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ELECTRIC POWER SUBSTATION
An assembly of equipment in an electric power system through which electric energy is
passed for transmission, transformation, distribution, or switching. Also known as
Electric power substation
An assembly of equipment in an electric power system through which electrical energy is
passed for transmission, distribution, interconnection, transformation, conversion, or
switching. See also Electric power systems.
Specifically, substations are used for some or all of the following purposes: connection of
generators, transmission or distribution lines, and loads to each other; transformation of
power from one voltage level to another; interconnection of alternate sources of power;
switching for alternate connections and isolation of failed or overloaded lines and
equipment; controlling system voltage and power flow; reactive power compensation;
suppression of overvoltage; and detection of faults, monitoring, recording of information,
power measurements, and remote communications. Minor distribution or transmission
equipment installation is not referred to as a substation.
Substations are referred to by the main duty they perform. Broadly speaking, they are
classified as: transmission substations, which are associated with high voltage levels; and
distribution substations, associated with low voltage levels. See also Electric distribution
Substations are also referred to in a variety of other ways:
1. Transformer substations are substations whose equipment includes transformers.
2. Switching substations are substations whose equipment is mainly for various
connections and interconnections, and does not include transformers.
3. Customer substations are usually distribution substations on the premises of a
larger customer, such as a shopping center, large office or commercial building,
or industrial plant.
4. Converter stations are complex substations required for high-voltage direct-
current (HVDC) transmission or interconnection of two ac systems which, for a
variety of reasons, cannot be connected by an ac connection. The main function of
converter stations is the conversion of power from ac to dc and vice versa. The
main equipment includes converter valves usually located inside a large hall,
transformers, filters, reactors, and capacitors.
5. Most substations are installed as air-insulated substations, implying that the bus-
bars and equipment terminations are generally open to the air, and utilize
insulation properties of ambient air for insulation to ground. Modern substations
in urban areas are esthetically designed with low profiles and often within walls,
or even indoors.
6. Metal-clad substations are also air-insulated, but for low voltage levels; they are
housed in metal cabinets and may be indoors or outdoors.
7. Acquiring a substation site in an urban area is very difficult because land is either
unavailable or very expensive. Therefore, there has been a trend toward
increasing use of gas-insulated substations, which occupy only 5–20% of the
space occupied by the air-insulated substations. In gas-insulated substations, all
live equipment and bus-bars are housed in grounded metal enclosures, which are
sealed and filled with sulfur hexafluoride (SF6) gas, which has excellent insulation
8. For emergency replacement or maintenance of substation transformers, mobile
substations are used by some utilities.
An appropriate switching arrangement for “connections” of generators, transformers,
lines, and other major equipment is basic to any substation design. There are seven
switching arrangements commonly used: single bus; double bus, single breaker; double
bus, double breaker; main and transfer bus; ring bus; breaker-and-a-half; and breaker-
and-a-third. Each breaker is usually accompanied by two disconnect switches, one on
each side, for maintenance purposes. Selecting the switching arrangement involves
considerations of cost, reliability, maintenance, and flexibility for expansion.
A substation includes a variety of equipment. The principal items are transformers, circuit
breakers, disconnect switches, bus-bars, shunt reactors, shunt capacitors, current and
potential transformers, and control and protection equipment. See also Bus-bar; Circuit
breaker; Electric protective devices; Electric switch; Relay; Transformer; Voltage
Good substation grounding is very important for effective relaying and insulation of
equipment; but the safety of the personnel is the governing criterion in the design of
substation grounding. It usually consists of a bare wire grid, laid in the ground; all
equipment grounding points, tanks, support structures, fences, shielding wires and poles,
and so forth, are securely connected to it. The grounding resistance is reduced enough
that a fault from high voltage to ground does not create such high potential gradients on
the ground, and from the structures to ground, to present a safety hazard. Good overhead
shielding is also essential for outdoor substations, so as to virtually eliminate the
possibility of lightning directly striking the equipment. Shielding is provided by overhead
ground wires stretched across the substation or tall grounded poles. See also Grounding;
Lightning and surge protection.
A link between power systems enabling them to draw on one another's reserves in time of
need and to take advantage of energy cost differentials resulting from such factors as load
diversity, seasonal conditions, time-zone differences, and shared investment in larger
A voltage greater than that at which a device or circuit is designed to operate. Also
known as overpotential.
• What is a substation transformer?
• What is the difference between current transformer and voltage transformer?
• Why substation is called so?
• What is the difference between a power station and a substation?
• What is the difference between isolation transformer and step up or step down
• Can you use a step down transformer as step up transformer by reversing the
primary voltage as a secondary voltage?
• What is pulse transformer?
• What is a booster transformer?
• What is isolation transformer?
• How transformer works?
• What is the uses of core transformer?
• What is transformer turns ratio?
• What happens when DC supply is given to transformer?
• How do you identify a liquid transformer?
• How did the Transformer franchise begin?
• Operation of an isolation transformer?
• What is the difference between amplifier and transformer?
• How do you measure core loss in a transformer?
• What is the input side of a transformer called?
• How many bushing do you get on delta to star transformer?
• What is the differnce of a two winding transformer and autotransformer?
• What is the difference between electronic and magnetic transformer?
• What is the name of the company that makes all of the Transformer toys ?
• Explain center-tapped transformer?
• What is the disadvantage of using dry type transformer?
• How do you work out the volt-amp in a transformer?
• Will a transformer within a light make a buzzing sound?
• Does Megatron have weapons in the 2007 Transformer movie?
• How do you calculate the length of the wire conductor in a transformer?
• Turns ratio of a single phase transformer?
• How do you figure out maximum current of a 240V transformer circuit?
• Can you use a 120V to 240V step up transformer for an electric range?
• How to determine the right transformer in three phase system say it should be
delta or wye?
• Why do a Voltage transformer give me a CLIPPED output signal?
• What is the best transformer to purchase to operate a sewing machine purchased
in the US in the UK?
• How do you calculate transformer turns ratio given primary and secondary
• Is it possible to use a 120-240V step up transformer with an electric range when a
house is only wired for 120V and if so how?
• Can you use the NTSC selectable option on the European television and a step up
transformer to make it work in the US?
• Do you have parts for a wall furnance Transformer part no AT30 Model s7035st
• Will a 240V water heater operate normally with a 120V 208V service coming off
a three phase transformer?
• How do you wire a boost transformer for 240v with 32v boostwhat is x1 x4 h1
• If you live in Israel can you use a transformer to step down the 240V power to
120V and then use an American appliance or do you also have a cycle problem
and if so can it be cured?
• Does varying the resistance in the secondary circuit cause a change in both the
input and output currents to keep the power on both sides equal on a transformer?
• Can a transformer autobot or decepticon survive falling throught the Earths
atmosphere or would the heat and friction instantly kill them?
• Could you run your in-house electricity on a 12V system since all your appliances
and radios and tvs mostly have a built-in transformer to reduce the current to a
much lower voltage?
• How do you produce 120V lines from 240V lines without using a transformer?
• Is it safe to run a 220volt ac- 12volt dc step down transformer in a 110v outlet-
will it transform it to 6v or will it run fine?
• If you have a 120V Sony stereo amplifier that you plugged into a wall transformer
to step down from 240V in Denmark and it worked fine then zapped can you
replace the fried component to accept 240V?
• How many turns do you need on a ferrite core transformer with a 1cm by 1cm
square center leg at 14 volts and 100Khz switching speed to maintain a Tesla
equal to or less than 0.15?
• What is the difference between a step up and a step down transformer?
1. It is a large encased group of metal plates about the size of a small house with 2
sets of insulated copper wires wrapped around the plates -an input and output - to
convert a very high supply line voltage - maybe 44 thousand volts to a lower
voltage of 2200 volts that runs down the power line poles in your neighborhood.
Look at your street's power poles and you will see a mini substation that converts
he 2200 volt to 220 volts and may supply several homes.
2. Actually...NONE. A transformer is a device that steps up, or steps down voltage.
During this process current is also stepped up or down, however, voltage and
current are inversely proportional ( meaning an increase in voltage results in a
decrease in current and vice versa ) As an example: A step up transformer of 10:1
ratio with 12 volts and 10 ampere of current applied to the primary will have ten
times the voltage ( 120 volts ) and ten times less current ( 1 ampere ) at the
secondary...and a step down transformer with the same turns ratio with 120 volts
and 1 ampere applied to the primary will have 12 volts and ten ampere available
at the secondary. The electricity supplied into homes and business uses wires
carrying very high voltage and low current over long distances, then uses step
down transformers to step down the voltage and step up the current.
3. The noun substation has one meaning - a subsidiary station where electricity is
transformed for distribution by a low-network.
4. Power station is where they actually generate the electricity on an industrial scale.
Substation is a subsidiary of a power station typically used to step down the
voltage for more local use of electricity.
5. Isolation transformer: is a transformer with two separate windings, the primary and
the secondary. There is an electrical isolation between the primary and the
secondary. There are also transformers with one winding and connections for
input and output.
If the input is low and the output high you can say STEP Up.
If the input is high and the output low you can say STEP DOWN.
This transformers can not by used where safety necessary.
6. If the transformer has two separate windings, then, YES a step-down can be used as a
step-up, and vice-versa.
It is more correct to say you are reversing the high and low sides.
By definition Primary is the "IN" side and Secondary is the "OUT" side.
The Primary could be the high voltage side if it is a step-down,
or it could be the low voltage side, if it is a step-up.
Care must be taken when reversing the operation of a step-down transformer to insure
that it does not exceed the ratings of the transformer. For example, let's say we have a 12
VA step-down transformer that takes 120Vac in and is rated to provide 12V at 1 amp at
the secondary. If we were to reverse it and apply 12Vac to the new primary (the old
secondary), we would have 120Vac at the new secondary, but to keep within the original
ratings it could only be loaded to 0.1A @ 120Vac.
As long as you treat the output rating of the step-down transformer as the input rating as a
step-up transformer, and don't try to draw current beyond what would normally have
been applied to the high voltage primary, you should be fine. Potentially one could push
up the voltage on such a transformer beyond what its stated application specified, while
making sure not to exceed the power rating of the transformer and not exceeding the
breakdown voltage of the transformer's insulation, for example driving the above
transformer at 24V to get 0.05A @ 240Vac. It is important to realize though that the rated
input voltage for a step-down transformer will likely not be a safe input voltage if you use
it as a step-up transformer - for example, applying 120V to the above transformer with
the windings reversed would generate 1.2KV!
7. A transformer not intended for power conversion, but for galvanically isolating
electrical signals - usually digital, therefore "pulse".
8. Normally used in public address systems, where the audio have to travel long
distances and have to drive more than one speaker then a booster transformer is
inserted in the audio line, also called audio transformer
9. An isolation transformer does not have a direct electrical path from the power input
side to the power output side. The term is also used to define how much electrical
isolation exists between the input and output windings. For example when using line-
voltage input transformers to power low volatge device handled by humans, a high
degree of isolation is required for safety.
Isolated transformers often use separate bobbins for the primary and secondary coil
windings, but usually the windings are just wound on top of each other with insulation in
Non-Isolated transformers are becoming rare. A common example is the "Variac" which
is a non-isolated variable transformer. Usually the term "auto-transformer" is used to
describe non-isolated transformers. They are rarely found in consumer products.
10. #1...Wire produces a magnetic field when current is passed through it. If you wrap the
wire around something (a core) to make a coil, it concentrates that field. The core isn't
actually necessary but it helps concentrate the field and make the transformer more
#2...If you pass a magnetic field through a wire, it produces electron flow.
If you make a coil with 100 wraps and pass current through it, it will produce the
magnetic field. If you have another coil close enough to be IN that magnetic field, and it
has 10 turns, you will get about 1/10 the voltage from the second coil that you put into
the first one.
It gets a lot more complicated than that with formulas and all kinds of mathematics, but
that is the basics of a "step down" transformer.
13. Nothing noticeable. DC power is not transmitted between the coils of a
transformer. There would be no current on the other side of the transformer,
unless the power of the source was constantly modulated.
14. An amplifier is what increases a rock band's speaker sound output into a ear
splitting experience and small radio signals in a radio until it makes it out to the
speaker. A transformer converts AC electricity up or down to a desired level for a
required project. Example: A battery charger plugged into a wall socket will
transformed to about 18 volts to do the charging
15. Core loss is also called “No-Load” loss. To measure the core lose simply you
need variable AC supply, Wattmeters, Ampere meters and Voltmeters. The basic
principle to perform this measurement is to supply the transformer with its
nominal voltage and then record the Watt or Kilo watt values. You will need
precision current and voltage transformers to supply your wattmeters, voltmeters
and ampere meters. There is a term called “Form Factor” which should be
measured in order to identify how sinusoidal is your supply voltage and based on
the value of this Form Factor you need to apply relevant corrections to the
measured values. For power transformers normally we record no load values
between 90% and 110% of rated voltage. Based on the test circuit configuration
you may use “2 or 3 wattmeter” arrangements for 3 phase transformers. For
further information you can check out my website at www.eonce.com and if you
need more information you can simply fill the form in “Contact” page on the
website. Hope this helps. B.M. Mirzaei, P.Eng.
Electricity distribution is the penultimate stage in the delivery (before retail) of
electricity to end users. It is generally considered to include medium-voltage (less than 50
kV) power lines, electrical substations and pole-mounted transformers, low-voltage (less
than 1000 V) distribution wiring and sometimes electricity meters.
In the early days of electricity generation to about 1900, direct current DC generators
were connected to loads at the same voltage. The generation, transmission and loads had
to be of the same voltage because there was no way of changing DC voltage levels, other
than inefficient motor-generator sets. Low DC voltages were used (on the order of 100
volts) since that was a practical voltage for incandescent lamps, which were then the
primary electrical load. The low voltage also required less insulation to be safely
distributed within buildings.
The losses in a cable are proportional to the square of the current, the length of the cable,
and the resistivity of the material, and are inversely proportional to cross-sectional area.
Early transmission networks were already using copper, which is one of the best
economically feasible conductors for this application. To reduce the current and copper
required for a given quantity of power transmitted would require a higher transmission
voltage, but no convenient efficient method existed to change the voltage level of DC
power circuits. To keep losses to an economically practical level the Edison DC system
needed thick cables and local generators. Early DC generating plants needed to be within
about 1.5 miles of the farthest customer to avoid the need for excessively large and
Introduction of alternating current
The adoption of alternating current (AC) for electricity generation following the War of
Currents dramatically changed the situation. Power transformers, installed at substations,
could be used to raise the voltage from the generators and reduce it to supply loads.
Increasing the voltage reduced the current in the transmission and distribution lines and
hence the size of conductors required and distribution losses incurred. This made it more
economical to distribute power over long distances. Generators (such as hydroelectric
sites) could be located far from the loads.
In North America, early distribution systems used a voltage of 2200 volts corner-
grounded delta. Over time, this was gradually increased to 2400 volts. As cities grew,
most 2400 volt systems were upgraded to 4160/2400 volt, three-phase systems. Some city
and suburban distribution systems continue to use this range of voltages, but most have
been converted to 7200/12470Y, 7620/13200Y, 14400/24940Y, and 19920/34500Y.
European systems used 3300 volts to ground, in support of the 220/380Y volt power
systems used in those countries. In the UK, urban systems progressed to 6.6 kV and then
11 kV (phase to phase), the most common distribution voltage.
North American and European power distribution systems also differ in that North
American systems tend to have a greater number of low-voltage, step-down transformers
located close to customers' premises. For example, in the US a pole-mounted transformer
in a suburban setting may supply 1-3 houses, whereas in the UK a typical urban or
suburban low-voltage substation might be rated at 2 MW and supply a whole
neighbourhood. This is because the higher voltage used in Europe (380 V vs 230 V) may
be carried over a greater distance with acceptable power loss. An advantage of the North
American setup is that failure or maintenance on a single transformer will only affect a
few customers. Advantages of the UK setup are that the transformers may be fewer,
larger and more efficient, and due to diversity there need be less spare capacity in the
transformers, reducing power wastage. In North American city areas with many
customers per unit area, network distribution will be used, with multiple transformers and
low-voltage busses interconnected over several city blocks.
Rural Electrification systems, in contrast to urban systems, tend to use higher voltages
because of the longer distances covered by those distribution lines (see Rural
Electrification Administration). 7200, 12470 and 25000 volt distribution is common in
the United States; 11 kV and 33 kV are common in the UK, New Zealand and Australia;
11 kV and 22 kV are common in South Africa. Other voltages are occasionally used.
In New Zealand, Australia, Saskatchewan, Canada, and South Africa, single wire earth
return systems (SWER) are used to electrify remote rural areas.
While power electronics now allow for conversion between DC voltage levels, AC is still
used in distribution due to the economy, efficiency and reliabilty of transformers. High-
voltage DC is used for transmission of large blocks of power over long distances, or for
interconnecting adjacent AC networks, but not for distribution to customers.
Distribution network configurations
Distribution networks are typically of two types, radial or interconnected (see Spot
Network Substations). A radial network leaves the station and passes through the network
area with no normal connection to any other supply. This is typical of long rural lines
with isolated load areas. An interconnected network is generally found in more urban
areas and will have multiple connections to other points of supply.
These points of connection are normally open but allow various configurations by the
operating utility linemen carefully closing and opening switches. The benefit of the
interconnected model is that in the event of a fault or required maintenance a small area
of network can be isolated and the remainder kept on supply.
Within these networks there may be a mix of overhead line construction utilizing
traditional utility poles and wires and, increasingly, underground construction with cables
and indoor or cabinet substations. However, underground distribution can cost as much as
11 times as much as overhead construction. In part to reduce this cost, underground
power lines are sometimes colocated with other utility lines in what are called Common
utility ducts. Distribution feeders emanating from a substation are generally controlled by
a circuit breaker or fuse which will open when a fault is detected. Automatic Circuit
Reclosers may be installed to further segregate the feeder thus minimising the impact of
Long feeders experience voltage drop requiring capacitors or voltage regulators to be
installed, and the phase physical relationship to be interchanged.
Characteristics of the supply given to customers are generally mandated by contract
between the supplier and customer. Deviations from the normal usage pattern usually
invoke monthly surcharges. Variables include:
• AC or DC - Virtually all public electricity supplies are AC today. Users of large
amounts of DC power such as some electric railways, telephone exchanges and
industrial processes such as aluminium smelting either operate their own or have
adjacent dedicated generating equipment, or use rectifiers to derive DC from the
public AC supply
• Voltage, including tolerance (usually +10 or -15 percentage)
• Frequency, commonly 50 & 60 Hz, 16-2/3 Hz for some railways and, in a few
older industrial and mining locations, 25 Hz
• Phase configuration (single phase, polyphase including two phase and three
• Maximum demand (usually measured as the largest amount of power delivered
within a 15 or 30 minute period during a billing period)
• Load Factor, expressed as a ratio of average load to peak load over a period of
time. Load factor indicates the degree of effective utilization of equipment (and
capital investment) of distribution line or system.
• Power factor of connected load
• Earthing arrangements - TT, TN-S, TN-C-S or TN-C
• Maximum prospective short circuit current
• Maximum level and frequency of occurrence of transients
See List of countries with mains power plugs, voltages and frequencies.
Modern Distribution Systems
The modern distribution system begins as the primary circuit leaves the sub-station and
ends as the secondary service enters the customers meter socket. A variety of methods,
materials, and equipment are used among the various utility companies across the U.S.,
but the end result is similar. First, the energy leaves the sub-station in a primary circuit,
usually with all three phases.
The most common type of primary is known as a wye configuration (so named because
of the shape of a "Y".) The wye configuration includes 3 phases (represented by the three
outer parts of the "Y") and a neutral (represented by the center of the "Y".) The neutral is
grounded both at the substation and at every power pole. In a typical 12470Y/7200 volt
system, the pole mount transformer's primary winding is rated for 7200 volts and is
connected across one phase of power and the neutral. The primary and secondary (low
voltage) neutrals are bonded (connected) together to provide a path to blow the primary
fuse if any fault occurs that allows primary voltage to enter the secondary lines. An
example of this type of fault would be a primary phase falling across the secondary lines.
Another example would be some type of fault in the transformer itself.
Electric distribution substations transform power from transmission voltage to the lower
voltage used for local distribution to homes and businesses.
The other type of primary configuration is known as delta, this method is older and less
common. Delta is so named because of the shape of the Greek letter delta, a triangle.
Delta has only 3 phases and no neutral. In delta there is only a single voltage, between
two phases (phase to phase), while in wye there are two voltages, between two phases
and between a phase and neutral (phase to neutral). Wye primary is safer because if one
phase becomes grounded, that is makes connection to the ground through a person, tree,
or other object, it should trip out the fused cutout similer to a household circuit breaker
tripping. In delta, if a phase makes connection to ground it will continue to function
normally. It takes two or three phases to make connection to ground before the fused
cutouts will open the circuit. The voltage for this configuration is usually 4800 volts.
Transformers are sometimes used to step down from 7200 or 7600 volts to 4800 volts or
to step up from 4800 volts to 7200 or 7600 volts. When the voltage is stepped up, a
neutral is created by bonding one leg of the 7200/7600 side to ground. This is commonly
used to power single phase underground services or whole housing developments that are
built in 4800 volt delta distribution areas. Step downs are used in areas that have been
upgraded to a 7200/12500Y or 7600/13200Y and the power company chooses to leave a
section as a 4800 volt setup. Sometimes power companies choose to leave sections of a
distribution grid as 4800 volts because this setup is less likely to trip fuses or reclosers in
heavily wooded areas where trees come into contact with lines.
Economic and Political
In the United States, Electric industry "deregulation" reform, started in the mid-1990s,
has led to the creation of electricity markets through the elimination of the former natural
monopoly of generation, transmission, and distribution. As a consequence, electricity has
become more of a commodity. The separation has also led to the development of new
terminology to describe the business units, e.g. line company, wires business and network
Electric Power Distribution
A distribution system originates at a distribution substation and includes the lines, poles,
transformers and other equipment needed to deliver electric power to the customer at
the required voltages. Customers are classed as:
• Industrial Customer
• Commercial Customer
• Residential Customer
• Transportation Customer
Distribution Systems TOP
A distribution system consists of all the facilities and equipment connecting a transmission
system to the customer's equipment.
A typical distribution system can consist of:
• Distribution Feeder Circuits
• Protective Equipment
• Primary Circuits
• Distribution Transformers
• Secondaries, and
Figure 1. Energy flow through a typical substation
The following are examples of distribution systems components. Collectively they
constitute a typical distribution system. These typically deliver voltages as high as 34,000
volts (34 kV) and as low as 120 volts.
Figure 2. Typical residential service drop Figure 3. Substation pull-off structure
Figure 5. Substation underground distribution
Figure 4. Substation pull-off structure
(connects substation busswork to overhead
Figure 6. Distribution primaries and
secondaries Figure 7. Distribution underbuild
on subtransmission pole
• The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth
Edition 2002, McGraw-Hill.
Industrial Customer TOP
Most industries need 2,400 to 4,160 volts to run heavy machinery and usually their
own substation or substations to reduce the voltage from the transmission line to the
desired level for distribution throughout the plant area. They usually require 3-phase
lines to power 3-phase motors.
Figure 8. Industrial facility distribution transformer
• The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E.,
Tenth Edition 2002, McGraw-Hill.
Commercial Customer TOP
Commercial customers are usually served at distribution voltages, ranging from 14.4 kV
to 7.2 kV through a service drop line which leads from a transformer on or near the
distribution pole to the customer's end use structure. They may require 3-phase lines to
power 3-phase motors.
Figure 10. Commercial service drop
Figure 9. Distribution transformer to
3-phase service - commercial facility
Residential Customer TOP
The distribution electricity is reduced to the end use voltage (120/240 volts single phase)
via a pole mounted or pad-mounted transformer. Power is delivered to the residential
customer through a service drop line which leads from the distribution pole transformer to
the customer's structure, for overhead lines, or underground.
Figure 11. Residential distribution transformer
Figure 12. Pad-mounted residential distribution
and service drop
Transportation Customer TOP
Currently the only electric transportation systems are light rail and subway systems. A small
distribution substation reduces the local distribution voltage to the transportation system
requirements. The overhead lines supply electric power to the transportation system motors
and the return current lines are connected to the train tracks.
Figure 13. Public transit train powered by overhead electric lines
Figure 14. Substation where electricity is conditioned for powering commuter trains
Figure 15. Power runs from the substation Figure 16. Electric cables carry electricity to
underground to the poles where power is power the train's motors
delivered to the power lines. The circuit is
completed through the train tracks, with lines
returning to the substation.
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
• Substation Types: Although, there are generally four types of substations there are substations that are a
combination of two or more types.
Step-up Transmission Substation
Step-down Transmission Substation
Underground Distribution Substation
Step-up Transmission Substation TOP
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
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
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
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): ±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 Figure 3. Step-up transmission substation to
AC transmission lines
Step-down Transmission Substation TOP
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
Figure 5. Step-down power transformer
Distribution Substation TOP
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 8. Distribution substation
Figure 9. Distribution substation
Figure 7. Distribution substation
Underground Distribution Substation TOP
Figure 10. Underground Distribution Substation
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:
• Duct Runs
• High-Voltage Underground Cables
• Transformer Vault
From here the power is distributed to industrial, commercial, and residential customers.
Substation Functions TOP
Substations are designed to accomplish the following functions, although not all substations have all these
• Change voltage from one level to another
• Regulate voltage to compensate for system voltage changes
• Switch transmission and distribution circuits into and out of the grid system
• Measure electric power qualities flowing in the circuits
• Connect communication signals to the circuits
• Eliminate lightning and other electrical surges from the system
• Connect electric generation plants to the system
• Make interconnections between the electric systems of more than one utility
• Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuits
• The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002,
Substation Equipment TOP
The major components of a typical substation are:
Air Circuit Breaker Distribution Bus Potheads
Batteries Duct Runs Power-line Carrier
Bus Support Insulators Frequency Changers Power Transformers
Capacitor Bank Grounding Resistors Rectifiers
Circuit Switchers Grounding Transformers Relays
Concrete Foundation High-Voltage Underground Cables SF6 Circuit Breakers
Conduits High-Voltage Fuses Shunt Reactors
Control House Lightning Arresters Steel Superstructures
Control Panels Manholes Supervisory Control
Control Wires Metal-clad Switchgear Suspension Insulators
Converter Stations Meters Synchronous Condensers
Coupling Capacitors Microwave Transmission Bus
Current Transformers Oil Circuit Breakers Vacuum Circuit Breakers
Disconnect Switches Potential Transformers
• The Lineman's and Cableman's Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002,
Air Circuit Breakers
Air circuit breakers are used to interrupt circuits while current flows through them.
Compressed air is used to quench the arc when the connection is broken.
Figure 1. Air circuit breaker
Batteries are used in the substation control house as a backup to power the control systems
in case of a power blackout.
Figure 1. Backup batteries in the control house
Bus Support Insulators
Bus support insulators are porcelain or fiberglass insulators that serve to isolate the bus bar
switches and other support structures and to prevent leakage current from flowing through
the structure or to ground. These insulators are similar in function to other insulators used in
substations and transmission poles and towers.
Figure 1. Bus support insulators
Capacitors are used to control the level of the voltage supplied to the customer by reducing
or eliminating the voltage drop in the system caused by inductive reactive loads.
Figure 1. Capacitor bank, end view Figure 2. Capacitor bank, side view
Circuit switchers provide equipment protection for transformers, lines, cables, and capacitor banks.
They also are used to energize and deenergize capacitor banks and other circuits.
Figure 1. Circuit switchers Figure 2. Circuit switcher
The substation control house contains switchboard panels, batteries, battery chargers, supervisory
control, power-line carrier, meters, and relays. The control house provides all weather protection
and security for the control equipment. It is also called a doghouse.
Figure 1. Control house
Figure 2. Substation control house Figure 3. Control house
Control panels contain meters, control switches and recorders located in the control building,
also called a doghouse. These are used to control the substation equipment, to send power
from one circuit to another or to open or to to shut down circuits when needed.
Figure 1. Substation control panel
Figure 2. Substation control panel, detail
Converter stations are located at the terminals of a DC transmission line. Converter stations
can change alternating current into direct current or change direct current to alternating
current. Sometimes converter stations are located at a generation power plant or at
transmission substations. Two unsynchronized AC transmission systems can be connected
together with converter stations.
Converter stations are also found in most substations for converting the emergency battery
back-up system to AC power for use in an emergency.
Figure 1. Converter station in battery room
Coupling capacitors are used to transmit communication signals to transmission lines. Some
are used to measure the voltage in transmission lines. In signal transmission the coupling
capacitor is part of a power line carrier circuit as shown in the schematic below. A coupling
capacitor is used in this circuit in conjunction with a line trap. Line traps can be installed at
the substation or on a transmission line tower.
Figure 1. Power line carrier schematic showing use of coupling capacitors
Figure 3. Substation line traps
Figure 2. Primary coupling capacitor
Current transformers can be used to supply information for measuring power flows and the
electrical inputs for the operation of protective relays associated with the transmission and
distribution circuits or for power transformers. These current transformers have the primary
winding connected in series with the conductor carrying the current to be measured or
controlled. The secondary winding is thus insulated from the high voltage and can then be
connected to low-voltage metering circuits.
Current transformers are also used for street lighting circuits. Street lighting requires a
constant current to prevent flickering lights and a current transformer is used to provide that
constant current. In this case the current transformer utilizes a moving secondary coil to
vary the output so that a constant current is obtained.
Figure 2. Pole type
Figure 1. Metering current transformers
Figure 3. 400 kV current transformer
Disconnect switches or circuit breakers are used to isolate equipment or to redirect current in a
substation. Many different types of disconnect switches are shown below.
Figure 1. Disconnect switches on an outgoing Figure 2. Motorized disconnect switch (circuit
distribution circuit breaker)
A distribution bus is a steel structure array of switches used to route power out of a
Figure 2. Distribution bus
Figure 1. Distribution bus
A frequency changer is a motor-generator set that changes power of an alternating current
system from one frequency to one or more different frequencies, with or without a change in
the number of phases, or in voltage. Sometimes a converter is used to accomplish this.
Figure 1. Frequency changers at a transportation substation
Grounding Resistors are designed to provide added safety to industrial distribution systems
by limiting ground fault current to reasonable levels. They are usually connected between
earth ground and the neutral of power transformers, power generators or artificial neutral
transformers. Their main purpose is to limit the maximum fault current to a value which will
not damage generating, distribution or other associated equipment in the power system, yet
allow sufficient flow of fault current to operate protective relays to clear the fault.
Figure 1. Grounding resistor
A grounding transformer is intended primarily to provide a neutral point for grounding
purposes. It may be provided with a delta winding in which resistors or reactors are
Figure 1. Grounding transformer - front view Figure 2. Grounding transformer - back view
High-Voltage Underground Cables
High-Voltage underground cables are constructed in many different ways, but are usually shielded
cables. They are made with a conductor, conductor-strand shielding, insulation, semi-conducting
insulation shielding, metallic insulation shielding, and a sheath. The sheath can be metallic and may
then serve as the metallic insulation shielding and be covered with a nonmetallic jacket to protect
the sheath. This sheath helps to reduce or eliminate inductive reactance. Such cables are commonly
used in circuits operating at 2400 volts or higher.
Figure 1. High-voltage underground cables Figure 2. High-voltage underground cables
High Voltage Fuses
High voltage fuses are used to protect the electrical system in a substation from power
transformer faults. They are switched for maintenance and safety.
Figure 1. High voltage fuses in a switch box Figure 2. External switch for high voltage fuses
Lightning arresters are protective devices for limiting surge voltages due to lightning strikes or
equipment faults or other events, to prevent damage to equipment and disruption of service. Also
called surge arresters.
Lightning arresters are installed on many different pieces of equipment such as power poles and
towers, power transformers, circuit breakers, bus structures, and steel superstructures in
Figure 2. Lightning arrester on distribution pole
Figure 1. Lightning arresters on bus structures
Figure 3. Lightning arresters
Figure 4. Lightning arrester on substation power
A manhole is the opening in the underground duct system which houses cables splices and which
cablemen enter to pull in cable and to make splices and tests. Also called a splicing chamber or
Figure 1. Manholes
Figure 2. Manhole cover
Various types of meters are found in substation control houses. They all are measuring
devices and can be an indicating meter or a recording meter. An indicating meter shows on a
dial the quantity being measured. A recording meter makes a permanent record of the
quantity being measured, usually by tracing a line on a chart or graph. Newer recording
meters store the information electronically. The photo below left is an indicating amperage
meter. On the right is a recording meter.
Figure 2. Recording power meter
Figure 1. An indicating AC amperes meter
Substations commonly use microwave communication equipment for communication with
local and regional electric power system control centers. This system allows for rapid
communication and signaling for controlling the routing of power.
Electric power for microwave transmission comes from special transformers that reduce
incoming transmission voltage to that required for the microwave system.
Figure 1. Substation microwave communication
Figure 2. Microwave power
Oil Circuit Breakers
Oil circuit breakers are used to switch circuits and equipment in and out of a system in a
substation. They are oil filled to provide cooling and to prevent arcing when the switch is
Figure 2. Oil circuit breakers in a distribution
Figure 1. Oil circuit breakers in a 41 kV
Potential transformers are required to provide accurate voltages for meters used for billing
industrial customers or utility companies.
Figure 1. Potential transformers Figure 2. Potential transformer
A type of insulator with a bell or pot-like shape used to connect underground electrical cables
to overhead lines. It serves to separate the bunched-up conductors from one another in the
cable to the much wider separation in the overhead line. It also seals the cable end from the
weather. Potheads are mounted on a distribution pole and the assembly is called a riser
Figure 1. Three conductor potheads on pole
Figure 2. Three conductor pothead Figure 3. Potheads on pole
A power line carrier is communication equipment that operates at radio-frequencies,
generally below 600 kilohertz, to transmit information over electric power transmission
lines. A high frequency signal is superimposed on the normal voltage on a power circuit. The
power line carrier is usually coupled to the power line by means of a coupling capacitor in
conjunction with a line trap.
A device for producing radio-frequency power for transmission on power lines.
Figure 1. Power-line carrier schematic
Figure 2. Power-line carrier device in
Power transformers raise or lower the voltage as needed to serve the transmission or distribution
Figure 3. Power Transformer, front view
Figure 1. Power transformer, back view
Figure 2. Large power transformers
Figure 4. Step-up transformer diagram
A rectifier is a device used to convert alternating current to direct current.
Figure 1. Full wave rectifier circuit diagram
Figure 2. Rectifier
A relay is a low-powered device used to activate a high-powered device. Relays are used to
trigger circuit breakers and other switches in substations and transmission and distribution
Figure 1. Substation control panel relays
Figure 2. Relay and control panel
SF6 Circuit Breakers
SF6 circuit breakers operate to switch electric circuits and equipment in and out of the
system. These circuit breakers are filled with compressed sulfur-hexafluoride gas which acts
to open and close the switch contacts. The gas also interrupts the current flow when the
contacts are open.
Figure 1. SF6 gas power circuit breaker
Figure 2. SF6 gas power circuit breaker
Shunt reactors are used in an extra high-voltage substation to neutralize inductive reactance
in long EHV transmission lines. The photo shows an installation of both an older version and
a newer version of the reactor.
Figure 1. Shunt reactors in a substation
Steel superstructures are used to support equipment, lines, and switches in substations as
well as transmission and distribution line towers and poles.
Figure 1. Steel superstructure for circuit breakers
Figure 2. Substation with many steel superstructures for equipment and connection supports
Supervisory control refers to equipment that allows for remote control of a substation's
functions from a system control center or other point of control. Supervisory control can be
operate circuit breakers,
operate tap changers on power transformers,
supervise the position and condition of equipment, and
telemeter the quantity of energy in a circuit or in substation equipment.
Figure 1. Supervisory control room
Figure 2. Supervisory control panel
An insulator type usually made of porcelain that can be stacked in a string and hangs from a
cross arm on a tower or pole and supports the line conductor. Suspension insulators are
used for very high voltage systems when it is not practical or safe to use other types of
insulators. They have an advantage in that one or more of the insulators in a string can be
changed out without replacing the entire string.
Figure 1. Suspension insulators
Figure 2. Suspension insulators Figure 3. Suspension insulators
A synchronous condenser is a synchronous machine running without mechanical load and
supplying or absorbing reactive power to or from a power system. Also called a synchronous
capacitor, synchronous compensator or rotating machinery.
In November 1995, the first static synchronous compensator began operating at a TVA
substation in Knoxville, Tennessee. This compensator can regulate voltage without expensive
external capacitors or reactors.
Figure 1. Synchronous condenser
Transmission buses are steel structure arrays of switches used to route power into a
Figure 1. Transmission bus
Figure 2. Transmission bus from inside
Vacuum Circuit Breakers
A circuit breaker is a device used to complete, maintain, and interrupt currents flowing in a
circuit under normal or faulted conditions. A vacuum circuit breaker utilizes a vacuum to
extinguish arcing when the circuit breaker is opened and to act as a dielectric to insulate the
contacts after the arc is interrupted. One type of circuit breaker is called a recloser. A
vacuum recloser is designed to interrupt and reclose an AC current circuit automatically, and
can be designed to cycle a set number of times before it must be reset manually.
Figure 1. Vacuum circuit breaker, inside Figure 2. Vacuum circuit breaker, outside
A transformer vault is a structure or room in which power transformers, network protectors,
voltage regulators, circuit breakers, meters, etc. are housed.
Figure 1. An underground transformer vault
Transformer - Underground
An underground transformer is essentially the same as an aboveground transformer, but is
constructed for the particular needs of underground installation. Vault type, pad-mounted,
submersible, and direct-buried transformers are used in underground systems. Pad-mounted
transformers are installed on a concrete pad on the surface near the end-user.
Figure 1. Pad-mounted transformer for
Figure 2. Transformer in underground vault
Distribution Feeder Circuits
Distribution feeder circuits are the connections between the output terminals of a distribution
substation and the input terminals of primary circuits. The distribution feeder circuit
conductors leave the substation from a circuit breaker or circuit recloser via underground
cables, called substation exit cables. The underground cables connect to a nearby overhead
primary circuit outside the substation. This eliminates multiple circuits on the poles adjacent
to the substations thereby improving the overall appearance of the substation.
Several distribution feeder circuits can leave a substation extending in different directions to
serve customers. The underground cables are connected to the primary circuit via a nearby
The distribution feeder bay routes power from the substation to the distribution primary
In the photo of the distribution main feeder the primary circuit is fed underground to a
nearby distribution system overhead line. The yellow cables are the primary feeder lines
Figure 1. 3-phase distribution feeder bay
Figure 3. Distribution feeder recloser
Figure 2. Distribution main feeder
Distribution transformers reduce the voltage of the primary circuit to the voltage required by
customers. This voltage varies and is usually:
120/240 volts single phase for residential customers,
480Y/277 or 208Y/120 for commercial or light industry customers.
Three-phase pad mounted transformers are used with an underground primary circuit and three
single-phase pole type transformers for overhead service.
Network service can be provided for areas with large concentrations of businesses. These are
usually transformers installed in an underground vault. Power is then sent via underground cables
to the separate customers.
Figure 3. Residential distribution transformer
Figure 1. Air Distribution transformer -
Figure 2. Industrial facility distribution Figure 4. Pad-mounted residential distribution
Primary circuits are the distribution circuits that carry power from substations to local load areas.
They are also called express feeders or distribution main feeders. The distribution feeder bay routes
power from the substation to the distribution primary feeder circuits.
In the photo of the distribution main feeder the primary circuit is fed underground to a nearby
distribution system overhead line. The yellow cables are the primary feeder lines going
Figure 1. 3phase distribution feeder bay
Figure 2. Distribution main feeder
Figure 3. Overhead primary feeder Figure 4. Distribution primary feeder underbuild
Protective equipment in a distribution system consists of protective relays, cutout switches,
disconnect switches, lightning arresters, and fuses. These work individually or may work in
concert to open circuits whenever a short circuit, lightning strikes or other disruptive event
When a circuit breakers opens, the entire distribution circuit is deenergized. Since this can
disrupt power to many customers, the distribution system is often designed with many
layers of redundancy. Through redundancy, power can be shut off in portions of the system
only, but not the entire system, or can be redirected to continue to serve customers. Only in
extreme events, or failure of redundant systems, does an entire system become
deenergized, shutting off power to large numbers of customers.
The redundancy consists of the many fuses and fused cutouts throughout the system that
can disable parts of the system but not the entire system. Lightning arresters also act locally
to drain off electrical energy from a lightning strike so that the larger circuit breakers are not
Figure 1. Substation bus lightning arresters Figure 4. Pole mounted type - lightning arrester
Figure 5. Air-break isolator switch
Figure 2. Fused cut-out
Figure 6. Non load-break fuse
Figure 3. Substation disconnect switch
Figure 7. Load-break fuse
Secondaries are the conductors originating at the low-voltage secondary winding of a distribution transformer
for residential service are three-wire single-phase circuits. They extend along the rear lot lines, alleys, or stre
customer's premises. The secondaries can be overhead lines or underground lines.
Overhead secondary lines are usually strung below the primary lines and typically in a vertical plane. When se
strung in a vertical plane, they are directly attached to the support pole one above the other. This is in contra
primary lines which are often strung on a cross bar or other attachment in a horizontal or "V" shaped plane.
Figure 2. Secondaries in a vertical plane
Figure 1. Cabled secondaries Figure 3. Cabled seconda
in a "V" plan
The wires extending from the secondaries or distribution transformer to a customer's location are
called a service. A service can be above or below ground. Underground services have a riser
connection at the distribution pole. Commercial and residential services are much the same and
can be either 120 or 220 or both.
Figure 2. Service line to residence
Figure 1. Distribution system lines and associated
Figure 3. Commercial service
Figure 4. Secondary to underground service via a
Distribution systems have switches installed at strategic locations to redirect or cut-off power flows
for load balancing or sectionalizing. Also, this permits repairing of damaged lines or equipment or
upgrading work on the system. The many types of switches include:
single-pole disconnect switches
three-pole group-operated switches
Figure 2. Air-break isolator switch
Figure 1. Air circuit-breaker switches
Figure 4. Single-pole disconnect switch combined
Figure 3. Circuit switchers
with a fuse is called a fused cutout
Figure 6. Pad mounted switchgear
Figure 5. Circuit breakers
Figure 7. Group-operated three-pole air break switch