Control Scheme of Multi Level Cascaded H - Bridge STATCOM

This paper presents a 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, capacitor voltage balancing between phase clusters is particularly important. Solving this problem, a technique using zerosequence voltage and negative-sequence current is proposed.By this scheme, the STATCOM is allowed to operate under asymmetrical conditions by power system faults. The validity is examined by digital simulation under one line and two-lines fault circuit condition. IndexTerms: Capacitor voltage balancing, cascaded Hbridge, multilevel converter, negative-sequence current, STATCOM, zerosequence voltage.