Analysis of fault current contribution of Doubly-Fed Induction Generator Wind Turbines during unbalanced grid faults

The fault current contribution of the Doubly-Fed Induction Generator Wind Turbines (DFIG-WTs) under unbalanced grid faults did not draw researchers' attention due to the absence of any related requirements in the former grid codes. However, in 2015 new requirements were established dictating the DFIG-WT/generating unit to remain connected to the grid during phase-to-phase short-circuit and to perform reactive power provision. Thereupon, the knowledge of the fault current magnitude as well as the dynamic response of the DFIG-WT are crucial for the power system component design and for the validation of its commitment to the grid codes requirements. There are concrete control objectives regarding negative sequence control that cannot be fulfilled simultaneously but rather set according to the owner's preferences. Therefore, in this paper, a detailed analysis of the negative sequence current response under each control objective were carried out, considering all the controller parameters. The influence of each controller structure on the short-circuit quantities is investigated and based on it an approximated mathematical models of the short-circuit quantities and currents were proposed. Finally, a brief analysis, due to space limitation, of the response during the voltage limitation period is also performed where the influences of voltage limitation on the negative sequence current were shown.