one Event process
On May 20 of one year, 2eh oil pump of a unit was running, EH oil pressure was low, and 1eh oil pump was started by interlock. 11: At 28:58, the steam turbine tripped, the first output was low EH oil pressure, the boiler MFT, the generator transformer unit tripped, and the trip process curve is shown in Figure 1.
two Cause finding and analysis
1) Check the situation
Through the inspection of DEH curve and DCS curve, it is found that before the trip, the unit load was 142MW, the No.2 fire-resistant oil pump was running, the current was 22.5a, and the main pipe oil pressure was 13.54mpa. 11: At 28:48, when the fire-resistant oil pressure was low, the No.1 fire-resistant oil pump was started by interlock, the current of No.1 fire-resistant oil pump was 25.4a, the current of No.2 fire-resistant oil pump was 24.7a, the oil pressure of fire-resistant oil main pipe dropped to 3.97mpa, and the steam turbine tripped at 11:28:58.
Figure 1 trip process curve
Before the unit trip, No.2 fire-resistant oil pump runs stably, and the pressure of fire-resistant oil system is normal without fluctuation. The fire resistant oil system has no maintenance and operation records.
When the oil pressure of the fire-resistant oil main pipe is low, the No.1 fire-resistant oil pump is started by interlock, and the two pumps are running at the same time, and the main pipe pressure is 3.98mpa. In the idling stage after the unit trip, the output of the two fire-resistant oil pumps is normal when switching, which indicates that the two fire-resistant oil pumps have no fault. When a single fire-resistant oil pump is running, the fire-resistant oil pressure does not rise, so there is no fault that the check valve at the outlet of the fire-resistant oil pump is not tight.
After R6 (48V shutdown relay) was handled by thermal control, ETS first alarm was reset, and the fire-resistant oil main pipe pressure rose to normal value, and no fire-resistant oil pressure fluctuation was found.
After the unit was restarted, the following inspections were carried out: the 12 valves of the main engine of the steam turbine were inspected in turn, no obvious leakage was heard, and the internal leakage of the fire-resistant oil system was preliminarily eliminated; the temperature of the oil return pipe of the relief valve of the fire-resistant oil main pipe was not high, which could eliminate the fault of the relief valve and cause the low oil pressure of the fire-resistant oil main pipe.
2) Cause investigation
Due to the inconsistency of unit clock, incomplete SOE points, incomplete historical trend points and inconsistent acquisition time, it is difficult to investigate the accident. According to the relevant historical data of DCS and DEH and the drawings of regulating and security system, the analysis is carried out.
If the unit trip is only caused by R6 relay and there is no other fault superposition, and the first trip out of steam turbine is low EH oil pressure, then the trip process should be: R6 is damaged, the trip relay jr1-jr5 is charged, and the ast solenoid valve is de energized. Since the command circuit does not go through the normal ETS circuit, the unit does not trip. At this time, AST solenoid valve loses power and high-pressure safety oil pressure, and turbine control valve and main steam valve start to close. Since the unit has not tripped, the command of control valve still exists. In order to maintain the existing valve opening, the servo valve offset increases, and the EH oil main pipe oil pressure is continuously pulled down until the protection action value is reached. Therefore, the first occurrence is low EH oil pressure tripping, as shown in Fig. 2, Fig. 3, FIG. 4, FIG. 5, FIG. 6.
Fig. 2 Relay condition
Figure 3 relay circuit
Figure 4 relay circuit
Figure 5 relay circuit
Figure 6 servo valve command return to zero after EH oil pressure trip
(1) If the unit trip is only caused by R6 relay, there is no other fault superposition, and the servo valve command returns to zero after tripping due to low EH oil pressure, then there is no fire-resistant oil leakage point in the system, and the fire-resistant oil pressure should return to the normal value of about 14MPa. However, according to the historical trend, after the normal tripping, the two fire-resistant oil pumps operate at the same time, and the fire-resistant oil pressure is only 3.97mpa, so there should be fire-resistant oil in the system Fuel leakage point.
(2) According to the schematic diagram of steam turbine governing and security system of the unit, the fire-resistant oil main pipe pressure kept low (3.97mpa) after the unit trip. The most likely reason is that the throttle on the pipeline from the fire-resistant oil main pipe to the trip solenoid valve (for the control valve, it is called the over speed limit solenoid valve) failed, and it was blocked during the normal operation of the unit In case of R6 relay failure, the main protection of turbine will act when the main oil pressure of fire-resistant oil main pipe will be directly unloaded if the solenoid valve is energized to open and drain oil in case of R6 relay failure.
Therefore, the most likely direct cause of the turbine trip event is the failure of tb7r6 relay in DEH system, which causes the ast solenoid valve to lose power and trip. According to the previous analysis, when the restrictor on the main oil pipe of fire-resistant oil motor to trip solenoid valve fails, and R6 relay fails, the oil pressure of fire-resistant oil main pipe will be directly unloaded, resulting in low pressure of fire-resistant oil main pipe, and the main protection of steam turbine will act.
three . event handling and Prevention
1) Check all relays to see if there are relay faults caused by equipment aging and dust.
2) Check all control valve servo valves for jamming.
3) The clock of DCS and DEH controller is inconsistent. According to 18.104.22.168 of technical guidelines for reliability evaluation of thermal automation system in thermal power plant (DL / t261-2012), DCS shall provide "digital master clock", and all station clocks on data communication bus shall be synchronized, and "digital master clock" shall be synchronized with GPS clock signal, and can be set by keyboard in man-machine interface station.
4) There are no necessary measuring points in SOE, which can not analyze the problems. It is suggested to improve SOE measuring points. The maximum scanning resolution of historical trend is 1s, and the sequence of events can not be distinguished, so the software should be upgraded.
5) It is not equipped with the function of real-time monitoring and monitoring of the oil pressure of the unit, which is convenient to analyze and record the emergency situation of the unit.