Circulating current derivation and comprehensive compensation of cascaded STATCOM under asymmetrical voltage conditions

Cascaded static synchronous compensator (STATCOM) with delta-connected configuration is an effective solution to compensate reactive and negative-sequence currents. This study focuses on the circulating current and compensation method of delta-connected STATCOM under asymmetrical voltage conditions. According to the phasor analysis, constrains of voltage phasors and current phasors under reactive current compensation mode, negative-sequence current compensation mode and comprehensive compensation mode are deduced. On the basis of this, the analytical expressions of circulating current under the three modes are derived. Furthermore, a comprehensive compensation method for delta-connected STATCOM is proposed. In the proposed method, phase current references are calculated directly with the instantaneous voltage and current signals, which could enhance the dynamic behaviour and cluster-balancing control performance especially under asymmetrical voltage conditions. Finally, simulation and experiment results have been given to verify the effectiveness and feasibility of the proposed control method.