Unbalanced Current Control Strategy Based on Complex-coefficient Proportional Integrator Controller Implemented in Double Synchronous Reference Frames

Unbalanced grid voltages caused by unbalanced grid faults will detriment the operation of grid-connected voltage source converters. Controlling the converters to inject suitable negative-sequence currents into the grid can help converters to withstand unbalanced grid voltages and stay connected with the grid. This paper proposed an unbalanced current control strategy based on the complex-coefficient proportional integrator controller implemented in double synchronous reference frames. The proposed method can fast track positive-and negative-sequence sinusoidal currents with zero steady-state error and without requiring current sequence decomposition. Moreover, the proposed method can realize decoupling control on direct and quadrature axis currents of both positive-and negative-sequence simultaneously. And through considering and compensating the effect of the control delay during controller parameter design, the proposed method can achieve good stability and dynamic performance for applications with low switching/sampling frequency. Experimental tests were implemented on a prototype with 2kHz switching frequency to verify the feasibility and performance of the proposed method.