Structure, principle and characteristics of vacuum circuit breaker

First, the vacuum circuit breaker structure

The structure of the vacuum circuit breaker is mainly composed of three parts: vacuum interrupter, operating mechanism, support and other components.

1.Vacuum arc extinguishing chamber (Vacuum interrupter)

Vacuum interrupter, also known as vacuum switch tube, is the core component of vacuum circuit breaker. Its main function is that through the excellent insulation performance of the vacuum in the tube, the middle and high voltage circuit can quickly extinguish the arc and suppress the current after cutting off the power supply, and avoid accidents and accidents. Vacuum interrupter is divided into glass vacuum interrupter and ceramic vacuum interrupter according to the shell.

​The vacuum interrupter is mainly composed of an airtight insulating shell, a conductive circuit, a shielding system, a contact, a bellows and other parts.

• Airtight insulation system: The airtight insulation system is composed of glass or ceramic airtight insulation shell, moving end cover plate, fixed end cover plate and stainless steel bellows. In order to ensure good air tightness between glass, ceramics and metal, in addition to strict operation skills when sealing, it is also required that the permeability of the material itself is as small as possible and the internal exhaust gas is limited to a minimum. The stainless steel bellows can not only isolate the vacuum state in the vacuum extinguishing chamber from the external atmospheric state, but also make the moving contacts together with the moving conductive rod move within the specified range to complete the switching and breaking operation of the vacuum switch.

• Conductive system: The conducting system of the arc extinguishing chamber consists of a fixed conductor rod, a fixed running arc surface, a fixed contact, a moving contact, a moving running arc surface and a moving conductor rod. The fixed conductor rod, the fixed running arc and the fixed contact are called the fixed electrode; Moving contact, moving arc surface and moving conductive rod are called moving electrode. When the vacuum circuit breaker, vacuum load switch and vacuum contactor assembled by the vacuum interrupter are closed, the operating mechanism makes the two contacts close through the movement of the moving conductor rod, and the circuit is completed. In order to make the contact resistance between the two contacts as small as possible and maintain stability, the arc interrupter has good mechanical strength when it is subjected to dynamic stable current, the vacuum switch is provided with a guide sleeve at one end of the dynamic conductive rod, and uses a set of compression springs to maintain a rated pressure between the two contacts. When the vacuum switch breaks the current, the two contacts of the arc extinguishing chamber are separated and an arc is generated between them, until the arc is extinguished when the current is naturally zero, and the circuit is broken.

• Shielding system: The shielding system of vacuum interrupter is mainly composed of shielding cylinder, shielding cover and other parts. The main functions of the shielding system are:
  • Prevent the contact from producing a large number of metal vapor and liquid droplets during the arc burning process, polluting the inner wall of the insulation shell, resulting in a decrease in insulation strength or flashover.
  • Improving the electric field distribution in the vacuum interrupter chamber is conducive to the miniaturization of the vacuum interrupter enclosure, especially for the miniaturization of the high-voltage vacuum interrupter.
  •  Absorb part of the arc energy and condense the arc product. Especially when the vacuum interrupter breaks 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 recovery strength of the medium between the contacts. The greater the amount of arc product absorbed by the shielding system, the greater the energy absorbed by it, which plays a good role in increasing the breaking capacity of the vacuum arc extinguishing chamber.

• Contact system: The contact is the part that generates arc and extinguishes arc, and the requirements for material and structure are relatively high.
  • Contact material, There are the following requirements for contact materials:
    • high breaking ability: The material itself is required to have large conductivity, small thermal conductivity, large thermal capacity and low thermal electron emission capacity.
    •  high breakdown voltage: The breakdown voltage is high, the recovery strength of the medium is high, and it is advantageous for arc extinguishing.
    •  high resistance to electrical corrosion: That is, it can withstand the ablation of the arc, and the amount of metal evaporation is small.
    • anti-fusion welding ability.
    •  low cut-off current value, required to be below 2.5A.
    • low gas content: Low gas content is a requirement for materials used in all vacuum arc-extinguishing chambers. In particular, copper must be oxygen-free copper treated by a special process with low gas content. Solder, etc., requires the use of silver, copper alloys.
    •  The contact material of the vacuum arc extinguishing chamber for circuit breakers is mostly copper-chromium alloy, with copper and chromium accounting for 50% each. Weld a copper-chromium alloy sheet on the butt surface of the upper and lower contacts, with a general thickness of 3mm. The rest is called the contact holder and can be made of oxygen-free copper.
  • Contact structure: The structure of the contact has a great influence on the breaking ability of the isolation chamber. The arc suppression effect produced by using different structure contacts is different. There are three kinds of contacts commonly used: helical trough type contacts, chamfered cup structure contacts and longitudinal magnetic field cup structure contacts, among which the longitudinal magnetic field cup structure contacts are mainly used.

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

2.Operate the structure

Vacuum circuit breaker adopts mature and reliable electric energy storage spring control mechanism, which has six functions of electric closing, electric breaking, manual energy storage, manual closing, manual breaking and over current (or short circuit) automatic trip breaking. The mechanism is composed of ratchet, CAM, closing spring, opening spring, manual tripping link, tripping device and auxiliary switch.

Second, the principle of vacuum circuit breaker

The vacuum interrupter, the core component of vacuum circuit breaker, is a kind of electric vacuum device which uses the high vacuum degree insulating interrupter medium and uses a pair of contacts sealed in the vacuum to realize the power circuit on-off function. When it breaks off a certain value of current, at the moment of separation of the static and static contacts, the current shrinks to a certain point or point where the contacts have just been separated, resulting in a sharp increase in the resistance between the electrodes and a rapid increase in temperature until the evaporation of the electrode metal occurs, and at the same time, a very high electric field strength is formed, resulting in a violent emission of electric field strength and gap breakdown, resulting in a vacuum arc. When the working current is close to zero, and the contact spacing increases, the ionization of the vacuum arc quickly spreads around.

After the arc current crosses zero, the medium in the contact gap rapidly changes from a conductor to an insulator, so the current is broken. Due to the special structure of the contacts, the gap between the contacts during the arc burning will produce an appropriate longitudinal magnetic field, which can make the arc evenly distributed on the surface of the contacts and maintain a low arc voltage, so that the vacuum arc extinguishing chamber has a high strength recovery rate of the backarc medium, a small arc energy and a small corrosion rate. In this way, the breaking current ability and service life of the vacuum interrupter are improved.

Third, the characteristics of vacuum circuit breaker

1. Advantages

1) Quenching in a sealed container, the arc and hot gas are not exposed. As an independent component, the arc extinguishing chamber is easy to install and debug.

2) The contact gap is small, generally about 10mm, the closing power is small, the mechanism is simple, and the service life is long.

3) Short quenching time, low arc, small arc energy, small contact loss, more times of breaking.

4) The inertia of the moving guide rod is small, suitable for frequent operation.

5) Small operating mechanism, small overall volume, light weight.

6) Small control power, small noise during switch operation.

7) The arc extinguishing medium or insulating medium does not use oil, and there is no danger of fire and explosion.

8) The contact head is divided into a completely sealed structure, which will not reduce its performance due to the influence of moisture, dust, harmful gases, etc., reliable work and stable on-off performance.

9) After the vacuum circuit breaker is broken, the medium between the breaks recovers quickly, and the medium does not need to be replaced.

10) Within the service life of the vacuum switch tube, the contact part does not need maintenance inspection, generally up to about 20 years without maintenance. The maintenance workload is small and the maintenance cost is low.

11) With multiple reclosing function, suitable for the application requirements in the distribution network.

2. Disadvantages

1) When the inductive breaking current is small, the contact material with strong arc extinguishing ability of the circuit breaker is easy to produce cut-off, causing overvoltage, and corresponding overvoltage protection should be taken.

2) The price is higher, mainly depends on the professional production of vacuum interrupter and the requirements of institutional reliability.

Thanks for reading this article

Steven Wong