VePi Newsletters The Electrical Power Systems Division The Switchyards (outdoor substations) section Number: 1Introduction:A substation is a high-voltage electric system facility. It is usedto switch generators, equipment, and circuits or lines in and out ofa system. It also is used to change AC voltages from one level toanother, and/or change alternating current to direct current ordirect current to alternating current. Some substations are smallwith little more than a transformer and associated switches. Othersare very large with several transformers and dozens of switches andother equipment. An electrical substation is a subsidiary station ofan electricity generation, transmission and distribution system wherevoltage is transformed from high to low or the reverse usingtransformers. Electric power may flow through several substationsbetween generating plant and consumer, and may be changed in voltagein several steps.A substation that has a step-up transformer increases the voltagewhile decreasing the current, while a step-down transformer decreasesthe voltage while increasing the current for domestic and commercialdistribution. The word substation comes from the days before thedistribution system became a grid.Substations are designed to accomplish the following functions,although not all substations have all these functions: 1) Change voltage from one level to another 2) Regulate voltage to compensate for system voltage changes 3) Switch transmission and distribution circuits into and out of thegrid system 4) Measure electric power quantities flowing in the circuits 5) Connect communication signals to the circuits
6) Eliminate lightning and other electrical surges from the system 7)Connect electric generation plants to the system 8) Make interconnections between the electric systems of more thanone utility 9) Control reactive kilovolt-amperes supplied to and the flow ofreactive kilovolt-amperes in the circuitsSubstations generally have switching, monitoring, protection andcontrol equipment and one or more transformers. In a largesubstation, circuit breakers are used to interrupt any short-circuitsor overload currents that may occur on the network. Smallerdistribution stations may use recloser circuit breakers or fuses forprotection of distribution circuits. Substations do not usually havegenerators, although a power plant may have a substation nearby.Other devices such as power factor correction capacitors and voltageregulators may also be located at a substation.Substations may be on the surface in fenced enclosures, underground,or located in special-purpose buildings. High-rise buildings may haveseveral indoor substations. Indoor substations are usually found inurban areas to reduce the noise from the transformers, for reasons ofappearance, or to protect switchgear from extreme climate orpollution conditions.Where a substation has a metallic fence, it must be properly grounded(UK: earthed) to the ground mat underneath the substation to protectpeople from high voltages that may occur during a fault in thenetwork. Earth faults at a substation can cause a ground potentialrise (GPR). Currents flowing in the earths surface during a faultcan cause metal objects to have a significantly different voltagethan the ground under a persons feet; this touch potential presentsa hazard of electrocution as the current passes through the bodyresistance (through the heart).Transmission substations:A transmission substation connects two or more transmission lines.The simplest case is where all transmission lines have the samevoltage. In such cases, the substation contains high-voltage switches(and or circuit breakers) that allow lines to be connected orisolated for fault clearance or maintenance. A transmission stationmay have transformers to convert between two transmission voltages,voltage control devices such as capacitors, reactors or Static VARsand equipment such as phase shifting transformers to control powerflow between two adjacent power systems.Terminal substations:A terminal substation is a facility that forms a strategic node pointin an interconnected electricity transmission system.A terminal substation fulfils either or both roles:1) It provides a connection point where transmission lines of thesame voltage may be joined to enable an electricity supply to beestablished to a new demand centre or centres (for example, thetransmission line voltage is 230 KV and the transformer station has
the 230 KV disconnect switch plus the transformer rated 230/27.6 KV,where 27.6 KV can be the sub-transmission voltage level or thedistribution level), or to achieve a greater degree ofinterconnection within the existing system (in which it comes closerto a non-transformation transmission substation). It is a bulk supplypoint in the electrical grid, where it may serve a significant areawithin the metropolitan area and/or some country areas.2) It is a transformation point where lower voltages are produced tosupply the metropolitan transmission system.Transformer substations:A transformer substation is a point where the transmission voltagelevel is stepped down to the sub-transmission voltage level. Thelatter voltage is then either used to feed a distribution substationto further reduce the voltage level to the distribution level oritself used as an input to distribution transformers (eg. 27.6 KV/600 V or 208 V) i.e. power is tapped from the sub-transmission linefor use in an industrial facility along the way, otherwise, the powergoes to a distribution substation. . Thus the major components insuch a station will be: one or two high voltage disconnect switches,one or two power transformers, one or two medium voltage switchgearlineups with their breakers, instrument transformers, relays,communication and control networks.Distribution Substation:Distribution substations are located near to the end-users.Distribution substation transformers change the transmission or sub-transmission voltage to lower levels. Typical distribution voltage is4,160Y/2400 volts. From here the power is distributed to industrial,commercial, and residential customers through distributiontransformers, pad mounted, overhead pole mounted, vault installed,the secondary of which is 600/347 V or 120/208 V.Unit substations:A unit substation would typically consist of a load break switch witha set of power or current limiting fuses, in series with it,connected to the high voltage winding of a distribution (or a powertransformer), the low voltage winding of the transformer would beconnected to the main circuit breaker plus the feeder circuitbreakers, motor contactors plus disconnect switch and fuses, or loadbreak switches in the switchgear lineup. Within the lineup, therewould be the utility metering compartment with the current andvoltage transformers approved for utility meter application as wellas the user instrument transformers, meters, protection and control.Collector substation:In distributed generation projects such as a wind farm, a collectorsubstation may be required. It somewhat resembles a distributionsubstation although power flow is in the opposite direction, frommany wind turbines up into the transmission grid. Usually for economyof construction the collector system operates around 35 kV, and thecollector substation steps up voltage to a transmission voltage forthe grid.
Double-ended unit substationTypical dimensions and weights for an indoor unit substationTransmission line substation Terminal substation
Single line transformer substation, sectionalized (H-type) busFor the other issues of The Switchyards (outdoor substations), click here VePi