ‘Friends of the Supergrid’ will be followed, which is also supported by many big players in the energy sector [4, 5]. According to these future scenarios, offshore wind farms in the North Sea and the electrical grid of countries in the region, e. g. Great Britain, Scandinavia, Germany and Netherlands, will be strongly interconnected via HVDC transmission lines. This allows for novel approaches in order to balance the electrical grid in terms of generation and demand and is believed to be a critical step in integrating renewable energy, especially offshore wind power. Additionally, more trade of electrical power between countries is encouraged [4]. The first interconnection between European countries via offshore wind farms has already been planned between Denmark, Germany and Sweden. It is to be commissioned in 2020. However, the distances for this project are comparatively low. Therefore, it is realised in HVAC transmission technology [6]. Within the offshore overlay grid, connections from the wind farms to the long-distance HVDC lines will be necessary. One possibility worth investigating is a concept often referred to as ‘Supernodes’. In this report, they are called AC Hubs. AC Hubs connect the long-distance HVDC lines with the offshore wind farms by means of power converters and AC networks. Taking this approach, there will be offshore AC Hubs connecting a number of offshore wind farms with at least two power converters for HVDC transmission in parallel [4]. The control of the AC Hub might not be a straight forward problem if fast and reliable communication is not available. The following assumptions are made within this report: Page 8 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 2.3. Control of VSCs Figure 5 HVDC link as in [14] (top); below how it is modeled within the simulations carried out in this report Figure 5 shows a typical configuration of VSCs used in power systems as described in [14]. One converter regulates the DC line voltage, whereas the other converter regulates the power reference, which is transferred via the DC link. This configuration is also used in the model of the AC Hub for the HVDC links and the wind farms. As the DC transmission system is assumed not to have a significant impact on the control of the AC Hub, the converter responsible for the DC voltage reference is modelled as DC voltage source. AC DC VSC for Power Reference DC AC VSC for DC Voltage Reference L1 L1 Onshore Grid Impedance R1 R1 AC Hub Impedance AC VSC for Power Reference R1 AC Hub Impedance DC DC AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 9 The control model is based on the control approaches outlined in [13] and [14]. Detailed lowlevel control issues for the case of MMC converters are neglected as this report is focussing on the control strategy applied to AC Hubs in general, instead of dealing with the low-level control of the converters. Figure 6 shows a generic control structure used for VSCs. The reference block is comprised of a power reference and/or a voltage control loop, depending on the task of the VSC within the AC Hub. The gray parts indicate the control structure introduced in Chapter 3. The resistance and reactance and , respectively, mainly serve the following three purposes: controlling the power by means of voltage magnitude and phase angle at the converter side, secondly, enabling the connection of the VSC to a voltage source without violating fundamental circuit theory, and finally, limiting the short circuit current at the AC side of the converter [14]. Figure 6 Control structure for the VSC model in this report, the final control structures as realised in the simulation are shown in Figure 16 and Figure 17 vz rlll L1 DC AC Voltage control loop PLL/ T1 TPark Current loop v*l vl Reference Droop i*q i*d id iq q vz i f E P* Q* PMeas QMeas/E* TPark iL Cf DC Page 10 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 2.3.1. Park Transformation In order to simplify the control design, electrical quantities are transformed from the frame into the frame by applying the following transformation law, as in [14]: [ ] [ ] [ ] (1) [ ( ) ( ) ] (2) where is the transformation matrix for the Park transformation; is the electrical angle; and is an electrical property, in particular current or voltage. 2.3.2. Instantaneous Power in the Park Reference Frame According to [13], [14] the instantaneous power of a three phase system in the frame can be expressed as (3) for active power and (4) for reactive power. Where is voltage in frame; and is current in frame. 2.3.3. Idealised VSC Model Figure 7 shows the idealised model of a VSC used in the further simulations. The switching states of the converter and its losses are not considered, as the focus of this work is in controlling the AC hub within an offshore HVDC transmission grid, not the converter itself. Therefore, the following relation between AC and DC power is made as in [14]: (5) AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 11 Figure 7 Idealised VSC Model 2.3.4. Voltage Equations in the qd0 Frame Following [14], the equations for the balanced three phase system in Figure 8 can be expressed in the frame as [ ] [ ] [ ][ ] [ ] [ ] (6) where is the voltage at the bus connecting the VSC to the system; is the voltage at the converter output, before the impedance of the connection to the bus; and form the output impedance; is the output current of the VSC; and is the electrical angular velocity. Figure 8 AC side of VSC, simplified DC AC AC AC AC DC = IDC DC DC L1 a b c L1 L1 a b c vlvz rl rl rl ll ll ll Page 12 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 2.3.5. Phase Locked Loop A phase locked loop is used to synchronise the control to the reference voltage. Such a loop is realised as shown in Figure 9 [14]. The transfer function of the used PI controller is defined as ( ) (7) where and are the proportional gain and the time constant, respectively, of the PI controller. In order to obtain the controller parameters Kp and , the equations √ (8) and √ (9) have to be evaluated, with peak voltage , electrical angular velocity and damping ratio . Figure 9 PLL loop assuring a synchronous voltage angle q Kf(s) 1 s Tpark(q) 0+_wq vd vq vavbvc AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 13 2.3.6. Current Reference As the component of the voltage is forced to be zero by the PLL, the current reference can be derived using equations (3) and (4), which are describing the instantaneous power in the frame: (10) (11) where is the reference vector for the output of the VSC; is the voltage at the bus the VSC is connected to; and and are the reference values for active and reactive power, respectively. Thus, the current can be limited according to the power rating of the converter. The current reference for active power as described here is applied only if the converter is controlling the power reference, not if the VSC regulates the DC-line voltage, nor if voltage control is applied, as described later. 2.3.7. Current Loop As stated before, the PLL is forcing the voltage component . By decoupling the and d components of the voltages and currents in equation (6) with [ ] [ ] (12) the output of the current controllers is [ ] [ ][ ] [ ] [ ] (13) where is the decoupled voltage vector output of the controller, which is then re-substituted according to equation (12) in order to get the output voltages of the VSC, shown in Figure 10. Page 14 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems The gains of the PI controllers for the current loops with their PI controller functions are calculated as following [14] (14) (15) where is the closed loop time constant of the current loop, chosen faster than the converter frequency. Figure 10 shows the current controller realized in the model. Figure 10 Current loop of VSC with converter voltage as output 2.3.8. Voltage Control For cases where a PLL is not used, voltage control may be necessary. In [13], voltage control is achieved by adding an additional, outer control loop to the above described current loop by means of a PI controller and additional capacitors after the impedance in parallel. Figure 11 shows the simplified control scheme. The closed loop transfer function of the inner current loop for and chosen as defined above is (16) GcI(s) _ +vlq + _ Iq Iq* vzq GcI(s) +_vld Id* Id X X ll + _ we _ AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 15 By using the same approach as in the current control loop above, the and components can be decoupled from each other. The approximated transfer function of the voltage to be controlled over the decoupled input signal is [13]. (17) Figure 11 Simplified scheme for outer voltage control loop The design of the PI controller is achieved by evaluating the equations below. The transfer function of the PI controller is (18) being the proportional gain. Phase margin , the frequency at , , and proportional gain are calculated using ( ) (19) √ (20) (21) where capacitance of the output capacitor of the VSC, and the crossover gain frequency [13]. k(s) 1 tis + 1 _ +vzq v^q vzq*1 Cfs eq k(s) 1 tis + 1 _ +vzd v^d vzd*1 Cfs ed Page 16 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 3. Proposed Control Approach for AC Hub In order to avoid the use of fast and reliable communication, a novel control scheme is proposed. The power reference of the VSCs connected in parallel to the AC Hub is determined by following a droop scheme. Therefore, the power reference is automatically set, depending on the current power generation by the connected offshore wind plants. Droop control is typically applied to power generation units using synchronous generators and also to distributed systems or microgrids, where frequency and voltage are used to regulate the active and reactive power in the system [15, 16]. A typical droop scheme is applied to the VSCs of the HVDC transmission system, which are connected in parallel to the AC Hub. Figure 12 and equations (22), (23) show the method typically applied for microgrids [15]: (22) (23) where and are the droop gains for active and reactive power and , respectively and is the magnitude of the voltage at the bus where the VSC is connected (the voltage to be controlled by the VSC). Power sharing between the VSCs depends on the values of their droop gains and . If power converters have different ratings, proper power sharing can be achieved by applying different droop gains for each converter. A similar approach is also found in [17]. Figure 12 – and – graphs of a classic droop scheme for generators; the point of operation is marked with a black dot P in MW f in Hz Df DP= m DP Df P* P f f* Q in Mvar E in V DE DQ= n DQ DE Q* Q E E* AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 17 3.1. Active and Reactive Power Sharing The incoming active power is measured and the frequency is adjusted according to the active power reference and the droop gain constant m, as shown in the equations (22) and (23). Figure 13 shows the block diagram of the realised droop scheme. Figure 13 Realised droop scheme for active (top) and reactive (bottom) power sharing Reactive power sharing can be achieved in a similar way. Instead of using an droop, a droop is applied, as shown in Figure 13. The output of this droop scheme is then fed into a voltage control loop, which then sets the current reference for the current loop, as explained above. In the case that no real PCC exists for the droop-controlled VSCs in parallel, e. g. when a tie cable is used to connect to AC systems, the voltage drop over this cable can significantly influence reactive power sharing. _ +fout + _ P* Pmeas f* m _ +Eout + _ Q* Qmeas E* m Page 24 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 4.2. Model of AC Lines The AC lines modelled according to Figure 20. According to the ratings of the lines and the specifications of the cable, their resistances, inductances and capacitances are calculated as described in equations (31), (32) and (33). The specifications for the cables are shown in Table 1 and Table 2 below, the underlined cross-section of 1400 mm² was chosen. Reactive power compensation is achieved by connecting a reactance to the PCC. (31) (32) (33) Table 1 Current ratings for XLPE cables [20]: Crosssection in mm² Current rating for spaced laying in A Current rating for closed laying in A 800 1180 830 1000 1290 895 1200 1400 960 1400 1510 1025 1600 1620 1090 Table 2 Capacitance and Inductance of XLPE cables [20]: Cross-section in mm² Capacitance in uF/km Inductance in mH/km 800 0,17 1,37 1000 0,18 1,35 1200 0,19 1,33 1400 0,2 1,32 1600 0,21 1,31 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 25 4.3. Transformer and Converter Output Impedance Model A simple model is established for the transformers from MV to HV and the HV transformers. It consists of impedance including winding resistance and leakage reactance. Magnetising losses will be neglected. In order to insure a limitation of the short-circuit current; the leakage impedance of a transformer is usually chosen somewhere between 5 to 20% of the impedance base, or even higher for high voltage transformers [21]. Furthermore, the short-circuit current of the converters needs to be limited. This is done by properly sizing the output impedance, which is also used for controlling current and voltage of the converter. Thus, reactance and resistance of the transformer model and converter output impedance is calculated as (34) √ ( ) (35) ( ) (36) where is the leakage or output impedance, is the output resistance, is the output inductance, is the percentage of the short-circuit voltage, is the impedance base 4.4. Wind Farm Model In order to simulate the inertia of the wind farm, a simple high level approximation is taken. A Laplace filter for 1st order response is used to simulate the inertia of the wind turbines, as in [17]. The procedure is further explained in Section 6.5. (37) Page 26 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 5. Simulation in MATLAB/Simulink The above introduced model is simulated using the SimPowerSystems package within the MATLAB/Simulink environment. The simulation parameters can be found in Table 3. As the simulations focus on the AC Hub, the model is simplified for the sake of computational performance. 5.1. Model with Central Platform and PCC Figure 21 shows the layout of the system as modelled in MATLAB/Simulink. Note that the HVDC transmission lines are not modelled. Instead, a DC voltage source is connected on the DC side of the HVDC VSCs. Also, the wind farms are modelled with a power reference VSC only. In order to emulate the inertia of the wind turbines, a filter can be added to the power reference. Figure 21 Model of the AC Hub with PCC as in the simulation with SimPowerSystems package of MATLAB/Simulink L1L1 L1 L1 L1 L1 L1 L1 33kV 10km 33kV/220kV 220kV/33kV 33kV 10km 220kV 20km 220kV/400kV 2x1250MVA PCC VSC4 1250MVA 1250MVA1250MVA 1250MVA AC DC HVDC 1000MW HVDC 1000MW AC AC DC 220kV 20km VSC6 1250MVA DC DC DC AC DC Source WF I DC Source WF II AC DC 1250MVA L1 Reactance AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 27 Input parameters are: active and reactive power generated by wind farms, times of converter loss, desired power transmission to a certain node and wind velocity data. Output parameters are the transient responses of the electrical quantities within the system, especially power fed into the HVDC VSCs, frequency, voltages and currents. A first evaluation of the stability and feasibility of the droop control scheme for the AC Hub is done by analysing these transient responses. 5.2. Model with Separate Platforms and Tie Cable The only difference of the model in Figure 22 and the one above is that it does not have a real PCC. The wind farms and VSCs are connected via a tie cable. This gives different circumstances when applying the droop scheme, as there is a certain voltage drop over the tie cable. Reactive power sharing is therefore not a straightforward problem anymore. Additionally, the impedances of the cables and the transformers where changed for the branch connected to wind farm II (WF II). This was done in order to get information about the system performance under unsymmetrical conditions. Figure 22 Model of AC Hub with tie cable as in the simulation L1 L1 L1 L1 L1 L1 L1 L1 AC DC 33kV 5km 33kV/220kV 220kV/33kV 33kV 10km 220kV 20km 220kV/400kV HVDC 1000MW HVDC 1000MW 1250MVA 1250MVA1250MVA 1250MVA AC AC DC L1 220kV 10km Tie Cable 1250MVA DC DC DC Source WF II AC DC DC DC Source WF I AC L1 L1 Reactance Reactance 220kV 5km 220kV/400kV 1250MVA VSC4 1250MVA VSC6 1250MVA AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 29 6. Results Different scenarios are tested within the AC Hub model, the most important ones being power sharing, power transmission and converter loss. Finally, wind data input is given in order to evaluate the performance of the control approach under more realistic circumstances. All results are obtained using the AC Hub model with PCC, except for the one in 6.1.2, where the model with tie cable is used. 6.1. Standard Operation – Power Sharing Depending on the rating of the converters and the final design of the AC Hub, power sharing between converters is achieved by choosing their droop constants accordingly. In this model only two HVDC lines are connected to the AC Hub. Both HVDC VSCs have the same droop constants, aiming for equally distributing the generated power over the two HVDC lines. 6.1.1. AC Hub Model with PCC According to Figure 23, the power sharing for both VSCs works. The active power generated by both wind farms is distributed evenly among the two VSCs of both HVDC lines, as the droop constants are the same for every VSC. When the generated power changes, the power fed into the VSCs is adjusted accordingly. Also, the reactive power is evenly distributed. The response is quite quick and the overshoot within acceptable limits. The frequency response in Figure 25 correlates perfectly to the expected behaviour. Given a droop gain of 0.05 Hz/GW, steady state values of 50.0125 Hz, 50.0175 Hz, and 50.0350 Hz are reached for a VSC power of 250 MW, 350 MW, and 700 MW, respectively. The same applies for the droop voltage response. Given a reference voltage Eref of 179,629.24V and a voltage droop of -17.4 V/Gvar and reactive power values of -150 Mvar, -60 Mvar,-20 Mvar, and -160 Mvar, steady state values for VSC output voltage of 179631.8 V, 179630.3 V, 179629.6 V, and 179632.0 V, respectively, are reached. The waveforms of the active and reactive power oscillate with amplitudes of about 0.5 to 1 MW/Mvar. Page 30 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 23 Wind farm I and II generated power (generated active power fed into the AC Hub is negative, inductive reactive power is negative) Figure 24 Power fed into the HVDC links (active power fed into the VSCs is positive, inductive reactive power is positive), note that power sharing is achieved exactly, as both reactive and active power have the same values in both converters 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 -8 -6 -4 -2 0 2x 108 Time in s P, Q in W, var PWFI QWFI PWFII QWFII 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 -2 -1 0 1 2 3 4 5 6 7 8 x 108 Time in s P, Q in W, var PVSC4 QVSC4 PVSC6 QVSC6 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 31 Figure 25 Droop frequency in the converters of the HVDC link with Figure 26 Droop voltage reference for reactive power sharing in the converters of the HVDC links with 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 50 50.005 50.01 50.015 50.02 50.025 50.03 50.035 Time in s f in Hz fVSC4 fVSC6 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 179627 179628 179629 179630 179631 179632 179633 Time in s E in V EVSC4 EVSC6 Page 32 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 27 Oscillations of active power for VSC4 Figure 28 Oscillations of reactive power for VSC4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 6.9 6.9002 6.9004 6.9006 6.9008 6.901 6.9012 6.9014 6.9016 6.9018 x 108 Time in s P, Q in MW, Mvar 4 4.2 4.4 4.6 4.8 5 5.2 5.4 1.946 1.948 1.95 1.952 1.954 1.956 1.958 1.96 1.962 1.964 1.966 x 107 Time in s P, Q in MW, Mvar AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 33 6.1.2. AC Hub Model with Tie Cable The AC Hub model using a tie cable to connect the HVDC VSCs and offshore wind farms gives slightly different conditions for power sharing than the model with a PCC. There is a voltage drop over the impedance of the tie cable, which leads to different voltages at the output of the HVDC VSCs. Figure 29 shows the power input by the wind farms. When the generated power of wind farm I is high, the power for wind farm II is low, and vice versa. This is done to test the model under unsymmetrical conditions. Furthermore, the cables that connect wind farm II are assumed to be shorter, as indicated in Figure 22. Also, the transformer impedances in the branch of wind farm II and at VSC6 are multiplied with a factor of 1.5. At , the generated power of wind farm I decreases and the power of wind farm II increases. Figure 30 gives an impression of the power sharing response. While active power sharing is achieved after a transient of about one second, there is an offset between the reactive power absorbed by VSC4 and VSC6. Details on the responses are shown in Figure 31, Figure 32 and Figure 33. Figure 29 Wind farm power input for model with tie cable 1 1.5 2 2.5 3 3.5 4 -12 -10 -8 -6 -4 -2 0 2 4x 108 Time in s P, Q in W, var PWFI QWFI PWFII QWFII Page 40 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 40 Frequency response in the control loop of the HVDC VSCs Figure 41 Frequency response in PLL of wind farm VSCs 1.5 1.6 1.7 1.8 1.9 2 50 50.005 50.01 50.015 50.02 50.025 50.03 50.035 50.04 50.045 50.05 Time in s f in Hz fVSC4 fVSC6 1.4 1.5 1.6 1.7 1.8 1.9 2 44 46 48 50 52 54 Time in s f in Hz fVSC8 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 41 6.4. Response of Voltages and Currents Figure 42 to Figure 48 show the responses of voltages and currents in and frame for the simulations above. Reference and measured values of voltages and currents in the frame are matching, which can be observed in Figure 43 and Figure 44. Moreover, voltages and currents follow a sinusoidal waveform (Figure 45 and Figure 46). During transition to a different reference value, the sinusoidal shape is conserved and a smooth transient behaviour is achieved, as shown in Figure 48. Figure 42 Current in VSC6 in frame, no deviation from the reference values can be observed 1.5 2 2.5 3 3.5 4 4.5 5 -3000 -2000 -1000 0 1000 2000 Time in s iq, id in A iq* iq id* id Page 42 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 43 Current in VSC6 in frame in detail, the current reference is shown with a dotted line Figure 44 Output voltage(dotted line) and reference (solid line) of VSC6 in frame 1.52 1.54 1.56 1.58 1.6 1.62 1.64 1.66 -3000 -2000 -1000 0 1000 2000 3000 4000 -3000 -2000 -1000 Time in s iq, id in A iq* iq id* id 1 1.2 1.4 1.6 1.8 2 1.76 1.77 1.78 1.79 1.8 1.81 1.82 1.83 1.84 x 105 Time in s vq, vd in V vzq* vzq AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 43 Figure 45 VSC6 output voltage in frame Figure 46 VSC6 converter voltage in frame 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.2 -1.5 -1 -0.5 0 0.5 1 1.5 x 105 Time in s vz abc frame in V vza vzb vzc 1.7 1.75 1.8 1.85 1.9 -1.5 -1 -0.5 0 0.5 1 1.5 x 105 Time in s vl abc frame in V vla vlb vlc Page 44 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 47 VSC6 ouput current in frame Figure 48 VSC6 output current in frame in detail, smooth transition and sinusoidal shape are fulfilled 1.5 2 2.5 3 3.5 4 4.5 5 5.5 -3000 -2000 -1000 0 1000 2000 3000 Time in s Irl abc frame in A irla irlb irlc 2.92 2.94 2.96 2.98 3 3.02 3.04 3.06 3.08 3.1 -1500 -1000 -500 0 500 1000 1500 Time in s IL abc frame in A iLa iLb iLc AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 45 6.5. Response to more realistic Wind Data Input The dotted line in Figure 49 shows a wind speed profile over a time span of 15 s. By means of the interpolated power curve of the Enercon E-126 wind turbine (Appendix, Figure 53), this wind profile is converted into a power profile in the pu unit system (solid line in Figure 49). For the simulations carried out here, this power time series is fed into a laplace filter as described in Section 4.4. The output of the filter is multiplied by the rated power of the plant and serves as power reference for the VSCs of the wind farms. Both wind farms are assumed to have the same power profile. Figure 50 to Figure 52 show a satisfying response the HVDC VSCs. Figure 49 Wind speed (dotted line) and power input of wind plants in pu (solid line) relative to their rated power 0 2 4 6 8 10 12 14 16 12 14 16 18 Time in s Wind Speed in m/s 0246810 12 14 16 0.8 0.9 1 1.1 Power in pu Power Wind Speed Page 46 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 50 Power generated by wind farm I and II Figure 51 Power sharing of HVDC VSCs, note that from until the droop constants are modified 2 4 6 8 10 12 14 -12 -10 -8 -6 -4 -2 0 2 4x 108 Time in s P, Q in W, var PWFI QWFI PWFII QWFII 2 4 6 8 10 12 14 -2 0 2 4 6 8 10 x 108 Time in s P, Q in W, var PVSC4 QVSC4 PVSC6 QVSC6 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 47 Figure 52 Frequency reference within the control loops of the HVDC VSCs 2 4 6 8 10 12 14 49.99 50 50.01 50.02 50.03 50.04 50.05 50.06 Time in s f in Hz fVSC4 fVSC6 Page 48 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems 7. Future Work This work is only a first step, a feasibility study on a technical level for AC Hubs using a droop control approach. The model is comprised of high-level approximations for power converters and control issues. All the controls within the converter models are well-known concepts, the novelty being the applied droop scheme and the offshore generation scenario. Therefore, the results give indications on the general feasibility and further research needs to be carried out. An exact mathematical description of the variable frequency system and its transients is desired as well as a stability analysis, resulting in suggested parameters for tuning the control gains, depending on system architecture. The feedback loop utilised for power transmission via the AC Hub requires further investigation in terms of control loop design and optimisation. Moreover, a power flow monitoring and optimisation algorithm taking into account variable frequency and high-level control, giving optimised parameters for frequency and voltage of the VSCs is desired. It would be interesting to implement voltage control for reactive power sharing in the HVDC VSCs, the reference voltage coming from a higher level system analysis, e. g. a power flow monitoring system. Also, a comparison of the droop approach without using fast communication and a system running fully optimised with fast communication would be helpful for the evaluation of the different control approaches. Finally, the whole concept of the HVDC-overlay grid needs to be developed further in order to know more about the design of nodes within the system. How offshore wind farms, HVDC lines and different countries’ grids will be interconnected in future is not clear at the moment. AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 49 8. Conclusion Within this work, a control approach for VSCs in parallel in the absence of communication was developed and verified by simulation in MATLAB/Simulink. This concept could be applied to nodes in the planned HVDC-overlay grid in the North Sea, which are interconnecting offshore wind farms and HVDC links. Chapter 2 gives an overview on the problem, introduces the technology and the control theory behind the model of the proposed control scheme, which is later described in Chapter 3 in more detail. The parts of the system, including AC lines, transformers, converters, output filters as well as wind farm and converter power ratings are designed and sized in Chapter 4. MATLAB/Simulink simulation of the system verifies the feasibility of the chosen control approach afterwards. Although further research is necessary, the droop control scheme presented in this report is promising. The technical model can serve as basis for further research in this field, e. g. building a test bench. Although the detailed design of nodes within an HVDC-overlay grid in the North Sea is not clear yet, the results indicate that choosing a droop control scheme and AC Hubs is a feasible option. Two different topologies to connect wind farms and HVDC VSCs were analysed: a system with PCC and a system using a tie cable. This leads to the conclusion that the final layout of such nodes will have significant influence on the control approach. Furthermore, it is assured that power transmission and power sharing between converters in the AC Hub can be adjusted without using fast, reliable communications, nor needing predictions of the power generation by the offshore wind farms. AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 57 References [1] Patricia Weis-Taylor. (2012, July) IEA Wind Annual Report for 2011. [Online]. Available: http://www.ieawind.org/annual_reports_PDF/2011.html [July 03, 2013]. Print: Boulder, Colorado, 2012. [2] P. Bresesti, W. L. 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[July 08, 2013] AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Page 59 Appendix Table 3 Parameters used in the simulation: Description Name Value Unit Power reference for pu system Sb 1000 MVA Voltage reference for pu system Ub 220 kV Voltage level at short circuit Ecc 10 % Inductance over resistance ratio for transformer and converter impedance XoverR 20 - Rated frequency f 50; Hz Electrical angular velocity w 2*pi*f; 1/s DC capacitor C 50 mF AC capacitor filter at Droop VSC output CfAC 5 F Norm of reactive power to be compensated Qc 750 Mvar Current limitation for Pmax=1000MVA iqMAX 3712 A Current limitation for Qmax=500 Mvar idMAX 1856 A Leakage inductance of transformers and VSC output impedance Ltrafo 15,4 mH Resistance of transformers Rtrafo 0,2417  DC voltage of wind farm I EDC1 1000 V DC voltage of HVDC VSC4 EDC3 300 kV DC voltage of wind farm II EDC5 300 kV DC voltage of HVDC VSC6 EDC7 1000 V Droop constant for power m -0,05 Hz/GW Droop constant for reactive power n -17,4 V/Gvar Droop frequency reference fref 50 Hz Droop voltage reference Eref 179,63 kV Resistance of 1km of cable R1km 3,21 m Inductance of 1km of cable L1km 0,33 mH Capacitance of 1km of cable C1km 0,8 F Chosen cross-section of HVAC cables 1400 mm² HVAC cables in parallel 4 The MVAC cables were assumed to have the same pu impedance values for the sake of lower system complexity Compensating reactor at PCC Lc 0.1926 H Changes in branch Sampling time for data logging Ts 1 ms Page 60 AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems Figure 53 Power curve of chosen Enercon E-126 wind turbine, the data points marked with an x are taken from the online datasheet [22] 0 5 10 15 20 25 0 1000 2000 3000 4000 5000 6000 7000 8000 Wind speed in m/s Power in kW