A Generalized Control Strategy of Per-Phase DC Voltage Balancing for Cascaded Multilevel Converter-based STATCOM

Because of its modularity, scalability and good power quality, the cascade multilevel converter (CMC), comprising a number of modular H-bridge voltage source converter (VSC) in each phase, is considered as one of the most promising topologies for STATCOM application. However, how to maintain a balanced DC-bus voltage is not well known. In this paper, a generalized per-phase DC-bus voltage balancing scheme of a CMC-based STATCOM is presented. STATCOM small signal model and existing control scheme are reviewed first. The concept of proposed per-phase DC-bus voltage balancing scheme based on the loop-gain shaping control design method and an "imaginary" dq/abc park transformation, is described. Secondly, the open-loop transfer function is derived from STATCOM small-signal model in dqO coordinates so that the compensator parameters of the proposed control strategy can be easily designed to achieve a flexible control bandwidth and desired stability margins without any tuning. The simulation results on the detailed switching model show the consistency of the proposed control scheme. Finally, the control is verified experimentally using a STATCOM transient network analyzer (TNA).