Requirements and precautions for coordination between upper and lower levels of circuit breakers
1. Requirements for selective coordination of circuit breakers
If the inter level coordination of Circuit Breakers is replaced with national product standards, it is selective protection. The definition and requirements in the product standards are as follows.
1) Fully selective
In the case of two series connected overcurrent protection devices jointly protecting, the load side protection device implements protection without causing the overcurrent selective protection of the other protection device to operate.
2) Local selectivity
In the case of two series connected overcurrent protection devices jointly protecting, the load side protection device implements overcurrent selective protection at the specified overcurrent level without causing the other protection device to operate. The overcurrent limit value is called the selective limit current Is.
In the protection circuit of the control cabinet, when using circuit breakers as upper and lower level protection, their actions should be selective, and each level should coordinate and cooperate with each other. The selective coordination between the upper and lower levels of the circuit breaker must have "selectivity, speed, and sensitivity". Selectivity is related to the coordination between the upper and lower circuit breakers, while speed and sensitivity are respectively related to the characteristics of the circuit breaker itself and the operation mode of the line. If the upper and lower circuit breakers are properly coordinated, the faulty circuit can be selectively cut off to ensure that other fault free circuits of the control system continue to work normally. Otherwise, it will affect the reliability of the control system. Cascade protection is a specific application of the current limiting characteristics of circuit breakers. Its main principle is to utilize the current limiting effect of the upper level circuit breaker. When selecting the lower level circuit breaker, its breaking capacity can be reduced, which not only ensures reliable protection but also reduces costs.
The selectivity of circuit breakers
The selectivity of circuit breakers can be divided into two areas, one is the selectivity of overload zone, and the other is the selectivity of short-circuit zone.
In order to reliably cut off the grounding fault circuit of the circuit breaker, the requirements for grounding fault protection vary for different grounding forms and the use of electrical equipment. For TN systems, when overcurrent protection can meet the requirement of cutting off the ground fault line within the specified time, it is advisable to use overcurrent protection as well as ground fault protection; In three-phase four wire distribution lines, when overcurrent protection cannot cut off the grounding fault line within the specified time and zero sequence current protection can meet the requirements, it is advisable to use zero sequence current protection, but its protection setting value should be greater than the maximum unbalanced current of the distribution line; When both of the above protections cannot meet the requirements, residual current operated circuit breakers should be used.
The current intelligent circuit breakers generally have zone selective interlocking (ZSI) function, which uses microelectronics technology to make protection more complete. They are mainly divided into short-circuit fault interlocking and grounding fault interlocking, which play an important role in graded distribution systems and can effectively solve the problem of inter level coordination of circuit breakers, ensuring sensitivity and selectivity of action.
2. Issues to be noted in selective cooperation
When a circuit breaker is used as upper and lower level protection, its action should be selective, and the upper and lower levels should cooperate with each other, while paying attention to the following issues.
1) When the upper and lower level actions of a circuit breaker require selectivity, attention should be paid to the coordination between the current release setting value and time. The setting current for overload long delay and short-circuit short delay of the upper level circuit breaker should not be less than 1.3 times the setting current of the lower level circuit breaker to ensure the selectivity of action between the upper and lower levels. In general, the first level circuit breaker (such as the low-voltage side incoming line of a transformer) should use overload long delay and short-circuit short delay (adjustable delay from 0 to 0.5s) protection characteristics, and no short-circuit instantaneous release device should be installed. The second level circuit breaker should be equipped with overload long delay, short-circuit short delay, short-circuit instantaneous, and ground fault protection. Bus tie circuit breakers should be equipped with overload long delay and short-circuit short delay protection.
2) When the previous level is a selective circuit breaker and the next level is a non selective circuit breaker, in order to prevent short-circuit current from occurring in the circuit protected by the next level circuit breaker (due to the insufficient sensitivity of the instantaneous action of this level, the previous level short delay overcurrent release will act first, causing it to lose selectivity), the short-circuit short delay release setting current of the previous level circuit breaker should not be less than 1.3 times the short-circuit instantaneous release setting current of the next level circuit breaker; The setting current of the instantaneous release of the previous level circuit breaker should be greater than 1.2 times the single-phase short-circuit current at the output end of the next level circuit breaker.
3) When both upper and lower levels use selective circuit breakers, in order to ensure selectivity, there should be a step difference time for short-circuit short delay. The short delay action time of the upper level circuit breaker should be at least 0.2s longer than that of the lower level circuit breaker.
4) When both the upper and lower levels are non selective circuit breakers, the difference in the setting current of the release of the upper and lower level circuit breakers should be increased. The setting current of the upper level circuit breaker long delay release should not be less than twice the setting current of the lower level circuit breaker long delay release; The instantaneous release setting current of the higher-level circuit breaker should not be less than 1.4 times the instantaneous release setting current of the lower level circuit breaker.
5) When the short-circuit current at the outlet of the lower level circuit breaker is greater than the instantaneous release setting current of the upper level circuit breaker, it is advisable to use a current limiting circuit breaker for the lower level circuit breaker to ensure selectivity requirements.
6) When the distance between the upper and lower circuit breakers is very close and the expected short-circuit current difference at the output end is small, the upper circuit breaker should be selected with a short delay release to delay the action and ensure selective cooperation.
7) The setting of the release and time limit of the circuit breaker can generally refer to the following principles:
The setting current of the long delay release can be determined by 0.9~1.1 times the rated current of the release, and the time limit can be selected as 15 seconds. The setting current of the short delay release can be selected as 3-5 times the rated current of the release, and the time limit can be selected as 0.1s, 0.2s, and 0.4s. The setting current of the instantaneous release can be selected as 10-15 times the rated current of the release.