Editing Substation (section)

==Types==
[[File:Kaanaan polttolaitos ja Kemiran putkia.jpg|thumb|High-voltage substation in [[Kaanaa]], [[Pori]], Finland]]
[[File:Switchgear_110_kV.jpg|thumb|Substation in Russia]]

Substations typically serve at least one of the following purposes:{{sfn|Finn|2019|pp=7
8}}
*Increasing the voltage produced by [[electric power generation]] for efficient [[electric power transmission|transmission]] over long distances, using step-up transformers{{sfn|Finn|2019|p=7}}{{sfn|Blume|2016|p=54}}
*[[Interconnection]] of different power grids{{sfn|Finn|2019|p=7}}
*Reducing the voltage from transmission to lower-voltage [[electric power distribution|distribution]] lines that supply individual homes or businesses{{sfn|Finn|2019|p=7}}{{sfn|Blume|2016|p=54}}
*Converting from [[alternating current]] (AC) to [[direct current]] (DC){{sfn|Finn|2019|p=7}}

===Transmission substation===
A ''transmission substation'' connects two or more transmission lines.<ref name="Design Guide for Rural Substations" /> The simplest case is where all transmission lines have the same voltage. In such cases, substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have [[transformer]]s to convert between two transmission voltages, [[voltage compensation|voltage control]]/[[Power factor#Power factor correction (PFC) in non-linear loads|power factor correction]] devices such as capacitors, reactors or [[static VAR compensator]]s and equipment such as [[quadrature booster|phase shifting transformers]] to control power flow between two adjacent power systems.
[[File:Dülmen, Umspannstation -- 2014 -- 0005.jpg|thumb|Minimal HV station in Germany]]
Transmission substations can range from simple to complex. A small "switching station" may be little more than a [[Busbar|bus]] plus some [[circuit breaker#High-voltage circuit breakers|circuit breakers]]. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers, and a large amount of protection and control equipment ([[Instrument transformer|voltage and current transformers]], [[digital protective relay|relays]] and [[SCADA]] systems). Modern substations may be implemented using international standards such as [[IEC 61850|IEC Standard 61850]].

===Distribution substation===
[[File:Turmstation.jpg|thumb|upright|Transformer tower in Germany. Medium voltage supply at the front, low voltage output on the side.]]
[[File:SubstationHouseScarborough.jpg|thumb|A distribution substation in [[Toronto]], Canada, disguised as a house, complete with a driveway, front walk and a mown lawn and shrubs in the front yard. A warning notice can be clearly seen on the "front door". Disguises for substations are common in many cities.<ref>{{cite web |last=Steinberg |first=Neil |title=Lights On but Nobody Home: Behind the Fake Buildings that Power Chicago |date=13 December 2013 |url=http://www.messynessychic.com/2013/12/13/lights-on-but-nobody-home-behind-the-fake-buildings-that-power-chicago/ |access-date=14 December 2013}}</ref> ]]
A ''distribution substation'' transfers power from the transmission system to the distribution system of an area.<ref name="Design Guide for Rural Substations">{{cite web |last1=Stockton |first1=Blaine |title=Design Guide for Rural Substations |url=http://www.rd.usda.gov/files/UEP_Bulletin_1724E-300.pdf |website=USDA Rural Development |publisher=United States Department of Agriculture |access-date=4 February 2016}}</ref> It is uneconomical to directly connect electricity consumers to the main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a level suitable for local distribution.

The input for a distribution substation is typically at least two transmission or sub-transmission lines. Input voltage may be, for example, 115&nbsp;kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4&nbsp;kV and 33&nbsp;kV, depending on the size of the area served and the practices of the local utility. The feeders run along streets overhead (or underground, in some cases) and power the distribution transformers at or near the customer premises.

In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems. Distribution substations are typically the points of [[voltage regulation]], although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line.

The downtown areas of large cities feature complicated distribution substations, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

===Collector substation===
In [[distributed generation]] projects such as a [[wind farm]] or [[photovoltaic power station]], a collector substation may be required. It resembles a distribution substation although power flow is in the opposite direction, from many [[wind turbine]]s or [[inverters]] up into the transmission grid. Usually for economy of construction the collector system operates around 35&nbsp;kV, although some collector systems are 12 kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide [[power factor correction]] if it is needed, metering, and control of the wind farm. In some special cases a collector substation can also contain an HVDC converter station.

Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity. Examples for such substations are [[Brauweiler]] in Germany and [[Hradec substation|Hradec]] in the Czech Republic, where power is collected from nearby [[lignite]]-fired power plants. If no transformers are required for increasing the voltage to transmission level, the substation is a switching station.

===Converter substations===
Converter substations may be associated with [[High-voltage direct current|HVDC]] converter plants, [[traction current]], or interconnected non-synchronous networks. These stations contain power electronic devices to change the frequency of current, or else convert from alternating to direct current or the reverse. Formerly [[rotary converter]]s changed frequency to interconnect two systems; nowadays such substations are rare.

===Switching station===
A switching station is a substation without transformers and operating only at a single voltage level. Switching stations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for parallelizing circuits in case of failure. An example is the switching stations for the [[Inga–Shaba|HVDC Inga–Shaba]] transmission line.

A switching station may also be known as a switchyard, and these are commonly located directly adjacent to or nearby a [[power station]]. In this case the generators from the power station supply their power into the yard onto the generator bus on one side of the yard, and the transmission lines take their power from a Feeder Bus on the other side of the yard.

An important function performed by a substation is [[switch#Power switching|switching]], which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be planned or unplanned. A transmission line or other component may need to be de-energized for maintenance or for new construction, for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, companies aim at keeping the system up and running while performing maintenance. All work to be performed, from routine testing to adding entirely new substations, should be done while keeping the whole system running.
<gallery mode="packed">
File:Grand Coulee Dam electrical switchyward.jpg|Switchyard at [[Grand Coulee Dam]], United States, 2006. This is a 500 kV switchyard.<br/>
File:Umspannwerk Stuttgart-Seewiesen.jpg|Former high-voltage substation in Stuttgart, Germany, now 110&nbsp;kV switching station. The 220&nbsp;kV level is eliminated for grid simplification.<br/>
File:E11445 - DPLA - 59d7213736b68ece2fbcf348197aecda.jpg|230&nbsp;kV High-voltage switching station in Washington, United States
</gallery>
Unplanned switching events are caused by a [[fault (power engineering)|fault]] in a transmission line or any other component, for example:
*a line is hit by lightning and develops an [[Electric arc|arc]],
*a [[Transmission tower|tower]] is blown down by high wind.
The function of the switching station is to isolate the faulty portion of the system in the shortest possible time. De-energizing faulty equipment protects it from further damage, and isolating a fault helps keep the rest of the electrical grid operating with stability.<ref>{{cite web |title=Transformer Fire Video |url=http://www.metacafe.com/watch/39058/transformer_fire_video/ |website=metacafe |publisher=User Eagle Eye |access-date=4 February 2016}}</ref>

===Railways===
{{Main|Traction substation}}
Electrified railways also use substations, often distribution substations. In some cases a conversion of the current type takes place, commonly with [[rectifier]]s for [[direct current]] (DC) trains, or [[rotary converter]]s for trains using [[alternating current]] (AC) at frequencies other than that of the public grid. Sometimes they are also transmission substations or collector substations if the railway network also operates its own grid and generators to supply the other stations.

===Mobile substation===
A ''mobile substation'' is a substation on wheels, containing a transformer, breakers and buswork mounted on a self-contained [[semi-trailer]], meant to be pulled by a [[Tractor unit|truck]]. They are designed to be compact for travel on public roads, and are used for temporary backup in times of [[natural disaster]] or [[war]]. Mobile substations are usually rated much lower than permanent installations, and may be built in several units to meet road travel limitations.<ref name="Mobile Substations">{{cite web |last1=Boyd |first1=Dan |last2=Rampaul |first2=Glen |title=Mobile Substations |url=http://sites.ieee.org/winnipeg-pes/files/2015/01/Mobile_Substation_Presentation.pdf |website=IEEE Winnipeg PES Chapter |publisher=IEEE Power and Energy Society |access-date=11 October 2017}}</ref>