Method to Minimize the Low-Frequency Neutral-Point Voltage Oscillations With Time-Offset Injection for Neutral-Point-Clamped Inverters

This paper proposes a method to reduce the low-frequency neutral-point voltage oscillations. The neutral-point voltage oscillations are considerably reduced by adding a time offset to the three-phase turn-on times. The proper time offset is simply calculated considering the phase currents and dwell time of small- and medium-voltage vectors. However, if the power factor is lower, there is a limitation to eliminate neutral-point oscillations. In this case, the proposed method can be improved by changing the switching sequence properly. Additionally, a method for neutral-point voltage balancing is also proposed. The change in switching frequency by the proposed method and its effects on the outputs are negligible. Simulation and experimental results verify the validity and feasibility of the proposed strategy.