Active power decoupling methods for three-phase grid-connected converters under unbalanced grid conditions

The grid-voltage unbalance can result in a double-frequency component in the output power of a three-phase grid-connected converter, which significantly degrades the system performance. To address this issue, the power decoupling concept, which was previously developed for the single-phase system to handle the double-frequency power oscillation, is applied to the three-phase system in this paper. To begin with, three topologies are proposed to implement the power decoupling function in the three-phase converter system, which can maintain dc-link voltage constant while ac grid current balanced under unbalanced grid voltage through absorbing the double-frequency power component. Then, the control system realization of each power decoupling topology is presented and their working principles are described in detail. Furthermore, three prototypes are developed under different types of unbalanced grid voltage condition to evaluate the performance of the proposed methods. Experimental results are eventually provided to validate the effectiveness of the three proposed schemes.