LVRT Control Strategy for Three-Phase CHB PV Inverter Based on Coordinated Compensation of Fundamental-Frequency Adaptive Zero-Sequence Voltage and its Odd Harmonics

During asymmetric faults in power grid, the negative-sequence modulation voltage decomposed by three-phase isolated cascaded H-bridge photovoltaic grid-connected inverter will redistribute three-phase active powers, which may cause active power backflow problem, that is, the output power of one phase in the inverter to be less than zero. The backflow power will cause H-bridge dc-bus voltages to pump up, triggering overvoltage protection and shutting down the inverter from the grid. Although existing methods effectively address single-phase and two-phase short-circuit faults, there is still larger room for improvement in inter-phase short-circuit faults. Therefore, this article proposes an adaptive zero-sequence voltage square wave compensation strategy, which reduces the modulation voltage amplitude by compensating for all odd harmonics of the line-frequency adaptive zero-sequence voltage, thus reducing the risk of overmodulation. On this basis, an active power backflow suppression strategy for mode switching is proposed. According to the real-time operating status of the inverter, the most suitable control strategy is selected to minimize the active power backflow range during inter-phase short-circuit faults. The proposed method reduces the active power backflow area by 22.08% compared to the existing AZSVCS. Finally, experimental results validate the effectiveness and feasibility of the proposed control method.