Capacitor Voltage Control of MMC-STATCOM during Unbalanced AC System Fault

This paper describes a capacitor voltage control method of a modular multi-level cascade (MMC) converter configuring delta connection for a STATCOM. Each converter cell has its capacitor in DC-side, therefore this circuit configuration requires controlling the capacitor voltage level individually. Especially, controlling the voltage of each phase is important because the grid voltage can be a sudden unbalance condition in a transient event such as a system fault. The proposed method of the phase balance of cell capacitor voltages adjusts both the negative-sequence output current and the circulating current during unbalanced AC power system faults. The 5 kVA experimental system shows that the proposed method is effective during such a system fault.