By www.eectech.cn | 16 March 2020 | 0 Comments
The composition principle and application of vacuum interrupter (vacuum interrupter)
The vacuum switch tube, also known as the vacuum interrupter, is the core component of the medium and high voltage power switch. Its main function is to quickly extinguish the arc and suppress the current after the medium and high voltage circuit is cut off through the excellent insulation of the vacuum in the tube, so as to avoid accidents and Accidents are mainly used in power transmission and distribution control systems, but also in metallurgy, mining, petroleum, chemical, railway, broadcasting, communications, industrial high-frequency heating and other power distribution systems. It has the characteristics of energy saving, material saving, fire prevention, explosion protection, small size, long life, low maintenance cost, reliable operation and no pollution. The vacuum interrupter is divided into arc interrupter for circuit breaker and arc interrupter for load switch. The interrupter for circuit breaker is mainly used for substations and power grid facilities in the power sector, and the arc interrupter for load switch is mainly used for power grid. End users.
structure type
It is mainly composed of airtight insulating shell, conductive loop, shielding system, bellows and other parts.
1, airtight insulation system
The airtight insulation shell, moving end cover plate, fixed end cover plate made of glass or ceramics, and stainless steel bellows constitute the airtight insulation system. In order to ensure good airtightness between glass, ceramics and metals, in addition to strict operation techniques during sealing, the air permeability of the material itself is also required to be as small as possible and the internal outgassing volume is limited to a minimum. The function of the stainless steel bellows can not only isolate the vacuum state inside the vacuum interrupter from the outside atmosphere, but also make the movable contact and the movable conductive rod move within a specified range to complete the closing and breaking of the vacuum switch operating.
2, conductive system
Fixed conductive rod, fixed arc surface, fixed contact, moving contact, moving arc surface, and movable conductive rod constitute the conductive system of the arc extinguishing chamber. Among them, the fixed conductive rod, fixed arc surface, and fixed contact are collectively called the fixed electrode, and the movable contact, the movable arc surface, and the movable conductive rod are collectively called the movable electrode. The vacuum circuit breaker and vacuum load switch are assembled from the vacuum interrupter. When closing with the vacuum contactor, the operating mechanism closes the two contacts through the movement of the conductive rod, completing the circuit connection. In order to reduce the contact resistance between the two contacts as much as possible and keep it stable, and the arc extinguishing chamber has good mechanical strength when it bears dynamic and stable current, the vacuum switch is equipped with a guide sleeve at one end of the moving conductive rod and uses a set of compression springs. Keep 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 naturally crosses zero, and the circuit breaking is completed.
3, shielding system
The shielding system of the vacuum interrupter is mainly composed of shielding cylinder, shielding cover and other parts.
The main function of the shielding system is:
(1) Prevent the contacts from producing a large amount of metal vapor and droplets splashing during the arcing process, polluting the inner wall of the insulating housing, and avoiding the insulation strength of the vacuum interrupter housing or flashover.
(2) Improve the electric field distribution inside the vacuum interrupter, which is beneficial to the miniaturization of the vacuum interrupter insulation shell, especially for the miniaturization of the high-voltage vacuum interrupter.
(3) Absorb part of the arc energy and condense the arc products. Especially when the vacuum interrupter interrupts the short-circuit current, most of the heat generated by the arc is absorbed by the shielding system, which is beneficial to improve the recovery strength of the medium 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
The contact is the part where the arc is generated and extinguished, and the requirements for materials and structure are relatively high.
4.1 There are the following requirements for contact materials:
(1) High breaking capacity. The material itself is required to have high conductivity, low thermal conductivity, large heat capacity, and low thermionic emission capability.
(2) High breakdown voltage. If the breakdown voltage is high, the recovery strength of the medium is high, which is beneficial to arc extinguishing.
(3) High electrical corrosion resistance. That is, it can withstand the ablation of the arc, and the amount of metal evaporation is small.
(4) Resistance to welding.
(5) Low intercept current value, hopefully below 2.5A.
(6) Low air content. Among them, low air content is a requirement for all materials used inside the vacuum interrupter. Especially for copper, oxygen-free copper must be treated with a special process with low gas content. For solder, silver and copper alloys are used.
At present, most of the contact materials of vacuum interrupters for circuit breakers are copper-chromium alloys, with copper and chromium each accounting for 50%. A copper-chromium alloy sheet is welded on the butting surface of the upper and lower contacts, each with a thickness of 3mm. The remaining part is called the contact seat, which can be made of oxygen-free copper.
4.2 Contact structure
The contact structure has a great influence on the breaking capacity of the isolated room. The arc extinguishing effect produced by the contacts of different structures is different. The simple cylindrical contacts were used in the early days. Although the structure is simple, the breaking capacity cannot meet the requirements of the circuit breaker. It can only break the current below 10kA. Vacuum switch tubes are used for vacuum load switches and high-voltage vacuum contactors. At present, there are three types of contacts with spiral groove structure, cup-shaped structure with chute, and cup-shaped structure of longitudinal magnetic field, among which the cup-shaped structure of longitudinal magnetic field is mainly used.
5, bellows
The bellows of the vacuum interrupter is mainly responsible for ensuring that the movable electrode moves within a certain range and maintains the function of high vacuum for a long time, and ensures that the vacuum interrupter has a high mechanical life.
The bellows of the vacuum interrupter is a thin-walled 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 cross section of the bellows is subject to variable stress, so the life of the bellows should be determined according to the amount of repeated expansion and contraction and the use pressure.
The fatigue life of the bellows is related to the heating temperature of the working conditions. After the vacuum interrupter interrupts the large short-circuit current, the residual heat of the conductive rod is transferred to the bellows, which increases the temperature of the bellows. When the temperature rise reaches a certain level , This will affect the fatigue strength of the bellows.
6. Working principle of vacuum switch tube
The vacuum interrupter in the vacuum package is an electric vacuum device that uses high vacuum to work as an insulating arc extinguishing medium and a pair of contacts sealed in a vacuum to realize the on-off function of the power circuit. When it disconnects a certain value of current, at the moment of separation of the moving and static contacts, the current shrinks to a point where the contacts just separated, and the resistance between the electrodes increases sharply and the temperature rises rapidly, until the evaporation of the electrode metal occurs, and the formation The extremely high electric field strength leads to extremely strong emission and gap breakdown, resulting in a vacuum arc. When the power frequency current is close to zero, the contact distance increases, and the plasma of the vacuum arc quickly spreads around. After the arc current crosses zero, the medium in the contact gap quickly changes from a conductor to an insulator, and the current is broken. Due to the special structure of the contact, the contact gap will produce an appropriate longitudinal magnetic field during arcing. This magnetic field can evenly distribute the arc on the contact surface and maintain a low arc voltage, so that the vacuum interrupter has a higher arc. After the medium strength recovery speed, small arc energy and small corrosion rate. In this way, the current breaking capacity and service life of the vacuum interrupter are improved.
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