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Reactive Power Generation and Optimization during a Power System Fault in Wind Power Turbines Having a DFIG and Crowbar Circuit

For variable-speed constant-frequency (VSCF) wind power generation, a doubly fed induction generator (DFIG) equipped with a crowbar circuit is becoming more widely used in electrical transmission networks. This prevents potentially damaging over-voltage and/or over-current conditions in the DFIG during a power system fault. The response of a DFIG based wind turbine system to voltage dips at the point of common coupling is studied in this paper. A crowbar circuit can limit the surge current as well as protect the rotor side converter and DC-link capacitor. When the crowbar is cut in, the rotor side converter is disconnected from the rotor windings, which makes the DFIG behave as an asynchronous induction generator with high rotor impedance. It is suggested that use of a crowbar circuit can reduce the reactive power absorbed by the stator and also assist in voltage recovery of the grid by injecting reactive power into the grid. This paper analyzes the influence of the AC crowbar resistors or inductors on the reactive power consumption of the stator during grid voltage dips. In addition, the paper presents a current-limiting method to control the grid side converter and the rotor side converter, which provides reactive power to the grid and thus enhance the capability of low voltage ride-through (LVRT). In the simulation studies presented, AC crowbars with different resistor and inductor values are selected. The simulation results show that the stator reactive power absorption can be controlled, while the rotor side converter and grid side converter can be controlled to inject reactive power into the grid to enhance voltage restoration and fault recovery.