Vacuum interrupters, also known as vacuum switch tubes, are core components of medium- and high-voltage power switches. Their primary function is to rapidly extinguish arcs and suppress current flow in medium- and high-voltage circuits after power is disconnected, using the excellent insulation properties of the vacuum within the tube to prevent accidents and incidents. They are primarily used in power transmission and distribution control systems, as well as in power distribution systems for metallurgy, mining, petroleum, chemical engineering, railways, broadcasting, communications, and industrial high-frequency heating. They offer advantages such as energy conservation, material conservation, fire and explosion resistance, compact size, long lifespan, low maintenance, reliable operation, and zero pollution. Vacuum interrupters are categorized by their intended use into circuit breaker applications and load switch applications. Circuit breaker interrupters are primarily used in substations and power grid facilities within the power sector, while load switch interrupters are primarily used by end users of the power grid.
Its structure consists of an airtight insulating shell, a conductive circuit, a shielding system, a bellows and other parts.
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1. Exhaust pipe protective cover
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1. Airtight Insulation System
The airtight insulation system consists of an airtight insulating casing made of glass or ceramic, a movable end cover, a fixed end cover, and a stainless steel bellows.
To ensure good airtightness between the glass, ceramic, and metal, in addition to strict sealing procedures, the material's inherent air permeability must be minimized and internal outgassing must be kept to a minimum. The stainless steel bellows not only isolate the vacuum inside the vacuum interrupter from the external atmosphere but also allow the movable contact and the movable conductive rod to move within a specified range, completing the closing and opening operations of the vacuum switch.
2. Conductive System
The fixed conductive rod, fixed arcing surface, fixed contact, movable contact, movable arcing surface, and movable conductive rod constitute the conductive system of the interrupter. The fixed conductive rod, fixed arcing surface, and fixed contact are collectively called the fixed electrode, while the movable contact, movable arcing surface, and movable conductive rod are collectively called the movable electrode. When a vacuum circuit breaker, vacuum load switch, or vacuum contactor, assembled from a vacuum interrupter, is closed, the operating mechanism moves the movable conductive rod, closing the two contacts and completing the circuit. To minimize and maintain contact resistance between the two contacts and ensure good mechanical strength when the interrupter withstands dynamic steady-state current, a guide sleeve is installed at one end of the movable conductive rod in the vacuum switch. A set of compression springs maintains a rated pressure between the two contacts. When the vacuum switch interrupts current, the two contacts of the interrupter separate, generating an arc between them. This arc extinguishes when the current naturally crosses zero, thus breaking the circuit.
3. Shielding system
The shielding system of the vacuum interrupter is mainly composed of a shielding tube, a shielding cover and other parts.
The main functions of the shielding system are:
(1) Preventing the contacts from generating a large amount of metal vapor and liquid droplets during the arcing process, which will contaminate the inner wall of the insulating shell and avoid causing the insulation strength of the vacuum interrupter shell to decrease or flashover.
(2) Improving the electric field distribution inside the vacuum interrupter is conducive to the miniaturization of the insulating shell of the vacuum interrupter, especially for the miniaturization of high-voltage vacuum interrupters.
(3) Absorbing part of the arc energy and condensing arc products. In particular, when the vacuum interrupter is interrupting 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 more arc products the shielding system absorbs, the greater the energy it absorbs, which plays a good role in increasing the interrupting capacity of the vacuum interrupter.
4. Contacts
The contacts are the parts where arcs are generated and extinguished, and the requirements for materials and structures are relatively high.
4.1 The following requirements are placed on contact materials:
(1) High breaking capacity. The material itself is required to have high electrical conductivity, low thermal conductivity, high heat capacity, and low thermal electron emission capability.
(2) High breakdown voltage. A high breakdown voltage means a high dielectric recovery strength, which is beneficial for arc extinguishing.
(3) High resistance to electrical corrosion. That is, it can withstand arc erosion and has low metal evaporation.
(4) Resistance to welding.
(5) Low cutoff current value, preferably below 2.5A.
(6) Low gas content.
Currently, the contact material of the vacuum interrupter used in circuit breakers is mostly copper-chromium alloy, with copper and chromium each accounting for 50%. A copper-chromium alloy sheet is welded on the mating surface of the upper and lower contacts, and the thickness is generally 3mm. The remaining part is called the contact seat and can be made of oxygen-free copper.
4.2 Contact Structure
The contact structure significantly influences the arc quenching chamber's breaking capacity. Different contact structures produce varying arc quenching effects. Early designs employed simple cylindrical contacts. While simple, their breaking capacity did not meet circuit breaker requirements, limited to currents below 10kA. Currently, these contacts are only used in vacuum load switches and high-voltage vacuum contactors using vacuum interrupters. Currently, three common contact structures are used: spiral grooved contacts, cup-shaped contacts with inclined slots, and longitudinal magnetic field cup-shaped contacts. The longitudinal magnetic field cup-shaped contact is the most common.
5. Bellows
The bellows of a vacuum interrupter are primarily responsible for ensuring the movement of the moving electrode within a certain range and maintaining a high vacuum for a long period of time, thereby ensuring the long mechanical life of the vacuum interrupter.
The bellows of a vacuum interrupter are thin-walled components made of stainless steel with a thickness of 0.1-0.2 mm. During the opening and closing of a vacuum switch, the bellows of the interrupter are subject to expansion and contraction, resulting in variable stress on the cross-section of the bellows. Therefore, the life of the bellows should be determined based on the amount of repeated expansion and contraction and the operating pressure.
The fatigue life of the bellows is related to the operating temperature. After the vacuum interrupter interrupts a large short-circuit current, the residual heat from the conductive rod is transferred to the bellows, causing the temperature of the bellows to rise. When the temperature rise reaches a certain level, this will affect the fatigue strength of the bellows.
6. Operating Principle
A vacuum interrupter is an electrical device that uses a pair of contacts sealed in a vacuum to connect and disconnect power circuits. It utilizes a high-vacuum insulating medium. When interrupting a certain current, the current shrinks to a point or points immediately after the moving and fixed contacts separate. This causes a sharp increase in resistance and a rapid rise in temperature between the electrodes, ultimately leading to evaporation of the electrode metal. This creates an extremely high electric field, leading to intense field emission and gap breakdown, creating a vacuum arc. When the operating current approaches zero and the contact spacing increases, the vacuum arc plasma rapidly spreads. After the arc current passes zero, the medium in the contact gap rapidly changes from a conductor to an insulator, interrupting the current and ending the interruption.
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Our company produces various types of vacuum interrupters, which can be divided into vacuum interrupters for circuit breakers, vacuum interrupters for load switches, vacuum interrupters for contactors, vacuum interrupters for reclosers and vacuum interrupters for sectionalizers according to their uses, parameters and breaking capacity.If you are intrested,please contact us at:
Email:pannie@hdswitchgear.com
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