The open-phase faults of pumped storage units and gas-turbine units may easily occur due to the frequent open and close operations of generator circuit breaker(GCB). It cannot be detected by the conventional protection during the initial period of grid-connection and in the normal operation of grid-disconnection. This paper adopts the symmetrical component method to analyze the voltage characteristics of both sides of the breaking point in the open-phase fault of a single-machine infinite system. Furthermore

this paper proposes a new principle of open-phase protection based on the voltage phasor differences between two sides of the breaking point. And the dynamic experiment is carried out to verify the correctness of the new criterion. Independent of current values and three-phase discrepancy contact status of the breaker

the new protection principle can quickly detect the open-phase fault during the initial period of grid-connection and in the normal operation of grid-disconnection for generating units

and it will not be affected by the potential transformer(PT)circuit failure. The principle has been used on four 300 MW pumped storage units of Guangzhou

China

and the operation is good.