Many times, the lowest-level circuit breaker does not jump! But the one above level jumped! This caused a large-scale power outage! Why is this? Today we will talk about this issue.
main situations of tripping overstep
main switch load capacity is less than the total capacity of the split switch load.
main switch has leakage protection device, but the split switch does not. When the electrical leakage is greater than or equal to 30 mAh, the main switch trips.
two-stage circuit breaker protection does not match, so try to use circuit breakers of the same brand.
often operates the main switch with load, causing the resistance to increase and the current to increase after poor carbonization contact, and heat trip. The protection configured by the lower circuit breaker of
cannot correctly determine the fault (such as single-phase grounding fault but zero-sequence protection is not configured).
Aging of the circuit breaker causes the trip time of the split excitation to become longer. A split excitation trip time needs to be replaced with a split excitation trip time that is less than the previous switch.
Over-level tripping solution
If the superior circuit breaker trips over-level tripping, if it is found that there is a branch protection action, but the branch circuit breaker has not tripped, the circuit breaker at this level will be broken, and then the upper circuit breaker will be restored; if it is found that the branch protection has not been operated, the equipment within the power outage range should be checked for any faults. If there is no fault, the superior circuit breaker can be closed and each branch circuit breaker can be tried to send each branch circuit breaker one by one. When the power circuit breaker trips again when sent to a certain branch, it can be determined that the circuit breaker is a faulty circuit breaker. This line can be isolated, repaired and replaced. To trip the circuit breaker, two conditions must be met. The first is that the fault current reaches the set value, and the second is that the fault current duration reaches the set length of time. Therefore, to ensure that the circuit breaker does not trip overstep, it must be coordinated with the current setting value and the time setting value.
For example, the overcurrent protection setting value of the first-stage circuit breaker is 700A and the duration setting value is 0.6 seconds. Then the overcurrent protection setting value of the second-stage circuit breaker should be reduced according to a certain proportion, such as the current setting value is set to 630A and the time is set to 0.3 seconds. In this way, if a fault occurs within the protection of the second-level circuit breaker, regardless of whether the fault current reaches the fixed value of the first-level circuit breaker, because the fault current will be cut off by the second-level circuit breaker when it lasts to 0.3. seconds, and it will not reach 0.6 seconds of the first-level circuit breaker, so the first-level circuit breaker will not jump, which will avoid cross-step tripping.
Here we can extend several points:
The first is the protection type. Whether it is a short circuit fault or a ground fault, it is the same, and it is all staggered by the current size and the judgment of the time length.
is the second one. It should be said that the coordination in time is more important, because the fault current is likely to meet the protection fixed value of the multi-stage circuit breaker at the same time.
The third one is, the protection setting of the circuit breaker has been set, and I have also set the time. So can this ensure that it will not exceed the level? Not necessarily, why? From the setting value, as shown in the above example, I think it should be able to cooperate at first glance. But in fact, the time required for the circuit breaker to completely disconnect the fault current is not only the basic judgment time, but also the operation time of the machine itself. This time length varies according to the performance of equipment of different manufacturers. However, because the protection time of the circuit breaker is in milliseconds, this difference may also affect the coordination between the circuit breakers. What does it mean? That is to say, like the above example, the second-stage circuit breaker should have to cut off the fault current in 0.3 seconds, so that the first stage has no chance to operate, but the mechanical performance is too poor, and it takes 0.4 seconds to completely disconnect. 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 operate, which leads to tripping over the stage. Therefore, to ensure that there is no tripping overstep, the relay equipment must be used to test the actual operating time of the circuit breaker, and the length of the actual test results must be used to ensure the correct coordination.
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