substation design (1)

5/20/2018 Substation Design (1)

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The protection system is designed to limit the effects of disturbances in power

system, which when allowed persisting, may damage the substation and interrupt the

supply of electrical energy. It covers various types of protection used in substation for

220/132/33 ! transmission lines such as bus bar protection relays, auto reclosing

schemes, etc.,

The present day electrical power system is "# 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 $uite

away from the consumers. It is delivered to the consumers through a large networ% of

transmission and distribution. "t many places in the line of the power, it may be desirable

and necessary to change some characteristics of power supply. This is accomplished by

suitable apparatus called &ubstation.

'enerating voltage at the power station is stepped upto high voltage for

transmission of electric power. The assembly of apparatus used for this purpose is the

substation. &imilarly,near the consumers localities, the voltage may have to be stepped

down to utili(ation level. This )ob is again accomplished by a suitable apparatus called

substation. The type of e$uipment needed in the substation will depend upon the service

re$uirement.

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Introduction

"n electrical substation is a subsidiary station of an electricity generation,

transmission and distribution system where voltage is transformed from high to low or

the reverse using transformers. *lectric power flows through several substations between

generating plant and consumer changing the voltage level in several stages.

" substation that has a step+up transformer increases the voltage with decreasing

current, while a step+down transformer decreases the voltage with increasing the current

for domestic and commercial distribution. The word substation comes from the daysbefore the distribution system became a grid. "t first substations were connected to only

one power station where the generator was housed and were subsidiaries of that power

station.

2.2 Elements of Substation

&ubstations generally contain one or more transformers and have switching,

protection and control e$uipment. In a large substation, circuit brea%ers are used to

interrupt any short+circuits or overload currents that may occur on the networ%. &maller

distribution stations may use re+closer circuit brea%ers or fuses for protection of branch

circuits. " typical substation will contain line termination structures, high+voltage

switchgear, one or more power transformers, low voltage switchgear, surge protection,

controls, grounding earthing- system, and metering. ther devices such as power factor

correction capacitors and voltage regulators may also be located at a substation.

&ubstations may be on the surface in fenced enclosures, underground, or located in

special+purpose buildings.

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igh+rise buildings may have indoor substations. Indoor substations are usually

found in urban areas to reduce the noise from the transformers, to protect switchgear

from etreme climate or pollution conditions.

2.3 Types of Substation

&ubstations are of three types. They are

a- Transmission &ubstation

b- istribution &ubstation

c- #ollector &ubstation

a) Transmission Substation

" transmission substation connects two or more transmission 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 fault

clearance or maintenance. " transmission station may have transformers to convert the

voltage from voltage level to other, voltage control devices such as capacitors, reactors or

&tatic !" #ompensators and e$uipment such as phase shifting transformers to control

power flow between two ad)acent power systems. The largest transmission substations

can cover a large area several acres/hectares- with multiple voltage levels, many circuit

brea%ers and a large amount of protection and control e$uipment voltage and current

transformers, relays and "" systems-. 4odern substations may be implementedusing International &tandards such as I*#51670.

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b) Distribution Substation

" 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 high+voltage main transmission networ%, unless they use large amounts

of power. &o the distribution station reduces voltage to a value suitable for local

distribution. The input for a distribution substation is typically at least two transmission

or sub transmission lines. Input voltage may be, for eample, 220! or whatever is

common in the area. istribution voltages are typically medium voltage, between 33 and

55 %! depending on the si(e of the area served and the practices of the local utility.

8esides changing the voltage, the )ob of the distribution substation is to isolate

faults in either the transmission or distribution systems. istribution substations may also

be the points of voltage regulation, although on long distribution circuits several

%m/miles-, voltage regulation e$uipment may also be installed along the line.

#omplicated distribution substations can be found in the downtown areas of large

cities, with high+voltage switching and, switching and bac%up systems on the low+voltage

side. 4ost of the typical distribution substations have a switch, one transformer, and

minimal facilities on the low+voltage side.

c) Collector substation

In distributed generation pro)ects such as a wind farm, a collector substation may

be re$uired. It somewhat resembles a distribution substation although power flow is in

the opposite direction. 9sually for economy of construction the collector system operates

around 37 !, and the collector substation steps up voltage to a transmission voltage for

the grid. The collector substation also provides power factor correction, metering and

control of the wind farm.

2.4 Substation Transformer Type

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;urther, transmission substations are mainly classified into two types depending on

changes made to the voltage level.They are

a- &tep+9p Transmission &ubstations.

b- &tep+own Transmission &ubstations.

a) Step-Up Transmission Substation

" step+up transmission substation receives electric power from a near by

generating facility and uses a large power transformer to increase the voltage for

transmission to distant locations.

There can also be a tap on the incoming power feed from the generation plant to

provide electric power to operate e$uipment in the generation plant.

b) Step-Don Transmission Substation

&tep+down transmission substations are located at switching points in an electrical

grid. They connect different parts of a grid and are a source for sub transmission lines or

distribution lines.

2.! "eneral Considerations

The general considerations regarding the substation that are discussed are

functions,design and different layouts of the substation.

a) T#e $unctions of t#e substation are%

i. To #hange voltage from one level to another.

ii.To egulate voltage to compensate for system voltage changes.

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iii. To &witch transmission and distribution circuits into and out of the grid system.

iv. To 4easure electric power $uantity flowing in the circuits.

v. To #onnect communication signals to the circuits.

vi. To *liminate lightning and other electrical surges from the system.

vii. To #onnect electric generation plants to the system.

viii. To 4a%e interconnections between the electric systems of more than one utility.

b) Desi&n

The main issues facing a power engineer are reliability and cost. " good design

attempts to stri%e a balance between these two to achieve sufficient reliability without

ecessive cost. The design should also allow easy epansion of the station, if re$uired.

&election of the location of a substation must consider many factors. &ufficient

land area is re$uired for installation of e$uipment with necessary clearances for electrical

safety and for access to maintain large apparatus such as transformers.


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substation. This design also places minimum reliance on signaling for satisfactory

operation of protection. "dditionally there is the facility to support the economical

operation of future feeder bays.

$i& 2. s#os sin&le bus bar Substation

&uch a substation has the following characteristics.

a. *ach circuit is protected by its own circuit brea%er and hence plant outage does

not necessarily result in loss of supply.

b. " fault on the feeder or transformer circuit brea%er causes loss of the transformer

and feeder circuit, one of which may be restored after isolating the faulty circuit

brea%er.

c. " fault on the bus section circuit brea%er causes complete shutdown of the

substation. "ll circuits may be restored after isolating the faulty circuit brea%er.

d. " bus+bar fault causes loss of one transformer and one feeder. 4aintenance of one

bus+bar section or isolator will cause the temporary outage of two circuits.

e. 4aintenance of a feeder or transformer circuit brea%er involves loss of the circuit.

ii) *es# Substation

The general layout for a full mesh substation is shown in the schematic ;ig2.2

The characteristics of such a substation are as follows

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a. peration of two circuit brea%ers is re$uired to connect or disconnect a circuit,

and disconnection involves opening of a mesh.

b. #ircuit brea%ers may be maintained without loss of supply or protection, and no

additional bypass facilities are re$uired.

c. 8us+bar faults will only cause the loss of one circuit brea%er. 8rea%er faults will

involve the loss of a maimum of two circuits.

d. 'enerally, not more than twice as many outgoing circuits as infeeds are used in

order to rationalise circuit e$uipment load capabilities and rating.

4esh substation

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$i& 2.2 s#os mes# substation.

2.+ 'ayout

a) ,rinciple of Substation 'ayouts

&ubstation layout consists essentially in arranging a number of switchgear

components in an ordered pattern governed by their function and rules of spatial

separation.

b- Spatial Seperation

i. *arth #learance This is the clearance between live parts and earthed structures,

walls, screens and ground.

ii. >hase #learance This is the clearance between live parts of different phases.

iii. Isolating istance This is the clearance between the terminals of an isolator and

the connections.

iv. &ection #learance This is the clearance between live parts and the terminals of a

wor% section. The limits of this wor% section, or maintenance (one, may be the

ground or a platform from which the man wor%s .

c) Separation of maintenance ones

Two methods are available for separating e$uipment in a maintenance (one that

has been isolated and made dead.

i. The provision of a section clearance

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ii. 9se of an intervening earthed barrier

The choice between the two methods depends on the voltage and whether hori(ontal

or vertical clearances are involved.

i. " section clearance is composed of the reach of a man ta%en as 6 feet plus an

earth clearance.

ii. ;or the voltage at which the earth clearance is 6 feet the space re$uired will be the

same whether a section clearance or an earthed barrier is used .

2. *aintenance

4aintenance plays a ma)or role in increasing the efficiency and decreasing thebrea%down. The rules and basic principle are discussed.

&eparation by earthed barrier @ *arth #learance A 70mm for barrier A *arth #learance

&eparation by section clearance @ 2.::m A *arth clearance

i. ;or vertical clearances it is necessary to ta%e into account the space occupied by

the e$uipment and the need for an access platform at higher voltages.

ii. The height of the platform is ta%en as 1.3=m below the highest point of wor%.

*aintenance is done t#rou to ays%

a- 8y *stablishing 4aintenance Bones.

b- 8y *lectrical &eparations.

a) Establis#in& *aintenance /ones

&ome maintenance (ones are easily defined and the need for them is self evident

as in the case of a circuit brea%er. There should be a means of isolation on each side of

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the circuit brea%er, and to separate it from ad)acent live parts when isolated either by

section clearances or earth barriers

b) Electrical Separations

Together with maintenance (oning, the separation, by isolating distance and phase

clearances, of the substation components and of the conductors interconnecting them

constitute the main basis of substation layouts.

There are at least three such electrical separations per phase that are needed in a

circuit

i. 8etween the terminals of the bus bar isolator and their connections.

ii. 8etween the terminals of the circuit brea%er and their connections.

iii. 8etween the terminals of the feeder isolator and their connections.

2.0 Conclusion%


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connection of 220 ! line and also about the lines that feeds this substation from

generating units.

3.2 'ine dia&ram%

In power engineering, a one+line diagram or single+line diagram is a simplified

notation for representing a three+phase power system. The one+line diagram has its largest

application in power flow studies. *lectrical elements such as circuit brea%ers,

transformers, capacitors, bus bars, and conductors are shown by standardi(ed schematic

symbols. Instead of representing each of three phases with a separate line or terminal,

only one conductor is represented. It is a form of bloc% diagram graphically depicting the

paths for power flow between entities of the system. *lements on the diagram do not

represent the physical si(e or location of the electrical e$uipment, but it is a common

convention to organi(e the diagram with the same left+to+right, top+to+bottom se$uence as

the switchgear or other apparatus represented.

&1 and

the other from which have two lines, named as 4al%aram1 C 4al%aram 2.

The single line diagram of 220/132/33 %! &">9 D"'" sub station is

shown at the end of this report.

3.3 T#e interconnection of 221 "rid Substations

The interconnection of 220! to different grid substations is given below,

220 ! &">9D"'" + '"#I89D"'" + '"#I8


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circuit conductors leave the substation from a circuit brea%er via underground cables,

called substation eit cables. The underground cables connect to a nearby overhead

primary circuit outside the substation. This eliminates multiple circuits on the poles

ad)acent to the substations there by improving the overall appearance of the substation.

$i&.3. s#os 3-p#ase distribution feeder bay

This substation has two types of feeder i.e. 132 ! and 33 ! feeder. They are

12 feeders of 132 ! which are basically collector substation and it has 15 feeders of

33! which are industries and for domestic user.

a) T#e interconnection of 32 "rid Substations

The interconnection of 132! to different grid substations is given below,

i. &">9D"'" + 4*#"E+ I circuit Do.1.

ii. &">9D"'" + 4*#"E+I circuit Do. 2.

iii. &">9D"'" + .#.>9"4.

iv. &">9D"'" + D"&">9.v. &">9D"'" + "E*.

vi. &">9D"'" + '944"I "E".

vii. &">9D"'" + 8DI'I.viii. &">9D"'" + '9D#.

i. &">9D"'" + 49E"EI.

. &">9D"'" + I>E.i. &">9D"'" + &"D"TD"'" "IE9D"'" + 8EE""4.

b- T#e interconnection of 33 "rid Substations

The interconnection of 33! to different substations is given below,

i. &">9D"'" + &">9D"'"

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ii. &">9D"'" + G**I4*TE" circuit Do.1

iii. &">9D"'" + G**I4*TE" circuit Do.2

iv. &">9D"'" + G**I4*TE" circuit Do.3v. &">9D"'" + G**I4*TE" circuit Do. :

vi. &">9D"'" + &"TF"4 circuit Do. 1

vii. &">9D"'" + &"TF"4 circuit Do. 2viii. &">9D"'" + G"I"G circuit Do.1

i. &">9D"'" + G"I"G circuit Do. 2

. &">9D"'" + "I;#* "#"*4F circuit Do. 1i. &">9D"'" + "I;#* "#"*4F circuit Do. 2

ii. &">9D"'" + ##

iii. &">9D"'" + 8.>"EEIF

iv. &">9D"'" + .".Ev. &">9D"'" + I>E

vi. &">9D"'" + .4.T

3.! Conclusion


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" simplified transformer design is shown in ;ig :.1. " current passing through the

primary coil creates a varying magnetic field. The primary and secondary coils are

wrapped around a core of very high magnetic permeability, such as iron, This ensures that

most of the magnetic field lines produced by the primary current are within the iron core

and pass through the secondary coil as well as the primary coil. Transformers are

essential for high voltage power transmission, which ma%es long distance transmission

economically practical.

c) ,ractical Considerations

i. Effect of freuency

The time+derivative term in ;aradayHs Eaw shows that the flu in the core is the

integral of the applied voltage. ypothetically an ideal transformer would wor% with

direct+current ecitation, with the core flu increasing linearly with time. In practice, the

flu would rise to the point where magnetic saturation of the core occurs, causing a huge

increase in the magneti(ing current and overheating the transformer. "ll practical

transformers must therefore operate with alternating current.

ii. Transformer uni5ersal E*$ euation

If the flu in the core is sinusoidal, the relationship for either winding between its

!oltage of the winding E, and the supply fre$uency f, number of turns N, core cross+

sectional area a and pea% magnetic flu density B is given by the universal *4;

e$uation

The *4; of a transformer at a given flu density increases with fre$uency. 8y

operating at higher fre$uencies, transformers can be physically more compact because a

given core is able to transfer more power without reaching saturation and fewer turns are

needed to achieve the same impedance.

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owever properties such as core loss and conductor s%in effect also increase with

fre$uency. "ircraft and military e$uipment employ :00 ( power supply which reduce

core and winding weight.

iii. Ener&y 'osses

"n ideal transformer would have no energy losses, and would be 100 efficient.

In practical transformers energy is dissipated in the windings, core, and surrounding

structures. Earger transformers are generally more efficient, and those rated for electricity

distribution usually perform better than ?6.*perimental transformers using

superconducting windings achieve efficiencies of ??.67, while the increase in

efficiency is small, when applied to large heavily+loaded transformers the annual savings

in energy losses are significant.

Transformer losses are divided into losses in the windings, termed copper loss, and

those in the magnetic circuit, termed iron loss. Eosses in the transformer arise from

i.


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4.2 Construction

The constructional details of the transformer are

a.) Cores

i 'aminated steel cores

ii Solid cores

iii Toroidal cores

i5 6ir cores

b) 7indin&s

7indings are usually arranged concentrically to minimi(e flu lea%age.

$i&. 4.28i) s#os indin&s of transformer

The ;ig :.2 i- shows #ut view through transformer windings. rimary winding made of oygen+free

copper. ed &econdary winding. Top left Toroidal transformer. ight #+core, but *+

core would be similar. The blac% windings are made of film.

Top *$ually low capacitance between all ends of both the windings. &ince most

cores are at least moderately conductive they also need insulation at 8ottom.

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c) Coolant

The oil reservoir is visible at the top. adioactive fins aid the dissipation of heat

$i& 4.28ii) s#os coolant of transformer

igh temperatures will damage the winding insulation. >ower transformers rated

up to several hundred !" can be ade$uately cooled by natural convective air+cooling,

sometimes assissted by fans. &ome power transformers are immersed in transformer oil

that both cools and insulates the windings. The oil is a highly refined mineral oil that

remains stable at transformer operating temperature. The oil+filled tan% often hasradiators through which the oil circulates by natural convection some large transformers

employ forced circulation of the oil by electric pumps, aided by eternal fans or water+

cooled heat echangers.

il+filled transformers undergo prolonged drying processes to ensure that the

transformer is completely free of water vapuor before the cooling oil is introduced. This

helps to prevent electrical brea%down under load. il+filled transformers may be

e$uipped with 8uchhol( relays, which detect gas evolved during internal arcing and

rapidly de+energi(e the transformer to avert catastrophic failure.

*perimental power transformers in the 2 4!" range have been built with

superconducting windings which eliminates the copper losses, but not the core steel loss

but these are cooled by li$uid nitrogen or helium.

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d) Tappin&s

Do+load tap changers DET#- or load tap changers ET#- can be obtained on

power transformers.

The addition of no+load taps in the primary of a substation transformer ma%es it

possible to adapt the transformer to a range of supply voltages usually a 10 percent

overall range of which 7 percent is above nominal and 7 percent below nominal, usually

in 2.7 percent steps-. &ince no+load taps are not capable of interrupting any current

including transformer charging current, the transformers have to be de+energi(ed when

the manual no+load tap position is changed. "ll taps should have full capacity ratings.

"ny decision to use load tap changing transformers should be based on a careful

analysis of the particular voltage re$uirements of the loads served and consideration of

the advantages and disadvantages including costs of alternatives such as separate voltage

regulators.

e) Terminals

!ery small transformers will have wire leads connected directly to the ends of the

coils and brought out to the base of the unit for circuit connections. Earger transformers

may have heavy bolted terminals, bus bars or high+voltage insulated bushings made of

polymers or porcelain.

" large bushing can be of comple structure since it must provide careful control

of the electric field gradient without letting the transformer lea% oil.

4.3 Types and Classification $actors

" wide variety of transformer designs are used for different applications though

they share several common features. Important common transformer types include

a. "uto transformer

b. >oly >hase transformers

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c. Eea%age transformer

d. esonant transformers

e. Instrument transformers

Classification of Transformers is based on folloin& factors.

i. 8y power capacity from a fraction of a volt+ampere !"- to over a thousand

4!".

ii. 8y fre$uency range power, audio, or radio fre$uency.

iii. 8y voltage class from a few volts to hundreds of %ilovolts.

iv. 8y cooling type air cooled, oil filled, fan cooled, or water cooled.

v. 8y application such as power supply, impedance matching, output voltage and

current stabili(er, or circuit isolation.

vi. 8y end purpose distribution, rectifier, arc furnace, amplifier output.

vii. 8y winding turns ratio step+up, step+down, isolating e$ual or near+e$ual ratio-,

and variable.

"mong the above mentioned transformers only instrument transformers are widely

used in the sub station. ence only instrument transformers are discussed in this

section.

4.3. Instrument Transformer%

Instrument transformers are used to step+down the current or voltage to

measurable values. They provide standardi(ed, useable levels of current or voltage in a

variety of power monitoring and measurement applications.

8oth current and voltage instrument transformers are designed to have

predictable characteristics on overloads.

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>roper operation of over+current protection relays re$uires that current

transformers provide a predictable transformation ratio even during a short Jcircuit.

These are further classified into two types which are discussed below.

a- #urrent Transformers

b- !oltage Transformers

a) Current Transformers%

i. ,rinciple of 9peration

" current transformer is defined as as an instrument transformer in which the

secondary current is substantially proportional to the primary current under normal

conditions of operation- and differs in phase from it by an angle which is approimately

(ero for an appropriate direction of the connections. This highlights the accuracy

re$uirement of the current transformer but also important is the isolating function, which

means no matter what the system voltage the secondary circuit need to be insulated onlyfor a low voltage.

The current transformer wor%s on the principle of variable flu. In the ideal current

transformer, secondary current would be eactly e$ual when multiplied by the turns

ratio- and opposite to the primary current.

8ut, as in the voltage transformer, some of the primary current or the primary

ampere+turns are utili(ed for magneti(ing the core, thus leaving less than the actual

primary ampere turns to be transformed into the secondary ampere+turns. This naturally

introduces an error in the transformation. The error is classified into current ratio error

and the phase error.

ii. Definitions

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Typical terms used for specifying current transformer are,

:ated primary current%The value of current which is to be transformed to a

lower value. In #T parallence, the load of the #T refers to the primary current.

:ated secondary current%The current in the secondary circuit and on which the

performance of the #T is based. Typical values of secondary current are 1 " or 7 ".

:ated burden%The apparent power of the secondary circuit in !olt+amperes

epressed at the rated secondary current and at a specific power factor.

Composite Error% The 4& value of the difference between the instantaneous

primary current and the instantaneous secondary current multiplied by the turns ratio,

under steady state conditions.

6ccuracy limit factor%The value of primary current up to which the #T compiles

with composite error re$uirements. This is typically 7, 10 or 17, which means that the

composite error of the #T has to be within specified limits at 7, 10 or 17 times the rated

primary current.

S#ort time ratin&%The value of primary current in %"- that the #T should be able

to withstand both thermally and dynamically without damage to the windings with the

secondary circuit being short+circuited. The time specified is usually 1 or 3 seconds.

Class ,S; < CT% In balance systems of protection, #T s with a high degree of

similarity in their characteristics are re$uired. These re$uirements are met by #lass >&

K- #T s. Their performance is defined in terms of a %nee+point voltage >!-, the

magneti(ing current Image- at the %nee point voltage or 1/2 or 1/: the %nee+point

voltage, and the resistance of the #T secondary winding corrected to =7#. "ccuracy is

defined in terms of the turns ratio.

nee point 5olta&e%The point on the magneti(ing curve where an increase of

10 in the flu density voltage- causes an increase of 70 in the magneti(ing force

current-.

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Summation CT%


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i5. Typical specification for a @ CT

&ystem voltage11 %!

Insulation level voltage IE!- 12/26/=7 %!

atio 200/1 + 1 + 0.7== "

#ore 1 1", metering, 17 !"/class 1, I&;M10

#ore 2 1 ", protection, 17 !"/7>10

#ore 3 0.7== ",#lass >&, >!N@ 170 !,Img at !%/2 M@30 m", #T at =7 #M@2

&hort time rating20 %" for 1 second

#THs may be accommodate in one of si manners

a. ver #ircuit 8rea%er bushings or in pedestals.

b. In separate post type housings.

c. ver moving bushings of some types of insulators.

d. ver power transformers of reactor bushings.

e. ver wall or roof bushings.

f. ver cables.

In all ecept the second of the list, the #THs occupy incidental space and do not

affect the si(e of the layout. The #THs become more remote from the circuit brea%er in the

order listed above. "ccommodation of #THs over isolator bushings or bushings through

walls or roofs is usually confined to indoor substations.

b) olta&e Transformers

i. ,rinciple of operation

The standards define a voltage transformer as one in which the secondary

voltage is substantially proportional to the primary voltage and differs in phase from it by

an angle which is approimately e$ual to (ero for an appropriate direction of the

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connections. This in essence means that the voltage transformer has to be as close as

possible to the ideal transformer.

In an ideal transformer, the secondary voltage vector is eactly opposite and

e$ual to the primary voltage vector when multiplied by the turnLs ratio.

In a practical transformer, errors are introduced because some current is drawn for

the magneti(ation of the core and because of drops in the primary and secondary

windings due to lea%age reactance and winding resistance. ne can thus tal% of a voltage

error which is the amount by which the voltage is less than the applied primary voltage

and the phase error which is the phase angle by which the reversed secondary voltage

vector is displaced from the primary voltage vector.

ii. Definitions

Typical terms used for specifying a voltage transformer !T- are

a. :ated primary 5olta&e%This is the rated voltage of the system whose voltage is

re$uired to be stepped down for measurement and protective purposes.

b. :ated secondary 5olta&e%This is the voltage at which the meters and protective

devices connected to the secondary circuit of the voltage transformer operations.

c. :ated burden%This is the load in terms of volt+amperes !"- posed by the

devices in the secondary circuit on the !T. This includes the burden imposed by

the connecting leads. The !T is re$uired to be accurate at both the rated burden

and 27 of the rated burden.

d. :ated 5olta&e factor%epending on the system in which the !T is to be used,

the rated voltage factors to be specified are different. The table :.2 below is

adopted from Indian and International standards.

Table 4.2 s#os rated 5ota&e factor for Ts

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:ated

5olta&e

factor

:ated

time*et#od of connectin& primary indin& in

system

1.2 #ontinuous 8etween phases in any networ%.

8etween transformer star+point and earth in any

networ%.

1.2

1.7

#ontinuous 8etween phase and in an effectively earthed

neutral system.

1.2

1.?

#ontinuous

for 30

seconds

8etween phase and earth in a non+effectively

earthed neutral system with automatic fault

tripping.

1.2

1.?

#ontinuous

for 6 hours

8etween phase and earth in an isolated neutral

system without automatic fault tripping or in a

resonant earthed system without automatic fault

tripping.

e. Temperature class of insulation% The permissible temperature rise over the

specified ambient temperature. Typically, classes *, 8 and ;.

f. :esidual 5olta&e transformer 8:T)%!Ts are used for residual earth faultprotection and for discharging capacitor ban%s. The secondary residual voltage

winding is connected in open delta. 9nder normal conditions of operation, there is

no voltage output across the residual voltage winding.


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Table 4.3 s#os standard references for Ts

StandardStandard

>umber?ear

Indian I& 3175 1??2

8ritish 8& 36:1 1?=3

"merican "D&I #.7=.13 1?=6

Typical specification for a @ T

&ystem voltage 11 %!

Insulation level voltage IE!- 12 /26/=7 %!

Dumber of phases Three

!ector 'roup &tar / &tar

atio 11 %!/ 110 !

8urden 100 !"

"ccuracy #lass 0.7

!oltage ;actor 1.2 continuous and 1.7 for 30 seconds

with provision for fuse

c) Couplin& capacitor 5olta&e transformers

#oupling capacitor voltage transformers, commonly termed capacitor voltage

transformers #!Ts-, are devices used for coupling to a power line to provide low

voltage for the operation of relays and metering instruments.

>ower line carrier accessories or provisions for future installation of carrier

accessories may be included in the base. #oupling capacitor voltage transformers are

26


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commonly supplied without carrier accessories, especially at voltages above 11 %!, as a

more economical alternative to inductive voltage transformers. #oupling capacitor

voltage transformers can be provided with the same ratings and accuracy as inductive

voltage transformers

$i& 4.28iii)s#oin& Couplin& capacitor 5olta&e transformers

owever, because of the energy+storage capability of capacitors, sudden

reductions in the power line voltage may result in momentary distortion of the ##!T

secondary voltage. The amount of distortion is related to ##!T capacitance and the

burden secondary load- value and configuration. 4odern ##!T designs are available to

minimi(e this problem.

4.3.2 ,oer Transformers

>ower transformers convert power+level voltages from one level or phase

configuration to another. They can include features for electrical isolation, power

distribution, and control and instrumentation applications

*! power transformers are usually oil immersed with all three phases in one

tan%. "uto transformers can offer advantage of smaller physical si(e and reduced losses.

The different classes of power transformers are

i. .D. il immersed, natural cooling.

ii. .8. il immersed, air blast cooling.

iii. .;.D. il immersed, oil circulation forced.

iv. ;.". il immersed, oil circulation forced, air blast cooling.

2?


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>ower transformers are usually the largest single e$uipment in a substation. ;or economy

of service roads, transformers are located on one side of a substation and the connection to

switchgear is by bare conductors. 8ecause of the large $uantity of oil, it is essential to ta%e

precaution against the spread of fire. ence, the transformer is usually located around a sump

used to collect the ecess oil.

4.4 Tests

" number of routine and type tests have to be conducted on !T s and #Ts before

they can meet the standards specified above. The tests can be classified as

i. "ccuracy tests

ii. ielectric insulation tests

iii. Temperature rise tests

iv. &hort circuit tests.

4.! Commissionin&

nce the unit is received and pac%ing is opened first thing is to chec% whether

there are any transit damages.

In case of minor damages, such as loose screws or li%ewise, they can be attended

immediately. In case of ma)or damages, the report for this is to be sent to the supplier

who can immediately attend these.

nce the unit is found to have received in good condition, the following need to be

chec%ed

i. #hec% the primary terminals.

ii. #hec% the secondary terminals.

iii. #hec% *arthing.

iv. #hec% oil level

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v. #hec% Insulation esistance ;or primary .T- winding it should be minimum

700m ohms with 1000!..#.4eggar and for secondary E.T- winding. It should

be minimum 274 ohms with 700!..# 4erger.

vi. #hec% atio+ for this a- >ass the rated primary current through primary

b- #hec% the secondary current across the respective Terminals.

If everything is all right, put transformer into operation verification of terminal

mar%ings and polarity

4.+ 6pplications and "eneral Instructions

There are certain applications of transformers and general instructions for

erection, uses and maintenance.

a) 6pplications

" ma)or application of transformers is to increase voltage before transmitting

electrical energy over long distances through wires.


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iv. Transformers should be mounted on corresponding supports or base and firmly

tightened for this purpose.

v. #hec% up whether base to which transformer is fied is in hori(ontal position.

vi. #onnecting cables/conductors by means of which transformer is connected to

high voltage bus+bar or supply system should be correctly dimensioned placed

and mounted not to cause additional over stresses of transformer connections.

vii. >rior to the connection of transformer compare connection diagram with

indications on the transformer and carryout connection in compliance with

corresponding indications.

viii. >roperly carryout earthing on all intended spots on boes and or base

frame of transformers.

i. 9pon completion of above chec% up prior to putting in operation if assembly

properly done.

. >ut connected transformer on line.

i. #ompare instrument indicated with operational condition in supply system.

c) "eneral Instructions for use

i. egular periodical inspection

ii. #hec% up of all sealed spots in order to ascertain oil lea%, if any

iii. #leaning of insulator and possible painting of transformer.

iv. #hec% up of all placement of diaphragm and oil level in oil level indicators.

v. In case of damage of diaphragm or if there is no oil level indicators,

transformer should be thoroughly chec%ed up by the service mechanic since

probably more serious defect occurred. This should be carried out at least once

a year or in two.

vi. #hec% up of primary and secondary connections. their cleaning and tightening

is precaution.

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vii. #hec% up of sealed places consists of detection of oil around connections,

flanges etc. no case transformer should be opened.

viii. "ll earthed parts should be chec%ed and if re$uired, they should be cleaned

and tightened.

i. >ainting of originally painted transformer parts is advisable if re$uired during

regular chec%+ups.

. Transformer should not be opened barring in service wor%shop.

d) "eneral Instructions for *aintenance

The maintenance of transformer is usually done in speciali(ed wor%shops, but ifpossible also on the spot.

"fter the maintenance,

i. ;ollow all steps as said under erection, commissioning C inspection.

ii 4easure insulation resistance and loss angle after ma)or maintenance.

4. Conclusion


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!. Introduction


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$i& !.28i) litnin& arresters

The >ressure elief arrangement transfers the internal arc to outside in the remote event

of arrester failure.

ii. Installation of 'itnin& 6rresters

Three simple rules to be followed in installing lightning arresters for the effective

protection of the e$uipment

i. The arrester should be connected to a ground of low resistance for effective

discharge of the surge current.ii. The arrester should be mounted close to e$uipment to be protected and connected

with shortest possible leads. n both the line and ground side to reduce the

inductive effects of the leads while discharging large surge currents.

iii. To protect the transformer windings. It is desirable to interconnect the ground lead

of the arrester with the tan% and also the neutral of the secondary. This

interconnection reduces the stress imposed on the transformer winding by the

surge currents to the etent of the drop across the ground.

iii. *aAimum Continuous 9peratin& olta&e

9nder actual service conditions 4*T!" functions as insulators at the

maimum line to ground operating voltage. ;or each arrester rating there is a limit to the

magnitude of the voltage that may be continuously applied. There for 4.#..! is the

designated maimum permissible power fre$uency voltage that may be applied

continuously across the arrester terminal.

i5. Caution

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9nder no circumstances, the 4aimum #ontinuous >ower ;re$uency !oltage

between phase and ground appearing the arrester should eceed the arrester 4.#..! as

specified in the name plate.

5. ,ac@in&

*ach arrester is pac%ed in a wooden bo with proper cushioning material. The

terminal connectors are also pac%ed in the same wooden bo ta%en to see that the arrester

housing is not damaged due to rough handling

b) Control and :elay ,anel

The control and relay panel is of cubical construction suitable for floor mounting.

"ll protective, indicating and control elements are mounted on the front panel for ease of

operation and control. The hinged rear door will provide access to all the internal

components to facilitate easy inspection and maintenance. >rovision is made for

terminating incoming cables at the bottom of the panels by providing separate line+up

terminal bloc%s. ;or cable entry provision is made both from top and bottom.

The control and relay panel accepts #T, >T au 230 "# and 220!/10! #

connections at respective designated terminal points. 220!/10! # supply is used for

control supply of all internal relays and timers and also for energi(ing closing and

tripping coils of the brea%ers. 230! "# station auiliary supply is used for internal

illumination lamp of the panel and the space heater. >rotective # fuse are provided

with in the panel for >.T secondary. "u "# and battery supplies.

*ach #apacitor 8an% is controlled by brea%er and provided with a line ammeter with

selector switch for 3 phase system C ver current relay 2 phase and 1 *arth fault for 3

ph system-. 9nder voltage and over voltage elays.

Deutral #urrent 9nbalance elays are for both "larm and Trip facilities brea%er

control switch with local/remote selector switch, master trip relay and trip alarms

ac%nowledge and reset facilities.

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c) ,rotecti5e :elayin&

>rotective relays are used to detect defective lines or apparatus and to initiate the

operation of circuit interrupting devices to isolate the defective e$uipment. elays are

also used to detect abnormal or undesirable operating conditions other than those caused

by defective e$uipment and either operate an alarm or initiate operation of circuit+

interrupting devices. >rotective relays protect the electrical system by causing the

defective apparatus or lines to be disconnected to minimi(e damage and maintain service

continuity to the rest of the system

There are different types of relays.

i. ver current relay

ii. istance relay

iii. ifferential relay

iv. irectional over current relay

i. 95er Current :elay

The over current relay responds to a magnitude of current above a specified value.

There are four basic types of construction They are plunger, rotating disc, static, and

microprocessor type. In the plunger type, a plunger is moved by magnetic attraction when

the current eceeds a specified value. In the rotating induction+disc type, which is a

motor, the disc rotates by electromagnetic induction when the current eceeds a specified

value.

&tatic types convert the current to a proportional .# mill volt signal and apply it to a

level detector with voltage or contact output. &uch relays can be designed to have various

current+versus+time operating characteristics. In a special type of rotating induction+disc

relay, called the voltage restrained over current relay.

The magnitude of voltage restrains the operation of the disc until the magnitude of

the voltage drops below a threshold value. &tatic over current relays are e$uipped with

3=


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multiple curve characteristics and can duplicate almost any shape of electromechanical

relay curve. 4icroprocessor relays convert the current to a digital signal. The digital

signal can then be compared to the setting values input into the relay.


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The differential relay is used to provide internal fault protection to e$uipment such

as transformers, generators, and buses. elays are designed to permit differences in the

input currents as a result of current transformer mismatch and applications where the

input currents come from different system voltages, such as transformers. " current

differential relay provides restraint coils on the incoming current circuits. The restraint

coils in combination with the operating coil provide an operation curve, above which the

relay will operate. ifferential relays are often used with a loc%out relay to trip all power

sources to the device and prevent the device from being automatically or remotely re+

energi(ed. These relays are very sensitive. The operation of the device usually means

ma)or problems with the protected e$uipment and the li%ely failure in re+energi(ing the

e$uipment

i5. Directional 95er current :elay

" directional over current relay operates only for ecessive current flow in a given

direction. irectional over current relays are available in electromechanical, static, and

microprocessor constructions. "n electromechanical overcorrect relay is made directional

by adding a directional unit that prevents the over current relay from operating until the

directional unit has operated. The directional unit responds to the product of the

magnitude of current, voltage, and the phase angle between them or to the product of two

currents and the phase angle between them. The value of this product necessary to

provide operation of the directional unit is small, so that it will not limit the sensitivity of

the relay such as an over current relay that it controls-. In most cases, the directional

element is mounted inside the same case as the relay it controls. ;or eample, an over

current relay and a directional element are mounted in the same case, and the

combination is called a directional over current relay. 4icroprocessor relays often

provide a choice as to the polari(ing method that can be used in providing the direction of

fault, such as applying residual current or voltage or negative se$uence current or voltage

polari(ing functions to the relay.

d) Circuit (rea@ers

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" circuit brea@er is an automatically+operated electrical switch designed to

protect an electrical circuit from damage caused by overload or short circuit. Its basic

function is to detect a fault condition and these by interrupting continuity, to immediately

discontinue electrical flow.

i. ,rinciple of 9peration

"ll circuit brea%ers have common features in their operation, although details vary

substantially depending on the voltage class, current rating and type of the circuit brea%er.

The circuit brea%er must detect a fault condition in low+voltage circuit brea%ers this

is usually done within the brea%er enclosure. #ircuit brea%ers for large currents or high

voltages are usually arranged with pilot devices to sense a fault current and to operate the

trip opening mechanism. The trip solenoid that releases the latch is usually energi(ed by a

separate battery, although some high+voltage circuit brea%ers are self+contained with

current transformers, protection relays and an internal control power source.

nce a fault is detected, contacts within the circuit brea%er must open to interrupt the

circuit. &ome mechanically+stored energy using something such as springs or

compressed air- contained within the brea%er is used to separate the contacts, although

some of the energy re$uired may be obtained from the fault current itself. The circuit

brea%er contacts must carry the load current without ecessive heating, and must also

withstand the heat of the arc produced when interrupting the circuit. #ontacts are made of

copper or copper alloys, silver alloys and other materials. &ervice life of the contacts is

limited by the erosion due to interrupting the arc. 4iniature circuit brea%ers are usually

discarded when the contacts are worn, but power circuit brea%ers and high+voltage circuit

brea%ers have replaceable contacts.


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i. Eengthening of the arc

ii. Intensive cooling in )et chambers-

iii. ivision into partial arcs

iv. Bero point $uenching

v. #onnecting capacitors in parallel with contacts in # circuits

;inally, once the fault condition has been cleared, the contacts must again be

closed to restore power to the interrupted circuit.

ii. 6rc Interruption

4iniature low+voltage circuit brea%ers use air alone to etinguish the arc. Earger

ratings will have metal plates or non+metallic arc chutes to divide and cool the arc.

4agnetic blowout coils deflect the arc into the arc chute.

In larger ratings, oil circuit brea%ers rely upon vapori(ation of some of the oil to blast

a )et of oil through the arc.

'as usually sulfur heafluoride- circuit brea%ers sometimes stretch the arc using amagnetic field, and then rely upon the dielectric strength of the sulfur heafluoride &; 5-

to $uench the stretched arc.

!acuum circuit brea%ers have minimal arcing as there is nothing to ioni(e other than

the contact material-, so the arc $uenches when it is stretched a very small amount M2+3

mm-. !acuum circuit brea%ers are fre$uently used in modern medium+voltage switchgear

to 37,000 volts.

"ir circuit brea%ers may use compressed air to blow out the arc, or alternatively, the

contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air

thus blowing out the arc.

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#ircuit brea%ers are usually able to terminate all current very $uic%ly. Typically the

arc is etinguished between 30 ms and 170 ms after the mechanism has been tripped,

depending upon age and construction of the device.

iii. S#ort circuit current

" circuit brea%er must incorporate various features to divide and etinguish the arc.

The maimum short+circuit current that a brea%er can interrupt is determined by testing.

"pplication of a brea%er in a circuit with a prospective short+circuit current higher than

the brea%erHs interrupting capacity rating may result in failure of the brea%er to safely

interrupt a fault. In a worst+case scenario the brea%er may successfully interrupt the fault,

only to eplode when reset.

4iniature circuit brea%ers used to protect control circuits or small appliances may not

have sufficient interrupting capacity to use at a panelboard. These circuit brea%ers are

called Osupplemental circuit protectorsO to distinguish them from distribution+type circuit

brea%er.

i5. Bi-5olta&e circuit brea@ers

:00! &;5 circuit brea%ers

*lectrical power transmission networ%s are protected and controlled by high+voltage

brea%ers. The definition of Ohigh voltageO varies but in power transmission wor% is

usually thought to be =2,700 ! or higher according to a recent definition by the

International *lectro technical #ommission I*#-.

igh+voltage brea%ers are nearly always solenoid+operated, with current sensing

protective relays operated through current transformers. In substations the protection

relay scheme can be comple, protecting e$uipment and busses from various types of

overload or ground/earth fault.

:2


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igh+voltage brea%ers are broadly classified by the medium used to etinguish the

arc.

i. 8ul% oil

ii. 4inimum oil

iii. "ir blast

iv. &;5

$i&.!.2 8iii) s#os

circuit brea@er

igh+voltage circuit brea%ers used on transmission systems may be arranged to

allow a single pole of a three+phase line to trip, instead of tripping all three poles.;or

some classes of faults this improves the system stability and availability.

e) Conductor Systems

"n ideal conductor should fulfill the following re$uirements

i. &hould be capable of carrying the specified load currents and short time currents.

ii. &hould be able to withstand forces on it due to its situation. These forces comprise

self weight, and weight of other conductors and e$uipment, short circuit forces

and atmospheric forces such as wind and ice loading.

iii. &hould be corona free at rated voltage.

iv. &hould have the minimum number of )oints.

v. &hould need the minimum number of supporting insulators.

vi. &hould be economical.

:3


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vii. The most suitable material for the conductor system is copper or aluminum. &teel

may be used but has limitations of poor conductivity and high susceptibility to

corrosion.

$i&.!.28iii) s#os Conductor systems

In an effort to ma%e the conductor ideal, three different types have been utili(ed,

and these include

i. ;lat surfaced #onductors.

ii. &tranded #onductors.

iii. Tubular #onductors.

f ) DC ,oer Supply

i. DC (attery and C#ar&er

"ll but the smallest substations include auiliary power supplies. "# power is

re$uired for substation building small power, lighting, heating and ventilation, some

communications e$uipment, switchgear operating mechanisms, anti+condensation heaters

and motors. # power is used to feed essential services such as circuit brea%er trip coils

and associated relays, supervisory control and data ac$uisition ""- and

communications e$uipment. This describes how these auiliary supplies are derived and

eplains how to specify such e$uipment.

ii. (attery and C#ar&er confi&urations

::


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#apital cost and reliability ob)ectives must first be considered before defining the

battery and battery charger combination to be used for a specific installation. The

comparison given in Table 7.1 describes the advantages and disadvantages of three such

combinations.

Table 7.1 #apital cost and reliability ob)ectives must first be considered before defining

the battery/battery charger combination to be used for a specific installation. The

comparison given describes the advantages and disadvantages of three such combinations

Type Advantages Disadvantages

1. &ingle

100 battery

and 100

charger

Eow capital cost

Do standby # &ystem outage for

maintenance Deed to isolate battery/charger

combination from load under boost charge

conditions in order to prevent high boost

voltages appearing on # distribution

system

2. &emi+

duplicate

70 batteries

and

100

chargers

4edium capital cost &tandby

# provided which is 100capacity on loss of one

charger *ach battery or

charger can be maintained in

turn. *ach battery can be

isolated and...

++++++++++++++++++++++++++++++

iii. 221 DC (attery

4a%e *ide, #apacity 300 " at 2=P

Do. of #ells 110 Do. , ate of installation 05/200

4a%e 9niversal, &r. Do. 8# 1020/62

:7


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ate of installation 1?63

Input ating !oltage :17 ! A 10

;re$uency 70 (. 3 >hase

utput ating

;loat 220 !, 10 "mp $i&.!.28i5) s#os 221 (attery C#ar&er

8oost 160 !, 30"mp

&) 7a5e Trapper

This is relevant in >ower Eine #arrier #ommunication >E##- systems for

communication among various substations without dependence on the telecom company

networ%. The signals are primarily teleportation signals and in addition, voice and data

communication signals. Eine trap also is %nown as


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Deutral bus bars may also be insulated. *arth bus bars are typically bolted directly

onto any metal chassis of their enclosure. 8us bars may be enclosed in a metal housing,

in the form of bus duct or bus way, segregated+phase bus, or isolated+phase bus.

i. ,rotection

8us bars are vital parts of a power system and so a fault should be cleared as fast

as possible. " bus bar must have its own protection, although they have high degrees

of reliability. 8earing in mind the ris% of unnecessary trips, the protection should be

dependable, selective and should be stable for eternal faults, called Hthrough faultsH.

The most common fault is phase to ground, which usually results from human

error.

There are many types of relaying principles used in bus bar.

" special attention should be made to current transformer selection since

measuring errors need to be considered.

i) Isolators

Isolators are used to connect and disconnect high voltage power systems under no

load conditions.

These are essentially off load devices although they are capable of dealing with small

charging currents of bus bars and connections. The design of isolators is closely related to

the design of substations. Isolator design is considered in the following aspects

i. &pace ;actor

ii. Insulation &ecurity

iii. &tandardi(ation

iv. *ase of 4aintenance

:=


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v. #ost

Some types of isolators include%

i. ori(ontal Isolation types

ii. !ertical Isolation types

iii. 4oving 8ushing types

i. ,roperties

The isolators comprises three identical poles in the case of the three phase system

only- each pole consisting of

i. " 'alvani(ed ;abricated 8ase out of 4& #hannel having one supporting

insulation mounting stool.

ii. Three post insulators stac%s one for mounting one the centre rotating stool and

other two stac%s on both ends of the base channel.

iii. 4oving contact assembly for mounting on the centre rotating insulator stac% and

the fied contact assembly with terminal pad or two outer insulator stac%s.

iv. Tandem pipe for interlin%ing the three poles and operating down pipe to lin% the

tandem pipe with the bottom operating mechanism of 3 phase system.v. 8ottom operating mechanism bo.

vi. *arthing switch moving contact assembly

vii. *arthing switch fied contact assembly for fiing to the main switch fied

contacts.

viii. *arthing switch operating down pipe to lin% earth switch tandem pipe to the

bottom

i. 8ottom operating mechanism bo

. 4echanical interloc% between main switch and earthling switch.

!.3 Conclusion

:6


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The principle dielectric used on overhead power lines is air at atmospheric

pressure. The air surrounding the bare high voltage threshold. It is however necessary to

attach the conductors at certain points onto the cross arms of the pylons. The problem of

reliably suspending the conductors of high voltage transmission lines has therefore been

with us since the turn of the century. The tas% is particularly comple, bearing in mind the

multiple etreme stresses present are mechanical, electrical and environmental stresses.

+.2 Types of Insulators

a) Porcelain pin type

insulators

These were originally used for

telephone lines and lightning conductors, have

been adapted for power transmission and some

variations are still in use for medium voltage

systems. " pin+type insulator is shown

schematically in figure 5.2i-and 5.2ii-

Fig. 6.2(i)

Porcelain Insulator

b) Cap and Pin Type Insulators

The pin+type insulator is so called because in use it is screwed onto a galvani(ed

forged steel HpinH which mounted vertically on a metal or wooden cross arm.

;or low voltage systems, 5.5 to 11 %!, it is usual to have a one+piece insulator

shed in which the porcelain is loaded largely in compression. " typical pin+type insulator

is shown in ;igure 5.2ii-.The s%etches show that the top of the porcelain body is formed

into a groove into which the conductor is bound by means of wire or fied with the aid of

special clips. Toughened glass pin+type insulators re$uire a metal capQ this holds together

the HdicedH pieces of glass which result if the glass becomes shattered.

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Fig.62 (ii): shows cap & pin insulator

c) ,ost Type Insulator

These insulators consist of a solid porcelain cylinder, corrugated to increase the

lea%+ age length, with metalware on each end. They are used to support the high voltage

conductor and are mounted on pedestals or on the power line cross arms. >ost insulators

are tall and are mainly used in substations. These insulators are #lass "Q the shortest

distance through the porcelain eceeds 70 of the shortest distance through air between

the electrodes. They are therefore un puncturable. " typical eample of a post insulator is

shown schematically in figure 5.2iii-

$i&.+.28iii)post insulator

d) Porcelain ong !od Insulators

Eong rod insulators are similar to post insulators but are lighter, slimmer and are

71


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used as suspension insulators.

Eong rod insulators have the apparent advantage over cap and pin insulators in

that metal fittings eist only at the ends of the insulators.

+.3 (us#in&s

8ushings are used to insulate the conductors of the high voltage terminals

of a transformer as is shown schematically in figure 7. 3 Traditionally, transformer

bushings are manufactured using porcelain. #apacitive grading, using foil

cylinders is often used to improve the aial and radial field distribution.

$i&.+.3 s#os bus#in&s

+.4 Terminolo&y


53/69

pulled piece of string.

Inter s#ed spacin&%The distance between corresponding points on ad)acent sheds.

+.! ,ollution Deposition ,rocess

Insulators eposed to the environment collect pollutants from various sources.

>ollutants that become conducting when moistened are of particular concern.

Two ma)or sources are

i. #oastal pollution the salt spray from the sea or wind+driven salt laden solid

material such as sand collects on the insulator surface. These layers become

conducting during periods of high humidity and fog. &odium chloride is the mainconstituent of this type of pollution.

ii. Industrial pollution substations and power lines near industrial complees are

sub)ect to the stac% emissions from nearby plants. These materials are usually dry

when depositedQ they may then become conducting when wetted. The materials

will absorb moisture to different degrees, and apart from salts, acids are also

deposited on the insulator.

a) T#e role of t#e eat#er


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flash over

6.6 Failure "odes o# Insulators

;lashovers, caused by air brea%down or pollution, generally do not cause physical

damage to the insulators and the system can often be restored by means of auto closing.

&ome other events, however cause ir+repairable damage to the insulators.

a) ,uncture

"s previously mentioned, porcelain pin+type and cap and pin insulators may suffer

punctures between the pin and the either the pin or the high voltage conductor.

These occurrences are usually caused by very steep impulse voltages, where the time

delay for air flashover eceeds that of puncture of porcelain. >unctures caused by severe

stress over dry bands also occur on composite insulators on sheds and through the sheath.

" puncture of the sheath is particularly serious as this eposes the glass fiber rod to the

environment .

b) S#atterin&

'lass insulators shatter when eposed to severe arcing or puncturing due to

vandalism. ne advantage is that they retain their mechanical integrity.

c) Erosion

>rolonged arcing of glass insulators leads to erosion of the surface layer of the

glass. This may lead to shattering of the glass discs + a result of the tempering processused during manufacture. "rcing and corona over long periods may cause removal of

shed or sheath material in the case of polymeric insulators. &evere erosion may lead to

the eposure of the glass fiber core.

d) Trac@in&

7:


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Trac%ing occurs when carboni(ed trac%s form because of arcing. These trac%s are

conductive. This phenomenon only occurs in carbon+based polymers.

e) (rittle $racture


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and protected from moisture. The disadvantage of greasing is that the spent grease must

be removed and new grease applied, usually annually.

c) C#oice of Creep a&e 'en&t#


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possible by good grounding, improves the overall safety and reliability of an electrical

system. Therefore, substation reliability must be as Obuilt+inO as possible because of the

high available fault current levels present and unli%ely occurrence of follow+up

grounding inspections.

$i&.. s#os &roundin&

=.2 Types and *et#ods of "roundin&

There are different types and methods of grounding which ensures the reliable

performance of a substation.

a) Types

'rounding of earth may be classified as i- *$uipment grounding ii- &ystem

groundingand iii- Deutral grounding.

*$uipment grounding deals with earthing the non current carrying metal parts of

the electrical e$uipment. n the other hand, system grounding means earthing some part

of the electrical system e.g. earthing of neutral point of star connected system in

generating stations and substations.

i. Euipment "roundin&

The process of connecting non current carrying metal parts of the electrical

e$uipment to earth in such a way that in case of insulation failure, the enclosure

effectively remains at earth potential is called *$uipment grounding.

ii. System "roundin&

7=


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The process of connecting some electrical part of the power system neutral point of

a star connected system, one conductor of the secondary of a transformer- to earth is

called &ystem grounding.

iii. >eutral "roundin&

The process of connecting neutral point of 3+phase system to earth either directly or

through some circuit element e.g. resistance or reactance etc.- is called Deutral

grounding.

Deutral grounding provides protection to personal and e$uipment. It is because

during earth fault the current path is completed through the earthed neutral and the

protective devices operate to isolate the faulty conductor from the rest of the system.

b) *et#ods of "roundin&

The methods commonly used for grounding the neutral point of a 3+phase system

are

i- &olid or effective grounding ii- esistance grounding

iii- eactance grounding iv- esonant grounding

i. Solid "roundin&

ermits the easy operation of earth fault relay.

Disad5anta&es%

a- It causes the system to become unstable.

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b- The increased earth fault current results in greater interference in the neighboring

communication lines.

ii. :esistance "roundin&


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b- igh transient voltages appear under fault conditions.

i5. :esonant "roundin&

eterson coil grounding as the arc suppression coil used here is the >eterson coil which is

an iron cored connected between the neutral and earth. The resultant current in the fault

will be (ero or can be reduced by ad)usting the tappings on the >eterson coil.

6d5anta&es%

The >eterson coil grounding has the following advantages

a- The >eterson coil is completely effective in preventing any damage by an arcingground.

b- This coil has the advantage of ungrounded neutral system.

Disad5anta&es%

The >eterson coil grounding has following disadvantages

a- ue to varying operational conditions, the capacitance of the networ% changes from

time to time. Therefore, inductance E of >eterson coil re$uires read)ustment.

b- The lines should be transposed.

.3 Eart#in& and (ondin&

The function of an earthing and bonding system is to provide an earthing system

connection to which transformer neutrals or earthing impedances may be connected in

order to pass the maimum fault current. The earthing system also ensures that no

thermal or mechanical damage occurs on the e$uipment within the substation, thereby

resulting in safety to operation and maintenance personnel.

The earthing system also guarantees e$ui+potential bonding such that there are no

dangerous potential gradients developed in the substation.

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a) Substation Eart#in& Calculation *et#odolo&y

#alculations for earth impedances and touch and step potentials are based on site

measurements of ground resistivity and system fault levels. " grid layout with particular

conductors is then analy(ed to determine the effective substation earthing resistance,

from which the earthing voltage is calculated. In practice, it is normal to ta%e the highest

fault level for substation earth grid calculation purposes.

To determine the earth resistivity, probe tests are carried out on the site. These tests

are best performed in dry weather such that conservative resistivity readings are obtained.

b) Eart#in& *aterials

i) Conductors8are copper conductor is usually used for the substation earthing grid. The

copper bars themselves usually have a cross+sectional area of ?7 s$uare millimeters, and

they are laid at a shallow depth of 0.27+0.7m, in 3+=m s$uares. In addition to the buried

potential earth grid, a separate above ground earthing ring is usually provided, to which

all metallic substation plant is bonded.

ii) Connections #onnections to the grid and other earthing )oints should not be soldered

because the heat generated during fault conditions could cause a soldered )oint to fail.

Goints are usually bolted, and in this case, the face of the )oints should be tinned.

iii) Eart#in& :ods The earthing grid must be supplemented by earthing rods to assist in

the dissipation of earth fault currents and further reduce the overall substation earthing

resistance. These rods are usually made of solid copper or copper clad steel.

i5) Sitc#yard $ence Eart#in&% The switchyard fence earthing practices are possibleand are used by different utilities. *tend the substation earth grid 0.7m+1.7m beyond the

fence perimeter. The fence is then bonded to the grid at regular intervals.

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>lace the fence beyond the perimeter of the switchyard earthing grid and bond the

fence to its own earthing rod system. This earthing rod system is not coupled to the main

substation earthing grid.

.4 Conclusion

In this chapter we have discussed about the various earthing /grounding techni$ue

used in substation for the protection of the e$uipment from the high voltage and eternal

faults.

0. Introduction

In this chapter we are going to discuss about the various power factor correction

techni$ue used in the substation and they mentions as well as protection of this

e$uipments.

9nder normal operating conditions certain electrical loads draw not only active

power from the supply %ilowatts


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This reactive power has no useful function, but is necessary for the e$uipment to operate

correctly. Eoads such as induction motors, welding e$uipment, arc furnaces and

fluorescent lighting would fall into this category.

a) Definition

The >ower ;actor of a load is defined as being the ratio of active power to total

demand. The uncorrected power factor of a load is cos S where S is the phase angle

between the uncorrected load and unity-, and the corrected power factor is cos S2 where

S2 is the phase angle between the corrected load and unity-. "s cos S approaches to

unity, reactive power drawn from the supply is minimi(ed

0.2 Compensatin& Capacitor

" capacitor inside an op+amp that prevents oscillations is called compensating

caacitor.. "lso any capacitor that stabili(es an amplifier with a negative+feedbac% path.

ower factor will be improved by connecting capacitors in parallel to the

load.

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0.3 ,oer factor correction

In electric power distribution, capacitors are used for power factor correction. &uch

capacitors often come as three capacitors connected as a three phase load. 9sually, the

values of these capacitors are given not in farads but rather as a reactive power in volt+

amperes reactive !"-. The purpose is to counteract inductive loading from devices li%e

electric motors and transmission lines to ma%e the load appear to be mostly resistive.

Individual motor or lamp loads may have capacitors for power factor correction, or larger

sets of capacitors usually with automatic switching devices- may be installed at a load

center within a building or in a large utility substation.

$i&.0.3 ,.$ Correction


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If possible, capacitors should be located at position 2. This does not change the

current flowing through motor overload protectors. #onnection of capacitors at position 3

re$uires a change of overload protectors. #apacitors should be located at position 1 for

applications listed in paragraph 2 above. 8e sure bus power factor is not increased above

?7 under all loading conditions to avoid over ecitation.

The table 6.1 below shows the power factor correction.

riginal

>ower

;actor

>ercent

Desired ,oer $actor ,ercent

100 ?7 ?0 67 60

50 1.333 1.00: 0.6:? 0.=13 0.763

52 1.255 0.?3= 0.=62 0.5:5 0.715

5: 1.201 0.6=2 0.=1= 0.761 0.:71

55 1.136 0.60? 0.57: 0.716 0.366

56 1.0=6 0.=:? 0.7?: 0.:76 0.326

=0 1.020 0.5?1 0.735 0.:00 0.2=0

=2 0.?5: 0.537 0.:60 0.3:: 0.21:

=: 0.?0? 0.760 0.:27 0.26? 0.17?

=5 0.677 0.725 0.3=1 0.237 0.107

=6 0.602 0.:=3 0.316 0.162 0.072

=? 0.==5 0.::= 0.2?2 0.175 0.025

60 0.=70 0.:21 0.255 0.130 +

61 0.=2: 0.3?7 0.2:0 0.10: +

62 0.5?6 0.35? 0.21: 0.0=6 +

63 0.5=2 0.3:3 0.166 0.072 +6: 0.5:5 0.31= 0.152 0.205 +

67 0.520 0.2?1 0.135 + +

65 0.7?3 0.25: 0.10? + +

6= 0.75= 0.236 0.063 + +

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6? 0.712 0.163 0.026 + +

?0 0.:6: 0.177 + + +

?1 0.:75 0.12= + + +

?2 0.:25 0.0?= + + +

?3 0.3?7 0.055 + + +

?: 0.353 0.03: + + +

?7 0.32? + "ssume Total plant load is 100

< at 50 power factor.

#apacitor !" rating

necessary to improve power

factor to 60 is found by

multiplying < 100- by the

multiplier in table 0.763-

which gives !" 76.3-,

nearest standard rating 50

!"- should be used.

?5 0.2?2 +

?= 0.271 +

?? 0.1:3 +

The connection of a capacitor capable of OcorrectingO half of the reactive power

of a load leads to a reduction in the demand on the supply of approimately 17. This

results in the following

a- The load on the cables and switches is reduced.

b- The supply is now able to support additional load

c- The charges made by the electricity supply company are li%ely to be reduced

8y reducing the load on cables and switches, power loss is reduced and life is

etended. The facility to connect additional load is always useful to an epanding

company.

0.4 Conclusion

In this chapter we have discussed about the various power factor correction

techni$ues involved in substation and benefits of it.

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In our pro)ect we have studied about the operation of different e$uipments in

substation. It includes study of transmission lines, bus bars, circuit brea%ers, isolators,

earth switches, current transformers, voltage transformers, lightning arresters, wave traps

and grounding system of substation.


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(I('I9":6,B?

1. *lectric >ower &ubstations *ngineering 8y James C. Burke and Anne-Marie

Sahazizian.>ublisher ##.

2. *lectric >ower &ystems " #onceptual Introduction 8y "leandra von 4eier

>ublisher .!.'upta,9.&

8hatnagar.

>ublisher hanpat ai C #o

7. Transmission, istribution and 9tili(ation !olume III, 8y 8.E.T*"G" C

"..T*"G"

>ublisher &.#"D C #4>"DF ET. 200:

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