In recent years, wind power in China has been developed by leaps and bounds. By the end of 2011, China's wind power grid-connected capacity exceeded 45 million kW. With the rapid increase of wind power installed capacity, more and more attention has been paid to the operation safety problems caused by wind power connected to the power grid, among which the serious problem is the fan off-grid problem. At present, the focus of the research is mainly on the fan off-grid caused by the fault of the power grid, but the research on the fan off-grid caused by the transient disturbance as a result of the switching of the transformer is less.