Control strategy for cascade multilevel inverter based STATCOM with optimal combination modulation

Static synchronous compensation (STATCOM) is a flexible AC transmission system (FACTS) device connected in shunt to the network for generating or absorbing reactive power. The STATCOM is used to regulate voltage, control power factor and stabilize power flow. The transformerless STATCOM is attractive due to low cost and power losses. Cascaded multilevel inverters are viable topologies for the transformerless STATCOM since each power semiconductor device only sustains low voltage. To meet IEEE 519 standard and keep low switching frequency, an optimal combination modulation strategy is used, which leads to some challenges in designing the controller, such as extra switching and the balancing of individual dc capacitor voltages. In this paper, the design of the controller is specified. Simulation results and experimental results are shown to verify the performance of the proposed control strategy.