A Novel Combined Injection Method for Neutral Point Voltage Control of Three-Level NPC Inverter Under Carrier-Based VSVPWM

The three-level neutral point clamped (3L-NPC) inverter is widely used in high-power and high-voltage applications due to its low harmonic distortion and high voltage tolerance. However, neutral point (NP) imbalance in this topology degrades output power quality and damages switching devices. Virtual space vector pulse width modulation (VSVPWM) has shown potential in mitigating NP fluctuations but lacks the ability to correct NP offset and is complex to implement. To address this issue, a carrier-based VSVPWM with a combined injection method to achieve active NP balance is proposed. First, VSVPWM is equivalently transformed into carrier-based pulse width modulation (PWM) to simplify implementation. After that, the basic form of NP balance injection method is classified and analyzed. By incorporating an injection limiting factor, a combined injection method is then constructed to provide an active NP balancing strategy with improved adjustment margins and efficiency, whose adjustment capability and switching losses are discussed. Finally, the proposed method is validated using StarSim hardware-in-the-loop platform, which indicates fast NP balance recovery and stable operation.