Research On Strategy Of Voltage Feedforward Space Vector Modulation In STATCOM

In the applications of high-voltage and large-capacity inverter, cascade multilevel voltage source inverter is widely used. When H bridge topology is used as the cascaded unit in the high-voltage large-capacity static var compensator(STATCOM), the problem of unbalanced DC voltage among the cascaded units becomes common. In order to avoid the system instability caused by the unbalance or even divergent trend of unit DC voltages in cascade multilevel STATCOM, this paper proposes a feed-forward voltage space vector modulation(FFSVM) strategy for cascaded STATCOM to solve the unit DC voltage balance control problem based on the traditional voltage space vector modulation(SVM) strategy.The paper firstly introduces the algorithm of the new modulation strategy, and designs the control system and the modulation module, then applies the new modulation strategy in single-phase H-bridge cascaded in STATCOM with two units in each phase. Then, compared with traditional phase-shift pulse width modulation(PS-PWM), simulation results show that feed-forward voltage space vector modulation strategy can output voltage waveform with good quality under the condition of unbalance unit DC voltage and has the ability of balancing the unit DC voltages in STATCOM. Based on the new modulation strategy proposed in this paper, FFSVM is applied to the structure of the three-phase two H-bridge cascaded STATCOM. The paper builds simulation model with Matlab/Simulink to verify the correctness of the FFSVM modulation method and the feasibility in three-phase power system. The algorithm in the simulation process and the method of restraining the current and voltage unbalance are the same as in the single phase two H-bridge cascaded STATCOM. The results of the simulation with the same simulation parameters applied in phase shifted pulse width modulation(PS-PWM) method are compared with FFSVM modulation mode. This comparison further verifies that FFSVM modulation can make the inverter output sine wave voltage waveform in three-phase cascade multilevel circuit. And FFSVM modulation has the ability of forcing the unit DC voltages converge in the STATCOM, highlighting the FFSVM modulation mode is suitable for the conditions of unbalanced the DC voltages.