Decoupled Discontinuous Modulation for Cascaded H-Bridge StatCom with Star Configuration

This paper presents a novel zero-sequence voltage injection method for discontinuous modulation (DM) in star-connected cascaded H-bridge (CHB) static compensators (Stat-Coms). Conventional DM methods in CHB StatComs suffer from control interactions during unbalanced grid conditions. This paper proposes a DM that eliminates this control interaction by inserting a clamping transition within a third of the fundamental period. The transition instant is decided by comparing a duty cycle, which is calculated to mitigate the fundamental-frequency in the zero-sequence voltage for discontinuous operation, with a triangular carrier signal with a frequency of three times the fundamental. The clamping intervals under the proposed DM are affected by the phase-shift of this carrier. Specifically, the paper analyses the effects on the harmonic content of the proposed zero-sequence voltage, switching loss, twice-fundamental-frequency capacitor voltage ripple, and current total harmonic distortion, for different phase-shift angles of the DM carrier signal. Besides, the feasibility of the proposed DM in dealing with a grid voltage sag is also discussed.