1. Fault phenomenon: since the 125 MW unit of a power plant was put into operation, the feed pump occasionally trips when it is closed, and there is no signal relay dropping. After clearing the fault of the switch mechanism, Huludao water pump plant checks the cable, secondary circuit wiring, all relays and their settings are normal according to the conventional method, and it is often successful to start again. The latter suspicion is caused by the soft fault of DCS system, but it will still appear if the operation is changed on the control panel.
2. Experiment to find out the reason: in order to find out the reason of this phenomenon, check the changes of meters in the process of switch closing to confirm why the water pump trips. In the experiment, the voltmeter monitors the trip circuit of the microcomputer, the milliammeter monitors the action of the differential relay 1cj and 2cj, and the ammeter monitors the thermal maintenance circuit. After connecting the meter, start the feed water pump. After a period of experiment, the water pump trips at one start. At the same time, it is observed that the pointer of the milliammeter deflects for a while, and there is no response from other monitoring meters. The new xjl-0025 / 31 integrated block type signal relay 1xj also acts as a drop board, indicating that the differential maintenance action causes the water pump to trip.
3. Source analysis: for differential maintenance action, first suspect that there is something wrong inside the maintained equipment. After routine inspection, the motor and cable of feed water pump are normal, the differential relay is normal, and the polarity of current transformer is connected correctly. After clearing the causes of equipment faults and wiring errors, the differential maintenance operates during the motor startup process. Huludao water pump plant indicates that the differential current of the differential circuit exceeds the setting value of the differential relay in this process. There are two main reasons for causing differential current in differential circuit under normal condition. First, the ratio error of current transformer on both sides of motor head and tail is different, there is a very small difference current, which is less than 5% of rated current ID of motor. Second, the difference of the secondary load of the current transformer on both sides of the head and tail will also cause the difference of its transformation ratio, so there is a difference current. The load difference of current transformer in the differential maintenance circuit of feed water pump motor is only the difference of secondary cable length, about 50m. Moreover, under the rated current, the power consumption of differential relay is not more than 3VA, and the secondary load is not heavy. Through inspection, it is found that the model of current transformer at the head and tail side for differential maintenance of feed pump motor is lmzbj-10, class B 15 times of rated current, transformation ratio is 600 / 5, capacity is 40VA, which can completely meet the requirements of secondary load.