Hybrid UP-PWM for Single-phase transformerless photovoltaic inverter to improve Zero-Crossing Distortion

The zero-crossing distortion (ZCD) is a common problem in Single-phase transformerless photovoltaic (PV) inverter. By establishing the mathematical model of the HERIC type inverter which is proved to be a mainstream transformerless PV inverter, the result for causing the ZCD is presented. To suppressing the ZCD while achieve high efficiency and reactive power capability, a hybrid unipolar pulse width modulation (UP-PWM) is proposed. The proposed method adopts grid frequency unipolar pulse width modulation (GFUP-PWM) when outputting active power. While generating reactive power, the leg switches are all closed, and only the freewheeling switches work at high frequency. Aiming at reducing the ZCD, a modified high frequency unipolar pulse width modulation (HFUP-PWM) is proposed near the voltage and current zero-crossing point (ZCP) regions. And the duty cycle is adjusted to compensate the narrow pulse limit and dead zone in HFUP-PWM. The proposed scheme has high efficiency and good power quality while provide reactive power support. Finally, the effectiveness of the proposed modulation scheme is verified by the simulations results based on a 4-kW inverter at 20-kHz switching frequency.