Variable Switching Frequency PWM for Three-level NPC Inverter with Neutral Point Voltage Control

The neutral point voltage imbalance is an inherent problem of the three-level NPC inverter. In this paper, a carrier-based scheme with zero-sequence injection is used to eliminate low-frequency fluctuations of neutral point voltage (NPV). Then the high-frequency NPV prediction with equivalent time-domain model is presented and the variable switching frequency PWM (VSFPWM) based on this prediction is proposed to control the high-frequency components of NPV. With the proposed strategy, the switching frequency is allowed to change in a wide range and the switching losses and electromagnetic interference (EMI) will be improved relative to the constant switching frequency PWM (CSFPWM), while the low-frequency fluctuations are suppressed and high-frequency fluctuations are controlled within the requirement. Simulation and experimental results are provided to verify the proposed method.