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

A novel single-phase AC/AC converter with neutral-point diode-clamped scheme is proposed. 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 three-level PWM waveform to reduce the voltage harmonics compared with that of 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 analyzed. Simulation results were performed to verify the effectiveness of the proposed control strategies.