Pump Trip Troubleshooting 1: Symptom Meixian Power Plant 125 mw unit has been put into operation since the pump occasionally a closing or tripping problem, without any signal relay card. After excluding the switchgear failure, according to the conventional method of checking the cable, the secondary circuit wiring and the relay and its rating are normal, start again often successful. Dcs system after the suspicion of soft failure caused, but the change in the control panel operation, there will still be this phenomenon. 2: test find the reason In order to find out the reason for this phenomenon, observe the change of each meter in the process of closing the switch to confirm what caused it to trip. Test voltage meter which monitors the microcomputer trip circuit, mA watch differential relay 1cj, 2cj action, ammeter monitoring thermal protection circuit. Followed by a good meter to start the feed pump, after a period of testing, and finally there is a feedwater pump a start is tripped, while observing the milliammeter pointer deflection a little, other surveillance meter does not respond, put on the new xjl -0025 / 31-type integrated block signal relay lxj also action card, that is caused by the differential protection action trip. 3: root cause analysis Differential protection action, first of all suspect that there is a fault within the protected equipment. Through routine inspection, the pump motor and its cable are normal, the differential relay is calibrated, and the polarity of the current transformer is connected correctly. After eliminating the causes of equipment failure and wiring errors, differential protection operates during motor start-up, indicating that the differential current of the differential circuit exceeded the differential relay setting during this process. Under normal circumstances, the differential current caused by differential current loop there are two main reasons: First, both sides of the motor current transformer ratio error is different, there is a small difference current, the difference current is less than 5% of the motor rated current id . Second, both ends of the current transformer secondary load difference will cause the difference between the ratio, resulting in a poor current. The difference in current transformer load in the feed pump motor differential protection circuit is only a difference of the secondary cable length of about 50 m, and the power consumption of the differential relay is not more than 3 va at rated current and the secondary load is not weight. Check found to pump motor differential protection with the end of the current transformer models are lmzbj-10, b-level 15 times the rated current, the ratio of 600/5, the capacity of 40 va, fully able to meet the requirements of the secondary load. The above analysis is based on the normal operation of the condition, the motor starts, the situation is different. When the motor starts, the current is very large. The current transformer on both sides of the motor may be saturated. In this case, the secondary differential current may be very large due to the inconsistent magnetization characteristics of the current transformers. According to the relay factory in acheng lcd-12 differential relay setting instructions, the relay operating current setting izd = △ i1 × kk × in / n = 0.06 × 3 × 356/120 = 0.534a (where: △ i1- First, the end of the current transformer maximum error during normal operation, 0.04 ~ 0.06; kk-reliability factor, 2 to 3; in- motor rated current; n- current transformer ratio). Should be set at 1.0a position. The use of b-level transformer case, the differential relay operating current set at 1.5a, the braking factor of 0.4, the differential protection is still occasionally activated when the motor starts, is due to b-level current transformer magnetization saturation point Lower, anti-saturation capacity is low, can not meet the requirements of the differential relay. Usually require differential protection circuit current transformer using d-level, d-level transformer saturation point higher, not so easy to saturation, can reduce the difference between the current flowing through the differential loop when the motor starts. In the replacement of the d-level current transformer, while the differential relay operating current set at 1.0a, the braking factor of 0.4, no longer appeared to switch a switch that trip.