Structure, principle and characteristics of vacuum circuit breaker

Structure, principle and characteristics of vacuum circuit breaker

Structure of vacuum circuit breaker
The structure of vacuum circuit breaker is mainly composed of three parts: vacuum arc extinguishing chamber, operating mechanism, support and other components.

1. Vacuum interrupter
Vacuum interrupter, also known as vacuum switch tube, is the core component of vacuum circuit breaker. Its main function is to enable the medium and high voltage circuit to quickly extinguish the arc and suppress the current after cutting off the power supply through the excellent insulation performance of the vacuum in the pipe, so as to avoid accidents and accidents. Vacuum interrupters are divided into glass vacuum interrupters and ceramic vacuum interrupters according to their shells.

Vacuum arc extinguishing chamber is mainly composed of air tight insulating shell, conductive circuit, shielding system, contact, bellows and other parts.

1) Air tight insulation system
The air tight insulation system consists of an air tight insulation shell made of glass or ceramics, a moving end cover plate, a fixed end cover plate, and a stainless steel bellows. In order to ensure good air tightness between glass, ceramics and metal, in addition to strict operation process during sealing, the permeability of the material itself is required to be as small as possible and the internal air release is limited to a minimum. Stainless steel bellows can not only isolate the vacuum state inside the vacuum arc extinguishing chamber from the external atmospheric state, but also make the moving contact and the moving conductive rod move within the specified range to complete the connection and disconnection operation of the vacuum switch.

2) Conductive system
The conducting system of the arc extinguishing chamber consists of the fixed conducting rod, the fixed running arc surface, the fixed contact, the moving contact, the moving running arc surface and the moving conducting rod. Among them, the fixed conducting rod, the fixed running arc surface and the fixed contact are collectively referred to as the fixed electrode; Moving contact, moving arc surface and moving conductive rod are collectively referred to as moving electrode. When the vacuum circuit breaker, vacuum load switch and vacuum contactor assembled by the vacuum arc extinguishing chamber are closed, the operating mechanism closes the two contacts through the movement of the moving conductive rod, completing the connection of the circuit. In order to keep the contact resistance between the two contacts as small as possible and stable, and have good mechanical strength when the arc extinguishing chamber bears the dynamic stable current, the vacuum switch is equipped with a guide sleeve at one end of the dynamic conductive rod, and a set of compression springs are used to maintain a rated pressure between the two contacts. When the vacuum switch breaks the current, the two contacts of the arc extinguishing chamber separate and generate an arc between them until the arc goes out when the current naturally crosses zero, and the circuit breaking is completed.

3) Shielding system
The shielding system of vacuum arc extinguishing chamber is mainly composed of shielding cylinder, shielding cover and other parts. The main functions of the shielding system are:
(1) Prevent the contact from generating a large amount of metal vapor and liquid droplet splashing during arcing, polluting the inner wall of the insulating shell, causing the insulation strength to decline or flashover.
(2) Improving the electric field distribution inside the vacuum interrupter is conducive to the miniaturization of the insulation shell of the vacuum interrupter, especially for the miniaturization of the vacuum interrupter with high voltage.
(3) Absorb part of arc energy and condense arc products. Especially when the vacuum interrupter interrupts the short-circuit current, most of the heat energy generated by the arc is absorbed by the shielding system, which is conducive to improving the dielectric recovery strength between the contacts. The greater the amount of arc products absorbed by the shielding system, the greater the energy it absorbs, which plays a good role in increasing the breaking capacity of the vacuum interrupter.

4) Contact system
The contact is the part where the arc is generated and extinguished, and the requirements for materials and structures are relatively high.
(1) Contact material
There are the following requirements for contact materials:
a. High breaking capacity
It requires that the conductivity of the material itself is large, the thermal conductivity coefficient is small, the thermal capacity is large, and the thermal electron emission capacity is low.
b. High breakdown voltage
High breakdown voltage leads to high dielectric recovery strength, which is beneficial to arc extinguishing.
c. High electrical corrosion resistance
That is, it can withstand the ablation of electric arc and has less metal evaporation.
d. Resistance to fusion welding.
e. The low cut-off current value is required to be below 2.5A.
f. Low gas content
Low air content is the requirement for all materials used inside the vacuum interrupter. Copper, in particular, must be oxygen free copper treated by a special process with low gas content. And the alloy of silver and copper is required for solder.
g. The contact material of vacuum arc extinguishing chamber for circuit breaker mostly adopts copper chromium alloy, with copper and chromium accounting for 50% respectively. A copper chromium alloy sheet with a thickness of 3mm is welded on the mating surfaces of the upper and lower contacts respectively. The rest is called contact base, which can be made of oxygen free copper.

(2) Contact structure
The contact structure has a great influence on the breaking capacity of the arc extinguishing chamber. The arc extinguishing effect produced by using contacts with different structures is different. There are three kinds of commonly used contacts: spiral trough type structure contact, cup-shaped structure contact with chute and cup-shaped structure contact with longitudinal magnetic field, of which the cup-shaped structure contact with longitudinal magnetic field is the main one.

5) Bellows
The bellows of the vacuum arc extinguishing chamber is mainly responsible for ensuring the movement of the moving electrode within a certain range and maintaining a high vacuum for a long time, and is used to ensure that the vacuum arc extinguishing chamber has a high mechanical life. The bellows of the vacuum interrupter is a thin-walled element made of stainless steel with a thickness of 0.1~0.2mm. During the opening and closing process of the vacuum switch, the bellows of the arc extinguishing chamber is subject to expansion and contraction, and the section of the bellows is subject to variable stress, so the service life of the bellows should be determined according to the repeated expansion and contraction and the service pressure. The service life of the bellows is related to the heating temperature of the working conditions. After the vacuum arc extinguishing chamber breaks the large short-circuit current, the residual heat of the conductive rod is transferred to the bellows to raise the temperature of the bellows. When the temperature rises to a certain extent, it will cause the fatigue of the bellows and affect the service life of the bellows.