AC and DC fault ride through hybrid MMC integrating wind power

The 6th International Conference on Renewable Power Generation (RPG) 19 20 October 2017 AC and DC fault ride through hybrid MMC integrating wind power Shuai Cao 1, Wang Xiang 1, Liangzhong Yao 2, Bo Yang 2, Jinyu Wen 1 1 State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province, People s Republic of China 2 State Key Laboratory of Operation and Control of Renewable Energy & Storage Systems, China Electric Power Research Institute, Beijing 100192, People s Republic of China E-mail: xiangwang1003@foxmail.com Published in The Journal of Engineering; Received on 6th October 2017; Accepted on 2nd November 2017 Abstract: To transmit bulk wind power over long distance, using high-voltage direct current (HVDC) transmission technology with overhead lines is a preferred approach. Since direct current (DC) faults are frequent in overhead lines, this study adopts the hybrid modular multilevel converter (MMC) based on half bridge sub-module and full bridge sub-module to integrate wind power. The design of controllers, the alternating current (AC) and DC fault ride-through strategy of the hybrid MMC-HVDC system will be presented. Since the wind power continues injecting into the system during the faults, the damping resistor is adopted to dissipate the surplus wind power. The coordination control of the damping resistor and the chopper resistor inside the wind turbine is designed. Finally, the performance during AC, DC faults ride though is verified by extensive simulations. 1 Introduction In recent years, the energy crisis and environmental pollution have attracted the attention of many countries. To reduce carbon emission and the proportion of coal-fired power generation, wind power has become the most competitive clean energy [1]. The high voltage direct current (HVDC) transmission technology has been proved as an effective way to integrate wind farms. As a modular multilevel converter (MMC) has many advantages such as modular structure, low volume, self-commutation and no need to implement reactive power compensation devices and filters, it becomes the preferred approach to integrate wind power [2]. Since many large wind farms are far away from the load centres, power transmission using overhead lines is inevitable. However, the overhead lines are prone to faults, in order to deal with direct current (DC) faults, high power DC circuit breakers (DCCB) are necessary for the MMC based on half bridge sub-module (HBSM). However, the high-power DCCB is extremely expensive and not mature [3]. Some literature proposed newly topologies with DC fault blocking capability. By blocking the firing signals of insulated gate bipolar translator (IGBTs), the DC fault current is naturally blocked [4]. However, the method of blocking IGBTs leads to the shutdown of MMC and prolongs the speed of system recovery. The hybrid MMC composed of HBSM and full bridge sub-module (FBSM) can ride-through DC fault without blocking IGBTs and continually support the alternating current (AC) system during faults, which makes it a promising candidate for application in wind power integration over a long distance. However, the wind farm constantly produces power during the fault, measures should be taken to dissipate the surplus wind power. Otherwise, the sub-module (SM) capacitors will be overcharged and the power electronic devices might be damaged. To solve this problem, Yu et al. [5] proposed a power step-down control to reduce the output power of the wind farm. Although no hardware cost is increased, the effect is not as good as that of using the dissipative device. Dong et al. [6] designed a dissipation resistor on the DC lines to absorb the wind power during the fault, but the required resistance is so large that it needs a large area and high costs. To dissipate the surplus wind power, this study adopts a damping resistor on the AC side of wind farm side MMC (WFMMC) to absorb excessive wind power. In the meantime, a small chopper resistor is introduced on the DC side of the rotor side converter. By cooperating with hybrid MMC and dissipative resistors, the system can achieve AC and DC fault ride through. This paper is organised as follows. Section 2 outlines the layout of the wind power integrating system based on hybrid MMC. The normal operation and control strategies are analysed in Section 3. Section 4 discloses the fault ride through a strategy of hybrid MMC and coordination with dissipative resistors to ride through AC and DC fault. Simulation results are presented in Section 5 to demonstrate the proposed methods. Finally, Section 6 draws the conclusions. 2 Topology of the wind power integration system with hybrid MMC Fig. 1 shows the layout of the wind power integration system. In Fig. 1, the wind farm is composed of a permanent magnet synchronous generator (PMSG) and a full power converter (FPC). Both the WFMMC and grid side MMC (GSMMC) adopt the hybrid MMC topology. To dissipate surplus wind power during the faults, the damping resistor and chopper resistor are implemented at the AC side of the WFMMC and the DC side of the FPC, respectively. The topology of the hybrid MMC is shown in Fig. 2. Each arm of the hybrid MMC is composed of half the FBSMs and half the HBSMs. The topologies of FBSM and HBSM are also shown in Fig. 2. 3 Control strategy of the wind power integration system 3.1 Control of the GSMMC Since FBSMs have the ability to produce negative voltages, the DC line voltage of hybrid MMC can be regulated independently of the capacitor voltages [7]. Therefore, a DC modulation index M dc is

The DC control loop is used to control the DC voltage. Herein, the DC current loop control is M dc = K i (I dc ref I dc pu )dt + K p (I dc ref I dc pu ), (3) Fig. 1 Topology of wind power integration system where I dcref represents the DC current reference which is determined by the outer DC voltage control loop. 3.2 Control of the WFMMC Since the wind generator is a passive source, the AC side voltage should be supported by the WFMMC. The control structure of the WFMMC is shown in Fig. 4. Comparing with Fig. 3, the DC control loop in Fig. 4 is used to control the average capacitor voltage while the AC control loop is used to control the AC voltage. The AC voltage controller is detailed as follows: I sdref = K o p (U sdref U sd ) + K o i (Usdref U sd )dt, I sqref = 0, (4) m d = K i p (I sdref I sd ) + K i i (Isdref I sd )dt, w = 2pf 0 t + w 0, Fig. 2 Topology of hybrid MMC introduced. The upper arm voltage of phase A v pa can be described as v pa = V dc 2 v a = M V dcm dc 2 M V dcn 2 cos (vt + u), (1) where M is the AC modulation index of the hybrid MMC. Similarly, the lower arm output voltage v na can be expressed as v na = V dc 2 + v V a = M dcm dc 2 + M V dcn 2 cos (vt + u), (2) where U sdref is the AC voltage reference, I sdref and I sqref are the AC current references, m d is the d-axis component of the AC modulation ratio, w is the reference phase angle, and w 0 is the initial phase angle. 3.3 Control of the wind farm The wind farm control strategy is shown in Fig. 5. The wind turbine adopts the maximum power tracking control [8], and the output of the mechanical power is calculated as P mec = 0.5C p (l, b)prr 2 v 3, (5) where C p (λ,β) is the wind power utilisation coefficient, ρ is the air density, r is the blade radius, and v is the wind speed. At a certain wind speed, the wind power is only determined by C p (λ, β). If the pitch angle β is constant, C p (λ, β) is related to the tip speed λ. There is an optimum blade velocity ratio λ opt, which when M dc = 0, the arm voltage only contains a pure AC component. Thus, the hybrid MMC can operate at zero DC voltage to ride through DC short circuit fault without being blocked. The GSMMC is used to control the DC voltage of the HVDC system. The control structure of the GSMMC shown in Fig. 3 is composed of a DC control loop and an AC control loop. The AC control loop is used to control the average capacitor voltage of all the SMs to be a constant. Thus, at any instant, the active power between the AC and DC side can be achieved. Fig. 4 Control structure of WFMMC Fig. 3 Control structure of GSMMC Fig. 5 Wind farm control strategy

corresponds to the maximum wind power utilisation coefficient C pmax. The tip speed λ = ω m r/v, where ω m is the speed of the wind turbine. Assuming the wind turbine is running in the state of λ opt, the output P mec is ( ) 3 P mech opt = 0.5rC Pmax pr 2 v m r = K l optv 3 m. (6) opt Since the frequency of wind power is variable, it is necessary to utilise the back-to-back voltage source converter (VSC) to obtain a stable frequency consistent with the AC grid. In the FPC, its rectifier side uses active power control, while the inverter side uses the DC voltage control. As the PMSG does not need to be magnetised, the zero current control can be adopted on the rectifier side. The active power control is added to the q-axis outer loop, controlling the torque to achieve the maximum power tracking. 4 AC and DC fault ride-through strategy 4.1 DC fault ride-through To determine the low voltage operation capability of hybrid MMC, the MMC operating range is demonstrated as follows. The hybrid MMC arm voltages are v p = V dc 2 v, v n = V dc 2 + v. Denote the maximum and minimum values of the upper arm voltages are V p+, V p. According to (7), they can be described as ( V p = V dc v ) V 2V dcn V dcn = V dcn dcn 2 (V dcmin M), (8) ( V p+ = V dc + v ) V 2V dcn = V dcn dcn 2 (V dcmin + M). (9) V dcn Considering the most severe condition that only the FBSMs are inserted, the V p+ and V p are expressed as { V p = 0.5V dcn, (10) V p+ = 0.5V dcn, when the AC modulation M = 0.9, the operating range of DC voltage is 0.1 to 0.1V dcn. Thus, the hybrid MMC can ride through DC faults. When DC fault happens, the DC control loops of GSMMC and WFMMC switch to zero DC current control to reduce the value of M dc. Meantime, the damping resistor and chopper resistor are put into operation to absorb the surplus wind power, so that to protect the MMC and wind turbine from over-heat damage. After the DC fault is cleared, the GSMMC resumes to DC voltage control. When the DC voltage is resumed, the WFMMC recovers to SM average voltage control. As the AC voltage control is controlled to be constant, the damping resistor turns off and the wind power is restored. 4.2 AC fault ride-through When a single-phase to ground fault, F 1, happened as shown in Fig. 1, the GSMMC s AC voltage drops instantaneously. To dissipate the surplus wind power, the damping resistor and chopper resistor are inserted. As long as the surplus wind power is dissipated, the SM capacitor voltage can be controlled near the rated value, (7) Fig. 6 Control logical diagram of the dissipating resistors which makes DC voltage stable. Therefore, the wind turbine can still run normally without being cut off and the system can rapidly recover to normal operation. 4.3 Control and design of the dissipation and chopper resistors During the DC fault, the conventional half-bridge MMC is blocked, so the damping resistor cannot be installed on the AC side because of the breakdown of AC voltage. However, the hybrid MMC can support the AC voltage during the DC fault so that the damping resistor can be paralleled on the AC side of the WFMMC to absorb the wind power. Its resistance can be determined by the following formula: R = U 2 ac P out. (11) The control logical diagram of damping and chopper resistors is shown in Fig. 6. An appropriate threshold U dclim is set to prevent the malfunction caused by the normal fluctuation of the DC voltage. During the faults, the DC voltage of the FPC is detected to prevent potential overvoltage. If the overvoltage exceeds the threshold U clim, the chopper resistor is inserted to absorb the wind power to ensure the safety operation of MMC. The firing logic of the chopper resistor is shown as below. As shown in Fig. 7, the deviation of the DC voltage is compared with the triangular wave to determine the conduction duty cycle of the chopper circuit. Through the cooperation of the damping resistor and chopper resistor, the wind power is consumed in the process of faults so that to achieve the faults ride through without cutting off the wind turbines. 5 Simulation verification To verify the theoretical analysis and control strategies, the wind power integrating system is built based on the PSCAD/ Fig. 7 Firing logic the chopper resistor

Table 1 Parameters of the simulated model Parameter GSMMC and WFMMC converter nominal capacity, MVA 900 DC voltage, kv ±200 transformer capacity, MVA 900 turn-ratio of transformer 400/220 leak inductance, p.u. 0.15 number of SMs of each arm 95HBSM + 95FBSM capacitance of each SM, μf 9000 arm inductance, mh 31 DC line inductance, mh 40 5.1 Wind power fluctuation simulation During start-up, the GSMMC needs to start at first to establish the DC voltage. Then the WFMMC starts to establish the AC voltage. Once the AC voltage is established, the wind power starts to be transmitted. To verify the system control structure in normal operation, the wind power drops from 1.0 to 0.5 p.u. during 1.2 1.3 s. The simulation results are shown in Fig. 8. Fig. 8a shows that the DC voltage follows the reference value (U dcref ) well and Fig. 8b shows that the AC voltage of WFMMC gradually ramps up to 1 p.u. from 0.4 s. After 0.6 s, the AC voltage is well controlled at a rated value. Then, the wind power EMTDC. The parameters of GSMMC and WFMMC are shown in Table 1. The damping resistor is 176.6 Ω and chopper resistor is 3 Ω. U dclim = 0.95 p.u. and U clim = 1.1 p.u. In this study, the normal fluctuation range of DC voltage is set as ±3%. Fig. 8 Simulation waveforms during wind power fluctuation Fig. 9 Simulation waveforms during DC fault

begins to transmit, as shown in Fig. 8c. The system achieves rated operation and transmits full wind power at 1.1 s. With the wind power decreasing, the AC and DC voltages of the WFMMC are still controlled at the rated value, while the AC and DC currents are half of the rated value, as shown in Figs. 8d and e. Figs. 8f and g show that the average capacitor voltage can track the reference value well which verifies the effectiveness of the average capacitance voltage control. The simulation results show that the system can respond to the change of wind power and realise the stable operation under power fluctuation. 5.2 Pole to pole DC short circuit fault ride through To verify the capability of DC fault ride-through, a pole to pole DC short-circuit fault is applied at 1.5 s and lasted for 0.1 s. During the fault, MMC continually operates without blocking the IGBTs. The simulation results are shown in Fig. 9. In Fig. 9a, when the DC fault occurs, the DC voltage drops rapidly to zero. Meanwhile, the GSMMC and WFMMC switch to zero DC current control. Since hybrid MMC is not blocked during the fault, WFMMC can still control the AC voltage, as shown in Fig. 9b. In Figs. 9c e, although the instantaneous DC current has risen more than double the rated value, the arm currents of GSMMC and WFMMC are still within the safety range. Fig. 9f shows that when the dissipating resistors are inserted, the wind power received by WFMMC drops rapidly to 0. Thus the GSMMC and WFMMC SM capacitors can avoid over-voltage and current. After the fault is cleared, the DC voltage is resumed to the rated value within 0.2 s. Figs. 9g and h show that the SM capacitor voltages are kept within the safety range due to the power dissipation by damping and chopper resistors. As the DC voltage of FPC recovers the rated values, the wind power resumes full power transmission after 2.1 s. 5.3 AC single-phase grounding fault ride through To verify the capability of AC fault ride through, an AC singlephase grounding fault is applied at 2.5 s and lasted for 0.1 s. The other operations are consistent with Section 5.2, and the simulation results are shown in Fig. 10. In Fig. 10a, the AC voltage fluctuates immediately as the AC fault occurs. However, due to the SM average capacitor voltage control, the capacitors are not discharged, so it will only cause voltage ripples on the DC side. Thus, the DC voltage and current are maintained near the rated value, as shown in Figs. 10b and c. Figs. 10d and e show that the WFMMC AC voltage can be maintained during the fault. Also, the surplus wind power absorbed by damping and chopper resistors is reduced. Figs. 10f and g show that the SM capacitors of GSMMC and WFMMC do not experience overvoltage. After the fault is cleared at 2.6 s, the GSMMC and WFMMC resume to normal operation. The dissipative resistors are switched off and the wind power transmission is restored, which verifies that the system has the AC fault ride through capability. 6 Conclusions The wind power integration system based on hybrid MMC can ride through the AC and DC faults without blocking MMC. Through the co-operation of damping and chopper resistors, the surplus wind power can be dissipated and the wind turbines are not needed to be cut-off. Using the average capacitor voltage control, the SM capacitors are well maintained in the safety range. The simulation results validate the DC and AC fault ride through capability as well the fast recoverability. 7 Acknowledgement This work is sponsored by the State Grid Corporation of China Science and Technology Project (NYN17201600314). 8 References Fig. 10 Simulation waveforms during AC fault [1] Li C.H., Zhan P., Wen J.Y., ET AL.: Off-shore wind farm integration and frequency support control utilizing hybrid multiterminal HVDC transmission, IEEE Trans. Ind. Appl., 2014, 50, (4), pp. 2788 2797 [2] Zeng R., Xu L., Yao L.Z., ET AL.: Hybrid HVDC for integrating wind farms with special consideration on commutation failure, IEEE Trans. Power Deliv., 2016, 31, (2), pp. 789 796 [3] Xiang B., Liu Z.Y., Geng Y.S., ET AL.: DC circuit breaker using superconductor for current limiting, IEEE Trans. Appl. Supercond., 2015, 25, (2), pp. 1 7 [4] Xiang W., Lin W.X., Wen J.Y.: Equivalent electromagnetic model of self-blocking MMC with DC fault isolation capability. Proc. IEEE Power Energy Society General Meeting, 2016, pp. 1 5 [5] Yu Y., Xu Z., Xu Q., ET AL.: A control strategy for integration of permanent magnet direct-driven wind turbines through a hybrid HVDC system, Proc. CSEE, 2016, 36, (11), pp. 2863 2870 [6] Dong X., Zhang J.J., Wang F., ET AL.: AC and DC fault ride-through technology for wind power integration via VSC-HVDC overhead lines, Autom. Electr. Power Syst., 2016, 40, (18), pp. 48 55

[7] Lin W.X., Jovcic D., Nguefeu S., ET AL.: Full-bridge MMC converter optimal design to HVDC operational requirements, IEEE Trans. Power Deliv., 2016, 31, (3), pp. 1342 1350 [8] Putri R.I., Pujiantara M., Priyadi A., ET AL.: Optimum control strategy of grid connected PMSG wind turbine based on energy storage system. 2016 Int. Seminar on Intelligent Technology and Its Application, June 2016, pp. 623 628