Abstract: When a three-phase asymmetric fault occurs, the capacitor voltages between phases of the star-connected static synchronous compensator(STATCOM) valve group tend to be unbalanced, which is a key difficulty in the fault ride-through (FRT). To address this issue, based on the star-connected STATCOM topology, a single-phase instantaneous power model of the STATCOM is established to analyze the three-phase AC voltage characteristics under diverse grid faults, along with inter-phase power transfer and capacitor voltage variation patterns in the STATCOM. According to the integrated control idea of positive and negative sequence power, a capacitor voltage balancing control strategy combining open-loop and closed-loop control is proposed. An electromagnetic transient(EMT) simulation model built on RTDS is tested against various typical grid faults. Results demonstrate that the proposed strategy effectively balances capacitor voltages in STATCOM commutation valve. The capacitor voltage balancing control strategy using integrated control of positive and negative sequence power resolves limitations of conventional independent positive and negative sequence control methods and enhances STATCOM’s AC fault ride-through capability and operational safety margin.

Key words: STATCOM, SLCC, star-connected topology, three-phase asymmetry, fault ride-through(FRT), positive-sequence component, negative-sequence component, capacitor voltage balancing