Intro to Substations

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Introduction

Substation Types

Parts ofSubstation

Major Components

Of Substation

Substatio

ns


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When electricity leaves a power plant (1),

its voltage is increased at a step-up substation (2).

Next, the energy travels along a transmission line to thearea where the power is needed (3).

Once there, the voltage is decreased or stepped-down,at another substation (4),

and a distribution power line (5)carries the electricity until it reaches a home or business


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A substationis a high-voltage electric system facility.It is used to switch generators equipment andcircuits or lines in and out of a system. It also isused to change AC voltages from one level toanother and/or change alternating current to directcurrent or direct current to alternating current.Some substations are small with little more than atransformer and associated switches. Others arevery large with several transformers and dozens ofswitches and other equipment
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Substation Types: Although, there are generallyfour types of substations there are substations

that are a combination of two or more types.

Step-up Transmission Substation

Step-down Transmission Substation

Distribution Substation

Underground Distribution Substation

Substation Functions

Substation Equipment
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A step-up transmission substationreceiveselectric power from a nearby generating facilityand uses a large power transformer to increasethe voltage for transmission to distant locations.

A transmission bus is used to distribute electricpower to one or more transmission lines. Therecan also be a tap on the incoming power feedfrom the generation plant to provide electricpower to operate equipment in the generationplant.

A substation can have circuit breakers that are

used to switch generation and transmissioncircuits in and out of service as needed or foremergencies requiring shut-down of power to acircuit or redirection of power.
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The specific voltages leaving a step-uptransmission substation are determined by the

customer needs of the utility supplying powerand to the requirements of any connections toregional grids. Typical voltages are:

High voltage (HV) ac:69 kV, 115 kV, 138 kV, 161kV, 230 kVExtra-high voltage (EHV) ac:345 kV,500 kV, 765 kVUltra-high voltage (UHV) ac:1100kV, 1500 kVDirect-current high voltage (dc HV):250 kV, 400 kV, 500 kV

Direct current voltage is either positive or negativepolarity. A DC line has two conductors, so onewould be positive and the other negative.


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Step-down transmission substationsare

located at switching points in an electrical

grid. They connect different parts of a gridand are a source for subtransmission lines

or distribution lines. The step-down

substation can change the transmissionvoltage 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
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Distribution substationsare located near to

the end-users. Distribution substation

transformers change the transmission orsubtransmission voltage to lower levels for

use by end-users. Typical distribution

voltages vary from 34,500Y/19,920 volts to4,160Y/2400 volts.

34,500Y/19,920 volts is interpreted as athree-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


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Underground distribution substationsare

also located near to the end-users.

Distribution substation transformers

change the subtransmission voltage tolower levels for use by end-users. Typical

distribution voltages vary from

34,500Y/19,920 volts to 4,160Y/2400volts.


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An underground system may consist of these parts:

Figure 10. Underground Distribution Substation

Conduits

Duct Runs

Manholes

High-Voltage Underground Cables

Transformer Vault

Riser

Transformers

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:

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 thecircuits


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The major components of a typical substation are:

Air circuit breakersare used to interrupt

circuits while current flows through them.

Compressed air is used to quench the arc

when the connection is broken.


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Batteriesare used in the substation control

house as a backup to power the control

systems in case of a power blackout


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Bus support insulatorsare 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 insulatorsused in substations and transmission poles and towers.


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Capacitor Bank

Capacitorsare used to control the level of

the voltage supplied to the customer byreducing or eliminating the voltage drop in

the system caused by inductive reactive

loads.


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Circuit switchersprovide equipment

protection for transformers, lines, cables,

and capacitor banks. They also are used

to energize and deenergize capacitor

banks and other circuits.


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Concrete foundationsor pads are laid for

all large equipment, support structures,

and control buildings in a substation.


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Conduitsare hollow tubes running from

manhole to manhole in an underground

transmission or distribution system. Theycan contain one or more ducts (See Duct

Runs). They can be made of plastic

(PVC), fiberglass, fiber, tile, concrete, or

steel. PVC and fiberglass are most

commonly used.
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The substation control housecontains

switchboard panels, batteries, battery

chargers, supervisory control, power-linecarrier, meters, and relays. The control

house provides all weather protection and

security for the control equipment. It isalso called a doghouse.


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Control Panels

Control panelscontain meters, control

switches and recorders located in thecontrol building, also called a doghouse.

These are used to control the substation

equipment, to send power from one circuitto another or to open or to shut down

circuits when needed.


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Control wiresare installed connecting the

control house control panels to all the

equipment in the substation. A typicalsubstation control house contains several

thousand feet of conduit and miles of

control wire.


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Converter stationsare 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.


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Coupling capacitorsare 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.


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Current Transformers

Current transformerscan 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.


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Disconnect switchesor circuit breakers are used to isolate

equipment or to redirect current in a substation. Many different types

of disconnect switches are shown below.


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A distribution busis a steel structure

array of switches used to route powerout of a substation.


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A frequency changer is a motor-generator

set that changes power of an alternating

current system from one frequency to oneor more different frequencies, with or

without a change in the number of phases,

or in voltage. Sometimes a converter isused to accomplish this.


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High voltage fusesare used to protect the

electrical system in a substation from

power transformer faults. They areswitched for maintenance and safety.


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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

substations.

Intro to Substations

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