Practical operation range improvement of Voltage-Sourced Converter based STATCOM

Voltage-Sourced Converter (VSC) based STATCOM is used for voltage regulation in transmission and distribution systems. In order to lower the STATCOM losses and total system loss, switching devices in “angle controlled” STATCOM are fired once with the line frequency (50/60 Hz). Despite the superior features on voltage waveform quality and efficiency, the practical angle controlled STATCOMs suffer from the over current and possible saturation of the interfacing transformers caused by negative sequence generated by system faults and disturbances. In this paper, the current control scheme and Instantaneous Phase-Locked Loop (IPLL) for an “angle controlled” STATCOM are presented. From power electronic point of view, IPLL prevents over-current in the VSC under system faults and bus voltage distortions. The current control can further improve STATCOM operation with higher percentages of negative sequence voltage. In addition to improved system performance under fault, the system can be installed with regular transformers, not custom-designed or over-rated transformers. RTDS (Real Time Digital Simulator) results verify the applicability of the proposed IPLL based control methods.