View source for Switchgear

{{Use dmy dates|date=July 2023}}
{{Short description|Control gear of an electric power system}}
[[Image:High-voltage switchgear 01.jpg|thumb|[[High-voltage switchgear]]]]
[[Image:IndustrialSwitchgear.JPG|thumb|A section of a large switchgear panel.]]
[[Image:Tram switchgear.JPG|thumb|Tram switchgear]]
[[Image:Hybrid switchgear.jpg|thumb|This circuit breaker uses both SF<sub>6</sub> and air as insulation.]]

In an [[electric power system]], a '''switchgear''' is composed of electrical disconnect switches, [[fuse (electrical)|fuse]]s or [[circuit breaker]]s used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear [[fault (power engineering)|fault]]s downstream. This type of equipment is directly linked to the reliability of the [[electricity]] supply.

The earliest central power stations used simple open [[knife switch]]es, mounted on insulating panels of [[marble]] or [[asbestos]]. Power levels and voltages rapidly escalated, making opening manually operated switches too dangerous for anything other than [[Galvanic isolation|isolation]] of a de-energized circuit. Oil-filled switchgear equipment allows arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically operated switching elements using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or [[sulfur hexafluoride|SF<sub>6</sub>]] equipment, allowing large currents and power levels to be safely controlled by automatic equipment.

[[High-voltage switchgear]] was invented at the end of the 19th century for operating [[Electric motor|motors]] and other electric machines.<ref>[http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=GB189320069&F=0&QPN=GB189320069 British Pattern GB 20069 ''Improvements in Apparatus for Controlling the Application or Use of Electric Currents of High Tension and Great Quantity'' in 1893], on espacenet.com</ref> The technology has been improved over time and can now be used with voltages up to 1,100&nbsp;kV.<ref name="1100kV">Lin Jiming et al., ''Transient characteristics of 1 100 kV circuit-breakers'', ''International Symposium on International Standards for Ultra High Voltage'', Beijing, Juillet 2007.</ref>

Typically, switchgear in [[Electrical substation|substations]] is located on both the high- and low-voltage sides of large power [[transformer]]s. The switchgear on the low-voltage side of the transformers may be located in a building, with medium-voltage circuit breakers for distribution circuits, along with metering, control, and protection equipment. For industrial applications, a [[transformer]] and switchgear line-up may be combined in one housing, called a unitized [[electrical substation|substation]] (USS). According to the latest research by Visiongain, a market research company, the worldwide switchgear market is expected to achieve $152.5 billion by 2029 at a CAGR of 5.9%. Growing investment in renewable energy and enhanced demand for safe and secure electrical distribution systems are expected to generate the increase.<ref>{{Cite web|url=https://www.visiongain.com/the-worldwide-switchgear-market-is-expected-to-achieve-152-5bn-by-2029-says-visiongain-report/|title="The worldwide switchgear market is expected to achieve $152.5bn by 2029", says Visiongain report|date=2019-09-05|website=Visiongain|language=en-US|access-date=2019-09-06}}</ref>

==Components==
A switchgear assembly has two types of components:
*Power-conducting components, such as switches, circuit breakers, fuses, and lightning arrestors, that conduct or interrupt the flow of electrical power.
*[[Control system]]s such as control panels, current transformers, potential transformers, [[Protective relay|protective relays]], and associated circuitry, that monitor, control, and protect the power-conducting components.

==Functions==
One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear is further used to enhance system availability by allowing more than one source to feed a load.

==History==
[[Image:Schaltanlage um1910.jpg|thumb|Early switchgear (about 1910)]]
Switchgear is as old as [[electricity generation]]. The first models were very primitive: all components were simply fixed to a wall. Later they were mounted on wooden panels. For reasons of fire protection, the wood was replaced by [[slate]] or [[marble]]. This led to a further improvement, because the switching and measuring devices could be attached to the front, while the wiring was on the back.<ref>(German) Allgemeine Elektricitäts-Gesellschaft (ed) ''AEG Hilfsbuch für elektrische Licht- und Kraftanlagen 6th Ed.'', W. Girardet, Essen 1953</ref>

==Housing==
Switchgear for lower voltages may be entirely enclosed within a building. For higher voltages (over about 66 kV), switchgear is typically mounted outdoors and insulated by air, although this requires a large amount of space. Gas-insulated switchgear saves space compared with air-insulated equipment, although the equipment cost is higher. Oil insulated switchgear presents an oil spill hazard.

Switches may be manually operated or have motor drives to allow for remote control.

==Circuit breaker types ==
A switchgear may be a simple open-air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is the gas-insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulfur hexafluoride [[gas]] (SF<sub>6</sub>). Other common types are oil or vacuum insulated switchgear.

The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of thousands of amps. A [[circuit breaker]] (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the circuit breaker pulls apart the contacts (disconnects the circuit) requires careful design. Circuit breakers fall into these six types:

===Oil===
[[File:TMW 50981 Schnittmodell Hochspannung-Leistungschalter HPF500F.jpg|thumb|Cutaway model of an oil-filled high-voltage circuit breaker]]
Oil circuit breakers rely upon the vaporization of some of the oil to blast a jet of oil along the arc's path. The vapor released by the arcing consists of [[hydrogen gas]].
[[Mineral oil]] has better insulating properties than air. Whenever there is a separation of current-carrying contacts in the oil, the arc in the circuit breaker is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed to mostly [[hydrogen gas]] and ultimately creates a hydrogen bubble around the [[electric arc]]. This highly compressed gas bubble around the turn prevents the re-striking of the arc after the current reaches zero crossing of the cycle. The oil circuit breaker is one of the oldest types of circuit breakers.

===Air===
Air circuit breakers may use compressed air (puff) or the magnetic force of the arc itself to elongate the arc. As the length of the sustainable arc is dependent on the available voltage, the elongated arc will eventually exhaust itself. Alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current flow very quickly: typically between 30&nbsp;ms and 150&nbsp;ms depending upon the age and construction of the device.

===Gas===
{{main article|Sulfur hexafluoride circuit breaker}}
Gas (SF<sub>6</sub>) circuit breakers sometimes stretch the arc using a [[magnetic field]], and then rely upon the dielectric strength of the SF<sub>6</sub> gas to quench the stretched arc.

===Hybrid===
{{main article|Hybrid switchgear modules}}
Hybrid switchgear is a type which combines the components of traditional air-insulated switchgear (AIS) and SF<sub>6</sub> gas-insulated switchgear (GIS) technologies. It is characterized by a compact and modular design, which encompasses several different functions in one module.

===Vacuum===
Circuit breakers with [[vacuum interrupter]]s have minimal arcing characteristics (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched by a small amount (&lt;2–8&nbsp;mm). Near zero current the arc is not hot enough to maintain a plasma, and current ceases; the gap can then withstand the rise of voltage. Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 40,500 volts. Unlike the other types, they are inherently unsuitable for interrupting DC faults. The reason vacuum circuit breakers are unsuitable for breaking high DC voltages is that with DC there is no "current zero" period. The plasma arc can feed itself by continuing to gasify the contact material.

===Carbon dioxide ===
Breakers that use [[carbon dioxide]] as the insulating and arc extinguishing medium work on the same principles as a [[sulfur hexafluoride]] (SF<sub>6</sub>) breaker. Because SF<sub>6</sub> is a [[greenhouse gas]] more potent than CO<sub>2</sub>, by switching from SF<sub>6</sub> to CO<sub>2</sub> it is possible to reduce the greenhouse gas emissions by 10 tons during the product lifecycle.<ref>{{cite web |title= Switzerland : ABB breaks new ground with environment friendly high-voltage circuit breaker. |url= http://www.thefreelibrary.com/Switzerland+%3A+ABB+breaks+new+ground+with+environment+friendly...-a0300987882 |access-date= 9 July 2013 |archive-date= 24 December 2019 |archive-url= https://web.archive.org/web/20191224163718/https://www.thefreelibrary.com/Switzerland+:+ABB+breaks+new+ground+with+environment+friendly...-a0300987882 |url-status= dead }}</ref>

==Protective circuitry==

===Circuit breakers and fuses===
Circuit breakers and fuses disconnect when current exceeds a predetermined safe level.  However they cannot sense other critical faults, such as unbalanced currents—for example, when a transformer winding contacts ground.  By themselves, circuit breakers and fuses cannot distinguish between short circuits and high levels of electrical demand.

===Merz-Price circulating current scheme===
Differential protection depends upon [[Kirchhoff's circuit laws#Kirchhoff's current law (KCL)|Kirchhoff's current law]], which states that the sum of currents entering or leaving a circuit node must equal zero.  Using this principle to implement differential protection, any section of a conductive path may be considered a node.  The conductive path could be a transmission line, a winding of a transformer, a winding in a motor, or a winding in the stator of an alternator.  This form of protection works best when both ends of the conductive path are physically close to each other.  This scheme was invented  in Great Britain by [[Charles Hesterman Merz]] and [[Bernard Price]].<ref name="Black1983">{{cite book|author=Robert Monro Black|title=The History of Electric Wires and Cables|url=https://books.google.com/books?id=HUCieJjeQ-wC&pg=PA101|date=January 1983|publisher=IET|isbn=978-0-86341-001-7|pages=101–}}</ref>

Two identical current transformers are used for each winding of a transformer, stator, or other device.  The current transformers are placed around opposite ends of a winding.  The current through both ends should be identical.  A protective relay detects any imbalance in currents, and trips circuit breakers to isolate the device.  In the case of a transformer, the circuit breakers on both the primary and secondary would open.

===Distance relays===
A short circuit at the end of a long transmission line appears similar to a normal load, because the impedance of the transmission line limits the fault current.  A distance relay detects a fault by comparing the voltage and current on the transmission line.  A large current along with a voltage drop indicates a fault.

==Classification==
Several different classifications of switchgear can be made:<ref>Robert W. Smeaton (ed) ''Switchgear and Control Handbook 3rd Ed.'', McGraw Hill, New York 1997 {{ISBN|0-07-058451-6}}</ref>
{{div col}}
* By the current rating.
* By interrupting rating (maximum [[short circuit]] current kAIC that the device can safely interrupt)
** Circuit breakers can open and close on fault currents
** Load-break/Load-make switches can switch normal system load currents
** Isolators are off load disconnectors which are to be operated after Circuit Breakers, or else if the load current is very small
*By voltage class:
** Low voltage (less than 1 kV AC)
** Medium voltage (1 kV AC through to approximately 75 kV AC)
** High voltage (75 kV to about 230 kV AC)
** Extra high voltage, ultra high voltage (more than 230 kV)
* By insulating medium:
** Air
** Gas (SF<sub>6</sub> or mixtures)
** Oil
** Vacuum
** Carbon dioxide (CO<sub>2</sub>)
* By construction type:
** Indoor (further classified by IP ([[Ingress Protection]]) class or NEMA enclosure type)
** Outdoor
** Industrial
** Utility
** Marine
** Draw-out elements (removable without many tools)
** Fixed elements (bolted fasteners)
** Live-front
** Dead-front
** Open
** Metal-enclosed (ME) — A switchgear assembly completely enclosed on all sides and the top with sheet metal.<ref>IEEE Std C37.20.2-1999. IEEE Standard for Metal-Clad Switchgear.</ref>
** {{anchor|Metal-clad}} Metal-clad (MC) — A more expensive variety of metal-enclosed switchgear that has the following characteristics: the main switching and interrupting device of removable type; grounded metal barriers to separate compartments and enclose all major circuits and parts; mechanical interlocks; insulated [[busbar|bus conductors]] and other features.<ref>IEEE Std C37.100-1992. IEEE Standard Definitions for Power Switchgear.</ref><ref>{{cite web |title=Metal-Clad vs Metal-Enclosed |date=November 4, 2008 |publisher= Electrical Engineers and Master Electricians Portal |url=http://eeame.com/portal/contents-mainmenu-29/opinions-mainmenu-63/109-metal-clad-vs-metal-enclosed.html |access-date=June 28, 2016 |archive-url=https://web.archive.org/web/20160827002632/http://eeame.com/portal/contents-mainmenu-29/opinions-mainmenu-63/109-metal-clad-vs-metal-enclosed.html |archive-date=August 27, 2016}}</ref>
** {{vanchor|Cubicle switchgear|text=Cubicle}}
** Arc-resistant
* By IEC degree of internal separation<ref>IEC Standard EN 60439 part 1 Table 6A</ref>
** No Separation (Form 1)
** Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
** Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
** Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
** Functional units separated from each other (Form 3a, 3b, 4a, 4b)
** Terminals for external conductors separated from each other (Form 4a, 4b)
** Terminals for external conductors separate from their associated functional unit (Form 4b)
*By interrupting device:
** Fuses
** Air Circuit Breaker
** Minimum Oil Circuit Breaker
** Oil Circuit Breaker
** Vacuum Circuit Breaker
** [[Sulfur hexafluoride circuit breaker|Gas (SF<sub>6</sub>) Circuit breaker]]
** CO<sub>2</sub> Circuit Breaker
* By operating method:
** Manually operated
** Motor/stored energy operated
** Solenoid operated
* By type of current:
** Alternating current
** Direct current
* By application:
** Transmission system
** Distribution
* By purpose
** Isolating switches ([[disconnector]]s)
** Load-break switches.<ref>{{in lang|fr}} [http://domino.iec.ch/preview/info_iec60265-1%7Bed3%2E0%7Db.pdf ''Norme CEI 60265-1 Interrupteurs pour tension assignée supérieure à 1 kV et inférieure à 52 kV'']  {{webarchive |url=https://web.archive.org/web/20070930171034/http://domino.iec.ch/preview/info_iec60265-1%7Bed3%2E0%7Db.pdf |date=September 30, 2007 }}</ref><ref>{{in lang|fr}} [http://domino.iec.ch/preview/info_iec60265-2%7Bed3%2E0%7Db.img.pdf ''Norme CEI 60265-2 Interrupteurs pour tension assignée supérieure à 52 kV'']  {{dead link|date=July 2020|bot=medic}}{{cbignore|bot=medic}}</ref>
** Grounding (earthing) switches
{{div col end}}
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually operated switches may all exist in the same row of cubicles.

Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly [[IEEE]] and [[ANSI]] standards are used, much of the rest of the world uses [[International Electrotechnical Commission|IEC]] standards, sometimes with local national derivatives or variations.

==Safety==
[[Image:Disjoncteurs 245kV.jpg|thumb|245&nbsp;kV circuit breaker in air insulated substation]]
[[Image:GIS 420kV.jpg|thumb|420&nbsp;kV gas insulated switchgear]]
To help ensure safe operation sequences of switchgear, [[trapped-key interlocking]] provides predefined scenarios of operation. For example, if only one of two sources of supply are permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.

Indoor switchgear can also be type tested for internal arc containment (e.g., IEC 62271-200). This test is important for user safety as modern switchgear is capable of switching large currents.<ref>{{cite web |language=en |access-date=29 July 2023 |url=https://www.energy.siemens.com/cms/00000013/aune/Documents/Medium%20Voltage%20Arc%20Fault%20Containment.pdf |title=Medium Voltage Arc Fault Containment |website=[[Siemens]] |archive-url=https://web.archive.org/web/20090318070617/https://www.energy.siemens.com/cms/00000013/aune/Documents/Medium%20Voltage%20Arc%20Fault%20Containment.pdf |url-status=usurped |archive-date=March 18, 2009}}</ref>

Switchgear is often inspected using [[thermal imaging]] to assess the state of the system and predict failures before they occur. Other methods include [[partial discharge]] (PD) testing, using either fixed or portable testers, and [[acoustic emission]] testing using surface-mounted transducers (for oil equipment) or [[Ultrasonic testing|ultrasonic]] detectors used in outdoor switchyards. Temperature sensors fitted to cables to the switchgear can permanently monitor temperature build-up. SF<sub>6</sub> equipment is invariably fitted with alarms and interlocks to warn of loss of pressure, and to prevent operation if the pressure falls too low.

The increasing awareness of dangers associated with high fault levels has resulted in network operators specifying closed-door operations for earth switches and racking breakers. Many European power companies have banned operators from switch rooms while operating. [[Remote racking system]]s are available which allow an operator to rack switchgear from a remote location without the need to wear a protective arc flash hazard suit. Switchgear systems require continuous maintenance and servicing to remain safe to use and fully optimized to provide such high voltages.<ref>{{Cite web|url=https://johnsonphillips.co.uk/services/switchgear/|title=Switchgear Systems and Services|website=johnsonphillips.co.uk|language=en-GB|access-date=2018-05-15}}</ref>

==See also==
*[[Arc flash]]
*[[Circuit breaker]]
*[[Disconnector]]
*[[Electrical safety]]
*[[Electric arc]]
*[[High voltage]]
*[[Remote racking system]]
*[[Short circuit]]

==References==
{{Reflist}}

==External links==
{{Commons category}}

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[[Category:Electric power systems components]]
[[Category:Electric power infrastructure]]