The relay protection settings and problems of low voltage collector network of wind farm are discussed.A differential protection scheme of collector line in the wind farm covering wind turbine step-up transformer is proposed，with an optical Ethernet network consisting of cascade merging units based on the protocol IEC61850-9-2.It is realized with synchronous sampling and communication of the generator outgoing line current and the collector line current on the same collector bus.This method is also applicable to the data synchronous acquisition of similar distribution network and microgrid.The equations of differential protection are derived.The effect of the transformer and cable capacitance current on the differential protection is analyzed，and corresponding solutions are presented.