Power systems-II (Transmission & Distribution)

POWER SYSTEMS

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DEPARTMENT OF TECHNICAL EDUCATION
ANDHRAPRADESH
Name of Participants :G.Rakesh Babu
Designation :Lecturer
Branch : Electrical & Electronics Engineering.
Institute : Sir C.R.R polytechnic,Eluru
Year/semester : v semester
Subject : Power systems-II (Transmission & Distribution)
Subject code : EE 504.
Major Topics : Substations.
Sub Topic : Equipment used in substations
Duration :50min.
Teaching Aids : PPT, Photographs

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Recap
In the previous period we learned about
•Comparison between the indoor and outdoor substations

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Objectives
Upon the completion of this topic you would be able to know
•Equipment used in substation

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Known to unknown
we came to know about
1.Substation and its need
2.Its functions and classification
3.Merits and demerits of indoor and outdoor substation
But we don’t know what is the equipment used in
substation.

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Symbols used for equipment in Substation
Sl.No Circuit elementSymbol
1. Bus bar
2. Single break isolating switch
3. Double break isolating switch
4. On load switch with earth blade

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5. Lightening arrester (valve)
6. Arcing horn
7. 3- Power transformer
8. Over Current relay
9. Earth fault relay

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Equipment used in Substation :
The equipment used in Substation mainly depends upon
•The type of Substation
•The service requirement
•The degree of protection desired.

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In general, a Substation has the following main equipments.
1.Bus Bars
2.Insulators
3.Transformers
4.Switch gear
5.Indicating & metering equipment
6.Protective relays
7.Lightning arrestors
8.Cables
9.Firefighting equipment etc.

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Bus Bars
When a no. of lines operating at the same voltage have to be
directly connected electrically, bus bars are used as the
common electrical component.
•Bus bars are copper or Aluminum bars (or) Al. conductor steel
rein forced wires (A.C.S.R).
•The incoming & outgoing lines in a substation are connected to
the bus bars.

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•Most commonly used bus bar arrangements are
1.Single bus bar arrangement.
2.Single bus bar with sectionalisation.
3.Duplicate bus bar arrangement.

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Bus Bar Sizes

The diameters of the tubular bus bars normally employed for various voltages
are as follows :
11KV  (1½”)38mm  Aluminize tubes
33KV  (1½”)38mm  Aluminize tubes
66KV  (2½”) 63.5mm  Aluminize tubes
132KV  ( 3” ) 76mm  Aluminize tubes
220KV  ( 3” ) 76mm  Aluminize tubes

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The Size of the Conductor normally used for the strain bus are :
220KV  61/0.398(61/0.157)A. C. S. R
132KV  37/0.427(37/0.168)A. C. S. R
(or)
37/0.335(37/0.11)A. C. S. R
66KV  37/0.279(37/0.11) A. C. S. R
33KV  The size of the standard conductor of tubes
used depend upon the transformer capacity.
11KV  The size of the standard conductor of
tubes used depend upon the transformer capacity.

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Insulators :
The insulators have two purposes.
•They support the conductors.
•Confine the current to the conductors .
There are several Insulators used in Substations depending upon
the service requirement.
1.Pin type
2.Suspension type (or) Strain type (or) disc type.
3.Post insulator.
4.Bushing insulator.
5.Shackle insulator etc.

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•EE 504 / 5.3 of 5In pin type Insulator a grove is placed on top side of the
insulator for housing the conductor.
Fig.1

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• This type insulators are used up to 33KV . For operating
voltage above 33 KV the pin insulators are un economical.

Fig.2

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Suspension type (or) Strain type (or) disc type
•For operating voltage about 33KV the pin type insulators are
uneconomical.
•Suspension insulators are used for above 33KV.
•It consists of a number of disc insulators connected in series by
metal link in the form of a string.
•The conductor is suspended at the bottom end of string.

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POST INSULATOR :
It consists of porcelain body, cast in on cap and flanged cast
in on base.

•This type of insulators are directly connected to bus bar clamp.

Fig.3

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BUSHING INSULATOR :
It consists of a porcelain Shell body.
•It mainly consists of washers at the
top and bottom for fixing the position
of bus bar (or) rod in shell.
•Mounting flange with holes drilled for
fixing bolts and supplied with an earth
bolt as shown in Fig.5
Fig.4

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Shackle Insulator
•It is designed for low voltages.
•The insulator is round and has a through hole at the centre to receive a
long bolt.
•It is used where there is a considerable mechanical stress (or) when the
angle of transmission line changes.
•It is also used at the end of span of the serial wire (or) service connection
to a domestic house (or) factory.

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TRANSFORMERS
1.POWER TRANSFORMERS
Fig.5

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•A power transformer is used in a substation to step up (or) step down the
voltage.
•All power transformers except at generating station are step down
transformers.
•Now a days at substations a 3-phase transformer (or) 3 single phase bank of
transformers are using.

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•The use of 3-phase transformer instead of 3 single phase bank of
transformers permits two advantages.
1.Only one 3-phase load tap changing mechanism can be used.
2.Installation is simpler than 3 single phase transformers.
•For rating up to 10 MVA naturally cooled, oil immersed transformers are
used.
•For higher ratings, the transformers are generally air blast cooled.

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2. INSTRUMENT OF TRANSFORMERS
•The lines in substations operate at high voltages and carry current of
thousands of amperes.
•But the measuring instruments and protective devices and designed for
low voltages (generally 110V) and currents (about 5A).
•So instrument transformers are used on the power lines.

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

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INSTRUMENT TRANSFORMERS ARE USED :
•To protect personnel and apparatus from high voltage.
•To Permit the use of reasonable in insulation levels and currents carrying capacity in
relays & motors.

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•There are two types of instruments transformers.
1.current transformers (C.T).
2.pontential transformers (P.T)

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CURRENT TRANSFORMER (C.T) :
Fig.6

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•A current transformer is essential a step up transformer which step down the
current to a known value.
•Primary winding consists of one or more turns of thick wire is connected in
series with the power circuit.
•Secondary winding is consists large no. of turns is connected to the
indicating & metering instruments & relays.

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POTENTIAL TRANSFORMER (P.T) :
•It is essentially a step down transformer that step-downs the voltage in a
known ratio.
•The primary winding of PT consist of a large no. of turns.
•Secondary winding has few turns.

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SWITCH GEAR :
•It is required for controlling & protecting equipments during normal &
abnormal conditions.
•It comprises of
1. Isolators
2. Air break switches
3. Circuit breakers
4. Fuses etc.

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ISOLATOR
Fig.8

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•In sub stations it is desired to disconnect a part of system for general
maintenance and repair. This is accomplished by using an isolator
•An isolator is essentially a knife switch and is designed to open a
circuit under No load

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Circuit breaker
•A CB is a protective device which makes (or) break a circuit either
manually (or) automatically under no load, full load & short circuit
conditions.
•It performs the following functions
1.To carry full load current continuously
2.To open and close the circuit on no load
3.To make and brake the circuit under normal operations

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•FUSES :
A fuse is a short piece of metal, inserted in the circuit which melts
when excessive current flows through it & thus breaks the circuit.

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INDICATING & METERING EQUIPMENT :
Ammeters, voltmeters, pf meters, wattmeters, KWH meters, KVARH
meters are installed in the substations to control & maintain.

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PROTECTIVE RELAYS :
•A Protective relay is a device which detects the fault & initiate the
operation of CB to isolate the defective element from the rest of the
system.
•Different types
1.Over current relay
2.Earth fault relay
3.Buchholz relay
4.Differential relay

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•Over current relay provides protection mainly against phase to phase faults and
overloading
•Earth faults relays provide protection against earth fault
•Buchholz relay provide protection against all kinds of transformer internal faults
•Differential relays provide protection against earth and phase fault

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• LIGHTNING ARRESTERS :
•Lightning arrester is a protective device which conducts the high voltage surge on the power
system network to ground .
•These are two types
1.Valve type
2.Thyrite type

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•CABLES :
In substations all the relays & measuring instruments are
operated at 110v or 220v. For this purpose 600v cables
insulation grade are generally used & these are laid in trenches,
cable trays in sometimes in conduits usually P.V.C cables are
used in substations.

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• FIRE FIGHTING EQUIPMENT :
1.sand.
2.Soda acid type fire extinguisher
3.Foam type
4.Carbon tetra chloride type
5.Carbon dioxide type
6.Water

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Sand
• It is useful in forming dikes to prevent the spread of oil and in
smothering small fires as an oil fire on the floor

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Soda acid type fire extinguisher
•Soda bicarb solution in water is contained in the body and sealed
acid bottle is held in a cage under the plunger of the cap.
•When the extinguisher is turned upside down and the plunger is
struck , the bottle breaks.
•Soda bicarb and acid mixed together and produce co2 gas whose
pressure forces the solution out .

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•This type is suitable for class A fires that is fires in wood, paper,
textile rubbish etc.
•It is not suitable for fires in oils and paints and must not be used
on fires involving live electrical apparatus.
•Because the jet produced by it is electrically highly conductive.

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Foam type
•It is most suitable for class “B” fires that is fires in oils, fats and varnishes
etc.

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Carbon tetra chloride type
•It is a liquid and gibes off a heavier air than air vapour that does not conduct
electricity and does not support combustion. It is most suitable for small fires.

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Carbon dioxide type
•Carbon dioxide is easily liquefied under pressure, is therefore bottled under
pressure in high pressure steel cylinder.
•It is suitable for putting out quickly fires in delicate machinery or apparatus
such as electrical relays, switch gear and motors, bearings and machines

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Water
•It alone is the universal extinguisher and is probably the most useful for
large fires.
•It is safe to direct fresh water on apparatus live even up to 250kV. When
the nozzle is 10 feet away.
•At 4 feet the stream can be used on equipment up to 33kV.

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Summary
We have discussed about
The equipment used in substation.

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Quiz
1.In outdoor substations lightning arrestor is placed near to
a) The isolators
b) The current transformer
C) The potential transformer
D) Power transformer

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2.In order to disconnect a part of system for general maintenance and repair is
accomplished by
A)Circuit breaker
B) Lightning Arrestor
C) Fuse
D) Both a and b

Quiz

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Frequently asked questions
1.Write a short notes on
a) Lightning arresters
b) Indicating and metering equipment
c) Isolators
d) Protective relays
e) Fire fighting equipment