Active Power Backflow Suppression Strategy of Cascaded PV Solid-State Transformer Under Interphase Short-Circuit Fault Condition

Abstract

Currently, the grid-tied guidelines all require large and medium-sized photovoltaic (PV) power stations to stay connected to the grid during low-voltage ride through (LVRT). However, three-phase cascaded PV solid-state transformer (SST) has the inherent active power backflow problem during asymmetric voltage sags, causing that H-bridge dc-bus voltages are out of control and that the converter will be disconnected from the grid because of overvoltage fault. The existing control strategy is capable of avoiding active power backflow in theory by injecting appropriate zero-sequence voltage, but the overmodulation range is larger during interphase short-circuit fault, weakening active power backflow suppression effect. For this issue, this article proposes a combined control method of positive–negative-sequence maximum–minimum harmonic zero-sequence voltage injection strategy and adaptive zero-sequence voltage compensation strategy, which can shrink effectively active power backflow range of three-phase cascaded PV SST, increasing its adaptability to different output powers and different grid drop depths, and then improving LVRT capability of the PV power generation system. Finally, a low-voltage and low-power experimental prototype is made to verify the effectiveness and feasibility of the proposed control strategy.