Two-Degree-of-Freedom Current Controller to Mitigate Current Imbalance for Grid-Connected Inverters

This article proposes a current controller to mitigate current imbalance in grid-connected inverters. The proposed method avoids transforming unbalanced currents into their positive and negative sequence components. As a result, the number of current controllers is only two in d-q synchronous reference frame. At the same time, it can also eliminate the current harmonics effectively. Imbalance and harmonics are regarded as disturbance, which guarantee the mitigation to have no effects on the current transient response. In other words, the controller has two-degree-of-freedom capability. Experimental results verify that the current unbalance factor and total harmonic distortion can be reduced from 42.6% to 0.2% and from 14.2% to 1.6%, respectively, without causing transient overshoot and oscillations. It is also shown that the proposed controller has 8.29 μ s shorter execution time than the conventional one.