Interstage coordination of circuit breakers
In the design of low-voltage power distribution system, the selective cooperation between the upper and lower levels of the low-voltage circuit breaker must have "selectivity, rapidity and sensitivity". The selectivity is related to the cooperation between the upper and lower low-voltage circuit breakers, while the rapidity and sensitivity are related to the characteristics of the protective equipment itself and the operation mode of the line. If the upper and lower circuit breakers are properly coordinated, the faulty circuit can be selectively removed to ensure that other fault-free circuits in the power distribution system continue to work normally. On the contrary, it will affect the reliability of the power distribution system.
1)
When the expected short-circuit currents at the outlet ends of the upper and lower circuit breakers are quite different, and both are equipped with an instantaneous release, the instantaneous trip setting current of the upper circuit breaker should be greater than the expected short-circuit current of the lower circuit to ensure selective protection. .
2)
When the distance between the upper and lower circuit breakers is close, and the expected short-circuit current difference at the outlet end is small, the upper circuit breaker should be selected with a short-time delay release to delay action to ensure selective cooperation.
3)
When the upper and lower protection appliances use selective circuit breakers, in order to ensure the selectivity of action between the upper and lower levels, the setting current of the overload long delay and short circuit short delay of the upper circuit breaker should not be less than the corresponding protection setting value of the lower level. 1.3 times.
4)
When the upper-level protection is a selective circuit breaker and the lower-level protection is a non-selective circuit breaker, the following conditions shall be met:
a)
The setting current of the short-circuit short-time delay release of the upper-level protection circuit breaker shall not be less than 1.3 times the setting current of the short-circuit instantaneous release of the lower-level protection circuit breaker.
b)
The setting current of the instantaneous release of the upper-level protection circuit breaker should be greater than 1.2 times the single-phase short-circuit current at the outlet end of the lower-level protection circuit breaker.
5)
When both the upper and lower protection appliances choose non-selective circuit breakers, the step difference value of the tripper setting current of the circuit breaker between the upper and lower levels should be increased, which can generally be determined according to the following principles:
a)
The setting current of the long-delay release of the upper-level protection circuit breaker should not be less than twice the setting current of the next-level long-delay release.
b)
The setting current of the instantaneous release of the upper-level protection circuit breaker should not be less than 1.4 times the setting current of the lower-level instantaneous release.
c)
For the final stage non-selective circuit breaker, the short-circuit instantaneous release setting current should be as small as possible, but it should avoid the short-term overload peak current.
6)
When the short-circuit current at the outlet of the lower protection circuit breaker is greater than the set current of the instantaneous release of the upper level, in order to ensure the selectivity requirements, the lower protection circuit breaker should choose a current-limiting circuit breaker.
