Active Power Backflow Control Strategy for Cascaded Photovoltaic Solid-State Transformer During Low-Voltage Ride Through

Different from the conventional photovoltaic (PV) inverters, a three-phase PV solid-state transformer (SST) based on the cascaded H-bridge (CHB) topology can be regarded as consisting of three single-phase CHB inverters in essence. During the low-voltage ride through (LVRT), some phases of three-phase PV SST may inversely absorb active power from ac grid affected by negative-sequence voltages, resulting in that the dc bus voltages of all H-bridges in these phases continue to rise, and that the system will shut down due to overvoltage protection. To deal with this problem, in this article, we study the mechanism of active power backflow during LVRT, analyzes and compares the main methods for suppressing active power backflow, and then proposes an LVRT control strategy based on the zero-sequence voltage compensation, which not only ensures that all modules in three-phase PV SST transmit the almost same active power but also can avoid effectively active power backflow problem even if the total output active of PV array is relatively low. The validity of the proposed method is verified by the simulation and experimental results.