Troubleshooting of pump tripping faults

Water pump trip fault elimination method:

1. Symptom: Since the 125 mw unit of a power plant has been put into operation, the feed pump will occasionally trip or trip, and there is no signal relay drop. After eliminating the failure of the switching mechanism, the cables, secondary circuit wiring, and the relays and their relays are normally checked for normal methods, and they are often successful again. After the suspicion is caused by a soft fault in the dcs system, but the operation is changed on the control panel, this phenomenon still occurs.

2. Test to find out the reason: To find out the reason of this phenomenon, observe the changes of each meter in the switch closing process to confirm why the pump tripped. The voltage meter is used to monitor the trip circuit of the microcomputer. The milliampere meter monitors the operation of the differential relay 1cj and 2cj. The ammeter monitors the thermal protection circuit. After the meter is connected, the water pump is started. After a period of time, the pump is tripped as soon as it starts. At the same time, the pointer of the milliampere meter is observed to be deflected. The other monitoring meter does not respond and the new one is replaced with xjl-. The 0025/31 integrated block type signal relay 1xj also operated to drop the plate, indicating that the differential pump was tripped by the differential protection action.

3. Root Cause Analysis: Differential protection action. First of all, it is suspected that there is a fault inside the protected device. Through regular inspection, the feed pump motor and its cable are normal, the differential relay calibration is normal, and the current transformer polarity connection is correct. After eliminating the causes of equipment faults and wiring errors, the differential protection acts during motor start-up, indicating that the differential current of the differential circuit exceeds the differential relay setting value during this process. There are two main causes of differential loop current under normal conditions. First, the current transformers on both sides of the motor have different error ratios. There is a small difference current. The difference current is less than 5% of the rated motor current id. Second, the difference in the secondary load of the current transformers on both sides of the first and the tail will also cause the difference in the ratio of the transformers, so that there is a difference current. In the differential protection circuit of the feed pump motor, the current transformer load difference is only the difference of the length of the secondary cable. The difference is about 50m. Under the rated current, the power consumption of the differential relay is not more than 3va, and the secondary load is not heavy. The inspection found that the front and the rear side current transformers for the differential protection of the water pump motor are all lmzbj-10, and the class b has a rated current of 15 times, a transformation ratio of 600/5, a capacity of 40 va, and can fully meet the requirements of the secondary load.

The above analysis of the pump trip is based on the normal operating conditions. When the motor is started, the situation is different. When the motor starts, the current is very large, and the current transformers on both sides of the first and the tail may be saturated. At this time, the secondary differential current may be large because of the inconsistent magnetization characteristics of the current transformers. According to the setting of the lcd-12 type differential relay of Acheng Relay Factory, the relay's operating current setting value is izd=Δi1×kk×in/n=0.06×3×356/120=0.534a (where: Δi1— The maximum error of the first and end current transformers in normal operation, 0.04~0.06; kk—reliable coefficient, 2~3; in—motor rated current; n—current transformer ratio). Should be set at 1.0a. In the case of using b-class transformers, when the differential relay operating current is set at 1.5a and the braking coefficient is 0.4, the differential protection still occasionally acts when the motor is started, due to the saturation point of the magnetization characteristic of the b-class current transformer. Low, low saturation resistance, can not meet the requirements of differential relays. It is usually required that the current transformer of the differential protection circuit adopt d-class, and the saturation point of the d-class transformer is higher, and it is not so easy to saturate, and the differential current flowing through the differential circuit at the start of the motor can be reduced. In the replacement of d-class current transformer, while the differential relay operating current is set at 1.0a, the braking factor is 0.4, and then there has been no switch closure that is the fault of the pump trip.