1.

INTRODUCTION
INTRODUCTION :
The present-day electrical power system is A.C. i.e. electric power is generated, transmitted and
distributed in the form of alternating current. The electric power is produced at the power
stations which are located at favourable 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, Power factor etc.) of electric supply.
This is accomplished by suitable apparatus called sub-station. For example, generation voltage
(11KV or 6.6KV) at the power station is stepped up to high voltage (say 220KV or 132KV) for
transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this
purpose is the sub-station. Similarly, near the consumer’s localities, the voltage may have to be
stepped down to utilization level. This job is again accomplished by a suitable apparatus called
‘substation.

CONSTRUCTION OF A SUBSTATION

At the time of constructing a substation, we have to consider some factors which

affect the substation efficiency like selection of site

SELECTION OF SITE:

Main points to be considered while selecting the site for EHV Sub-Station are as
follows:

i) The site chosen should be as near to the load centre as possible.

ii) It should be easily approachable by road or rail for transportation of equipments.
iii) Land should be fairly levelled to minimize development cost.
iv) The source of water should be as near to the site as possible. This is because water is
required for various construction activities;
(Especially civil works,), earthing and for drinking purposes etc.
v) The sub-station site should be as near to the town / city but should be clear of public
places, aerodromes, and Military / police installations.

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 vi) The land should be have sufficient ground area to accommodate substation equipments,
buildings, staff quarters, space for storage of material, such as store yards and store sheds
etc. with roads and space for future expansion.
vii) Set back distances from various roads such as National Highways, State Highways
should be observed as per the regulations in force.
viii) While selecting the land for the substation preference to be given to the Govt. land over
Private land.
ix) The land should not have water logging problem.
x) The site should permit easy and safe approach to outlets for EHV lines.

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 2. CLASSIFICATION OF SUBSTATIONS

There are several ways of classifying sub-stations. However, the two most important
ways of classifying them are according to (1) service requirement and (2) constructional
features.

ACCORDING TO THE 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:

(i) 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 sub-stations. Most of the sub-stations in the power system are of this
type.
(ii) 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.
(iii) 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. These sub-stations generally
use synchronous condensers as the power factor improvement equipment.

(iv) 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.

(v) Converting sub-stations: Those sub-stations which change A.C. power into
D.C. power are called converting sub-stations. These sub-stations receive A.C. power and
convert it into D.C. power with suitable apparatus (e.g. ignitron) to supply for such purposes
as traction, electroplating, electric welding etc.

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 (vi) Industrial sub-stations:- Those sub-stations which supply power to individual
industrial concerns are known as industrial sub-stations.

ACCORDING TO THE 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.
According to constructional features, the sub-stations are classified as:
 Indoor sub-station
 Outdoor sub-station
 Underground sub-station
 Pole-mounted sub-station

(i) Indoor sub-stations:- For voltages up to 11KV, 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.

(ii) Outdoor sub-stations:- For voltages beyond 66KV, equipment is invariably

installed out-door.

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.

(iii) 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.

(iv) Pole-Mounted sub-stations:- This is an outdoor sub-station with equipment

installed over-head on H-pole or 4-pole structure. It is the cheapest form of sub- station for
voltages not exceeding 11KV (or 33 KV in some cases). Electric power is almost distributed
in localities through such sub-station.

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 3.SINGLE LINE DIAGRAM (SLD)

A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the
concerned Electrical System which includes all the required electrical equipment connection
sequence wise from the point of entrance of Power up to the end of the scope of the
mentioned Work. As in the case of 132KV Substation, the SLD shall show Lightening
Arrestor, C.T/P.T Unit, Isolators, Protection and Metering P.T &
C.T. Circuit Breakers, again Isolators and circuit Breakers, Main Power Transformer, all
protective devices/relays and other special equipment like CVT, GUARD RINGS, etc as per
design criteria. And the symbols are shown below. There are several feeders enter into the
substation and carrying out the power. As these feeders enter the station they are to pass
through various instruments.
FEEDER CERCUIT:
1. Lightening arrestors; 2. CVT; 3. Wave trap; 4. Isolators with earth switch
5. Current transformer; 6. Circuit breaker; 7. Feeder Bus isolator
8. BUS; 9. Potential transformer in the bus with a bus isolator
TRANSFORMER CIRCUIT:
i) HV side:
1. Transformer bus Isolator 3. Current transformer
2. Circuit breaker 4. Lightning Arrestors
5. Auto Transformer 100MVA (220/132KV)
ii) LV side:
1. Lightening arrestors 5. Bus
2. Current transformer 6. Potential transformer with a bus isolator
3. Circuit breaker 7. A capacitor bank attached to the bus
4. Bus Isolator.

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 AUXILIARY SUPPLY:
220V.Battery system: To control and protect the substation equipment the 220 volts
DC battery system is necessary. It is provided in the main control room. It will be discussed
below.

Fig: 3.1 SINGLE LINE DIAGRAM OF A 220/132KV SUBSTATION WARANGAL.

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 4.BRIEF DISCRIPTION OF INSTRUMENTS IN THE
SUBSTATION
LIGHTENING ARRESTORS:

Lightening Arrestors:
Lightening arrestors are the instruments that are used in the incoming feeders so that
to prevent the high voltage entering the main station. This high voltage is very dangerous to
the instruments used in the substation. Even the instruments are very costly, so to prevent
any damage lightening arrestors are used. The lightening arrestors do not let the lightening
to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it
to the earth. In any substation the main important is of protection which is firstly done by
these lightening arrestors. The lightening arrestors are grounded to the earth so that it can
pull the lightening to the ground.
These are located at the entrance of the transmission line in to the substation and as
near as possible to the transformer terminals.
 LA will be provided on the support insulators to facilitate leakage current
measurement and to count the no of surges discharged through the LA.
 LA bottom flange will be earthed via leakage ammeter and surge counter. Leakage
current is to be recorded periodically. If the leakage current enters into the red range from
the green range, the LA is prone for failure. Hence, it is to be replaced.
 There should be independent earth pit for LA in each phase so as to facilitate fast
discharging and to raise the earth potential.
The lightning arresters or surge diverters provide protection against such surges. A
lightning arrester or a surge diverter is a protective device, which conducts the high voltage
surges on the power system to the ground.

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 Fig.4.1 (i) Surge diverter

(ii)Characteristics of the non linear resister

 Fig 4(i) shows the basic form of a surge diverter. It consists of a spark gap in series with a
non-linear resistor. One end of the diverter is connected to the terminal of the equipment to
be protected and the other end is effectively grounded. The length of the gap is so set that
normal voltage is not enough to cause an arc but a dangerously high voltage will break
down the air insulation and form an arc. The property of the non- linear resistance is that its
resistance increases as the voltage (or current) increases and vice-versa. This is clear from
the volt/amp characteristic of the resistor shown in Fig 4 (ii).

Fig: 4.2 LIGHTENING ARRESTORS.

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The action of the Lightning Arrester or surge diverter is as under:

(i) Under normal operation, the lightning arrester is off the line i.e. it conducts no
current to earth or the gap is non-conducting.

(ii) On the occurrence of over voltage, the air insulation across the gap breaks down
and an arc is formed providing a low resistance path for the surge to the ground. In this
way, the excess charge on the line due to the surge is harmlessly conducted through the
arrester to the ground instead of being sent back over the line.
(iii) It is worthwhile to mention the function of non-linear resistor in the operation of
arrester. As the gap sparks over due to over voltage, the arc would be a short circuit on the
power system and may cause power-follow current in the arrester. Since the characteristic of
the resistor is to offer low resistance to high voltage (or current), it gives the effect of short
circuit. After the surge is over, the resistor offers high resistance to make the gap non
conducting.

Guide for selection of LA:

(iv)Before selecting the LA it should be ascertained whether the system is
effectively earthed, non-effectively earthed or having isolated neutral.

(v) The system neutrals are considered to be effectively earthed when the co-
efficient of earthing does not exceed 80%.

In this case, the reactance ratio X0/ X1 (zero sequence reactance/positive sequence
reactance) is positive and less than 3 and at the same time the resistance ratio RO/X1 (zero
sequence resistance/positive sequence reactance) is less than 1 at any point on the system.
For this system the arrestor rating will be 80% of the highest phase to phase system voltage.

(vi)The LA voltage rating corresponding to the system voltages normal are

indicated below :
Rated system Highest system Arrester rating in KV
Voltage (KV) Voltage (KV) Effectively earthed systems
11 12 9
33 36 30
66 72.5 60
132 145 120/132 (latex)

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 220 245 198/216 (latex)
400 420 336
Table: 4.1.3 LA voltage rating
LOCATION OF LIGHTING ARRESTORS:
The LAs employed for protecting transformers should be installed as close as
possible to the transformer. The electrical circuit length between LA and the transformer
bushing terminal should not exceed the limits given below:

Rated BIL Max. distance between L.A and

KV Transformer bushing terminal
system Peak (inclusive of lead length) (in
Voltage metres) Effectively earthed
KV
11 75 12.0
33 200 18.0
66 325 24.0
132 550 35.0
650 43.0
220 900 Closes to
1050 Transforme
400 1425 r
1550
Table: 4.1.4 The limits of LA and Transformers
EARTHING:
The earthing practice adopted at generating stations, sub-stations and lines should be
in such a manner as to provide:
a) Safety to personnel
b) Minimum damage to equipment as a result of flow of heavy fault currents
c) Improve reliability of power supply
4.1.1 The primary requirements are:
The impedance to ground (Resistance of the earthing system) should be as low as
possible and should not exceed,

Large sub-stations -1 ohm

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 Small sub-stations -2 ohms
Power stations -0.5 ohms
Distribution transformer stations- 5 ohms
All exposed steel earthing conductors should be protected with bituminous paint.
PLATE EARTHING:
i) EHT Substation - 1.3 M x 13 M.Ms cast iron plates 25mm thick Plates are to be
buried vertically in pits and surrounded by finely divided coke, crushed coal or char coal at
least 155 mm all round the plates. Plates should not be less than 15 m apart and should be
buried to sufficient depth to ensure that they are always surrounded by moist earth.

PIPE EARTHING:
a) EHT substations Cast iron pipes 125 mm in diameter 2.75 m long and not less than
9.5 mm thick pipes 50.8mm in dia and 3.05m long. Pipes are to be placed vertically at
intervals of not less than 12.2 m in large stations surrounded by finely broken coke crushed
coal and charcoal at least 150 mm around the pipe on the extra depth.
a) Peripheral or main earth mat- 100 x 16 m MS flat
b) Internal earth mat- 50 x 8m MS flat to be placed at 5m apart
c) Branch connections- Cross section not less than 64.5 square meters

Joints are to be kept down to the minimum number. All joints and connections in earth grid are
to be brazed, riveted, sweated, bolted or welded. For rust protection the welds should be treated
with barium chromate. Welded surfaces should be painted with red lead and aluminium paint in
turn and afterwards coated with bitumen. Joints in the earthing conductor between the switch
gear units and the cable sheaths, which may require to subsequently broken should be bolted and
the joint faces tinned. All joints in steel earthing system should be made by welding except the
points for separating the earthing mat for testing purposes which should be bolted. These points
should be accessible and frequently supervised.

In all sub-stations there shall be provision for earthing the following:

a) The neutral point of earth separate system should have an independent earth,
which in turn should be interconnected with the station grounding mat

b) Equipment frame work and other non-current carrying parts (two

connections)

c) All extraneous metallic frame work not associated with equipment (two

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 connections)

d) Lightning arrestors should have independent earths which should in turn be

connected to the station grounding grid.

e) Over head lightning screen shall also be connected to the main ground mat.

The earth conductor of the mat could be buried under earth to economical depth of
burial of the mat 0.5 meters.

CAPACITOR VOLTAGE TRANSFORMER (CVT):

A capacitor voltage transformer (CVT) is a transformer used in power systems to
step-down extra high voltage signals and provide low voltage signals either for
measurement or to operate a protective relay. These are high pass Filters (carrier frequency
50KHZ to 500 KHZ) pass carrier frequency to carrier panels and power frequency
parameters to switch yard. In its most basic form the device consists of three parts: two
capacitors across which the voltage signal is split, an inductive element used to tune the
device and a transformer used to isolate and further step-down the voltage.

Fig: CIRCUIT DIAGRAM OF CVT.

The device has at least four terminals, a high-voltage terminal for connection to the
high voltage signal, a ground terminal and at least one set of secondary terminals for
connection to the instrumentation or protective relay. CVTs are typically single-phase
devices used for measuring voltages in excess of one hundred KV where the use of voltage
transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by
a stack of capacitors connected in series. This results in a large voltage drop across the stack
of capacitors, that replaced the first capacitor and a comparatively small voltage drop across
the second capacitor, C2, and hence the secondary terminals.

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 Fig: 4.3.2 CAPACITOR VOLTAGE TRANSFORMER.

Specifications of CVT
CVT type : CVEB/245/1050
Weight : 665 kg
Total output simultaneous : 250 VA
Output maximum : 750 VA at 50O C
Rated voltage : A-N, 220/√3
Highest system voltage : A-N, 245/√3
Insulation level : 460/1050 KV
Rated frequency : 50Hz
Nominal intermediate voltage : A1-N, 20/√3 KV
Voltage factor : 1.2Cont. 1.5/30 sec
‘HF’ capacitance : 4400pF +10% -5%
Primary capacitance C1 : 4840pF +10% -5%
Secondary capacitance C2 : 48400 pF +10%-5%
Voltage ratio : 220000/√3/ 110/√3/110-110/√3
Voltage : 110/√3 110-110/√3
Burden : 150 100
Class : 0.5 3P
WAVE TRAP:

Wave trap is an instrument using for trapping of the wave. The function of this wave
trap is that it traps the unwanted waves. Its shape is like a drum. It is connected to the main
incoming feeder so that it can trap the
waves which may be dangerous to the instruments in the substation.

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 Generally it is used to exclude unwanted frequency components,
such as noise or other interference, of a wave.
Note: Traps are usually unable to permit selection of unwanted
or interfering signals.

Fig: 4.4.1 WAVE TRAP.

Line trap also is known as Wave trap. What it does is trapping the high frequency communication
signals sent on the line from the remote substation and diverting them to the telecom/tele protection
panel in the substation control room through coupling capacitor.

This is relevant in Power Line Carrier Communication (PLCC) systems for

communication among various substations without dependence on the telecom company
network. The signals are primarily tele protection signals and in addition, voice and data
communication signals. The Line trap offers high impedance to the high frequency
communication signals thus obstructs the flow of these signals in to the substation bus bars.
If these are not present in the substation, then signal loss is more and communication will be
ineffective/probably impossible.

ISOLATOR WITH EARTH SWITCHES (ES):

Isolators are the no load switches and used to isolate the equipment. (Either line
equipment, power transformer equipment or power transformer). With the isolators, we are
able to see the isolation of the equipment with our naked eye. The line isolators are used to
isolate the high voltage from flow through the line into the bus. This isolator prevents the
instruments to get damaged. It also allows the only needed voltage and rest is earthed by
itself.

Isolator is a type of switching device. It has non control devices. Isolator are

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 operated after the circuit breaker is opened. While closing the circuit, first close the isolator
and after the circuit breaker is closed. Strictly speaking Isolators are operated under no
current condition. In the following cases it is permissible to use isolator for making and
breaking of the circuits.

Fig: 4.5.1 ISOLATOR WITH EARTH SWITCH.

Air break isolators or disconnecting switches are not intended to break load though
these are meant for transfer of load from one bus to another and also to isolate equipment
for maintenance. These are available mainly in two types vertical break type and horizontal
break type. The later type requires larger width. However the space requirement can be
reduced in the horizontal break isolators by having double break with a centre rotating
pillar.
Pantograph and semi-pantograph disconnects involve vertical movements of contact
arm and therefore require less separation between phases and thereby require less separation
between phases and thereby help in reducing the sub-station area to a larger extent. The
isolators could be operated mechanically or hydraulically or pneumatically or by electric
motor. Earthing facility shall be provided wherever required.

INSTRUMENT TRANSFORMERS:
“Instrument Transformers are defined as the instruments in which the secondary
current or voltage is substantially proportional to the primary current or voltage and differs
in phase from it by an angle which is approximately zero for an appropriate direction of
connection”.

Basic Function of Instrument Transformers:

Direct measurement of current or voltage in high voltage system is not possible
because of high values and insulation problems of measuring instruments they cannot be

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 directly used for protection purposes.

Therefore an instrument transformer serves the purpose and performs the following
function:

 Converts the higher line voltages or line currents into proportionally reduced values
by means of electromagnetic circuit and thus these values can be measured easily.

 Protects measuring instruments and distribution systems by sensing the

abnormalities and signals to the protective relay to isolate the faulty system.

Types of Instrument Transformers:

Instrument transformers are of two types:

 Current Transformers

 Voltage Transformers

Current transformers:

Current transformer is a current measuring device used to measure the

currents in high voltage lines directly by stepping down the currents to measurable values
by means of electromagnetic circuit.

Basic Design Principle of Current Transformers:

The basic principle induced in designing of current transformers is

Primary ampere turns = Secondary ampere turns

Ip  Np = Is  Ns Where, Ip

- Primary current

Np - Primary Winding Turns

Is - Secondary Current; Ns - Secondary Winding Turns

 Ampere turns plays very important role in designing current transformers.

 Current transformers must be connected in series only.

 Current transformer has less no of turns in primary and more no of turns in

secondary.

 The secondary current is directly proportional to primary current.

 The standards applicable to CT's are IEC-60044-1 and IS – 2705.

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 Simple Line Diagram of Current Transformer:
The line diagram of a current transformer contains different components:

Fig: 4.6.1 LINE DIAGRAM OF CT.

 Primary Winding: It is the winding which is connected in series with the

circuit, the current of which is to be transformed.

These are of two types:

1. Single turn primary winding 2. Multi-turn primary winding

 Magnetic Core: Performance of any current transformer depends on its

accuracy of transformation and characteristics of the core material used.

Design of a current transformer depends on the frequency of excitation.

 Secondary Winding: The winding which supplies the current to the measuring
instruments, meters, relays, etc.

 Burden: The relay, instrument or other device connected to the secondary

winding is termed as 'burden' of a current transformer.

Ex. Burden for Metering is CT – 20 VA, 15 VA.

Tests generally to be conducted on CT:
 Insulation resistance values (IR values): Primary to earth, primary to secondary
core1, primary to secondary core2, core1 to earth, core2 to earth and core1 to core2. Primary
to earth and primary to secondary cores are to be checked with 5KV motor operated
insulation tester (megger) and secondary to earth values are to be checked with 1000V
insulation tester or preferably with 500V insulation tester.
 Ratio test: Primary injection test is to be conducted for this purpose
 TAN-DELTA test: on 132KV CTs and above
 Polarity test at the time of commissioning (at least on the CTs connected to
revenue meters)

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  Excitation (saturation) characteristic check
 Secondary and lead resistance check
 Secondary injection check
 Primary injection check

The accuracy of a CT is directly related to a number of factors including:

* Burden
* Burden class/saturation class
* Rating factor
* Load
* External electromagnetic fields
* Temperature and
* Physical configuration.
* The selected tap, for multi-ratio CTs
Number of secondary cores in the current transformer is based on its usage. CTs
used for 11KV and 33KV feeder will have 2 secondary cores. Core 1 is generally for Over
current and earth fault protection. Core 2 is for metering. Usage of core is decided by the
accuracy class of the CT .Core material decides the accuracy class
Core with accuracy class 1.0, 0.5 and latest is 2.0 is used for metering.
Allowable errors are +/-1.0% in case of 1.0 accuracy class CTs.

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 5.CIRCUIT BREAKERS
The circuit breakers are used to break the circuit if any fault occurs in any of the
instrument. These circuit breaker breaks for a fault which can damage other instrument in
the station. For any unwanted fault over the station we need to break the line current. This is
only done automatically by the circuit breaker.
These are load switches. It is able to make or break the normal load current as well
as the fault currents. The basic construction of any circuit breaker requires the separation of
contacts in an insulating fluid, which serves two functions. It extinguishes the arc drawn
between contacts when the CB opens and it provides adequate insulation between the
contacts and from each contact to earth. For successful operation of the circuit breaker, two
functions are to be performed.
a) Operating mechanism function, b) Arc quenching function. There
are

 various operating mechanisms:

Spring charge mechanism, Pneumatic mechanism, Hydraulic Mechanism
 Arc quenching medium:
 Bulk oil (called bulk oil circuit breakers-BOCB)
 Minimum oil (called minimum oil circuit breakers-MOCB)
 Natural air (called air circuit breakers-ACB) (415v)
 Forced air (called air blast circuit breaker-ABCB)
 Vacuum (called vacuum circuit breaker-VCB)
 SF6 gas (called Sulphur Hexafluoride-SF6 gas CB)
The present trend is up to 33KV, VCBs are preferred and beyond 33KV, SF6 gas circuit
breakers are preferred.
 VCB is to be meggered periodically to know the healthiness of the vacuum
interrupter and the insulating pull rod. Vacuum integrity test is the correct test to know the
healthiness of the vacuum interrupter.
 SF6 gas pressure is to be noted in log sheets at least twice in a day. If it is reaching to
SF6 gas pressure low alarm stage, it is to be brought to the notice of the maintenance
personnel.SF6 gas circuit breaker goes to lockout conditions after falling to lockout pressure
close and trip circuits will be blocked and circuit breaker operation can’t be performed N<0
contacts of 63GLX were used in close and trip circuits of the CB and 63GLX contactor is in

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 picked up conditions when the gas pressure is sufficient. Some of the SF6 gas circuit
breaker automatically trips while going to lockout stage N<C contacts of 63GLX contactor
were used in close and trip circuits and 63GLX is in drop off condition when the gas
pressure is sufficient.
Oil condition in the air compressor is to be checked periodically. And it is to be
replaced based on condition of oil.
There are mainly two types of circuit breakers used for any substations. They are
(a) SF6 circuit breakers;
(b) Vacuum circuit breakers.
SF6 circuit breakers:

Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc
extinguishing properties. The dielectric strength of the gas increases with pressure and is
more than the dielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in
electrical equipment like high voltage metal enclosed cables; high voltage metal clad
switchgear, capacitors, circuit breakers, current transformers, bushings, etc. The gas is
liquefied at certain low temperature, liquidification temperature increases with the pressure.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the
fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4
kg of sulphur. The steel box is made gas tight.

The use of SF6 circuit breaker is mainly in the substations which are having high
input KV, say above 220KV and more. The gas is put inside the circuit breaker by force i.e.
under high pressure. When if the gas gets decreases there is a motor connected to the circuit
breaker. The motor starts operating if the gas went lower than
20.8 bar. There is a meter connected to the breaker so that it can be manually seen if the gas
goes low. The circuit breaker uses the SF6 gas to reduce the torque produce in it due to any
fault in the line. The circuit breaker has a direct link with the instruments in the station,
when any fault occur alarm bell rings.

Some of the properties of SF6 are,

 Very high dielectric strength

 High thermal and chemical inertia

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  Superior arc extinguishing capability
 Low decomposition by arcing

Vacuum circuit breakers:

Vacuum type of circuit breakers is used for small KV rated stations below 33KV.
They are only used in low distribution side.

Control Circuit of Circuit Breakers:-

 In closing circuit of the Circuit Breaker there are no. of series inter locks we can say that it

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 is an AND Gate and tripping circuit there are no.of parallel paths it is an OR Gate.
 For ‘closing’ the Circuit Breaker following conditions are to be met.
a) Local/Remote selector shall be in ‘Remote’ for closing the CB from
remote and it shall be in ‘Local’ for closing the CB from Local.
b) Spring is in charged condition in spring type CBs, Air pressure shall be
sufficient in kinematic CBs and Hydraulic Pressure is sufficient in Aero shell
fluid CBs.
c) SF6 Gas pressure is sufficient.
d) Master Trip is resettled.
 For tripping the circuit breaker,
a) Local/Remote selector Switch shall be in ‘Remote’ for tripping the CB from
Remote and it shall be in ‘Local’ for tripping the CB from Local.
b) SF6 Gas pressure is sufficient.
c) Air Pressure is sufficient/Hydraulic Pressure is sufficient.
d) Protection trip bypasses the local/Remote selector switch.
 Trip circuit healthiness is to be ensured immediately after closing the circuit breaker. It is to
be ensured at regular intervals at least once shift, as there is no trip circuit supervision relay
and annunciation relay for 33KV feeders and in case of old panels of 132KV feeders If any
deviation is found it is to be brought to the notice of maintenance personnel.

BUS:

The bus is a line in which the incoming feeders come into and get into the
instruments for further step up or step down. The first bus is used for putting the incoming
feeders in la single line. There may be double line in the bus so that if any fault occurs in the
one the other can still have the current and the supply will not stop. The two lines in the bus
are separated by a little distance by a conductor having a connector between them. This is so
that one can work at a time and the other works only if the first is having any fault.

TRANSFORMERS:
Transformers come in a range of sizes from a thumbnail-sized coupling transformer
hidden inside a stage microphone to huge units weighing hundreds of tons used to
interconnect portions of national power grids. All operate with the same basic principles,
although the range of designsis wide. While new technologies have eliminated the need for
transformers in some electronic circuits, transformers are still found in nearly all electronic

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 devices designed for household ("mains") voltage. Transformers are essential for high
voltage power transmission, which makes long distance transmission economically
practical.

Fig: ELECTRICAL TRANSFORMER.

Basic Principle:
The transformer is based on two principles: firstly, that an electric current can
produce a magnetic field (electromagnetism) and secondly that a changing magnetic field
within a coil of wire induces a voltage across the ends of the coil (electromagnetic
induction).
Changing the current in the primary coil changes the magnetic flux that is developed.
The changing magnetic flux induces a voltage in the secondary coil.

Fig: IDEAL TRANSFORMER.

An ideal transformer is shown in the adjacent figure; Current passing through the

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 primary coil creates a magnetic field. The primary and secondary coils are wrapped around
a core of very high magnetic permeability, such as iron, so that most of the magnetic flux
passes through both primary and secondary coils.

Induction law:
The voltage induced across the secondary coil may be calculated from Faraday's law
of induction, which states that, where VS is the instantaneous voltage, NS is the number of
turns in the secondary coil and Φ equals the magnetic flux through one turn of the coil.

If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is
the product of the magnetic field strength and the area A through which it cuts. The area is
constant, being equal to the cross-sectional area of the transformer core, whereas the
magnetic field varies with time according to the excitation of the primary.

Fig: 4.9.1.3 MUTUAL INDUCTION.

Since the same magnetic flux passes through both the primary and secondary coils in
an ideal transformer, the instantaneous voltage across the primary winding equals Taking
the ratio of the two equations for VS and VP gives the basic equation for stepping up or
stepping down the voltage Ideal power equation The ideal transformer as a circuit element.
If the secondary coil is attached to a load that allows current to flow, electrical power
is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is
perfectly efficient; all the incoming energy is transformed from the primary circuit to the
magnetic field and into the secondary circuit. If this condition is met, the incoming electric
power must equal the outgoing power.
Giving the ideal transformer equation Transformers are efficient so this formula is a
reasonable approximation. If the voltage is increased, then the current is decreased by the
same factor. If an impedance ZS is attached across the terminals of the secondary coil, it
appears to the primary circuit to have an impedance of ZS = (VS/IS).

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LIET-EEE 20KD1A0224 Page 26
 6.TYPES OF CONTROL
VAR control is the natural means to control capacitors because the latter adds a
fixed amount of leading VARs to the line regard less of other conditions, and loss reduction
depends only on reactive current. Since reactive current at any point along a feeder is
affected by downstream capacitor banks, this kind of control is susceptible to interaction
with downstream banks. Consequently, in multiple capacitor feeders, the furthest
downstream banks should go on-line first and off-line last. VAR controls require current
sensors.

Current control is not as efficient as VAR control because it responds to total line
current, and assumptions must be made about the load power factor. Current controls
require current sensors. Voltage control is used to regulate voltage profiles; however it may
actually increase losses and cause instability from highly leading currents. Voltage control
requires no current sensors.

Fig: 5.1 TYPES OF CONTROL

CAPACITORS:

a) Before commissioning a capacitor bank, capacitance of each capacitor shall be

measured with a capacitance meter. These shall be compared with the value obtained
by calculation using the formula,
C = KVAR x 109 Micro Farads

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 2 ∏f (V) 2
Where V is the rated voltage of capacitor and KVAR is the rated KVAR of capacitor. As per
IS the tolerance in the capacitance value for a capacitor unit is + 10% to – 5%.
b) In the event of failure of one capacitor unit (say in R-phase) it is observed that
balancing is done by removing one capacitor each from Y and B-phases.
c) It is therefore necessary that number of capacitor units connected in parallel in each
series group in all the three phases on one star bank shall be same.

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 7. PROTECTION FOR VARIOUS EQUIPMENTS
TRANSFORMER PROTECTION:
a) Station Transformer: HG Fuse protection on HV side and fuse protection on LV side and
Vent pipe.
b) Power transformers up to 7.5MVA:
HV side: O/L & Directional E/L protection with highest element in O/L relays.
LV side: O/L & E/L protection Buchholz Relay OLTC Buchholz Relay OTI and WTI.
c) Power transformers from 8.0MVA and above: HV side O/L & Directional E/L protection
with high set element in O/L relays. LV side O/L & E/L protection: differential protection
Buchholz Relay OLTC Buchholz Relay OTI, WTI and PRV.
d) Power transformers from 31.5MVA and above: Over flux protection & LV WTI in addition
to protection.
e) 220/132KV power transformers: Over flux protection on both HV & LV sides LBB
protection on HV side OLTC Buchholz phase wise in addition to protection.
6.1 FEEDER PROTECTION:
a) 33KV feeders: Non directional O/L & E/L protection with highest and IDMT
characteristics.
b) 132KV feeders: Main protection: Distance protection.
Backup protection: Directional O/L & E/L protection.
c) 220KV feeders: Main-1 protection: Distance protection
Main-2protection: Distance protection, LBB protection, pole discrepancy Relay.

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 CONCLUSION

Transmission and distribution stations exist at various scales throughout a power

system. In general, they represent an interface between different levels or sections of the
power system, with the capability to switch or reconfigure the connections among various
transmission and distribution lines.
The major stations include a control room from which operations are coordinated.
Smaller distribution substations follow the same principle of receiving power at higher
voltage on one side and sending out a number of distribution feeders at lower voltage on the
other, but they serve a more limited local area and are generally unstaffed. The central
component of the substation is the transformer, as it provides the effective in enface between
the high- and low-voltage parts of the system. Other crucial components are circuit breakers
and switches. Breakers serve as protective devices that open automatically in the event of a
fault, that is, when a protective relay indicates excessive current due to some abnormal
condition. Switches are control devices that can be opened or closed deliberately to establish
or break a connection. An important difference between circuit breakers and switches is that
breakers are designed to interrupt abnormally high currents (as they occur only in those very
situations for which circuit protection is needed), whereas regular switches are designed to
be operable under normal currents. Breakers are placed on both the high- and low-voltage
side of transformers. Finally, substations may also include capacitor banks to provide
voltage support.

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 REFERENCES

[1] Principles of Power Systems by V.K. Mehtha

[2] Electrical Power Systems by C.L. Wadhwa
[3] Power System Engineering by ML. Soni

[4] www.littelfuse.com/.../Littelfuse-Protection-Relay-Transformer- Protection

[5]www.osha.gov/SLTC/etools/electric_power/.../substation.html.

[6]http://www.scribd.com/doc/13595703/Substation-Construction-and-
Commissioning.
[7]http://www.authorstream.com/Presentation/marufdilse-881803-electrieal-
power-trasmission/
[8]http://skindustrialcorp.tradeindia.com/Exporters_Suppliers/Exporter17825.
277078/66-KV-Disc-Insulator-Ball-Socket-Type.html.
[9]http://en.wikipedia.org/wiki/Electrical_substation.

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