Equivalent Model for Calculating Short Circuit Current of Doubly Fed Wind Generator under Uninterrupted Excitation

The fault ride-through ability of doubly fed wind generators (DFIG) has got more and more attention. New grid codes for interconnection of wind turbines required that wind turbines remain connected to the grid and actively contribute to the system stability during and after grid faults. Uninterrupted excitation of DFIG can both avoid running over-speed of wind turbines during grid faults and accelerate the recovery of system after clearance of fault. Calculation of DFIG fault current contribution will benefit the research on protection for distribution network including DFIG. In this paper, the relations between output of DFIG and exciting current are analysed. Based on the analysis, the fact that the exciting current is increased in order to improve the power output during grid faults and the maximum exciting current is limited by nominal power of converter is demonstrated. According to these conditions, an equivalent model for calculating fault current of DFIG is presented and the no-load exciting potential of equivalent model can be calculated by utilizing the maximum excitation conditions. A comparison between the proposed method and conventional method for calculating DFIG fault current is made in this paper. At the end, a DFIG test case is set up in PSCAD/EMTDC and the accuracy of the equivalent model for calculating fault current is verified by simulation.