A protection start discrimination is proposed according to the variation of bus differential protection braking current during fault. After start

the protection relay analyzes the amplitudes variation characteristics of three differential current sampling points continuously

based on which the internal fault and external fault can be discriminated. When the fault is judged to be internal

differential protection is opened; when external

the differential protection is blocked. In order to guarantee reliable operation of differential protection when external fault transforms to internal fault

the re-tripping discrimination of differential protection needs to be switched in. Two differential protection re-tripping schemes for external fault transforming to internal fault based on the differential current sampling value is proposed. One is based on differential sampling value

the other is based on the differentiated differential sampling value.