Neutral-Point Voltage Oscillation Mitigation Scheme for Transformerless Three-Level PV Inverter in LVRT Operation With Selective Space Vector Modulation

In transformerless photovoltaic (PV) generation system, the neutral-point (NP) voltage balancing and the ground current reduction of three-level inverter are carefully regulated using a space vector modulation (SVM) scheme. Nevertheless, in low-voltage ride-through (LVRT) operation, the simultaneous reduction of NP voltage oscillation and common-mode voltage (CMV) has not been researched yet. To overcome these issues, a selective SVM (SSVM) strategy is proposed in this article. First, the NP voltage oscillation of three-level inverter in LVRT operation is analyzed in detail, which reveals that the NP voltage is affected by phase currents and duty cycles. Based on the location of output voltage and the direction of grid currents, the correct small vectors are selected under different power factor angles. Then, the small vector force (SVF) including the grid currents and duty cycle on NP voltage is proposed for the first time. According to the value of SVF, the optimal small vector having the most effective impact on NP voltage among the correct vectors and switching sequence is determined. With the proposed method, the NP voltage oscillation and CMV are reduced simultaneously in LVRT operation. Finally, the effectiveness of the proposed method is verified with experimental results.