Behaviour and protection of doubly-fed induction generators during network faults

The paper considers the behaviour of the rotor current of a DFIG during a fault on the network. It is seen that a large disturbance of the stator voltage will cause high transient rotor currents which trigger the crowbar protection for the rotor side converter (RSC). Once the crowbar is applied it cannot be removed until its current reaches zero. During the period the crowbar is applied, the machine behaves as a conventional FSIG, losing control of the active and reactive power. Therefore the time taken for the crowbar current to reach zero is crucial as it determines when the RSC can regain power control and how soon the AC voltage can recover. It is seen that the crowbar current can take a long time to decrease to zero and that this has a significant impact on the voltage recovery after fault. The larger the crowbar resistor and the further the generator is from synchronous speed, the shorter the crowbar current transient is. Power and energy dissipation through the crowbar resistor are also considered, and it is observed that the power varies in accordance with the maximum power transfer theorem. The detailed impact of the timings of crowbar removal and RSC reactivation on system voltage recovery is also investigated. It is seen that reactive power control during fault clearance can assist AC voltage recovery.