In order to overcome the challenge of reactive power control under unbalanced voltage conditions

a reactive power control strategy is presented for delta-connected static synchronous compensator(STATCOM)to inject positive- and negative-sequence reactive power flexibly by controlling the ratio between positive- and negative-sequence reactive power. The voltage of the point of common coupling(PCC)can be boosted by injecting positive-sequence while the negative-sequence voltage can be suppressed by injecting negative-sequence. Then

circulating current and phase current references under the unbalanced voltage are derived based on the instantaneous reactive power theory. The dynamic performance is fairly good using instantaneous commands. Moreover

peak current is limited in real-time to guarantee safe operation of STATCOM during asymmetrical voltage fault condition. Thus

the performance of STATCOM in engineering practice is improved. Simulation results are given to verify the performance of the proposed method.