Abstract: In the unit of a power plant, the grid connecting signal of DEH system for the generator had disappeared. The target value of the DEH system could not set normally. Under the speed control mode, the opening of the turbine speed governing valve had been decreased gradually; and this had caused the generator reverse power protection to trip, and a splitting accident of the generator from the system had occurred. The reasons of this outage accident were analyzed, and the measures for prevention of similar accidents were put forward.

Key words: steam turbine trip, turbine digital electro-hydraulic (DEH) control system, grid connecting signal for generator, reverse power protection