Reduced-Dimensional Controllable Region-Assisted Optimal Rotor Current Control Strategy of DFIG-Based WTGs During Asymmetrical Faults

Abstract

Doubly-fed induction generator (DFIG)-based wind turbine generators (WTGs) are required to provide grid support during asymmetrical faults and the corresponding asymmetrical current control strategy (AsymCCS) is of great importance. However, the output capability of the rotor-side converter (RSC) under asymmetrical faults lacks a concise expression. This deficiency hinders the optimal allocation of current references in AsymCCSs and may further cause RSC overcurrent and over-modulation. To address these issues, a reduced-dimensional RSC asymmetrical controllable region (AsymCR) is firstly established in this paper, describing the characteristics of the RSC output capability. The AsymCR points out that the negative sequence current output should hold a higher priority to ensure the positive sequence current output capability. Based on an AsymCR-assisted optimal stator equivalent negative sequence reactance, a novel RSC AsymCCS is then proposed. The control strategy can offer sufficient negative sequence support to the grid while enlarging the positive sequence support capability, optimally allocating the positive and negnative sequence output and effectively avoiding overcurrent and over-modulation problems. Simulations are conducted in an electromagnetic transient test system to verify the effectiveness and improved performance of the proposed method.