University of Gondar

Department of Electrical Engineering

Power System Protection and Control (ECEg5151)

CHAPTER FOUR
Physical Layout/Substation physical arrangement

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 Outlines
• Introduction

• Selection of Switching Scheme

• Surge Arresters

• Voltage Transformers

• Current Transformers

• Switching Stations

• Grounding Switches

• Typical Bus Configurations

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 4.1 Introduction
• The electric power is produced at the power stations which are
located at favorable places, generally quite away from the
consumers.

• It is delivered to the consumers through a large network of

transmission and distribution.

• At many places in the line of the power system, it may be

desirable and necessary to change some characteristic (e.g.
voltage, a.c. to d.c., frequency, p.f. etc.) of electric supply.

• This is accomplished by suitable apparatus called substation.

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 4.1 Introduction
• While designing and building a sub-station important points
which must be kept in view are :

 It should be located at the center of the load.

 It should provide safe and reliable arrangement.

 It should be easily operated and maintained.

 It should involve minimum capital cost.

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 4.2 Substation

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 5.3 Classification of Substation
• There are several ways of classifying sub-stations.
• However, the two most important ways of classifying them are
according to:

 Service requirement and

 Constructional features.
• According to service requirement: A sub-station may be
called upon to change voltage level or improve power factor or
convert a.c. power into d.c. power etc.
• According to the service requirement, sub-stations may be
classified into :

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 5.3 Classification of Substation
• Transformer sub-stations. Those sub-stations which change the
voltage level of electric supply are called transformer sub-stations.
• These sub-stations receive power at some voltage and deliver it at
some other voltage.
• Obviously, transformer will be the main component in such
substations.
• Most of the sub-stations in the power system are of this type.
• It divided in to different sub types
 Transmission substation and
 Distribution substation
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 5.3 Classification of Substation
• Switching sub-stations. These sub-stations do not change the
voltage level i.e. incoming and outgoing lines have the same
voltage.

• However, they simply perform the switching operations of power

lines.

• Power factor correction sub-stations. Those sub-stations which

improve the power factor of the system are called power factor
correction sub-stations.

• Such sub-stations are generally located at the receiving end of

transmission lines.
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 5.3 Classification of Substation
• These sub-stations generally use synchronous condensers as the
power factor improvement equipment.

• Frequency changer sub-stations. Those sub-stations which change

the supply frequency are known as frequency changer sub-
stations.

• Such a frequency change may be required for industrial utilization.

• Converting sub-stations.

• Those sub-stations which change a.c. power into d.c. power are
called converting sub-stations.

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 5.3 Classification of Substation
• These sub-stations receive a.c. power and convert it into d.c.
power with suitable apparatus to supply for such purposes as
traction, electroplating, electric welding etc.

• Industrial sub-stations. Those sub-stations which supply power

to individual industrial concerns are known as industrial sub-
stations.

• According to constructional features. A sub-station has many

components (e.g. circuit breakers, switches, fuses, instruments
etc.) which must be housed properly to ensure continuous and
reliable service.
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 5.3 Classification of Substation
• According to constructional features, the sub-stations are
classified as :

– Indoor sub-station

– Outdoor sub-station

– Underground sub-station

– Pole-mounted sub-station

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 5.3 Classification of Substation

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 5.3 Classification of Substation
• Indoor sub-stations. For voltages up to 11 kV, the equipment of the
sub-station is installed indoor because of economic considerations.
• However, when the atmosphere is contaminated with impurities,
these sub-stations can be erected for voltages up to 66 kV.
• Outdoor sub-stations. For voltages beyond 66 kV, equipment is
invariably installed outdoor.
• It is because for such voltages, the clearances between conductors
and the space required for switches, circuit breakers and other
equipment becomes so great that it is not economical to install the
equipment indoor.

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 5.3 Classification of Substation
• Underground sub-stations. In thickly populated areas, the space
available for equipment and building is limited and the cost of
land is high.
• Under such situations, the sub-station is created underground.
• Pole-mounted sub-stations. This is an outdoor sub-station with
equipment installed overhead on H-pole or 4-pole structure.
• It is the cheapest form of sub-station for voltages not
exceeding11kV (or 33 kV in some cases).
• Electric power is almost distributed in localities through such
substation.
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Comparison between Outdoor and Indoor Sub-Stations
NO Particular Outdoor Sub-station Indoor Sub-station

1 Space required More Less

2 Time required Less More

for erection
3 Future extension Easy Difficult

4 Fault location Easier because the Difficult because the

equipment is in full view equipment is enclosed
5 Capital cost Low High

6 Operation Difficult Easier

7 Possibility of Less because greater More

fault escalation clearances can be provided

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 Substation Equipment's
• The equipment required for a sub-station depends upon the type of
sub-station, service requirement and the degree of protection
desired.
 Transformers  Circuit breaker

 Isolating Switch (Isolator)  Earth Switch's

 Instrument Transformers  Wave Trap

(CT&PT)  Insulators & Bus Bars
 Reactors and Capacitors  Metering, Indicating
 Surge Arrestors (Lightening Instruments and Control Panels
Arrestors)  Auxiliaries
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 Power Transformers

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 Power Transformers
• A power transformer:- is used in a sub-station to step-up or
step-down the voltage to a required level.
.
• Except power station, all subsequent substation use step-down
transformers.
• Capacity of a Transformer in a substation is chosen based on
the load requirement and anticipated load growth.
• Voltage control is done through tap changers provided in
power transformers.

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 Circuit breaker
• CB is advice capable of
making or breaking a circuit
under normal (for operation or
maintenance purpose) and
abnormal (fault) conditions.

• Under fault condition CB

operate automatically,
using relay operation.

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 Isolator/ Disconnecting Switch

• Isolator is essentially a knife

switch which is used to open
a circuit under no load
condition.

• Isolators help to ensure that

an electric circuit is
completely de-energized for
service, test or maintenance.

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 Isolator/ Disconnecting Switch
• Isolators are generally used on both ends of a CB to enable a
personnel carry out repair or replacement of CB without any
danger.

Types of Isolators
• Single Break Isolator: Only one terminal connects and
disconnects
• Double Break Isolator: Has locking mechanism
• Pantograph Isolator: work on force of stress or tension ,and
usually no locking mechanism is present

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 Earth Switch
• An Earth switch is a device used to
discharge to earth any residual
voltage trapped on a line after it’s
opening.
• An Earth switch are mounted on
the base of the line isolator. They
are normally vertical break switch
and have their contact arms aligned
horizontally in open position.

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 Earth Switch
• The sequence of operation while opening and closing a circuit
is :

While opening While Closing

1. Open CB 1. Open Earth Switch

2. Open Isolator 2. Closing Isolator
3. Closing Earth Switch 3. Closing CB

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Sequence of operation for earth switch looks like:
While Opening

While Closing

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 Instrument transformers
• The lines in sub-stations operate at high voltages and carry large
current.

• The measuring instruments and protective devices are designed for

low voltages and currents.

• Therefore, they will not work satisfactorily if mounted directly on

the power lines.

• This difficulty is overcome by installing instrument transformers

on the power lines.

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 Instrument transformers
• The function of these instrument transformers is to transfer
voltages or currents in the power lines to values which are
convenient for the operation of measuring instruments and relays.

• They are classified as

 Protective transformer and,

 Measuring transformer

• Normally both functions are usually combined in one hence the

general term INSTRUMENT TRANSFORMERs.

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 Instrument transformers
• There are majorly two main types namely: Current transformer
(C.T.) and Potential transformer (P.T.)

PT/VT

Current Transformer Potential Transformer

CT
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 Instrument transformers
• Current transformer (C.T.) A current transformer is essentially
a step-up transformer which steps down the current to a
known ratio.
• The primary of this transformer consists of one or more turns
of thick wire connected in series with the line.
• The secondary consists of a large number of turns of fine wire
and provides for the measuring instruments and relays a
current which is a constant fraction of the current in the line.
• Is used for the measurement of a.c. by taking samples of the
high currents of the system.
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 Instrument transformers

• These reduced samples are in accurate proportions

with the actual high currents of the system.
• These are used for installation and maintenance of the
current relays in substations for protection purpose
which are normally have low current rating for their
operation.
• CT are used with ammeter, watt-meters, power factor-
meters, watt-hour-meters, compensators, protective
and regulative relays and the trip coil of CB.
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 Instrument transformers
• Voltage transformer. It is essentially a step down

transformer and steps down the voltage to a known ratio.

• The primary of this transformer consists of a large number

of turns of fine wire connected across the line.

• The secondary winding consists of a few turns and provides

for measuring instruments and relays a voltage which is a

known fraction of the line voltage.

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 Instrument transformers
• PT is quite similar with CT but, it is used for taking samples of high

voltage of a system for providing low voltage to the relays of

protection system and also to the low rating meters for voltage

measurement.

• From this low voltage measurement, the actual systems high voltage

can be calculated with out measuring the high voltages directly to

avoid the cost of measurement system and injuries.

• PT are used with volt-meter, watt-meters, power factor-meters, watt-

hour-meters, frequency meters, protective and regulating relays and

the no voltage and overvoltage trip coil of automatic CB.

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 Wave Traps

• Wave traps are also

known as Line traps .
• They are connected in
series to transmission
lines and are a critical
component of Power Line
Carrier Communication
(PLCC) systems.

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 Wave Traps
• They are used to filter high frequency communication signals
sent on the line from remote substations and divert them to the
telecom/tele-protection panels in the substation control rooms.
• This is relevant in PLCC systems for communication among
various substation without dependence on the telecom
company network.
• PLCC is the technology used for communication, tele-
monitoring and protection between substations through high
voltage power lines.

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 Reactors and Capacitors
Shunt Reactor
• This are installed to control high
voltages that occur especially at night
due to the capacitive effect of lightly
loaded transmission lines and also
when sudden loss of a block of
customer load occurs.

Shunt Capacitors • These are installed to provide MVARs to

the system to help support voltage levels.
• It also helps to improve power factor by

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 Reactors and Capacitors
Series Reactor
• These are used as a current limiting
reactors (decrease short circuit
currents) to increase the impedance
of the line; thereby improving the
transmission capacity of power lines.

Series Capacitors • They are installed in a very long transmission

line to improve system stability, reduce

system loss and optimize power flow
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 Insulator and Bus bar
• Insulators: they are basically
used to support conductors (or
bus bars) and to confine the flow
of current to the conductor.

• The most commonly used materials for the manufacture of

insulator is porcelain.
• There are several type of insulators (e.g. pin type, disc type
,suspension type, post insulators etc.) and their use in the
substation depends on the service requirements.
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 Insulator and Bus bar

• Bus Bars: this is a conductor or an assembly of conductor used

to connect incoming and outgoing lines in a substation.
• They are copper or aluminum bars and operate at a constant
voltage.
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 Lightning Arrester (LAs)
• Electrical equipment and Device in power
station are subjected to overvoltage's which
affect their insulation.

• Electrical equipment and Device in power station are subjected to

overvoltage's which affect their insulation.
• These over voltage may be caused by system faults, switching on
and off of lines and equipment as well as lightning phenomenon.
• Lightning Arrester were basically used to protect system from
lightening in the past but in recent times, due to development in
power system, EHV and UHV system came in to existence.
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 Lightning Arrester (LAs)
• Over voltage are classified as:
 Atmospheric over voltages-caused by lightning.
 Switching over voltages- due to system operations like
switching off transformer on no load etc.
 Temporary over voltages- mainly due to load rejection and or
single phase to ground faults.

• Thus switching over voltages (opening and closing of breakers

etc.) became so sever and since LAs also protect system from
switching over voltages.
• The word surge arresters is now been widely used to refer to them.
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 Control Room

• There are several control, metering,

and indicating instruments being
used at substation.
• They are installed to enable
constant monitoring and control of
system parameters for reliability of
power supply.

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 Substation Layout
• Generally, substation layout should look like:

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 Substation Configurations
• There are several bus configurations that can be used in a substation.
• The choice of configuration depends upon various factors such as
voltage level, reliability, position of substation, cost etc.
• Certain configurations may be more suited for a specific task hence
the equipment in each type of substations may vary.
Typical Configuration
Single bus
 Single bus with Sectionalisation
 Duplicate bus-bar system
 Ring bus
 Breaker and a half
 Double breaker double bus
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 Single Bus Configuration

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 Single Bus Configuration
• This is the simplest type of bus bar configurations and has the least
reliability.
• It consists of a single bus-bar and all the incoming and outgoing
lines are connected to this bus.
• It can be constructed in either low or high profile arrangement
depending on the space available.
Advantages Disadvantage
 Low cost  Low reliability
 Less space  Complete interruption of supply
 Simplicity of operation and is needed for maintenance
maintenance  Can not be used for voltages
 Easily expandable exceeding 33KV
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 Single bus with Sectionalisation

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 Single bus with Sectionalisation
• This is basically an extension of the single bus configuration.

• Here two single bus sections are connected together with a center
circuit breaker(sometimes referred to as bus coupler) which may
be normally open or normally closed as the case may be and load
is often equally distributed on both sections.
Advantages Disadvantage
 Flexible operation.  Higher cost as compared to
 Isolation of bus section for single bus due to additional
maintenance. circuit breaker
 Only a section is affected in  Maintenances with out
event of breaker failure or bus interruption of supply is not
fault. possible
 Low maintenance cost.
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 Duplicate bus-bar system

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 Duplicate bus-bar system.
• This bus bar configuration system consists of two separate and
independent bus, a main bus and a spare/transfer bus.

• Normally, all circuits (incoming and outgoing) are connected to the

main bus.

• For the purpose of maintenance or repair, the associated circuit can be

connected to the transfer bus (the circuit has been ‘transferred’)while
the main bus is isolated from the system.
Advantage Disadvantage
 Low cost  An extra circuit breaker is required
 Small area of land is required for bus tie
 Maintenance can be carried out  Relay protection is often
with out total loss of supply. complicated
 Easily expandable  Bus fault causes entire loss of
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 Ring Bus Configuration

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 Ring Bus Configuration
• This bus configuration is an extension of the sectionalized bus
system.

• In this system, there is a closed loop on the bus with each section
separated by a circuit breaker.

• This provides greater reliability and allows for flexible operation.

Advantage Disadvantage
 Flexibility of operation  An extra circuit breaker is
 High reliability required for bus tie
 Each circuit is double fed  Complexity of protection
 Maintenance with out circuit  High cost
disruption
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 Breaker and a half configuration

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 Breaker and a half configuration
• This bus configuration has two buses that are both energized during
normal operation unlike the main and transfer bus scheme.

• In this system every two circuit has three CB’s with each circuit
sharing a common center breaker.

• This enables maintenance to be carried out with out total shut

down.
Disadvantage
Advantage  An extra CB is required
 Flexibility of operation  Complexity of protection
 High reliability  Each circuit requires its
 Each circuit is double fed own potential source for
 Maintenance without circuit relying
disruption  High cost of installation
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 Double Bus Double Breaker

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 Double Bus Double Breaker
• This bus configuration also has two buses that are both energized
during normal operation like the breaker and a half scheme above.

• In this system, each circuit requires two breakers unlike the breaker
and a half that shares a breaker.

• Incoming and outgoing feeders can be taken from any of the buses.
Advantage Disadvantage
 Flexibility of operation  The high reliability comes
 High reliability with the cost of additional
 Each circuit is double fed breakers thus, this bus scheme
 Maintenance without circuit is usually used at large
disruption generating stations.
 The most expensive bus
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• Group Assignment (5 members each) Total mark 15%.

Clearly describe the type of substation, the rating of the substation's

equipment, the compensation mechanism used in the substation, and
the substation's configuration with a neatly drawn general layout for
the following substation.

1. Azezo distribution substation

2. Lideta distribution substation

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 Thank YOU!

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