The following topics will be covered: the definition of a substation, an overview of the different type of substations and the functions of substations

1. VePi Newsletters
The Electrical Power Systems Division The Switchyards (outdoor substations)
section Number: 1
Introduction:
A substation is a high-voltage electric system facility. It is used
to switch generators, equipment, and circuits or lines in and out of
a system. It also is used to change AC voltages from one level to
another, and/or change alternating current to direct current or
direct current to alternating current. Some substations are small
with little more than a transformer and associated switches. Others
are very large with several transformers and dozens of switches and
other equipment. An electrical substation is a subsidiary station of
an electricity generation, transmission and distribution system where
voltage is transformed from high to low or the reverse using
transformers. Electric power may flow through several substations
between generating plant and consumer, and may be changed in voltage
in several steps.
A substation that has a step-up transformer increases the voltage
while decreasing the current, while a step-down transformer decreases
the voltage while increasing the current for domestic and commercial
distribution. The word substation comes from the days before the
distribution 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 the
grid system
4) Measure electric power quantities flowing in the circuits
5) Connect communication signals to the circuits

2. 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 than
one utility
9) Control reactive kilovolt-amperes supplied to and the flow of
reactive kilovolt-amperes in the circuits
Substations generally have switching, monitoring, protection and
control equipment and one or more 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 do not usually have
generators, although a power plant may have a substation nearby.
Other devices such as power factor correction capacitors and voltage
regulators 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 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 protect switchgear from extreme climate or
pollution conditions.
Where a substation has a metallic fence, it must be properly grounded
(UK: earthed) to the ground mat underneath the substation to protect
people from high voltages that may occur during a fault in the
network. Earth faults at a substation can cause a ground potential
rise (GPR). Currents flowing in the earth's surface during a fault
can cause metal objects to have a significantly different voltage
than the ground under a person's feet; this touch potential presents
a hazard of electrocution as the current passes through the body
resistance (through the heart).
Transmission substations:
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
(and or circuit breakers) 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 devices such as capacitors, reactors or Static VARs
and equipment such as phase shifting transformers to control power
flow between two adjacent power systems.
Terminal substations:
A terminal substation is a facility that forms a strategic node point
in an interconnected electricity transmission system.
A terminal substation fulfils either or both roles:
1) It provides a connection point where transmission lines of the
same voltage may be joined to enable an electricity supply to be
established to a new demand centre or centres (for example, the
transmission line voltage is 230 KV and the transformer station has

3. 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 the
distribution level), or to achieve a greater degree of
interconnection within the existing system (in which it comes closer
to a non-transformation transmission substation). It is a bulk supply
point in the electrical grid, where it may serve a significant area
within the metropolitan area and/or some country areas.
2) It is a transformation point where lower voltages are produced to
supply the metropolitan transmission system.
Transformer substations:
A transformer substation is a point where the transmission voltage
level is stepped down to the sub-transmission voltage level. The
latter voltage is then either used to feed a distribution substation
to further reduce the voltage level to the distribution level or
itself 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 line
for use in an industrial facility along the way, otherwise, the power
goes to a distribution substation. . Thus the major components in
such a station will be: one or two high voltage disconnect switches,
one or two power transformers, one or two medium voltage switchgear
lineups 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 is
4,160Y/2400 volts. From here the power is distributed to industrial,
commercial, and residential customers through distribution
transformers, 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 with
a set of power or current limiting fuses, in series with it
,connected to the high voltage winding of a distribution (or a power
transformer), the low voltage winding of the transformer would be
connected to the main circuit breaker plus the feeder circuit
breakers, motor contactors plus disconnect switch and fuses, or load
break switches in the switchgear lineup. Within the lineup, there
would be the utility metering compartment with the current and
voltage transformers approved for utility meter application as well
as the user instrument transformers, meters, protection and control.
Collector substation:
In distributed generation projects such as a wind farm, a collector
substation may be required. It somewhat resembles a distribution
substation although power flow is in the opposite direction, from
many wind turbines up into the transmission grid. Usually for economy
of construction the collector system operates around 35 kV, and the
collector substation steps up voltage to a transmission voltage for
the grid.

4. Double-ended unit substation
Typical dimensions and weights for an indoor unit substation
Transmission line substation Terminal substation

5. Single line transformer substation, sectionalized (H-type) bus
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