Combined Unipolar and Bipolar PWM for Current Distortion Improvement During Power Compensation

To eliminate leakage ground current while achieve high efficiency, many transformerless photovoltaic inverters with unipolar pulse width modulation (UP-PWM) have been proposed and verified with real power injection only. However, in 2011, German standard VDE-AR-N 4105 updated the requirement that the inverters should satisfy the power factor from 0.9 leading to 0.9 lagging at power level higher than 4.6 kVA. When compensating the reactive power under a leading power factor, the inverter with UP-PWM might not have sufficient voltage to magnetize the inductor, which results in current distortion at zero crossing. It is hard to comply with the requirement of 1% displacement-factor accuracy. The reactive power can be well compensated with a bipolar PWM (BP-PWM) scheme; however, this will result in high current ripple and high switching loss. In this study, for reducing the current distortion under leading power factors and increasing the inductor current in the negative power region, UP-PWM is changed to BP-PWM. While in the positive power region, the inverter operation is still kept with UP-PWM to yield higher efficiency and lower current ripple. The proposed combination of UP-PWM and BP-PWM can improve the current distortion and reduce THD, while still can sustain high efficiency even with the power factors other than unity. Experimental results measured from a 5 kVA PV inverter have verified the analysis and discussion of the proposed approach.