A substation is a part of an electrical generation,
transmission, and distribution system. Substations transform voltage from high
to low, or the reverse, or perform any of several other important functions.
Between the generating station and consumer, electric power may flow through
several substations at different voltage levels.
Substations may be owned and operated by an electrical
utility, or may be owned by a large industrial or commercial customer.
Generally substations are un-attended, relying on SCADA for remote supervision
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 arrester 9.Main transformer 10.Control
building 11.Security fence 12.Secondary power lines
Substations generally have switching, protection and control
equipment, and transformers. In a large substation, circuit breakers are used
to interrupt any short circuits or overload currents that may occur on the
network. Smaller distribution stations may use recloser circuit breakers or
fuses for protection of distribution circuits. Substations themselves do not
usually have generators, although a power plant may have a substation nearby.
Other devices such as capacitors and voltage regulators may also be located at
A transmission substation connects two or more transmission
lines.The simplest case is where all transmission lines have the same
voltage. In such cases, the substation contains high-voltage switches that
allow lines to be connected or isolated for fault clearance or maintenance. A
transmission station may have transformers to convert between two transmission
voltages, voltage control/power factor correction devices such as capacitors,
reactors or static VAr compensators and equipment such as phase shifting
transformers to control power flow between two adjacent power systems.
Transmission substations can range from simple to complex. A
small "switching station" may be little more than a bus plus some
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 (voltage and current
transformers, relays and SCADA systems). Modern substations may be implemented
using international standards such as IEC Standard 61850.
A distribution substation in Scarborough, Ontario 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
A distribution substation transfers power from the
transmission system to the distribution system of an area.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 main issues facing a power engineer are reliability and
cost. A good design attempts to strike a balance between these two, to achieve
sufficient reliability without excessive cost. The design should also allow
expansion of the station, when required.
Selection of the location of a substation must consider many
factors. Sufficient land area is required for installation of equipment with
necessary clearances for electrical safety, and for access to maintain large
apparatus such as transformers. Where land is costly, such as in urban areas,
gas insulated switchgear may save money overall. The site must have room for
expansion due to load growth or planned transmission additions. Environmental
effects of the substation must be considered, such as drainage, noise and road
traffic effects. A grounding (earthing) system must be designed. The total
ground potential rise, and the gradients in potential during a fault (called
"touch" and "step" potentials), must be calculated to
protect passers-by during a short-circuit in the transmission system. The
substation site must be reasonably central to the distribution area to be
served. The site must be secure from intrusion by passers-by, both to protect
people from injury by electric shock or arcs, and to protect the electrical
system from misoperation due to vandalism.
The first step in planning a substation layout is the
preparation of a one-line diagram which shows in simplified form the switching
and protection arrangement required, as well as the incoming supply lines and
outgoing feeders or transmission lines. It is a usual practice by many
electrical utilities to prepare one-line diagrams with principal elements
(lines, switches, circuit breakers, transformers) arranged on the page
similarly to the way the apparatus would be laid out in the actual station.
In a common design, incoming lines have a disconnect switch
and a circuit breaker. In some cases, the lines will not have both, with either
a switch or a circuit breaker being all that is considered necessary. A
disconnect switch is used to provide isolation, since it cannot interrupt load
current. A circuit breaker is used as a protection device to interrupt fault
currents automatically, and may be used to switch loads on and off, or to cut
off a line when power is flowing in the 'wrong' direction. When a large fault current
flows through the circuit breaker, this is detected through the use of current
transformers. The magnitude of the current transformer outputs may be used to
trip the circuit breaker resulting in a disconnection of the load supplied by
the circuit break from the feeding point. This seeks to isolate the fault point
from the rest of the system, and allow the rest of the system to continue
operating with minimal impact. Both switches and circuit breakers may be
operated locally (within the substation) or remotely from a supervisory control
Once past the switching components, the lines of a given
voltage connect to one or more buses. These are sets of busbars, usually in
multiples of three, since three-phase electrical power distribution is largely
universal around the world.
The arrangement of switches, circuit breakers and buses used
affects the cost and reliability of the substation. For important substations a
ring bus, double bus, or so-called "breaker and a half" setup can be
used, so that the failure of any one circuit breaker does not interrupt power
to other circuits, and so that parts of the substation may be de-energized for
maintenance and repairs. Substations feeding only a single industrial load may
have minimal switching provisions, especially for small installations.
Once having established buses for the various voltage
levels, transformers may be connected between the voltage levels. These will
again have a circuit breaker, much like transmission lines, in case a
transformer has a fault (commonly called a "short circuit").
Along with this, a substation always has control circuitry
needed to command the various circuit breakers to open in case of the failure
of some component.