Transient Analysis and Control Solutions for DFIG-based Wind Turbine Generators to Improve FRT Performance under Asymmetrical Faults

Modern grid codes require wind turbine generators (WTGs) to ride through asymmetrical faults and inject positive- and negative-sequence reactive currents within a defined response time. In doubly fed induction generator (DFIG) based WTGs, a coupling loop is introduced under asymmetrical fault by the grid impedance among phase-locked loop (PLL), and inner and outer control loops. As a result, DFIG-based WTGs may react much slower than expected depending on the design of control parameters, due to grid impedance and asymmetrical voltage dip. The impact of this coupling loop on the phase angle transient is analyzed in this paper. In order to speed up the transient response, a magnitude normalization element is employed in the PLL to eliminate the impact of voltage magnitude dip, and the PLL control parameters are redesigned to increase the damping ratio. The proposed solutions are validated by a detailed EMT-type simulation in the EPRI benchmark system. The comparisons show that the FRT performance of the DFIG-based WTGs is improved by decreasing the overshoot and settling/rising time of the phase angle transient.