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A substation is a part of an electrical

generation, transmission, and distributionsystem. Substations transform voltage

from high to low, or the reverse, or

perform any of several other importantfunctions. Electric power may flow

through several substations between

generating plant and consumer, and itsvoltage may change in several steps.

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Substations may be owned and

operated by a transmission orgeneration electrical utility, or may

be owned by a large industrial or

commercial customer. Generallysubstations are un-attended, relying

on SCADA for remote supervision

and control.

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A substation may include transformers to

change voltage levels between high transmission

voltages and lower distribution voltages, or at theinterconnection of two different transmission

voltages. The word substation comes from the days

before the distribution system became a grid. As

central generation stations became larger, smallergenerating plants were converted to distribution

stations, receiving their energy supply from a larger

plant instead of using their own generators. The

first substations were connected to only one power

station, where the generators were housed, and

were subsidiaries of that power station.

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Images

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Elements of a substation

Substations generally have switching,protection and control equipment, andtransformers. In a large substation, circuitbreakers are used to interrupt any short circuits or

overload currents that may occur on the network.Smaller distribution stations may use reclosercircuit breakers or fuses for protection ofdistribution circuits. Substations themselves do

not usually have generators, although a powerplant may have a substation nearby. Other devicessuch as capacitors and voltage regulators may alsobe located at a substation.

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Elements of a substation

A:Primary power lines' side B:Secondary power lines' side

1.Primary power lines 2.Ground wire 3.Overhead lines

4.Transformer for measurement of electric voltage 5.Disconnect

switch 6.Circuit breaker 7.Current transformer 8.Lightning arrester9.Main transformer 10.Control building 11.Security fence

12.Secondary power lines

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Substations may be on the surface in

fenced enclosures, underground, or

located in special-purpose buildings.

High-rise buildings may have several

indoor substations. Indoor substations

are usually found in urban areas to

reduce the noise from the transformers,

for reasons of appearance, or to protectswitchgear from extreme climate or

pollution conditions.

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Transmission substation

A transmission substation connects two or moretransmission lines. The simplest case is where alltransmission lines have the same voltage. In suchcases, the substation contains high-voltage switchesthat allow lines to be connected or isolated for faultclearance or maintenance. A transmission stationmay have transformers to convert between twotransmission voltages, voltage control/power factor

correction devices such as capacitors, reactors orstatic VAr compensators and equipment such asphase shifting transformers to control power flowbetween two adjacent power systems.

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Distribution substation

A distribution substation transferspower from the transmission system to the

distribution system of an area. It is

uneconomical to directly connectelectricity consumers to the main

transmission network, unless they use

large amounts of power, so the distributionstation reduces voltage to a value suitable

for local distribution.

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Distribution substations also isolate

faults in either the transmission or

distribution systems. Distribution

substations are typically the points of

voltage regulation, although on longdistribution circuits (of several

miles/kilometers), voltage regulation

equipment may also be installedalong the line.

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Collector substation

In distributed generation projects such as a windfarm, a collector substation may be required. Itresembles a distribution substation although powerflow is in the opposite direction, from many windturbines up into the transmission grid. Usually foreconomy of construction the collector systemoperates around 35 kV, and the collector substationsteps up voltage to a transmission voltage for thegrid. The collector substation can also provide

power factor correction if it is needed, metering andcontrol of the wind farm. In some special cases acollector substation can also contain an HVDC staticinverter plant.

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Stations with change of current type

Substations may be associated with

HVDC converter plants, traction current,

or interconnected non-synchronousnetworks. Formerly rotary converters

changed frequency to interconnect two

systems; such substations today are rare.

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Switching substation

A switching substation is a substation whichdoes not contain transformers and operates

only at a single voltage level. Switching

substations are sometimes used as collectorand distribution stations. Sometimes they

are used for switching the current to back-

up lines or for parallelizing circuits in case offailure. An example is the switching stations

for the HVDC Inga-Shaba transmission line.

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