Substation Design (1)
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Transcript of 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
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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
$i& 4.28iii)s#oin& Couplin& capacitor 5olta&e transformers
owever, because of the energy+storage capability of capacitors, sudden
reductions in the power line voltage may result in momentary distortion of the ##!T
secondary voltage. The amount of distortion is related to ##!T capacitance and the
burden secondary load- value and configuration. 4odern ##!T designs are available to
minimi(e this problem.
4.3.2 ,oer Transformers
>ower transformers convert power+level voltages from one level or phase
configuration to another. They can include features for electrical isolation, power
distribution, and control and instrumentation applications
*! power transformers are usually oil immersed with all three phases in one
tan%. "uto transformers can offer advantage of smaller physical si(e and reduced losses.
The different classes of power transformers are
i. .D. il immersed, natural cooling.
ii. .8. il immersed, air blast cooling.
iii. .;.D. il immersed, oil circulation forced.
iv. ;.". il immersed, oil circulation forced, air blast cooling.
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>ower transformers are usually the largest single e$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$uipped with
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multiple curve characteristics and can duplicate almost any shape of electromechanical
relay curve. 4icroprocessor relays convert the current to a digital signal. The digital
signal can then be compared to the setting values input into the relay.
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The differential relay is used to provide internal fault protection to e$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.
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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.
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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
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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
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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
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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
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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&
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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&
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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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5?