DC Current Offset Compensation Method for Grid-Connected Inverter

Grid-connected inverters without isolated transformers are widely used because of their high efficiency, small size and mass, however this converter has a negative effect on the grid due to the DC current offset occurred at the inverter output. One of the major problems to overcome in the control of grid-connected converters is the DC offset existed in measurement. Also, the current offset in one phase results in the one of the other phases due to the correlation among three phase currents unlike single-phase and accordingly, the current offset of each phase cannot be removed by only measuring. In this paper, a method for removing the DC current offset by using both observer and compensation method is proposed for three-phase grid-connected inverter. By using the observer, the current offset is obtained by filtering of the deference between the grid current and the output reference current extracted from the grid voltage and switching state. Also, without the conventional DC offset PI controller, the DC current offset directly inputs as the DC offset reference of the current controller in the rotating reference frame. The proposed DC offset compensation method is validated on a 10 kW grid-connected converter using MATLAB.