Illustrated Glossary: Substations
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. There are three aspects to substations:
Figure 1. Typical substation
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Substation Types: Although, there are
generally four types of substations there are substations that are a
combination of two or more types.
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A step-up transmission substation receives electric
power from a nearby generating facility and uses a large power transformer to
increase the voltage for transmission to distant locations. A transmission
bus is used to distribute electric power to one or more transmission
lines. There can also be a tap on the incoming power feed from the generation plant
to provide electric power to operate equipment in the generation
plant.
A substation can have circuit breakers that are used to switch generation and transmission circuits in
and out of service as needed or for emergencies requiring shut-down of power
to a circuit or redirection of power.
The specific voltages leaving a step-up transmission substation are
determined by the customer needs of the utility supplying power and to the
requirements of any connections to regional grids. Typical voltages are:
| High voltage (HV) ac: |
69 kV, 115
kV, 138
kV,
161
kV,
230
kV |
| Extra-high voltage (EHV) ac: |
345
kV,
500
kV,
765
kV |
|
Ultra-high voltage (UHV) ac: |
1100
kV,
1500
kV |
|
Direct-current high voltage (dc HV):
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±250 kV,
±400
kV,
±500
kV |
Direct current voltage is either positive or negative polarity. A
DC line has two conductors, so one would be positive and the other
negative.
Figure 2. Step-up AC transmission substation
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Figure 3. Step-up transmission substation to
AC transmission lines
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Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a
source for subtransmission lines or distribution lines. The step-down substation can change the transmission voltage to a subtransmission voltage,
usually 69 kV. The subtransmission voltage lines can then serve as a source to distribution substations. Sometimes, power is
tapped from the subtransmission line for use in an industrial facility along the way.
Otherwise, the power goes to a distribution substation.
Figure 4. Step-down transmission substation
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Figure 5. Step-down power transformer
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Distribution substations are located near to the end-users.
Distribution substation transformers change the transmission or subtransmission
voltage to lower levels for use by end-users. Typical distribution
voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.
34,500Y/19,920 volts is interpreted as a three-phase circuit with a
grounded neutral source. This would have three high-voltage
conductors or wires and one grounded neutral conductor, a total of
four wires. The voltage between the three phase conductors or wires
would be 34,500 volts and the voltage between one phase conductor
and the neutral ground would be 19,920 volts.
From here the power is distributed to industrial, commercial, and
residential customers.
Figure 6. Distribution substation
Figure 7. Distribution substation
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Figure 8. Distribution substation
Figure 9. Distribution substation
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Underground distribution substations are also located near to the end-users.
Distribution substation transformers change the subtransmission
voltage to lower levels for use by end-users. Typical distribution
voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.
An underground system may consist of these parts:
Figure 10. Underground Distribution Substation
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From here the power is distributed to industrial, commercial, and
residential customers.
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Substations are designed to accomplish the following functions, although not
all substations have all these functions:
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Change voltage from one level to another
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Regulate voltage to compensate for system voltage
changes
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Switch transmission and distribution circuits into and
out of the grid system
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Measure electric power qualities flowing in the
circuits
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Connect communication signals to the circuits
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Eliminate lightning and other electrical surges from
the system
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Connect electric generation plants to the system
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Make interconnections between the electric systems of
more than one utility
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Control reactive kilovolt-amperes supplied to and the
flow of reactive kilovolt-amperes in the circuits
Additional information:
- The Lineman's and Cableman's Handbook, Shoemaker, T.
M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.
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The major components of a typical substation are:
Additional information:
- The Lineman's and Cableman's Handbook, Shoemaker, T.
M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.
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