Vacuum Circuit Breaker Working Principle and Model Explanation
2024-09-10 11:03:58
"Vacuum circuit breaker" is named after its arc extinguishing medium and the insulating medium of the contact gap after arc extinguishing are both high vacuum; it has the advantages of small size, light weight, suitable for frequent operation, and no need for maintenance of arc extinguishing. The application in the power grid is more popular. The vacuum circuit breaker is an indoor power distribution device in a 3~10kV, 50Hz three-phase AC system. It can be used as the protection and control of electrical equipment in industrial and mining enterprises, power plants, and substations. For maintenance and frequent operation places, circuit breakers can be configured in centre cabinets, double-layer cabinets, and fixed cabinets for control and protection of high-voltage electrical equipment.
1. Working Principle of Vacuum Circuit Breaker
The working principle of the vacuum circuit breaker is: when the moving and static contacts are opened under the action of the operating mechanism, the arc is generated between the contacts, and the contact surface volatilises steam at a high temperature. Due to the special shape of the contact, a magnetic field is generated while the current passing through, the arc moves rapidly along the tangential direction of the contact surface under the action of this magnetic field, and part of the metal vapour is condensed on the metal cylinder (shield cover). The arc is extinguished when it naturally crosses zero and after that, the strength of the medium quickly recovered.
2. The Structure of the Vacuum Circuit Breaker
The vacuum circuit breaker is composed of the vacuum interrupter, the operating mechanism, the transmission mechanism, and the chassis. As shown in the figure, the vacuum arc extinguishing consists of static contact, moving contact, shielding cover, shell, bellows, protective tube, conductive rod, end cover plate, etc. The disconnection distance between the moving and static contacts of a 10kV vacuum circuit breaker is generally 10~15mm.
The contact of the vacuum interrupter generally adopts the butt type, which is prone to the bounce phenomenon. There are special requirements for the material of the contact to prevent the operation over-voltage caused by bounce, and it is also required that there are sufficient initial and final pressures during the closing process. Furthermore, the bounce time should not exceed 2ms.
The characteristics of resistance to welding, arc resistance, low water content, and low current interception level are required for the material of the vacuum circuit breaker contacts to reliably meet the short-circuit current. The contacts are generally made by multi-element alloys, such as copper-tungsten-bismuth-zirconium quaternary alloy, copper-tungsten-nickel-antimony quaternary alloy, and copper-tellurium-selenium, copper-bismuth-cerium, copper-bismuth-aluminium etc.
The components of the vacuum interrupter are sealed in the glass shell, and the glass shell itself is insulating as well. The bellows are used to seal the movable conductive rod, which is an elastic element, through which the vacuum interrupter can complete the opening and closing operations under the action of the operating mechanism without damaging the vacuum.
During the opening operation of the vacuum circuit breaker, as the high insulation strength in the high vacuum degree and the extremely rare gas, the charged particles generated by the arc between the contacts diffuse rapidly. Therefore, it will not reignite after the arc crosses zero and extinguishes. The metal vapour and charged particles in the arc combustion process are absorbed by the shield and condense during the intense diffusion. There are three Archimedes spiral grooves on the arcing surface of the contacts which make the arc current generate a transverse magnetic field between the contacts on the path through which the arc current passes and move quickly in the tangential direction on the main contacts, thereby reducing both the temperature and the burning of the contact.