In the power system, the circuit breaker is an important protective device, used to quickly cut off the current when a circuit fault occurs to avoid equipment damage and protect personal safety. However, in actual operation, circuit breakers sometimes override and trip, causing great trouble to the normal operation of the power system. In short, preventing circuit breakers from tripping is one of the important measures to ensure the normal operation of the power system. Measures such as strengthening equipment maintenance and repair, rationally configuring protection devices, adjusting relay protection settings and the coordination relationship between upper and lower circuit breakers, and using modern scientific and technological means for monitoring and management should be used to prevent and solve the problem of circuit breaker overstepping. Only in this way can the safe and stable operation of the power system and the reliability of power supply be guaranteed. The following article will briefly analyze the reasons for circuit breaker overstepping and propose corresponding solutions

▌01 The main reasons for circuit breaker override tripping

1. The load capacity of the main switch is less than the total load capacity of the separate switches.

2. The main switch has a leakage protection device but the branch switch does not. When the leakage of electrical appliances is greater than the residual current value of the main switch, the main switch trips.

3. The two-level circuit breaker protection does not match. Try to use circuit breakers of the same brand.

4. Frequent operation of the main switch with load leads to electric shock, carbonization, poor contact, increased resistance, increased current, and heating and tripping.

5. The protection configured in the lower-level circuit breaker cannot correctly determine the fault (such as a single-phase ground fault but no zero-sequence protection is configured).

6. The aging of the circuit breaker causes the shunt tripping time to become longer. It is necessary to replace a branch switch with a shunt tripping time shorter than that of the upper level switch.

▌02 Countermeasures for circuit breaker override tripping

If the upper-level circuit breaker oversteps and trips, if it is found that the branch circuit protection has acted but the branch circuit breaker has not tripped, disconnect the circuit breaker of that level and then restore the upper-level circuit breaker; if it is found that none of the branch circuit protections have acted, You should check whether there is any fault in the equipment within the power outage range. If there is no fault, you can close the superior circuit breaker and test each branch circuit breaker one by one. When the power circuit breaker is sent to a certain branch circuit and trips again, it can be determined that the circuit breaker is a faulty circuit breaker. The line can be isolated, repaired and replaced.

For a circuit breaker to trip, two conditions must be met:

The first one is when the fault current reaches the set value;

The second is when the fault current lasts for a set length of time.

Therefore, to ensure that the circuit breaker does not trip over level, the current setting value and time setting value must be well coordinated.

For example, if the overcurrent protection setting value of the first-level circuit breaker is 700A and the duration setting value is 0.6 seconds, then the overcurrent protection setting value of the second-level circuit breaker should be reduced according to a certain proportion. For example, the current setting value is set to 630A, time set to 0.3 seconds.

In this case, if a fault occurs within the protection of the second-level circuit breaker, regardless of whether the fault current reaches the set value of the first-level circuit breaker, because the fault current lasts for 0.3 seconds, it will be cut off by the second-level circuit breaker. , cannot reach the 0.6 second of the first-level circuit breaker, so the first-level circuit breaker will not trip, thus avoiding over-level tripping.

▌03 Expansion information

Several points can be drawn here:

The first is the type of protection. Whether it is a short circuit fault or a ground fault, it is the same. They are all staggered by the size of the current and the length of time.

Second, it should be said that timing coordination is more important, because the fault current is likely to meet the protection settings of the multi-stage circuit breaker at the same time.

Third, the protection setting value of the circuit breaker has been set, and the time has also been set. Does this ensure that it will not be overstepped? Not necessarily, why? Judging from the setting values, like the above example, it seems that it should be able to cooperate at first glance. However, in reality, the time required for the circuit breaker to completely disconnect the fault current requires not only the basic judgment time, but also the mechanical The action time itself varies depending on the performance of equipment from different manufacturers. However, because the protection time of circuit breakers is on the millisecond level, this difference may also affect the cooperation between circuit breakers.

What does that mean? That is to say, like the above example, the second stage circuit breaker was supposed to cut off the fault current in 0.3 seconds, leaving the first stage with no chance to operate. However, the mechanical performance was too poor and it took 0.4 seconds longer to completely disconnect. , and before the second stage is completely disconnected, the first stage circuit breaker has detected the fault current for 0.6 seconds, so the first stage will also act, which leads to override tripping. Therefore, to ensure that there is no over-level tripping, the actual total operating time of the circuit breaker must be tested with a relay protection tester, and the time length of the actual test results is used to ensure the correct coordination.