A control structure for line-frequency-switched STATCOMs under system faults

Voltage-Sourced Converter (VSC) based Static Synchronous Compensator (STATCOM) is used for voltage regulation in transmission and distribution systems. Comparing with PWM STATCOMs, Angle-controlled STATCOMs are fired at line frequency to lower the system losses. In recent years, angle-controlled STATCOMs have been deployed by utility owners for the purpose of voltage regulation, voltage stability improvement and increasing operational functionality. Despite the superior feature on voltage waveform quality and efficiency, the practical angle-controlled STATCOMs suffer from the over-current (and trips) and possible saturation of the interfacing transformers caused by negative sequence current during unbalanced conditions and system faults. This paper specifically proposes a control structure to improve the angle-controlled STATCOMs performance under unbalanced conditions and system faults. The main improvement is to decrease the negative sequence current and DC-link voltage oscillations substantially under power line faults through the control and not the components design. PSCAD/EMTDC and experimental results verify the validity of the proposed control structure under unbalanced conditions and system faults.