Control Scheme of Cascaded H-bridge Statcom: Zero-sequence Voltage and NegativeSequence Current

Control scheme of cascaded H-bridge Statcom in three-phase power systems. Cascaded H-bridge Statcom has merits in point of switching losses, output harmonics, and the number of circuit components. But every H-bridge cell has isolated dc capacitors. So the balancing problem of capacitor voltages exists. Since Statcom is often requested to operate under asymmetrical condition by power system faults such as one line grounding or two-line short circuit. Recently, several methods of voltage balancing between phase clusters are proposed. One method is based on zero-sequence voltage injection. However it needs wide margin of dc capacitor voltage compared with rated power system voltage when the unbalance of power system voltage is large. The other method handles the capacitor voltage unbalance by independently controlling active power of individual phase cluster, but unbalance of power system voltage is not considered.