With more and more AC excited wind turbines connecting to the power networks， the new gird code requires the generators to be capable of riding through a grid voltage dip. In order to protect the rotor-side converter and generator， a crowbar protection circuit is often connected to the rotor windings to limit peak currents in the rotor during a voltage dip. By analyzing the behaviors of an AC excited wind turbines during system faults， the effects of voltage dip and bypass resistor values on the Crowbar protection is analyzed. The performances of wind turbines with a Crowbar circuit are also discussed. The simulation results validate the effectiveness of Crowbar protection in terms of fault ride through capability.