Analysis of the by-pass resistance of an active crowbar for doubly-fed induction generator based wind turbines under grid faults

The doubly-fed induction generator (DFIG) based wind turbines of today are often equipped with a crowbar, which can be used to limit the inrush rotor current and protect the ac excitation converter from damage via a set of resistors that are connected to the rotor windings of the DFIG. When the rotor current becomes too high due to the severe grid voltage dips on the transmission line, the crowbar is activated and the rotor side converter is disconnected from the rotor windings at the same time, which makes the DFIG behave as a squirrel cage induction generator with increased rotor resistance. This paper analyzes the influence of the crowbar resistors on the electrical behavior of the DFIG during the grid voltage dips. Approximate expression of the fault stator and rotor currents are deduced from mathematical analysis and verified by simulation. Different crowbar by-pass resistances and their impacts on the maximal fault currents and electromagnetic torque are examined by simulations with different parameter sets of the DFIG. Based on the simulation results, the appropriate crowbar by-pass resistance can be effectively predicted.