Impact of wind farm integration on relay protection(3): Performance analysis for wind farm outgoing transformer protection

Simulative analysis shows that the short circuit current frequency of doubly-fed induction generator during low-voltage ride-through period deviates from the fundamental frequency according to the working conditions before the fault. When the transformer internal fault occurs, the differential action and braking currents of differential protection based on phasor values fluctuate widely and the action conditions of differential protection based on sample values are not satisfied at certain sampling points, resulting in the incorrect and instable operations. With an actual regional wind farm as an example, the off-line action performance of the main transformer protection of wind farm and the step-up transformer protection of wind power group is tested and analyzed on EMTDC platform, which shows that, the fast action of differential protection based on either phasor or sample values can not be ensured and the factors influencing the accurate operation of protections for wind farm outgoing transformers are generator type, operating condition, fault location, fault type and short circuit capacity ratio.