Neutral-Point Voltage Analysis and Suppression for NPC Three-Level Photovoltaic Converter in LVRT Operation Under Imbalanced Grid Faults With Selective Hybrid SVPWM Strategy

Balanced issues of neutral-point (NP) voltage in the neutral-point-clamped (NPC) three-level photovoltaic converter have been studied in depth. Numerous algorithms for NP voltage control have been proposed and proven to be effective in normal operations. However, most of the previous studies fail to consider the effect of imbalanced loads on NP voltage, especially under low-voltage-ride-through (LVRT) condition, which faces problems of imbalanced grid voltages and low power factor. In this paper, through derivation of NP current model for different loading conditions, the behavior of NP voltage under LVRT condition is comprehensively analyzed for the first time. Then, theoretical limitations of NP voltage control based on two common space-vector-pulsewidth modulations (SVPWMs) that three-nearest-vectors and symmetric SVPWM are investigated, and additional low-frequency oscillation in NP voltage is analyzed. This oscillation is dominated by fundamental frequency and has different generation mechanisms under different imbalanced grid faults. Then, a novel method of selective hybrid SVPWM, as selective combinations of SVPWM and virtual SVPWM according to different imbalanced grid faults, is proposed to effectively suppress NP voltage oscillations in LVRT operation without any complex improvements and increase of dc-link capacitor values. Finally, the analytical result and performance of the proposed method are confirmed in an existing 300 kW NPC converter.