Research On Control Strategy Of H-bridge Cascaded Multi-level STATCOM

STATCOM has been paid great attention because of its good compensation performance. With the expansion of the grid scale, STATCOM develops towards direction of high-voltage and high-power. Based on the characteristics of multi-level and easy to extend, H-bridge cascaded STATCOM has been a hot topic nowadays. But H-bridge cascaded STATCOM has some problems such as reactive power control strategy, voltage balancing control strategy and fault processing control strategy.Reactive power control strategy plays a most important role in the operation of STATCOM. Since it is a strongly coupled and nonlinear system, in order to eliminate coupling of active and reactive power, and improve the robustness, feedback linearization and sliding mode control theories are used. The simulation results show that the strategy has both better dynamic performance and robustness than the feed-forward decoupling control strategy, and is suitable for engineering application.As for voltage balancing control strategy, two methods are introduced:energy exchange with grid and energy exchange with external circuit. The paper proposes a kind of method by adjusting both modulation ratio and phase shift angle. Simulation results show it has good performance. Since the method of energy exchange with grid is coupled with reactive power control strategy, another voltage balancing method is proposed, which is independent of reactive power control strategy and has been verified by simulation in the Matlab/Simulink.The fault processing technologies are classed into three kinds:peer bypass technology, neutral point drifting technology and redundant fault-tolerant technology. Simulation results show that peer bypass technology has limited the reactive compensation capacity. Both neutral points of grid and STATCOM are not directly connected, which is the theoretical foundation of neutral point drifting technology. Simulation results show that it has greatly improved the utilization ratio of H-bridge power unit. If taken the variation of reference value of capacitor voltage as a standard, redundant fault-tolerant technology has three types. All three methods are simulated in the Matlab/Simulink. and the third method has advantage over the others.