Low-Capacitance Cascaded H-Bridge STA TCOM with Enhanced Control Scheme under Unbalanced Grid

Preserving the stable operation of STATCOMs under grid voltage sags has been a primary control challenge. This challenge is even more detrimental to the control system in the Low-Capacitance STATCOMs due to the relatively large capacitor voltage ripples. By utilizing the cluster voltage balancing control based on zero-sequence voltage injection, this paper proposes an optimal control method for the Low-Capacitance STATCOM to balance the capacitor voltages in the presence of grid voltage faults. In addition, the destructive effects of unbalanced capacitor ripples are canceled in the control system to maintain the proper current quality and power flow of the converter. The paper verifies the investigated analysis by providing simulation results under various grid voltage sag conditions.