Circuit breaker cascade selectivity and protection analysis
Introduction of Cascade
According to low-voltage power distribution design specifications, the main purpose of electrical system design is to ensure dynamic and thermal stability of the entire system in the event of a short circuit. Simply put, the selected circuit breaker can cut off the short-circuit current in time and reduce the impact of the short-circuit current on the distribution system.
The circuit breaker can provide three main protection functions. Overcurrent protection is used to cut off the overload current, short-circuit delay protection is used for impedance short-circuit protection, and transient protection is used for metallic short-circuit protection. The short-circuit breaking capacity of the circuit breaker is the core performance parameter.
These three basic protections provide reliable protection for the power distribution system, but the entire power distribution system can be protected by not a single circuit breaker, it has the coordination of the upper and lower levels. For example, low-voltage power distribution systems in civil buildings are generally divided into two levels of power distribution. The first-level distribution is distributed by the mains through the low-voltage distribution screen, and the second-level distribution is distributed through the power distribution box and the lighting distribution box to the load. If you have superiors and subordinates, you can't look at a single point, but you need to consider the cooperation of superiors and subordinates. Cascading technology can use a circuit breaker with a breaking capacity lower than the expected short-circuit current in the case where the current-limiting protection circuit breaker is used in the upper stage. Cascading is applicable to all upper and lower power distribution devices, not just the adjacent upper and lower power distribution.
2.The cascading mechanism
The cascade protection mechanism can be understood as the linkage protection between stages of upper and lower circuit breakers, which is based on the current limiting capability. For example, the lower circuit breaker encounters a short circuit, and the short circuit current generates electric repulsion. The upper and lower circuit breaker contacts are slightly repulsive. Two small arcs are connected in the circuit. The small arc limits the short circuit current in the circuit. The short-circuit current after the current limit is broken, and the upper circuit breaker contact is closed to resume normal connection.
The use of the circuit breaker's current-limiting capability to achieve cascade protection shows that the components in the system are not isolated and function independently, but are coordinated and linked with each other and are part of the system. There are three main benefits:
1. Due to the current limiting effect, the upper circuit breaker is away from the operating current range, providing reliable selective protection; 2. Due to the current limiting effect provided by the upper and lower stages, the actual breaking current of the lower circuit breaker is less than the expected short circuit current, Reduced the breaking capacity requirements of the lower circuit breaker, which can reduce the system cost; 3. Due to the common current limiting effect, the actual short-circuit current will be less than the calculated expected current, thus increasing the service life of electrical equipment in the loop. Refer to the figure below to clearly see the suppression of short-circuit current by the cascade.
3. Is the cascade reliable?
Cascading technology is an application of current limiting technology. It was developed in the 1970s after the successful development of current limiting circuit breakers in the 1970s. This technology is basically a derivative of current limiting technology. If you understand the principle of current limiting technology, you will have confidence in cascading technology.
There are three main methods of current limiting technology: 1. Electric power current limiting. A U-shaped circuit is designed in the circuit breaker breaking unit, which can generate electric repulsion when a short circuit occurs. , To reduce the short-circuit current; 2. Mechanical structure current limiting, when the mechanism design is overloaded, the moving contact is opened; 3. Arc extinction current limiting, a well-designed arc extinguishing capability effectively limits the short circuit current.
The principle of the cascade has been recognized by the IEC364-434.3 standard, and the cascade technology is written in Appendix A of GB 14048. 2 "Low-voltage switchgear and control equipment: Circuit Breakers": including the choice between upper and lower circuit breaker Sexual protection and backup protection.
The foreign low-voltage power supply method adopts two-way or multi-way parallel operation, which greatly improves the breaking performance of the lower circuit breaker, and the use of cascade can greatly reduce the cost of the system. Therefore, the cascade technology is widely used abroad.
The cascading technical capabilities of products can generally only be verified by the manufacturer in the laboratory, and the manufacturer provides the cascading combination table of the product. There are many products that can provide current limiting technology, such as Schneider, ABB, Siemens, Eaton, Mueller, etc., but there are few manufacturers that can provide complete cascading coordination tables, among which the more complete is Schneider low voltage circuit breaker, which provides a frame Circuit breakers, molded case circuit breakers to the end of the miniature circuit breaker series of cascading coordination table, as well as more actual project data, it will be easier to refer to.
4. Cascade application examples
Consider the power distribution system with three circuit breakers connected in series. In the 380 / 415V system, after calculating the rated current 220A of the upper circuit breaker A, the calculated expected short circuit current is 80kA, the rated current of circuit breaker B is 63A, and the expected short circuit current is 40kA Circuit breaker C has a rated current of 25A and an expected short circuit current of 26kA. Take the Compact NSX circuit breaker cascade table as an example. The upper circuit breaker A selects NSX250L, and the breaking capacity is 150kA. From the table, it can be found that the lower circuit breaker B can use the NSX100F with a breaking capacity of 36kA. After the combination, the breaking capacity can be enhanced to 150kA. The same third level can use the iC65L with a breaking capacity of 15kA, because the cascading cooperation with the NSX250L superior circuit breaker can increase the breaking capacity to 30kA. The so-called enhancement is not to say that the actual breaking capacity of this circuit breaker has been improved, but that the breaking capacity of the lower-level circuit breaker has been considerably enhanced due to the cooperation of the upper circuit breaker. This design greatly reduces the cost of the system and the volume of the system.
The cascading effect of low-voltage short-circuiters using current limiting technology brings new design ideas to low-voltage power distribution design, which not only meets the requirements of parameters, but also saves product volume and cost.