Implementation of a single-phase three-leg AC/AC converter with neutral-point diode-clamped scheme

A single-phase AC/AC power converter with a neutral-point diode-clamped scheme is proposed. A three-leg configuration, which has a common leg between the AC/DC and DC/AC converters, is adopted to reduce the number of switching devices compared with the conventional three-level AC/AC system. The common leg is operated in both rectifier and inverter modes. The rectifier is controlled to have a constant DC-link voltage and unity input power factor. The AC side of the rectifier has a three-level PWM waveform to reduce the voltage harmonics compared with that of a two-level scheme. The hysteresis current control scheme is used in the inner control loop to track the line current command. A proportional-integral (PI) control is adopted in the outer control loop to maintain the constant DC-link voltage. In the inverter side, a sinusoidal output voltage is generated to provide a stable and clean source voltage to the critical load. The fixed-frequency current controller is adopted to generate the properly PWM waveform. The output voltage is independent of the mains voltage to provide a stable AC power source. The neutral-point diode-clamped circuit configuration is used to reduce the voltage stress of power devices. The system model and control algorithm are described and analysed. Simulations and experimental results based on a laboratory prototype were performed to verify the effectiveness of the proposed control strategies.