Dynamic performance of PWM STATCOMs operating under unbalance and fault conditions in distribution systems

Static shunt compensation, based on PWM synchronous voltage sources, is becoming a viable solution for improving power quality in distribution systems, including voltage regulation at the load bus. This paper describes the operating principles and control techniques of PWM STATCOMs under nonideal conditions including faults. Negative and zero sequence components are taken into account, and compensator response time is addressed. Two voltage regulation schemes are proposed and evaluated: the first is the direct voltage control scheme, with a parallel current limiting loop; the second scheme is based on the current controlled voltage source inverter concept, which decouples the voltage and current control functions. Steady state and transient performance obtained on an experimental STATCOM prototype are given for balanced, unbalanced and fault conditions. It is demonstrated that both the proposed schemes can satisfactorily operate under most operating conditions found in distribution systems. The inner current loop scheme enhances system performance, however, the system dynamic response may exhibit instability under certain conditions