1. Reasons for overspeed accident of steam turbine:
(1) The steam turbine governing system has defects (the speed control system's retardation rate should not exceed 0.5% at most);
(2) Failure of the overspeed security system (the operating speed of the emergency security device is 110% to 112% of the rated speed);
(3) Improper operation, adjustment and maintenance.
2. Dealing with overspeed accident of steam turbine:
(1) When an overspeed accident occurs, the emergency safety device should be hit by hand to break the vacuum fault and stop. After the main gate, the automatic main steam valve, steam control valve and extraction check valve should be quickly closed, and the speed should be reduced;
(2) If the rotation speed exceeds 3360r / min and the emergency guard does not operate, the emergency guard should be immediately hit by hand to destroy the vacuum emergency shutdown;
(3) If the emergency safety device is activated and the automatic main steam valve, speed governing valve and extraction check valve are stuck or not tightly closed, try to close the above valves or close the electric main and extraction valves immediately;
(4) If the speed of the unit still does not decrease after taking the above measures, all steam valves connected to the steam turbine should be quickly closed to cut off the steam source;
(5) If necessary, the generator excitation can be put in to increase the braking force;
(6) After the unit is shut down, it is required to fully check and repair the defects of the adjustment and security system, otherwise the unit will not be allowed to start again;
(7) When the unit is restarted, pay attention to check the vibration of the unit. Before the grid connection, it is required to perform an action test of the emergency guard. After the operation speed is qualified, the unit is allowed to be connected to the power grid.