In the initial stage of generator load-shedding,generator power sag protection devices can judge quickly and shut down safely.Power oscillation was caused by a power plant outlet line failure.The transmission cycle of the generator power transmitter was not matched with the set of steam turbine digital electro-hydraulic control system,and the waveform was distorted.In turn,the unit control valve was shut down by mistake,and the action of power sag protection caused the generator-tripping.This paper analyzes the causes of this action and puts forward some suggestions on how to prevent and solve similar problems in large units.