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Generator Set Over Temperature Protection Trip

Sep. 25, 2021

Generator sets are the last guarantee for the power supply in accidents of power plants. Each power plant has established a corresponding periodic test system. The test run time is generally 5-10 minutes, and it is generally considered that it is sufficient to verify whether the unit is in good standby. However, we know , After the power plant has a full stop accident of the auxiliary power bus, the load time of the generator set with the safety section will far exceed 5-10 minutes. Therefore, the key to prevent major accidents such as turbine burnout, bending of the main shaft, hydrogen leakage and explosion, etc. when the auxiliary power bus is fully stopped, not only depends on whether the generator set can be started and connected to the safety section load during the accident, but also depends on the generator set can it run reliably for a long time. The following analysis of a case of over-temperature protection action tripping after 19 minutes of load connected to the safety section of a generator set that we actually encountered in our work points out a hidden danger that may exist in the traditional regular test run of the generator set.

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The generator set tripped and the security section lost power. At this point, the unit has lost all factory AC power, relying on 220V DC bus to connect with part of the security load. Fortunately, it was timely judged that the point of failure was not in the factory system, and the 6KVA, section B power supply was quickly restored, otherwise the safety of the generator set would be extremely dangerous. After inspection, the generator set was under "over-temperature protection" action, the engine body temperature reached above 100°C, the water tank was boiling, and other parts were normal. During the connection with the safety section, the voltage and frequency output are stable. Finding the cause of the fault and preliminary analysis: In terms of capacity, the accident load of the safety section of Unit 11 is about 250A, which is far less than the rated current of the generator set 1433A. From the perspective of the management and maintenance of the generator set, since it was put into production, in addition to the normal inspection and maintenance, the generator set has been regularly started once a month (each time 10 minutes), and no serious abnormality has been found in the generator set. From the perspective of theoretical design and maintenance management, the generator set should be very reliable. But the actual situation is that the generator set tripped after only 19 minutes after the load of the safety section was connected. So what causes the generator set to trip after connecting to the safety section? Preliminarily judged by trial run that the thermostat is faulty: We have carried out detailed inspection and maintenance of the generator set. The inspection results found no abnormalities in the lubricating oil, fuel oil, battery, water level, water pump, ventilation, measuring point, control and protection circuit and other systems of the unit. 11:40-12:10, we conducted a no-load start-up test on the generator set.

Trial run situation: 10 minutes after starting, the unit has no abnormal operation, the water temperature is 68℃, but the cooling water temperature has continued to rise since then. When running for 30 minutes, it has reached 98°C (close to the trip temperature set value of 103°C), and there is still an upward trend, so the operation should be stopped immediately. Trial analysis: After careful inspection, it is found that the temperature of the return water pipe is abnormally low. Obviously, the high temperature water does not flow back to the water tank for cooling, and it is judged that the diesel engine cooling water circuit is faulty. The characteristics of the temperature-saving failure are: the temperature of the cooling water inside the internal combustion engine is too high, while the temperature of the water inside the radiator is low. At this time, the throttle can be removed (or replaced) from the internal combustion engine, and then the engine is started. If the internal combustion engine water temperature is normal, it can be determined that the throttle valve is invalid. If the water tank pipe is partially blocked, the internal combustion engine water will rise too fast. The manufacturer further determined that the diesel engine cooling water circuit throttle valve failed. Conclusion: In the event of an accident, it cannot be reliably used as a backup for the security section. It is preliminarily judged that the cooling water circuit caused by the failure of the thermostat has not established a circulation. After replacing the thermostat, it was normal for a test run for 50 minutes: On December 27, after the manufacturer was dismantled, it was found that the thermostat installed inside the body had a serious jam and malfunction. After replacing the thermostat, a continuous trial run was carried out again for about 50 minutes at 15:20-16:10.

Test run situation: The thermostat opens normally when the water temperature is 80°C in the 15th minute after starting, and the water temperature has been stable at around 82°C for 35 minutes after running, and the power output of the generator set is normal. Conclusion: After replacing the thermostat, the fault is eliminated, and it can be reliably used as a backup for the safety section. At the same time, the thermostatic valve was disassembled and it was found that the valve body was stuck and stuck, which further verified the malfunction of the thermostatic valve. Fault confirmation: the failure of the thermostat valve and the failure to open in time to establish a water cycle are the reasons for the over-temperature protection action of the generator set to trip in the event of an accident.

Although the diesel generator set conducts regular test work according to the system once a month, and has never found any serious abnormality. However, a trip did occur in an emergency situation at the scene. Afterwards, we have checked that the thermostat has a jam failure, so why didn't we find this hidden danger every time we tried to run it regularly? It is understood that at present, most of the generator sets of many manufacturers take 10 minutes to test regularly. In the past, it is generally believed that 10 minutes is enough to check whether the generator set is in good standby (that is, it can start normally, operate normally, and output qualified power). At the same time, because the load test of the generator set can only be arranged during the overhaul of the generator set, it can only be checked for its goodness by regular work. From the introduction of "Cooling Water Circuit and Throttle Valve" in this article, we know that the throttle valve will open automatically when the cooling water temperature reaches between 75°C and 85°C after the generator set is running. From the 50-minute test run of the water temperature control after replacing the thermostat, we will find that the opening time of the thermostat of the generator set is about 12-15 minutes after starting. The water temperature of the generator set is regularly tested for 10 minutes: each test runs for 10 minutes, the water temperature generally only rises to 70 ℃, and it is still rising, obviously not the opening temperature of the thermostat. In this way, even if we have strictly adhered to regular test runs for a year or more, the thermostat will never be opened: the thermostat soaked in antifreeze is in a long-term inactive state, and it is also prone to corrosion, Malfunctions such as jamming. That is to say: it is impossible to find the throttle valve for 10 minutes every time it is started.

Starlight Power was founded in 1974 and is one of the earliest manufacturers of generators and diesel generator sets in China. If you want get more information, please feel free to send email to sales@dieselgeneratortech.com we will pay highly attention on your question.

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