vocational training wbsetcl

7/30/2019 vocational training wbsetcl

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WEST BENGAL STATE ELECTRICITY TRANSMISSION COMPANY

LIMITED (WBSETCL)

2012

VOCATIONAL

TRAINING REPORT

K A L Y A N I T R A N S M I S S I O N ( O & M ) S U B - D I V I S I O N , 1 3 2 K V S U B - S T A T I O N


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WEST BENGAL STATE ELECTRICITY TRANSMISSION COMPANY LIMITED

KALYANI 132/33/11 KV SUB-STATION

KALYANI TR. (O & M) SUB-DIVISION

KALYANI, NADIA

A

REPORT ON KALYANI 132KV SUB-STATION & ITS INSTRUMENTS

FOR

MR. SIBASISH GHOSH

ASSISTANT ENGINEER

KALYANI TRANSMISSION (O & M) SUB-DIVISION

WBSETCL

BY

STUDENT OF

ELECTRICAL ENGINEERING

JIS COLLEGE OF ENGINEERING


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SL. NO. TOPICS PAGE NO.

1 PREFACE 42 ACKNOWLEDGEMENT 5

3 INTRODUCTION 6

4 ELECTRIC SUBSTATION 7-8

5 TRANSMISSION TOWER 9-11

6 ELECTRICAL BUS SYSTEM 12-14

7 CONDUCTORS 15

8 INSULATORS 16-22

9 CAPACITOR BANK 23

10 BUS COUPLER 24

11 CIRCUIT BREAKERS 25-31

12 ARC IN CIRCUIT BREAKER 32-3313 LIGHTNING ARRESTORS 34

14 TRANSFORMER 35-40

15 INTRODUCTION OF INSULATING OIL 41-44

16 BUCHHOLZ RELAY 45-46

17 EARTHING TRANSFORMER 47-48

18 CONTROL ROOM 49

19 PLCC 50

20 ELECTRICAL SWITCHGEAR 51

21 ELECTRICAL PROTECTION RELAY 52

22 BATTERY 53

23 CONCLUSION 54

24 BIBLIOGRAPHY 55


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PREFACEI have experienced vocational training in W.B.S.E.T.C.L. Kalyani

sub-station from 25th

June to 7th

July, 2012.

I am very grateful to all of the officers who gave warm

reception & the valuable time for me. I have learnt many more

things while doing training in sub-station which has helped me

to enlarge my practical knowledge. By undergoing this training

program I am able to relate my bookish knowledge with its

practical application.


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ACKNOWLEDGEMENT

Before going into the report, I would like to thank to the H.R.D. department of the

W.B.S.E.T.C.L., Bidhyut Bhawan, Saltlake & Kalyani sub-station for providing me the

opportunity to do the vocational training at their sub-station. I am highly thankful to

Sri. Sibasish Ghosh (A.E.), Sri. Sadhan Ghosh (LNSS), Sri. Ashit Ghosh (A.E.) for their

kind attention. I am also thankful to the other officers for sharing their valuable

experiences & time with me during this training. In this training, I also got the

opportunity to understand the status of export-import of in W.B.S.E.T.CL. & the overall

view of the grid system.

For this constant inspiration & active supervision from the very beginning of the

training, I gratefully acknowledge their significant contribution to the successful

completion of my training.


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INTRODUCTION

Substation is a part of an electrical generation, transmission, &

distribution system. Substations transform voltage from high to low, or the

reverse, or perform any of several other important functions. Electric power may flow

through several substations between generating plant and consumer, and its voltage

may change in several steps.

Substations may be owned and operated by a transmission or generation electricalutility, or may be owned by a large industrial or commercial customer. Generally

substations are un-attended, relying on SCADA for remote supervision and control.

A substation may include transformers to change voltage levels between high

transmission voltages and lower distribution voltages, or at the interconnection of two

different transmission voltages. The word substation comes from the days before the

distribution system became a grid. As central generation stations became larger,

smaller generating plants were converted to distribution stations, receiving their

energy supply from a larger plant instead of using their own generators. The first

substations were connected to only one power station, where the generators were

housed, and were subsidiaries of that power station.


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ELECTRIC SUB-STATION

An Electric sub-station is an assembly of equipment in an electric power system through

which electrical energy is passed for transmission, distribution, interconnection,

transformation, conversion or switching.

TYPES OF SUB-STATION:

A.Transmission sub-station,

B.Distribution sub-station,

C.Collector sub-station,

A.Transmission sub-station: A transmission substation connects two or moretransmission lines. The simplest case is where all transmission lines have the same

voltage. In such cases, the substation

contains high-voltage switches that allow

lines to be connected or isolated for faultclearance or maintenance. A transmission

station may have transformers to convert

between two transmission voltages,

voltage control/power factor

correction devices such as capacitors,

reactors or static VAr compensators and

equipment such as phase shifting transformers to control power flow between two

adjacent power systems.B.Distribution sub-station: A distribution substation transfers power from the

transmission system to the distribution

system of an area. It is uneconomical to

directly connect electricity consumers to

the main transmission network, unless

they use large amounts of power, so the

distribution station reduces voltage to avalue suitable for local distribution. The

input for a distribution substation is

typically at least two transmission or sub
http://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Voltage_compensationhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Static_VAr_compensatorhttp://en.wikipedia.org/wiki/Static_VAr_compensatorhttp://en.wikipedia.org/wiki/Static_VAr_compensatorhttp://en.wikipedia.org/wiki/Quadrature_boosterhttp://en.wikipedia.org/wiki/Quadrature_boosterhttp://en.wikipedia.org/wiki/Static_VAr_compensatorhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Voltage_compensationhttp://en.wikipedia.org/wiki/Transformer


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transmission lines. Input voltage may be, for example, 115 kV, or whatever is

common in the area. The output is a number of feeders. Distribution voltages are

typically medium voltage, between 2.4 kV and 33 kV depending on the size of the

area served and the practices of the local utility.

C.Collector sub-station: In distributed generation projects such as a wind farm,a collector substation may be required. It resembles a distribution substation

although power flow is in the opposite

direction, from many wind turbines up

into the transmission grid. Usually for

economy of construction the collector

system operates around 35 kV, and the

collector substation steps up voltage to atransmission voltage for the grid. The

collector substation can also

provide power factor correction if it is

needed, metering and control of the wind farm. In some special cases a collector

substation can also contain an HVDC static inverter plant.Collector substations also

exist where multiple thermal or hydroelectric power plants of comparable output

power are in proximity. Examples for such substations are Brauweiler in Germany

and Hradec in the Czech Republic, where power is collected from nearby lignite-

fired power plants. If no transformers are required for increase of voltage to

transmission level, the substation is a switching station.
http://en.wikipedia.org/wiki/Distributed_generationhttp://en.wikipedia.org/wiki/Wind_farmhttp://en.wikipedia.org/wiki/Wind_turbinehttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Power_factor_correctionhttp://en.wikipedia.org/wiki/Wind_turbinehttp://en.wikipedia.org/wiki/Wind_farmhttp://en.wikipedia.org/wiki/Distributed_generation


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TRANSMISSION TOWER

The main supporting unit of overhead transmission line is transmission tower. Transmission

towers have to carry the heavy transmission conductor

at a sufficient safe height from ground. In addition to

that all towers have to sustain all kinds of natural

calamities. So transmission tower designing is an

important engineering job where all three basic

engineering concepts, civil, mechanical and electrical

engineering concepts are equally applicable. Main parts

of a transmission tower.

A power transmission tower consists of the following

parts,

1) Peak of transmission tower

2) Cross Arm of transmission tower

3) Boom of transmission tower

4) Cage of transmission tower

5) Transmission Tower Body

6) Leg of transmission tower

7) Stub/Anchor Bolt and Base plate assembly of transmission tower.

THE MAIN PARTS AMONG THESE ARE SHOWN IN THE PICTURES:


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Peak of transmission tower:

The portion above the top cross arm is called peak of transmission tower. Generally earth

shield wire connected to the tip of this peak.

Cross Arm of transmission tower:

Cross arms of transmission tower hold the transmission conductor. The dimension of cross

arm depends on the level of transmission voltage, configuration and minimum forming

angle for stress distribution.

Cage of transmission tower:

The portion between tower body and peak is known as cage of transmission tower. This

portion of the tower holds the cross arms.

Transmission Tower Body:

The portion from bottom cross arms up to the ground level is called transmission tower

body. This portion of the tower plays a vital role for maintaining required ground

clearance of the bottom conductor of the transmission line.

DESIGN OF TRANSMISSION TOWER:

To determine the actual transmission tower height by considering the above points, we have

divided the total height of tower in four parts -

i. Minimum permissible ground clearance(H1),

ii. Maximum sag of the conductors (H2),

iii. Vertical spacing between top & bottom

conductors (H3),

iv. Vertical clearance between ground wire and top

conductors (H4),


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TYPES OF TRANSMISSION TOWER:

According to different considerations, there are different types of transmission towers.

The transmission line goes as per available corridors. Due to unavailability of shortest

distance straight corridor transmission line has to deviate from its straight way whenobstruction comes. In total length of a long transmission line there may be several

deviation points. According to the angle of deviation there are four types of

transmission tower

A type tower angle of deviation 0o to 2o.

B type tower angle of deviation 2o to 15o.

C type tower angle of deviation 15o to 30o.

D type tower angle of deviation 30

o

to 60

o

.

As per the force applied by the conductor on the cross arms, the transmission towers

can be categorized in another way.

Tangent Suspension tower and it is generally A - type tower.

Angle tower or tension tower or sometime it is called section tower. All B, C and

D types of transmission towers come under this category.

Apart from the above customized type of tower, the tower is designed to meet special

usages listed below,

These are called special type tower

River Crossing Tower

Railway/ Highway Crossing tower

Transposition tower

Based on numbers of circuits carried by a transmission tower, transportation can be

classified as

Single Circuit tower

Double Circuit tower

Multi Circuit tower.


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ELECTRICAL BUS SYSTEM & SUBSTATION LAYOUT

There are many different electrical bus system schemes available but selection of a

particular scheme depends upon the system voltage, position of substation in electrical

power system, flexibility needed in system and cost to be expensed.

The main criterias to be considered during selection of one particular Bus Bar

Arrangement Scheme among others

Simplicity of system.

Easy maintenance of different equipments.

Minimizing the outage during maintenance.

Future provision of extension with growth of demand.

Optimizing the selection of bus bar arrangement scheme so that it gives maximum

return from the system.

Some very commonly used bus bar arrangement are discussed below

SINGLE BUS SYSTEM:

Single Bus System is simplest and cheapest one. In this scheme all the feeders and

transformer bay are connected to only one single bus as shown.

Advantages of single bus system:

This is very simple in design

This is very cost effective scheme

This is very convenient to operate

Disadvantages of single bus system:

One but major difficulty of these type of

arrangement is that, maintenance of

equipment of any bay cannot be

possible without interrupting the feeder or transformer connected to that bay. The

indoor 11KV switchboards have quite often single bus bar arrangement.


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Single Bus System with Bus Sectionalizer:

Some advantages are realized if a single

bus bar is sectionalized with circuit

breaker. If there are more than one

incoming and the incoming sources and

outgoing feeders are evenly distributed

on the sections as shown in the figure,

interruption of system can be reduced to

a good extent.

DOUBLE BUS SYSTEM:

In double bus bar system two identical bus bars are used in such a way that any

outgoing or incoming feeder can be taken from any of the bus. Actually every feeder is

connected to both of the buses in parallel through individual isolator as shown in the

figure. By closing any of the isolators one

can put the feeder to associated bus.

Both of the buses are energized and

total feeders are divided into two

groups, one group is fed from one bus

and other from other bus. But any

feeder at any time can be transferred

from one bus to other. There is one bus

coupler breaker which should be kept

close during bus transfer operation. For transfer operation, one should first close the

bus coupler circuit breaker then close the isolator associated with the bus to where

the feeder would be transferred and then open the isolator associated with the bus

from where feeder is transferred. Lastly after this transfer operation he or she should

open the bus coupler breaker.

Advantages of Double Bus System:

Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System:

The arrangement does not permit breaker maintenance without interruption.


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ONE AND A HALF BREAKER BUS SYSTEM:

This is an improvement on the double breaker scheme to effect saving in the number

of circuit breakers. For every two circuits only one spare breaker is provided. The

protection is however complicated

since it must associate the central

breaker with the feeder whose own

breaker is taken out for

maintenance. For the reasons given

under double breaker scheme and

because of the prohibitory costs of

equipment even this scheme is not

much popular. As shown in the figure

that it is a simple design, two feeders

are fed from two different buses through their associated breakers and these two

feeders are coupled by a third breaker which is called tie breaker. Normally all the

three breakers are closed and power is fed to both the circuits from two buses which

are operated in parallel. The tie breaker acts as coupler for the two feeder circuits.

MAIN AND TRANSFER BUS SYSTEM:This is an alternative of double bus system. The main conception of Main and Transfer

Bus System is, here every feeder line is directly connected through an isolator to a

second bus called transfer bus. The

said isolator in between transfer bus

and feeder line is generally called

bypass isolator. The main bus is as

usual connected to each feeder

through a bay consists of circuit

breaker and associated isolators at

both side of the breaker. There is

one bus coupler bay which couples

transfer bus and main bus through a

circuit breaker and associated isolators at both sides of the breaker. If necessary the

transfer bus can be energized by main bus power by closing the transfer bus coupler

isolators and then breaker. Then the power in transfer bus can directly be fed to thefeeder line by closing the bypass isolator.


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CONDUCTORS

TYPES OF CONDUCTORS:

For 400 kV transmission line MOOSE wire is used.

For 132 kV transmission line PANTHER wire is used. The diameter is 3mm.

For 220 kV transmission line DEER wire is used. The diameter is 3.45mm.

For 220kV transmission line ZEBRA wire is used. The diameter is 3.15mm.

For 66 kV transmission line DOG wire is used.

TABLE OF CONDUCTORS:

NAMEVOLTAGE

LEVEL

WIRE IN

CONDUCTORWEIGHT

DIAMETER

OF SINGLE

WIRE

Moose 400kV 7/54 2.00kg/m N/A

Deer 220kV 7/30 1.977kg/m 3.54mm

Zebra 220kV 7/54 1.6kg/m 3.15mm

Panther 132kV 7/30 0.976kg/m N/A

Dog 66kV N/A N/A N/A


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ELECTRICAL INSULATOR

Electrical Insulator must be used in electrical system to prevent unwanted flow of

electric current to the earth from its supporting points. Example: Porcelain insulator,

glass insulator, polymer insulator.

Properties of insulating material:

a. It must be mechanically strong enough to carry tension and weight of conductors.

b. It must have very high dielectric strength to withstand the voltage stresses in

High Voltage system.

c. It must possess high Insulation Resistance to prevent leakage current to the

earth.

d. The insulating material must be free from unwanted impurities.

PORCELAIN INSULATOR:

Porcelain in most commonly used material for over head insulator in present days.

The porcelain is aluminium silicate. The aluminium silicate is mixed with plastic

kaolin, feldspar and quartz to obtain final hard and glazed porcelain

insulator material. The surface of the

insulator should be glazed enough so that

water should not be traced on it. Porcelain

also should be free from porosity since

porosity is the main cause of deterioration

of its dielectric property. It must also be

free from any impurity and air bubble

inside the material which may affect the

insulator properties.

PROPERTY VALUE

Dielectric strength 60kV/cm

Compressive strength 70,000kg/cm2

Tensile strength 500kg/cm2


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GLASS INSULATOR:

Nowadays glass insulator has become popular in transmission and distribution

system. Annealed tough glass is used for insulating purpose. Glass insulator has

numbers of advantages over conventional porcelain insulator.

Advantages of glass insulator:

It has very high dielectric strength compared to porcelain.

Its resistivity is also very high.

It has low coefficient of thermal expansion.

It has higher tensile strength compared to porcelain insulator.

As it is transparent in nature it is not heated up in sunlight as porcelain.

The impurities and air bubble can be easily detected inside the glass insulator body

because of its transparency.

PROPERTY VALUE

Dielectric strength 140kV/cm

Compressive strength 10,000kg/cm2

Tensile strength 35,000kg/cm

2


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POLYMER INSULATOR:

In a polymer insulator has two parts, one is glass fiber reinforced epoxy resin rod

shaped core and other is silicone rubber or EPDM (Ethylene Propylene Diene

Monomer) made weather sheds. Rod shaped core is covered by weather sheds.Weather sheds protect the insulator core from outside environment. As it is made of

two parts, core and weather sheds, polymer insulator is also calledcomposite

insulator. The rod shaped core is fixed with Hop dip galvanized cast steel made end

fittings in both sides.

Advantages of polymer insulator:

It is very light weight compared to porcelain

and glass insulator.

As the composite insulator is flexible the

chance of breakage becomes minimum.

Because of lighter in weight and smaller in

size, this insulator has lower installation cost.

It has higher tensile strength compared to

porcelain insulator.

Disadvantages of polymer insulator:

Moisture may enter in the core if there is any

unwanted gap between core and weather sheds.

This may cause electrical failure of the insulator.

Over crimping in end fittings may result to

cracks in the core which leads to mechanical failure

of polymer insulator.

Types of Insulator:

There are mainly three types of insulator used as overhead insulator likewise

Pin Insulator

Suspension Insulator Stray Insulator


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PIN INSULATOR:

Pin Insulator is earliest developed overhead insulator, but still popularly used in

power network up to 33KV system.Pin type insulator can be one part,

two parts or three parts type,

depending upon application voltage.

In 11KV system we generally use

one part type insulator where whole

pin insulator is one piece of properly

shaped porcelain or glass. As the

leakage path of insulator is through

its surface, it is desirable to increase

the vertical length of the insulator

surface area for lengthening leakage

path. In order to obtain lengthy

leakage path, one, two or more rain

sheds or petticoats are provided on

the insulator body. In addition to that rain shed or petticoats on an insulator serve

another purpose. These rain sheds or petticoats are so designed, that during raining

the outer surface of the rain shed becomes wet but the inner surface remains dry

and non-conductive. So there will be discontinuations of conducting path through

the wet pin insulator surface.

Designing consideration of Electrical Insulator:

The live conductor attached to the top of the pin insulator is at a potential and

bottom of the insulator fixed to supporting structure of earth potential. Theinsulator has to withstand the potential stresses between conductor and earth.

The shortest distance between conductor and earth, surrounding the insulator

body, along which electrical discharge may take place through air, is known as

flash over distance.

1. When insulator is wet, its outer surface becomes almost conducting. Hence

the flash over distance of insulator is decreased. The design of an electrical

insulator should be such that the decrease of flash over distance is minimum

when the insulator is wet. That is why the upper most petticoat of a pininsulator has umbrella type designed so that it can protect, the rest lower part

of the insulator from rain. The upper surface of top most petticoat is inclined as

less as possible to maintain maximum flash over voltage during raining.


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2. To keep the inner side of the insulator dry, the rain sheds are made in order

that these rain sheds should not disturb the voltage distribution they are so

designed that their subsurface at right angle to the electromagnetic lines of

force.

POST INSULATOR:

Post insulator is more or less similar to Pin insulator but former is suitable for higher

voltage application. Post insulator has higher numbers of petticoats and has greater

height. This type of insulator can be mounted on supporting structure horizontally as

well as vertically. The insulator is made of one piece of porcelain but has fixing clamp

arrangement are in both top and bottom end.

The main differences between pin insulator and post insulator are:

SUSPENSION INSULATOR:

In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator

because size, weight of the insulator become more. Handling and replacing bigger

size single unit insulator are quite difficult task. For overcoming these

difficulties, suspension insulator was developed.

In suspension insulator numbers of insulators are connected in series to form a

string and the line conductor is carried by the bottom most insulator. Each

insulator of a suspension string is called disc insulator because of their disc like

shape

SL. NO. PIN INSULATOR POST INSULATOR

1It is generally used up to 33KV

system.

It is suitable for lower voltage and also

for higher voltage.

2It is single stag. It can be single stag as well as multiple

stags.

3

Two insulators cannot be fixed

together for higher voltage

application.

Two or more insulators can be fixed

together one above other for higher

voltage application.

4

Metallic fixing arrangement

provided only on bottom end of

the insulator.

Metallic fixing arrangement provided

on both top and bottom ends of the

insulator.


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Advantages of Suspension Insulator:

1. Each suspension disc is designed for normal voltage rating 11KV(Higher voltage

rating 15KV), so by using different numbers of discs, a suspension string can be

made suitable for any voltage level.

2. If any one of the disc insulators in a suspension string is damaged, it can be

replaced much easily.

3. Mechanical stresses on the suspension insulator is less since the line hanged on

a flexible suspension string.

4. As the current carrying conductors are suspended from supporting structure by

suspension string, the height of the conductor position is always less than the total

height of the supporting structure. Therefore, the conductors may be safe from

lightening.

Disadvantages of Suspension Insulator:

Suspension insulator string costlier than pin and post type insulator.

Suspension string requires more height of supporting structure than that for pin

or post insulator to maintain same ground clearance of current conductor.

The amplitude of free swing of conductors is larger in suspension insulator

system, hence, more spacing between conductors should be provided.


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STRAIN INSULATOR:

When suspension string is used to sustain

extraordinary tensile load of conductor it is

referred as string insulator. When there is a deadend or there is a sharp corner in transmission line,

the line has to sustain a great tensile load of

conductor or strain. A strain insulator must have

considerable mechanical strength as well as the

necessary electrical insulating properties.

There are other two types of insulator for low voltage application. Those are i.Stay

Insulator ii.Shackle Insulator.

STAY INSULATOR:

For low voltage lines, the stays

are to be insulated from ground

at a height. The insulator used

in the stay wire is called as

the stay insulator and is usually

of porcelain and is so designed

that in case of breakage of the

insulator the guy-wire will not fall to the ground.

SHACKLE INSULATOR:

Theshackle insulatororspool insulatoris usually used

inlow voltage distribution network. It can be used

both in horizontal and vertical position.

Rated

SystemVoltage

Number of disc insulator used in strain

type tension insulator string

Number of disc insulator

used in suspensioninsulator string

33KV 3 3

66KV 5 4

132KV 9 8

220KV 15 14


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CAPACITOR BANK

Acapacitorbank is a grouping of several identical capacitors interconnected in parallel or in

series with one another. These groups of capacitors are typically used to correct or

counteract undesirable characteristics, such aspower factorlag or phase shifts inherent in

alternating current (AC) electricalpower supplies. Capacitor banks may also be used in

direct current (DC) power supplies to increase stored energy and improve the ripple current

capacity of thepower supply.

Single capacitors are electrical or electronic components which storeelectrical energy.

Capacitors consist of two conductors that are separated by an insulating material or

dielectric. When an electrical current is passed

through the conductor pair, a staticelectric

fielddevelops in the dielectric which represents

the stored energy. Unlike batteries, this stored

energy is not maintained indefinitely, as the

dielectric allows for a certain amount of current

leakage which results in the gradual dissipation

of the stored energy.

The energy storing characteristic of capacitors is

known ascapacitanceand is expressed or

measured by the unit farads. These

characteristics also allow capacitors to be used

in a group or capacitor bank to absorb and correct AC power supply faults.

The use of a capacitor bank to correct AC power supply anomalies is typically found in heavy

industrial environments that feature working loads made up of electric motors and

transformers. This type of working load is problematic from a power supply perspective aselectric motors and transformers represent inductive loads, which cause a phenomenon

known as phase shift or power factor lag in the power supply.

The use of a capacitor bank in the power supply system effectively cancels out or

counteracts these phase shift issues, making the power supply far more efficient and cost

effective. The installation of a capacitor bank is also one of the cheapest methods of

correcting power lag problems and maintaining a power factor capacitor bank is simple and

cost effective. One thing that should always be kept in mind when working withany capacitor or capacitor bank is the fact that the stored energy, if incorrectly discharged,

can cause serious burns or electric shocks.
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BUS COUPLER

Bus coupler is a device which is used to switch from one bus to the other without anyinterruption in power supply and without creating hazardous arcs. It is achieved with the

help of circuit breaker and isolators.

Bus coupler configurations are available as non-terminated or internally terminated. If two

or more non-terminated couplers are used on a bus, then the couplers at each end of the

bus must be terminated externally with 78 ohm terminators on the unused bus connections

of the end couplers. Alternately, internally single terminated couplers (with or without the

non-functional bus connectors) can be supplied.

Even if only one non-terminated coupler acts as the bus because all devices (bus controller,

remote terminals, etc.) are connected to the couplers stubs, the external bus connections

of the coupler must be terminated. A dual-terminated coupler (with or without non-

functional bus connectors) can be employed where the coupler acts as the bus without

other couplers.

COMPONENTS OF BUS COUPLER:

Main bus isolator

Current transformer

Circuit breaker

Line isolator

Supporting insulator

Potential transformer


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CIRCUIT BREAKERS

Electrical Circuit Breaker is a switching device which can be operated manually as well as

automatically for controlling and protection of electrical power system respectively.

TYPES OF CIRCUIT BREAKER:

According different criteria there are different types of circuit breaker.According to

their arc quenching media the circuit breaker can be divided as

SF6 CIRCUIT BREAKER

VACCUM CIRCUIT BREAKER

OIL CIRCUIT BREAKER

GAS CIRCUIT BREAKER

SF6 CIRCUIT BREAKER:

A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or

SF6 gas is known as an SF6 Circuit Breaker.

SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has

high affinity of absorbing free electron. Whenever a free electron collides with the SF6

gas molecule, it is absorbed by that gas molecule and forms a negative ion.

The attachment of electron with SF6 gas molecules may occur in tow different ways,

1) SF6 +e=SF6

2) SF6 + e = SF5-+ F

These negative ions obviously much heavier than a free electron and therefore over all

mobility of the charged particle in the SF6 gas is much less as compared other common

gases. We know that mobility of charged particle is majorly responsible for conducting

current through a gas.

WORKING OF SF6 CIRCUIT BREAKER:

The working of SF6 circuit Breaker of first generation was quite simple it is some extent

similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a highpressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is

released through the arc and collected to relatively low pressure reservoir and then it

pumped back to the high pressure reservoir for reutilize.


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The working of SF6 circuit breaker is little bit different in modern time. Innovation of

puffer type design makes operation of SF6 circuit breaker much easier. In buffer type

design, the arc energy is utilized to develop pressure in the arcing chamber for arc

quenching.

Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact

fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The

cylinder can axially slide upward and downward along the contacts. There is one

stationary piston inside the cylinder which is fixed with other stationary parts of the

breaker, in such a way that it can not change its position during the movement of the

cylinder. As the piston is fixed and cylinder is movable or sliding, the internal volume of

the cylinder changes when the cylinder slides.

During opening of the breaker the cylinder moves downwards against position of the

fixed piston hence the volume inside the cylinder is reduced which produces

compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which

were blocked by upper fixed contact body during closed position. As the cylinder move

further downwards, these vent openings cross the upper fixed contact, and become

unblocked and then compressed SF6 gas inside the cylinder will come out through this

vents in high speed towards the arc and passes through the axial hole of the both fixed

contacts. The arc is quenched during this flow of SF6 gas.

During closing of the breaker, the sliding cylinder moves upwards and as the position of

piston remains at fixed height, the volume of the cylinder increases which introduces

low pressure inside the cylinder compared to the surrounding. Due to this pressure

difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure

gas will come through the axial hole of both fixed contact and enters into cylinder via

vent and during this flow; the gas will quench the arc.


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VACUUM CIRCUIT BREAKER:

A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in

vacuum. The technology is suitable for mainly medium voltage application. For higher voltage

Vacuum technology has been developed but not commercially viable. The operation of

opening and closing of current carrying contacts and associated arc interruption take place in

a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter

consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The

vacuum pressure inside a vacuum interrupter is normally maintained at 10- 6

bar.

The material used for current carrying contacts plays an important role in the performance of

the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. Vacuum

interrupter technology was first introduced in the year of 1960. But still it is a developing

technology. As time goes on, the size of the vacuum interrupter is being reducing from its

early 1960s size due to different technical developments in this field of engineering. The

contact geometry is also improving with time, from butt contact of early days it gradually

changes to spiral shape, cup shape and axial magnetic field contact. The vacuum circuit

breaker is today recognized as most reliable current interruption technology for medium

voltage system. It requires minimum maintenance compared to other circuit breaker

technologies.

Advantages of vacuum circuit breaker:

Service life of Vacuum Circuit Breaker is much longer than other types of circuit breakers.There is no chance of fire hazard as oil circuit breaker. It is much environment friendly

than SF6 Circuit breaker. Beside of that contraction of VCB is much user friendly. Replacement

of Vacuum Interrupter (VI) is much convenient.

Operation of Vacuum Circuit Breaker:

The main aim of any circuit breaker is to quench arc during current zero crossing, by

establishing high dielectric strength in between the contacts so that reestablishment of arc

after current zero becomes impossible. The dielectric strength of vacuum is eight times

greater than that of air and four times greater than that of SF6 gas. This high dielectric

strength makes it possible to quench a vacuum arc within very small contact gap. For short

contact gap, low contact mass and no compression of medium the drive energy required in

vacuum circuit breaker is minimum. When two face to face contact areas are just being

separated to each other, they do not be separated instantly, contact area on the contact face

is being reduced and ultimately comes to a point and then they are finally de-touched.

Although this happens in a fraction of micro second but it is the fact.

At thid instant of de-touching of contacts in a vacuum, the current through the contacts

concentrated on that last contact point on the contact surface and makes a hot spot. As it isvacuum, the metal on the contact surface is easily vaporized due to that hot spot and create

a conducting media for arc path. Then the arc will be initiated and continued until the next

current zero. At current zero this vacuum arc is extinguished and the conducting metal

vapour is re-condensed on the contact surface. At this point, the contacts are already


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separated hence there is no question of re-vaporization of contact surface, for next cycle of

current. That means, the arc cannot be reestablished again. In this way vacuum circuit

breaker prevents the reestablishment of arc by producing high dielectric strength in the

contact gap after current zero.

There are two types of arc shapes. For interrupting current up to 10kA, the arc remains

diffused and the form of vapour discharge and cover the entire contact surface. Above 10kA

the diffused arc is constricted considerably by its own magnetic field and it contracts. The

phenomenon gives rise over heating of contact at its center. In order to prevent this, the

design of the contacts should be such that the arc does not remain stationary but keeps

travelling by its own magnetic field. Specially designed contact shape of vacuum circuit

breaker make the constricted stationary arc travel along the surface of the contacts, thereby

causing minimum and uniform contact erosion.

OIL CIRCUIT BREAKER:

Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and

moving contact are immerged inside the insulating oil. Whenever there is a separation of

current carrying contacts in the oil, the arc is initialized at the moment of separation of

contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas

and ultimately creates a hydrogen bubble around the arc. This highly compressed gas

bubble around the arc prevents re-striking of the arc after current reaches zero crossing of

the cycle. The Oil Circuit Breaker is the one of the oldest type of circuit breakers.

Operation of Oil Circuit Breaker:

The operation of oil circuit breaker is quite simple lets have a discussion. When the current

carrying contacts in the oil are separated an arc is established in between the separated

contacts. Actually, when separation of contacts has just started, distance between the

current contacts is small as a result the voltage gradient between contacts becomes high.

This high voltage gradient between the contacts ionized the oil and consequently initiates

arcing between the contacts. This arc will produce a large amount of heat in surrounding oil

and vaporizes the oil and decomposes the oil in mostly hydrogen and a small amount of

methane, ethylene and acetylene. The hydrogen gas can not remain in molecular form and

its is broken into its atomic form releasing lot of heat. The arc temperature may reach up to

50000K. Due to this high temperature the gas is liberated surround the arc very rapidly and


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forms an excessively fast growing gas bubble around the arc. It is found that the mixture of

gases occupies a volume about one thousand times that of the oil decomposed. From this

figure we can assume how fast the gas bubble around the arc will grow in size. If this

growing gas bubble around the arc is compressed by any means then rate of de ionization

process of ionized gaseous media in between the contacts will accelerate which rapidly

increase the dielectric strength between the contacts and consequently the arc will bequenched at zero crossing of the current cycle. This is the basic operation of oil circuit

breaker. In addition to that cooling effect of hydrogen gas surround the arc path also helps,

the quick arc quenching in oil circuit breaker.

Types of oil circuit breakers:

There are mainly two types of oil circuit breaker.

Bulk Oil Circuit Breaker:

Bulk Oil Circuit Breaker is such types of circuitbreakers where oil is used as arc quenching

media as well as insulating media between

current carrying contacts and earthed parts of

the breaker. The oil used here is same as

transformer insulating oil.

Minimum Oil Circuit Breaker:

These types of circuit breakers utilize oil as theinterrupting media. However, unlike bulk oil

circuit breaker, a minimum oil circuit

breaker places the interrupting unit in insulating

chamber at live potential. The insulating oil is

available only in interrupting chamber. The

features of designing MOCBare to reduce

requirement of oil, and hence these breaker are

called minimum oil circuit breaker.

AIR CIRCUIT BREAKER:

An air circuit breaker is that kind of circuit breaker which operates in air at atmospheric

pressure. After development of oil breaker, the medium voltage air circuit breaker is

replaced completely by oil circuit breaker in different countries. But in countries like France

and Italy, air circuit breakers are still preferable choice up to voltage 15 KV. It is also good

choice to avoid the risk of oil fire, in case of oil circuit breaker. In America air circuit

breakers were exclusively used for the system up to 15 KV until the development of new

vacuum and SF6 circuit breakers.

Working principle of Air Circuit Breaker:

The working principle of air circuit breaker is rather different from those in any other types

of circuit breakers. The main aim of all kind of circuit breaker is to prevent the


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reestablishment of arcing after current zero by creating a situation where in the contact gap

will withstand the system recovery voltage. The air circuit breaker does the same but in

different manner. For interrupting arc it creates an arc voltage in excess of the supply

voltage. Arc voltage is defined as the minimum voltage required maintaining the arc. This

circuit breaker increases the arc voltage by mainly three different ways.

It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma

is decreased, the mobility of the particle in arc plasma is reduced; hence more voltage

gradient is required to maintain the arc.

It may increase the arc voltage by lengthening the arc path. As the length of arc path is

increased, the resistance of the path is increased, and hence to maintain the same arc

current more voltage is required to be applied across the arc path. That means arc voltage

is increased. Splitting up the arc into a number of series arcs also increases the arc voltage.

Types of air circuit breaker:

There are mainly two types of air circuit breaker are available.

1) Plain air circuit breaker

2) Air blast Circuit Breaker

Operation of Air Circuit Breaker:

The first objective is usually achieved by forcing the arc into contact with as large an area as

possible of insulating material. All the air circuit breakers are fitted with a chamber

surrounding the contact. This chamber is called 'arc chute'. The arc is driven into it. If inside

of the arc chute is suitably shaped, and if the arc can be made conform to the shape, the

arc chute wall will help to achieve cooling. This type of arc chute should be made from

some kind of refractory material. High temperature plastics reinforced with glass fiber and

ceramics are preferable materials for making arc chute.

The second objective that is lengthening the arc path, is achieved concurrently with fist

objective. If the inner walls of the arc chute is shaped in such a way that the arc is not onlyforced into close proximity with it but also driven into a serpentine channel projected on

the arc chute wall. The lengthening of the arc path increases the arc resistance.

The third technique is achieved by using metal arc slitter inside the arc chute. The main arc

chute is divided into numbers of small compartments by using metallic separation plates.

These metallic separation plates are actually the arc splitters and each of the small

compartments behaves as individual mini arc chute. In this system the initial arc is split into

a number of series arcs, each of which will have its won mini arc chute. So each of the split

arcs has its won cooling and lengthening effect due to its won mini arc chute and hence

individual split arc voltage becomes high. These collectively, make the over all arc voltage,

much higher than the system voltage.


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This was working principleof air circuit breaker now we will discuss in details the operation

of air circuit breaker in practice.

The air circuit breaker, operated within the voltage level 1KV, does not require any arc

control device. Mainly for heavy fault current on low voltages (low voltage level above 1 KV)

air circuit breakers with appropriate arc control device, are good choice. These breakers

normally have two pairs of contacts. The main pair of contacts carries the current at normal

load and these contacts are made of copper. The additional pair is the arcing contact and is

made of carbon. When circuit breaker is being opened, the main contacts open first and

during opening of main contacts the arcing contacts are still in touch with each other. As

the current gets, a parallel low resistive path

through the arcing contact during opening of

main contacts, there will not be any arcing in

the main contact. The arcing is only initiated

when finally the arcing contacts are separated.

The each of the arc contacts is fitted with an

arc runner which helps, the arc discharge to

move upward due to both thermal and

electromagnetic effects as shown in the figure.

As the arc is driven upward it enters in the arc

chute, consisting of splitters. The arc in chute

will become colder, lengthen and split hence arc voltage becomes much larger than systemvoltage at the time of operation of air circuit breaker, and therefore the arc is quenched

finally during the current zero.


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ARC IN CIRCUIT BREAKER

Before going through details arc quenchingorarc extinction technologies employed in

circuit breaker we should know first what is arc actually.

Definition of arc:

During opening of current carrying contacts in a circuit breaker the medium in between

opening contacts become highly ionized through which the interrupting current gets low

resistive path and continues to flow through this path even the contacts are physically

separated. During the flowing of current from one contact to other the path becomes so

heated that it glows. This is called arc.

Arc in Circuit Breaker:

Whenever, on load current contacts of circuit breaker open there is an arc in circuit

breaker, established between the separating contacts. As long as this arc is sustained in

between the contacts the current through the circuit breaker will not be interrupted finally

as because arc is itself a conductive path of electricity. For total interruption of current the

circuit breaker it is essential to quench the arc as quick as possible. The main designing

criteria of a circuit breaker is to provide appropriate technology of arc quenching in circuit

breaker to fulfill quick and safe current interruption. So before going through different arc

quenching techniques employed in circuit breaker, we should try to understand "what is

arc" and basic theory of arc in circuit breaker, lets discuss.

Thermal Ionization of gas:

There are numbers of free electrons and ions present in a gas at room temperature due to

ultraviolet rays, cosmic rays and radioactivity of the earth. These free electrons and ions are

so few in number that they are insufficient to sustain conduction of electricity. The gas

molecules move randomly at room temperature. It is found an air molecule at a

temperature of 300oK (Room temperature) moves randomly with an approximate average

velocity of 500 meters/second and collides other molecules at a rate of 1010times/second.

These randomly moving molecules collide each other in very frequent manner but the

kinetic energy of the molecules is not sufficient to extract an electron from atoms of the

molecules. If the temperature is increased the air will be heated up and consequently the

velocity on the molecules increased. Higher velocity means higher impact during

intermolecular collision. During this situation some of the molecules are disassociated in to

atoms. If temperature of the air is further increased many atoms are deprived of valence

electrons and make the gas ionized. Then this ionized gas can conduct electricity because of

sufficient free electrons. This condition of any gas or air is called plasma. This phenomenon

is called thermal ionization of gas.


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Ionization due to electron collision:

As we discussed that there are always some free electrons and ions presents in the air or

gas but they are insufficient to conduct electricity. Whenever these free electrons come

across a strong electric field, these are directed towards higher potential points in the

field and acquire sufficiently high velocity. In other words, the electrons are accelerated

along the direction of the electric field due to high potential gradient. During their travel

these electrons collide with other atoms and molecules of the air or gas and extract

valance electrons from their orbits. After extracted from parent atoms, the electrons will

also run along the direction of the same electric field due to potential gradient. These

electrons will similarly collide with other atoms and create more free electrons which will

also be directed along the electric field. Due to this conjugative action the numbers of

free electrons in the gas will become so high that the gas stars conducting electricity. This

phenomenon is known as ionization of gas due to electron collision.

Deionization of gas:

If all the cause of ionization of gas are removed from an ionized gas it rapidly come back

to its neutral state by recombination of the positive and negative charges. The process of

recombination of positive and negative charges is known as deionization process. In

deionization by diffusion, the negative ions or electrons and positive ions move to the

walls under the influence of concentration gradients and thus completing the process of

recombination.

Role of arc in Circuit Breaker:

When two current contacts just open, an arc bridges the contact gap through which the

current gets a low resistive path to flow so there will not be any sudden interruption of

current. As there is no sudden and abrupt change in current during opening of the

contacts, there will not be any abnormal switching over voltage in the system. If i is the

current flows through the contacts just before they open, L is the system inductance,

switching over voltage during opening of contacts, may be expressed as V = L.(di/dt)

where (di/dt) rate of change of current with respect to time during opening of the

contacts. In the case of alternating current arc is monetarily extinguished at every currentzero. After crossing every current zero the media between separated contacts gets

ionized again during next cycle of current and the arc in circuit breaker is reestablished.

To make the interruption complete and successful, this re-ionization in between

separated contacts to be prevented after a current zero.

If arc in circuit breaker is absence during opening of current carrying contacts, there

would be sudden and abrupt interruption of current which will cause a huge switching

over voltage sufficient to severely stress the insulation of the system. On the other hand,

the arc provides a gradual but quick, transition from the current carrying to the current

breaking states of the contacts.


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LIGHTNING ARRESTORS (L.A)

A lightning arrester is a device used on electrical power systems

& telecommunications systems to protect the insulation and conductors of thesystem from the damaging effects of lightning. The typical lightning arrester has a

high-voltage terminal and a ground terminal. When a lightning surge (or switching

surge, which is very similar) travels along the power line to the arrester, the current

from the surge is diverted through the arrestor, in most cases to earth.

In telegraphy and telephony, a lightning

arrestor is placed where wires enter a

structure, preventing damage to

electronic instruments within andensuring the safety of individuals near

them. Smaller versions of lightning

arresters, also called surge protectors,

are devices that are connected between

each electrical conductor in power and

communications systems and the Earth.

These prevent the flow of the normal

power or signal currents to ground, but provide a path over which high-voltage

lightning current flows, bypassing the connected equipment. Their purpose is to limitthe rise in voltage when a communications or power line is struck by lightning or is

near to a lightning strike.

If protection fails or is absent, lightening that strikes the electrical system introduces

thousands of kilovolts that may damage the transmission lines, and can also cause

severe damage to transformers and other electrical or electronic devices. Lightning-

produced extreme voltage spikes in incoming power lines can damage electrical

home appliances.


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TRANSFORMER

Electrical Power Transformer is a static device which transforms electrical energy from onecircuit to another without any direct electrical connection and with the help of mutual

induction between to windings. It transforms power from one circuit to another without

changing its frequency but may be in different voltage level.

Working Principle of transformer:

The working principle of transformer is very simple. It depends upon Faraday's laws of

Electromagnetic Induction. Actually mutual induction between two or more winding is

responsible for transformation action in an electrical transformer.

Faraday's laws of Electromagnetic Induction:

According to these Faraday's laws, "Rate of change of flux linkage with respect to time is

directly proportional to the induced EMF in a conductor or coil".

Basic Theory of Transformer:

Say you have one winding which is supplied by an alternating electrical source. The

alternating current through the winding produces a continually changing flux or alternating

flux surrounds the winding. If any other winding is brought nearer to the previous one,

obviously some portion of this flux will link with

the second. As this flux is continually changing in

its amplitude and direction, there must be a

change in flux linkage in the second winding or

coil. According to Faraday's laws of

Electromagnetic Induction, there must be an EMF

induced in the second. If the circuit of the latter

winding is closed, there must be current flows

through it. This is the simplest form of electricalpower transformer and this is most basic

ofworking principle of transformer.

The winding which takes electrical power from the source, is generally known as Primary

Winding of transformer. Here it is first winding. The winding which gives the desired output

voltage due to mutual induction in the transformer, is commonly known as Secondary

Winding of Transformer. Here it is second winding.


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The above mentioned form of transformer is theoretically possible but not practically,

because in open air very tiny portion of the flux

of the first winding will link with second so the

current flows through the closed circuit of latter,

will be so small that it may be difficult to

measure.

The rate of change of flux linkage depends upon

the amount of linked flux, with the second

winding. So it desired to be linked almost all flux

of primary winding, to the secondary winding.

This is effectively and efficiently done by placing

one low reluctance path common to both the

winding. This low reluctance path is core of

transformer, through which maximum number of flux produced by the primary is passedthrough and linked with the secondary winding. This is most basic theory of transformer.

Main constructional parts of transformer:

So three main parts of a transformer are,

1. Primary Winding of transformer - which produces magnetic flux when it is connected to

electrical source.

2. Magnetic Core of transformer - the magnetic flux produced by the primary winding, will

pass through this low reluctance path linked with secondary winding and creates a closedmagnetic circuit.

3. Secondary Winding of transformer - the flux, produced by primary winding, passes

through the core, will link with the secondary winding. This winding is also wound on the

same core and gives the desired output of the transformer.

Definition of Instrument Transformer:

Instrument transformers means current transformer & voltage transformer are used

in electrical power system for stepping down currents and voltages of the system for

metering and protection purpose. Actually relays and meters used for protection and

metering, are not designed for high currents and voltages.

High currents or voltages of electrical power system cannot be directly fed to relays and

meters. Current transformer steps down rated system current to 1 Amp or 5 Amp

similarly voltage transformer steps down system voltages to 110V. The relays and meters

are generally designed for 1 Amp, 5 Amp and 110V.

Definition of current transformer (CT):

A current transformer (CT) is an instrument transformer in which the secondary current issubstantially proportional to primary current and differs in phase from it by ideally zero

degree.


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Theory of Current Transformer or CT:

A current transformer functions with the same basic working principle of electrical power

transformer, as we discussed earlier, but here is some difference. If a electrical power

transformer or other general purpose transformer, primary current varies with load or

secondary current. In case of current transformer, primary currentis the system current and this primary current or system current

transforms to the CT secondary, hence secondary current or

burden current depends upon primary current of the current

transformer.

In a power transformer, if load is disconnected, there will be only

magnetizing current flows in the primary. The primary of the power

transformer takes current from the source proportional to the load

connected with secondary. But in case of Current transformer, theprimary is connected in series with power line. So current through

its primary is nothing but the current flows through that power

line. The primary current of the CT, hence does not depend upon whether the load or

burden is connected to the secondary or not or what is the impedance value of burden.

Generally current transformer has very few turns in primary where as secondary turn is large

in number. Say Np is number of turns in CT primary and Ip is the current through primary.

Hence the primary AT is equal to NpIp AT.

If number of turns in secondary and secondary current in that CT are Ns and Is respectively

then Secondary AT is equal to NsIs AT.

In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT.

So from the above statement it is clear that if a CT has one turn in primary and 400 turns in

secondary winding, if it has 400 A current in primary then it will have 1A in secondary

burden. Thus the turn ratio of the CT is 400/1A.

Error in Current Transformer:

But in an actual current transformer, errors with which we are connected can best be

considered through a study of phasor diagram for a CT,

Is - Secondary Current

Es - Secondary induced emf

Ip - primary Current

Ep - primary induced emf

KT - turns ratio = numbers of secondary turns/number of primary turns

Io - Excitation Current

Im - magnetizing component of IoIw - core loss component of Io

m - main flux.


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Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of

the passers Es and Ep are proportional to secondary and primary turns. The excitation

current Io which is made up of two components Im and Iw. The secondary current Io lags

behind the secondary induced emf Esby an angle s. The secondary current is now

transferred to the primary side by reversing Is and multiplied by the turns ratio KT. The total

current flows through the primary Ip is then vector sum of KT Is and Io.

The Ratio Error in CurrentTransformer:

From above passer diagram it is clear that primary current Ip is not exactly equal to the

secondary current multiplied by turns ratio, i.e. KTIs. This difference is due to the primary

current is contributed by the core excitation current. The error in current

transformer introduced due to this difference is called current error of CT or Current error

of current transformer or some times Ratio Error in Current Transformer.

Hence, the percentage current error = {(|Ip| |KT.Is|)/Ip} X 100 %

Phase Angle Error in CurrentTransformer:

For a ideal current transformer the angle between the primary and reversed secondary

current vector is zero. But for an actual current transformer there is always a difference in

phase between two due to the fact that primary current has to supply the component of

the exiting current. The angle between the above two phases in termed as Phase Angle

Error in Current Transformer or CT. Here in the pharos diagram it is the phase angle error

is usually expressed in minutes.

Cause of error in current transformer:

The total primary current is not actually transformed in CT. One part of the primary current

is consumed for core excitation and remaining is actually transformers with turns ratio of

CT so there is error in current transformer means there are both Ratio Error in Current

Transformer as well as a Phase Angle Error in Current Transformer .

How to reduce error in current transformer:

It is desirable to reduce these errors, for better performance. For achieving minimum error

in current transformer, one can follow the following,

1) Using a core of high permeability and low hysteresis loss magnetic materials.

2) Keeping the rated burden to the nearer value of the actual burden.

3) Ensuring minimum length of flux path and increasing cross sectional area of the core,

minimizing joint of the core.

4) Lowering the secondary internal impedance.

Potential Transformer or Voltage Transformer are used in electrical power system for

stepping down the system voltage to a safe value which can be fed to low ratings meters

and relays. Commercially available relays and meters used for protection and metering, are

designed for low voltage. This is a simplest form of Potential Transformer Definition.


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Voltage Transformer or Potential Transformer Theory:

A Voltage Transformer theory or Potential Transformer theory is just like theory of general

purpose step down transformer. Primary of this transformer is connected across the phases

or and ground depending upon the requirement. Just like the transformer, used for

stepping down purpose, potential transformer i.e. PT has lowers turns winding at itssecondary. The system voltage is applied across the terminals of primary winding of that

transformer, and then proportionate secondary voltage appears across the secondary

terminals of the PT. The secondary voltage of the PT is generally 110V. In an ideal Potential

Transformer or Voltage Transformer when rated burden connected across the secondary

the ratio of primary and secondary voltages of transformer is equal to the turns ratio and

furthermore the two terminal voltages are in precise phase opposite to each other. But in

actual transformer there must be an error in the voltage ratio as well as in the phase angle

between primary and secondary voltages.

The errors in Potential Transformer or Voltage Transformer can best be explained by phesor

diagram, and this is the main part of Potential Transformer theory

Error in Potential Transformer:

Is - Secondary Current

Es - Secondary induced emf

Vs - Secondary terminal voltage

Rs - Secondary winding resistance

Xs - Secondary winding reactanceIp - Primary current

Ep - primary induced emf

Vp - Primary terminal voltage

Rp - Primary winding resistance

Xp - Primary winding reactance

KT - turns ratio = numbers of primary turns/number of

seconadary turns

Io - Excitation Current

Im - magnetizing component of IoIw - core loss component of Io

m - main flux

- phase angle error

As in the case of Current Transformer and other purpose

Electrical Power Transformer, total primary current Ip is the vector sum of excitation current

and the current equal to reversal of secondary current multiplied by the ratio 1/KT

Hence, Ip = Io + Is/KT

If Vp is the system voltage applied to the primary of the PT then voltage drops due to

resistance and reactance of primary winding due to primary current Ip will comes into

picture. After subtracting this voltage drop from Vp, Ep will appear across the primary


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terminals. this Ep is equal to primary induced emf. This primary emf will transform to the

secondary winding by mutual induction and transformed emf is E s. Again this Es will be

dropped by secondary winding resistance and reactance, and resultant will actually appear

across the burden terminals and it is denoted as Vs

So if system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in realityactual secondary voltage of PT is Vs.

Voltage error in Potential Transformer(PT):

The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio

error in a potential transformer, it can be expressed as ,

Percentage voltage error = {(Vp KT.Vs)/Vp} X 100 %

Phase angle error in potential transformer:

The angle between the primary system voltage Vp and the reversed secondary voltage

vectors KT.Vs is the phase error

Cause of error in Potential Transformer:

The voltage applied to the primary of the potential transformer first drops due to internal

impedance of primary. Then it appears across the primary winding and then transformed

proportionally to its turns ratio, to secondary winding. This transformed voltage acrosssecondary winding will again drops due to internal impedance of secondary, before

appearing across burden terminals. This is the reason of errors in potential transformer.


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INTRODUCTION OF INSULATING OIL

Insulating oil in an electrical power transformer is commonly known as Transformer Oil. It is

normally obtained by fractional distillation and subsequent treatment of crude petroleum.

That is why this oil is also known as Mineral Insulating Oil. Transformer Oil serves mainly

tow purposes one it is liquid insulation in electrical power transformer and two it dissipates

heat of the transformer e.i. acts as coolant. In addition to these, this oil serves other two

purposes, it helps to preserve the core and winding as these are fully immersed inside oil

and another important purpose of this oil is, it prevents direct contact of atmospheric

oxygen with cellulose made paper insulation of windings, which is susceptible to oxidation.

Types of Transformer Oil:

Generally there are two types of Transformer Oil used in transformer,

1. Paraffin based Transformer Oil

2. Naphtha based Transformer Oil

Naphtha oil is more easily oxidized than Paraffin oil. But oxidation product i.e. sludge in the

naphtha oil is more soluble than Paraffin oil. Thus sludge of naphtha based oil is not

precipitated in bottom of the transformer. Hence it does not obstruct convection

circulation of the oil, means it does not disturb the transformer cooling system. But in the

case of Paraffin oil although oxidation rate is lower than that of Naphtha oil but the

oxidation product or sludge is insoluble and precipitated at bottom of the tank and obstruct

the transformer cooling system. Although Paraffin based oil has above mentioned

disadvantage but still in our country it is generally used because of its easy availability.

Another problem with paraffin based oil is its high pour point due to the wax content, but

this does not effect its use due to warm climate condition of India .

Properties of Transformer Insulating Oil:

Some specific parameters of insulating oil should be considered to determine the

serviceability of that oil.

Parameters of Transformer Oil:

The parameters of Transformer Oil are categorized as,

1. Electrical Parameters Dielectric Strength, Specific Resistance, Dielectric Dissipation

Factor.

2. Chemical Parameter - Water Content, Acidity, Sludge Content.

3. Physical Parameters - Inter Facial Tension, Viscosity, Flash Point, Pour Point.


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Dielectric Strength of Transformer Oil:

Dielectric Strength of Transformer Oil is also knlown as Breakdown Voltage of transformer

oil orBDV of transformer oil. Break down voltage is measured by observing at what voltage,

sparking strants between two electrods immerged in the oil, separated by specific gap. low

value of BDV indicates presence of moisture content and conducting substances in the oil.For measuring BDV of transformer oil, portable BDV

measuring kit is generally available at site. In this kit, oil is

kept in a pot in which one pair of electrodes are fixed

with a gap of 2.5 mm (in some kit it 4mm) between them.

Now slowly rising voltage is applied between the

electrodes. Rate of rise of voltage is generally controlled

at 2KV/s and observe the voltage at which sparking starts

between the electrodes. That means at which voltage

Dielectric Strength of transformer oil between theelectrodes has been broken down. Generally this

measurement is taken 3 to 6 times in same sample of oil and the average value of these

reading is taken. BDV is important and popular test of transformer oil, as it is primary

indication of health of oil and it can be easily carried out at site.

Dry and clean oil gives BDV results, better than the oil with moisture content and other

conducting impurities. Minimum Breakdown Voltage of transformer oil or Dielectric

Strength of transformer oil at which this oil can safely be used in transformer, is considered

as 30 KV.

Specific Resistance of Transformer Oil:

This is another important property of transformer oil. This is measure of DC resistance

between two opposite sides of one cm3

block of oil. Its unit is taken as ohm-cm at specific

temperature. With increase in temperature the resistivity of oil decreases rapidly. Just after

charging a transformer after long shut down, the temperature of the oil will be at ambient

temperature and during full load the temperature will be very high and may go upto 90oC at

over load condition. So resistivity of the insulating oil must be high at room temperature

and also it should have good value at high temperature as well. That is why specific

resistance or resistivity of transformer oil should be measured at 27o

C as well as 90o

C.

Minimum standard Specific Resistance of Transformer oil at 90oC is 35X 10

12ohm cm and

at 27oC it is 1500X10

12ohm cm.

Dielectric Dissipation Factor of Transformer oil:

Dielectric Dissipation Factor is also known as loss factor or tan delta of transformer oil.

When a insulating materials is placed between live part and grounded part of an electrical

equipment, leakage current will flow. As insulating material is dielectric in nature the

current through the insulation ideally leads the voltage by 90 o. Here voltage means the

instantaneous voltage between live part and ground of the equipment. But in reality no

insulating materials are perfect dielectric in nature. Hence current through the insulator will

lead the voltage with an angle little bit shorter than 90o. Tangent of the angle by which it is

short of 90o

is called Dielectric Dissipation Factor or simply tan delta of transformer oil.


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More clearly, the leakage current through an insulation does have two component one is

capacitive or reactive and other one is resistive or active. Again it is clear from above

diagram, value of which is also known as loss angle, is smaller, means resistive

component of the current IR is smaller which indicates high resistive property of the

insulating material. High resistive insulation is good insulator. Hence it is desirable to have

loss angle as small as possible. So we should try to keep the value of tan as small aspossible. High value of this tan is an indication of presence of contaminants in transformer

oil. Hence there is a clear relationship between tan and resistivity of insulating oil. If

resistivity of the insulating oil is decreased, the value of tandelta increases and vice versa.

So both resistivity test and tan delta test of transformer oil are not normally required for

same piece of insulator or insulating oil.

In one sentence it can be said that, tan is measure of imperfection of dielectric nature of

insulation materials like oil.

Water Content in Transformer Oil:

Moisture or Water Content in Transformer Oil is highly undesirable as it affects adversely

the dielectric properties of oil. The water content in oil also affects the paper insulation of

the core and winding of transformer. Paper is highly hygroscopic in nature. Paper absorbs

maximum amount of water from oil which affects paper insulation property as well as

reduced its life. But in loaded transformer, oil becomes hotter, hence the solubility of water

in oil increases as a result the paper releases water and increase the water content in

transformer oil. Thus the temperature of the oil at the time of taking sample for test is very

important. During oxidation acid are formed in the oil the acids give rise the solubility of

water in the oil. Acid coupled with water further decompose the oil forming more acid and

water. This rate of degradation of oil increases. The water content in oil is measured as

pm(parts per million unit).

Water content in oil is allowed up to 50 ppm as recommended by IS 335(1993). The

accurate measurement of water content at such low levels requires very sophisticated

instrument like Coulometric Karl Fisher Titrator.

Acidity of Transformer Oil:

Acidity of transformer oil, is harmful property. If oil becomes acidic, water content in the oilbecomes more soluble to the the oil. Acidity of oil detoriates the insulation property of

paper insulation of winding. Acidity accelerates thee oxidation process in the oil. Acid also

includes rusting of iron in presence of moisture. The acidity of transformer oil is measure of

its acidic constituents of contaminants. Acidity of oil is express in mg of KOH required to

neutralize the acid present in a gram of oil. This is also known as neutralization number.

Inter Facial Tension of Transformer Oil:

Inter Facial Tension between the water and oil interface is the way to measure molecular

attractive force between water and oil. It is measured in Dynes/cm or miliNeuton/meter.

Inter facial Tension is exactly useful for determining the presence of polar contaminants

and oil decay products. Good new oil generally exhibits high inter facial tension. Oil

oxidation contaminants lower the IFT.


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Flash Point of Transformer Oil:

Flash point of transformer oil is the temperature at which oil gives enough vapors to

produce a flammable mixture with air. This mixture gives momentary flash on application of

flame under standard condition. Flash point is important because it specifies the chances of

fire hazard in the transformer. So it is desirable to have very high flash point of transformeroil. In general it is more than 140

o(>10

o).

Pour Point of Transformer Oil:

It is the minimum temperature at which oil just start to flow under standard test condition.

Pour Point of Transformer Oil is an important property mainly at the places where climate is

extremely cold. If the oil temperature falls bellow the pour point, transformer oil stops

convection flowing and obstruct cooling in transformer. Paraffin based oil has higher value

of pour point, compared to Naphtha based oil, but in India like country, it does not effectthe use of Paraffin oil due tits warm climate condition. Pour Point of transformer oil mainly

depends upon wax content in the oil. As Paraffin based oil has more wax content, it has

higher pour point.

Viscosity of Transformer Oil:

In few wards, Viscosity of Transformer Oil can be said that Viscosity is the resistance of

flow, at normal condition. Obviously resistance to flow of transformer oil means

obstruction of convection circulation of oil inside the transformer. A good oil should have

low viscosity so that it offers less resistance to the convectional flow of oil thereby notaffecting the cooling of transformer. Low viscosity of transformer oil is essential, but it is

equally important that, the viscosity of oil should increase as less as possible with decrease

in temperature. Every liquid becomes more viscous if temperature decreases.


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BUCHHOLZ RELAY

Construction of Buchholz Relay:

Buchholz Relay in transformer is an oil container housed the connecting pipe from main

tank to conservator tank. It has mainly two elements. The upper element consists of a float.

The float is attached to a hinge in such a way that it can move up and down depending

upon the oil level in the Buchholz Relay Container. One mercury switch is fixed on the float.

The alignment of mercury switch hence depends upon the position of the float.

The lower element consists of a baffle plate and mercury switch. This plate is fitted on a

hinge just in front of the inlet (main tank side) of Buchholz Relay in transformer in such a

way that when oil enters in the relay from that inlet in high pressure the alignment of the

baffle plate along with the mercury switch attached to it, will change. In addition to these

main elements a Buchholz Relay has gas release pockets on top. The electrical leads from

both mercury switches are taken out through a molded terminal block.

Buchholz Relay principle:

The Buchholz Relay working principle of is very simple. Buchholz Relay function is based on

very simple mechanical phenomenon. It is mechanically actuated. Whenever there will be a

minor internal fault in the transformer such as an insulation faults between turns, break

down of core of transformer, core heating, the transformer insulating oil will bedecomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to

decomposition of transformer insulating oil will accumulate in the upper part the Buchholz

Container which causes fall of oil level in it. Fall of oil level means lowering the position of

float and thereby

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