Research on DC Component Suppression Method of Non-isolated Grid-Connected Inverter

The zero drift occurring to the sampling conditioning circuit of the non-isolated grid-connected inverter will make the output develop a DC component, thus resulting in system failure and posing safety risks. According to the IEEE standard 1547-2003, the DC component injected into the grid side should be less than 0.5% of the rated current. In this paper, a moving average filter is proposed to extract the DC component of the three-phase AC output current. The filter has a very strong attenuation ability to the fundamental integer multiple harmonics, and can accurately extract the DC component. Then the proportional integral resonant controller (PIR) is used to control the system. The control system has sufficient bandwidth to avoid the stability problem caused by frequency offset. Through the above methods, the purpose of accurately suppressing the DC component in the non-isolated grid-connected inverter is realized. Also, a 50 kVA prototype is built in this study. The experimental results show that the moving average filter is advantageous over the conventional low-pass filter method in extracting the DC component, and the PIR controller used in the closed-loop control system outperforms the proportional integral and proportional resonant controllers. Under the strategy proposed in this study, the DC component is reduced to less than 0.5% of the rated current, and the THD of the grid-connected current falls below 5%.