In electrical engineering, a vacuum interrupter is a switch which uses electrical contacts in a vacuum. It is the core component of medium-voltage circuit-breakers, generator circuit-breakers, and high-voltage circuit-breakers. Separation of the electrical contacts results in a metal vapour arc, which is quickly extinguished. Vacuum interrupters are widely used in utility power transmission systems, power generation unit, and power-distribution systems for railways, arc furnace applications, and industrial plants. Since the arc is contained within the interrupter, switchgear using vacuum interrupters are very compact compared with switchgear using air, sulfur hexafluoride (SF6) or oil as arc-suppression medium. Vacuum interrupters can be used for circuit-breakers and load switches. Circuit-breaker vacuum interrupters are used primarily in the power sector in substation and power-generation facilities, and load-switching vacuum interrupters are used for power-grid end users.
History The use of a vacuum for switching electrical currents was motivated by the observation that a one-centimeter gap in an X-ray tube could withstand tens of thousands of volts. Although some vacuum switching devices were patented during the 19th century, they were not commercially available. In 1926, a group led by Royal Sorensen at the California Institute of Technology investigated vacuum switching and tested several devices; fundamental aspects of arc interruption in a vacuum were investigated. Sorenson presented the results at an AIEE meeting that year, and predicted the switches' commercial use. In 1927, General Electric purchased the patent rights and began commercial development. The Great Depression and the development of oil-filled switchgear caused the company to reduce development work, and little commercially important work was done on vacuum power switchgear until the 1950s. In 1956, Hugh C. Ross at Jennings Radio Manufacturing Corporation revolutionized the high-frequency-circuit vacuum switch and produced a vacuum switch with a rating of 15 kV at 200 A. Five years later, Thomas H. Lee at General Electric produced the first vacuum circuit breakers with a rated voltage of 15 kV at short-circuit breaking currents of 12.5 kA. In 1966, devices were developed with a rated voltage of 15 kV and short-circuit breaking currents of 25 and 31.5 kA. After the 1970s, vacuum switches began to replace the minimal-oil switches in medium-voltage switchgear. In the early 1980s, SF6 switches and breakers were also gradually replaced by vacuum technology in medium-voltage application. As of 2018, a vacuum circuit-breaker had reached 145 kV with a short-circuit rating of 200 kA. In 2019, a research team in China tested a vacuum high-voltage circuit-breaker with 12 interrupters, for a rated voltage of 363 kV and a short-circuit rating of 63 kA.
Classification Vacuum interrupters may be classified by enclosure type, by application, and by voltage class. Experimental, radio-frequency, and early power-switching vacuum interrupters had glass enclosures. More recently, vacuum interrupters for power switchgear are made with ceramic envelopes. Applications and uses include circuit-breakers, generator circuit-breaker, load switches, motor contactors, and reclosers. Special-purpose vacuum interrupters are also manufactured, such as those used in transformer tap changers or in electrical arc furnaces.
Generator circuit-breaker Research and investigation in the early 1990s allowed the employment of vacuum switching technology for generator applications. Generator switching applications are well known for their higher strains on interrupting devices, such as high fault current of high asymmetry or high and steep transient recovery voltage; the standard IEC/IEEE 62271-37-013 (former and still valid IEEE C37.013, 1997) was introduced to address such requirements on circuit-breakers used in generator applications. Vacuum circuit-breakers can be qualified as a generator circuit-breakers (GCB) according to IEC/IEEE 62271-37-013. Compared to circuit-breakers using other quenching media (such as SF6, air-blast or minimum oil), vacuum circuit-breakers have the advantages of:
Great recovery strength, eliminating the need for capacitors to reduce the steepness of the transient recovery voltage (as required in most SF6 generator circuit-breakers); High mechanical and electrical durability with significantly higher numbers and frequency of possible switching operations without maintenance; and Environmental-friendliness by not using SF6. Vacuum GCBs are suitable for frequent switching duty and for interrupting low-frequency currents as found in pumped storage power plants.
Structure A vacuum interrupter generally has one fixed and one moving contact, a flexible bellows to allow movement of that contact, and arc shields enclosed in a hermetically-sealed glass, ceramic or metal housing with a high vacuum. The moving contact is connected by a flexible braid to the external circuit, and is moved by a mechanism when the device is required to open or close. Since air pressure tends to close the contacts, the operating mechanism must hold the contacts open against the closing force of air pressure on the bellows.
Airtight enclosure The interrupter's enclosure is made of glass or ceramic. Hermetic seals ensure that the interrupter vacuum is maintained for the life of the device. The enclosure must be impermeable to gas, and must not give off trapped gas. The stainless-steel bellows isolates the vacuum inside the interrupter from the external atmosphere and moves the contact within a specified range, opening and closing the switch.
Shielding A vacuum interrupter has shields around the contacts and at the ends of the interrupter, preventing any contact material vaporized during an arc from condensing on the inside of the vacuum envelope. This would reduce the insulation strength of the envelope, ultimately resulting in the arcing of the interrupter when open. The shield also helps control the shape of the electric-field distribution inside the interrupter, contributing to a higher open-circuit voltage rating. It helps absorb some of the energy produced in the arc, increasing a device's interrupting rating.
Contacts
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