Optimal control strategy for cascaded STATCOM under unsymmetrical power system conditions

This paper presents a optimal control method for cascaded STATCOM under unsymmetrical conditions by power faults. Cascaded H-bridge multilevel converter is considered suitable for power system application. Since unsymmetrical faults have much higher probability than symmetrical faults in power system, it is importance for STATCOM to operate continuously under unsymmetrical conditions. While isolated capacitors in each H-bridge makes it more difficult to balancing 3-phase dc voltage. The mathematical model of cascaded STATCOM and its operation principle under unsymmetrical power conditions are analyzed. A optimal control method based on energy balance of 3-phase dc capacitors, including selection method of 3-phase current reference to ensure dc voltage stability and continuous operation. The validity is examined by digital simulation under ±50Mvar cascaded STATCOM connected to 110kV power grid.