Active Power Backflow Suppression Strategy Based on Improved Zero-Sequence Voltage Injection for Three-Phase CDCB-CHB inverter under Inter-Phase Short-Circuit Fault

Three-phase cascaded H-bridge (CHB) inverter can connect medium-voltage power grid without the bulky power-frequency transformer, and can realize multi-level output, reducing the volume of the filter. In addition, when the common DC-bus structure is adopted, the power imbalance problem of the CHB inverter can be completely avoided. Therefore, the three-phase CHB inverter based on the common DC bus structure (CDCB-CHB) receives more attention in applying high-voltage photovoltaic (PV) power generation. However, the three-phase CDCB-CHB inverter has the active power backflow (APB) problem during asymmetric voltage sags, which will cause the DC voltage of the H-bridge (HB) to rise, triggering inverter overvoltage protection. Some APB suppression strategies have been proposed, but the existing strategies will lead to overmodulation in some cases during the inter-phase shortcircuit fault. This paper proposes an APB suppression strategy based on the improved zero-sequence voltage injection method, it can reduce the overmodulation region of the system further, and expand the operational range of the CDCB-CHB inverter under the condition of inter-phase short-circuit fault. A down-scaled experimental platform is made to verify the effectiveness of the proposed strategies.