Abstract:The forced power oscillations have become one of the important problems that affect the safety and stability of the power grid. Forced power oscillations have happened frequently because there were defects of unit control system in the power system.In order to prevent the defects of unit control system from causing forced power oscillations that affect the safety and stability of the power grid, the paper analyzed an event of forced power oscillation by means of the PMU curve and operating condition of the plant unit.The defects of the electro-hydraulic servo valve was the cause of the oscillation,which caused the output of the unit to fluctuate greatly and the system to be disturbed. The oscillation calmed down when the disturbance source was removed.The paper put forward corresponding preventive measures for the problems and suggested to check the unit control system in power plant, in order to prevent this kind of forced power oscillation from happening again. Besides, it suggested to improve and perfect the module of power oscillation in WAMS system, in order to improve the accuracy of identifying disturbance source,which can find and remove the disturbance source quickly.