Accurate Neutral Current Control for Neutral Point Voltage Balancing in Three-Level Inverters Considering Digital Control and PWM Delay

This paper presents an accurate control method of neutral current for neutral point voltage balancing in three-level neutral point-clamped (NPC) inverters considering digital control and PWM delay. A stable and reliable operation of NPC inverters demands balancing in two DC capacitor voltages. Since the voltages are determined by the neutral current, the neutral current should be controlled accurately. The neutral current is synthesized with three-phase currents over a PWM cycle. Due to the digital control and PWM, their delays are included in the synthesis of the neutral current. These delays make the neutral current control inaccurate and affect voltage balancing control in DC link. In this paper, the delays were analyzed and compensated so that the accurate control of neutral current was achieved. Simulations and experimental results verify the effectiveness of the proposed method.