EARTHING
EARTHING
TRANSFORMERS
TRANSFORMERS
FOR
FOR
POWER SYSTEMS
POWER SYSTEMS
ABSTRACT:- Normally power systems and net works are operated under variable complex stresses.n power systems t!e "aults are not avodable even a"ter takin# utmost care at every sta#e-"rom plannin# to maintainance. T!e #roundin# o" a circuit reduces potential stresses under "ault condition. $ower "eedin# "rom delta delta or i" t!ere is no accessibility "or star connected trans"ormers occasionally s!orted to #round is very common-un-intentional #roundin# occurs any w!ere "rom t!e "eedin# system to utili%ation e&uipment T!e main ob'ective o" #roundin# neutral is to make a s!ort circuit current su""icient in ma#nitude "or t!e relay action. T!is article restricted to %i#-%a# type wit! oil "illed trans"ormers. T!e neutral point is usually available at every volta#e level "rom #enerator to trans"ormers. n t!e absence o" a power trans"ormer o" suitable capacity( connection and desi#n a separate #roundin# trans"ormer can be used .
T!ey are inductive devices intended primarily to provide a neutral point "or #roundin# purpose. T!e main purpose o" t!is paper is to discuss about t!e di""erence between )art!in# trans"ormer and $ower trans"ormer and to discuss about t!e desi#nin# o" eart! trans"ormers.
NTR*+,CT*N:-A trans"ormer is a device w!ic! electroma#netically trans"ers NTR*+,CT*N:-A.C. volta#e "rom one level to anot!er level. An eart!in# trans"ormer is a trans"ormer primarily to provide a neutral point "or #roundin# purpose. T!e sole duty o" t!e #roundin# trans"ormer is to pass #round current durin# an eart! "ault.( and it carries no use"ul load.
T!e desirable &uantities o" an eart!in# trans"ormer are low %ero impedance and low losses no load losses.
/,NCT*N */ )ART0N1
TRANS/*R2)RS:-• Apart "rom providin# an easy pat! to #round current durin# an eart! "ault( t!e
"ollowin# additional "unctions are also to be ac!ieved
• 0oldin# t!e neutral s!i"t wit! in t!e limits
• $ermittin# t!e circulation o" unbalanced load current in t!e neutral • To limit t!e current durin# line to eart! "aults
• To eart! t!e system
• To provide sin#le p!ase line to neutral load
• Can be used wit! resistance 3reactance3arc suppression coil
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)ART0N1
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TRANS/*R2)RS:-T!e selection any trans"ormer mainly by two "actors-namely( primary volta#e and load ./or an eart!in# trans"ormer load is s!ort time or s!ort duty
only at t!e time o" "ault (normally "rom a "ew seconds to one minute
COMPARISION CHARACTERISTICS
2.29MVA (S.C) 6.6KV 600 Amps(ET@) 2MVA110.!""KV #$%& 1 (PT@)
)T5 $T5
$urpose To provide a neutral point
"or eart!in# Raidin# volta#es or lowerin# 67A ratin# S!ort time Continuous
7olta#es in #eneral ,p to 88kv 0i#!er volta#eseven 9967 and above
+irection o" power "low ,ni direction B-directional
S.C wit! stand duration ;sec.to ; min 9 sec
<osses No load losses No load losses and load losses
2a#neti%in# current 7ery less Normal
$riority "or impedance =ero se&uence impedance S!ort circuit impedance
S!ort circuit current +epends on system "rom "ew !undred amperes to "ew t!ousand amperes
,p to 9> times normal current
<eaka#e reactance <ittle 2ore
Type o" volta#e mixed Constantin #eneral Constant3variable3re#ulation
Transportation Road Road3Rail
S!ort circuit temperature
n de#rees centi#rade 9> 9> 0armonic residuals "ree exist
<oad "actor <ow 0i#!
nsulation ,ni"orm ,ni"orm3#raded
)""iciency 2ore <ess
Cost <ess 2ore
• 4ei#!ts( losses( dimensions and cost "actors are muc! lower. • Rise o" oil 3windin# temperature is very less.
• 0i#!er speci"ic loadin# and e""iciency.
• 2ore si#ni"icant "actor is %ero se&uence impedance.
Significance of zero sequence parameters
n symmetrical components t!e positive se&uence is one !avin# normal p!ase se&uence? t!e second is o" ne#ative se&uence wit! reversed p!ase se&uence? and t!e t!ird t!at !as no se&uence and is called %ero se&uence @ are important parameters. An eart! "ault is one t!at part o" t!e current in t!e "aulty p!ase returns to t!e supply t!rou#! t!e eart!. T!is is due to a symmetrical component consistin# o" t!ree sin#le p!ase currents - one in eac! line( and all bein# in p!ase.
n #eneral( t!e impedance o" t!e trans"ormer is t!e main "actor w!ic! in"luences applications. T!e %ero se&uence impedance o" a standard eart!in# trans"ormer can be calculated "rom t!e "ollowin# "ormula:
8) =s
n
4!ere =smpedance in o!ms 3 p!ase
)<ine-to-line volta#e in 67 nneutral current in amps
n a certain system it is necessary to insert resistance or reactance to restrict t!e "ault current. T!e %ero se&uence impedance is used in s!ort circuit calculations. 4!en a %ero se&uence component o" t!ree line currents are compounded( t!ey will add to#et!er to produce a resultant called a residue( w!ilst t!e positive and ne#ative se&uence components #et cancelled in t!e trans"ormer.
T!e ma#netic "ield produced by a %ero-se&uence set o" currents is radically di""erent "rom t!ose produced by ne#ative or positive se&uence currents( and t!ere"ore %ero se&uence impedance is #enerally very di""erent "rom positive and
ne#ative impedances. A %ero se&uence reactance is e&ual to or less t!an a positive se&uence and it depends on t!e "orm o" core construction and disposition o" t!e windin#s
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T!e purpose to establis! a suitable #round pat! can be ac!ieved wit! t!e !elp o" t!ree pairs o" concentric coils connected to oppose ampere turns. As t!e "luxes oppose( t!e trans"ormer takes a very small ma#neti%in# current durin# normal condition. <ines o" "orce enclosin# bot! coils on one limb are t!ere"ore impossible. T!e eart! "ault current "inds little impedance. T!is connection !as uni&ue c!aracteristics( and is pre"erred to ot!er types o" neutral derivin# trans"ormers.
T!e main "eatures are:
• 4indin# !as muc! lower impedance to %ero se&uence currents. • /ree "orm !armonic residuals.
• Stable neutral can be obtained. • 2ost economical and practical.
• Can be used wit! t!ree p!ase system wit!out secondary windin#. • $ermittin# sin#le p!ase line to neutral.
• Can be economically used wit! resistance3reactance3are suppression coil. • )&uivalent 67A is approximately >D.>E o" star-delta trans"ormer.
• Avoidance o" undesirable stresses in t!e insulation.
• Can be used wit! eit!er delta or star connected windin# to "eed desired load. • t keeps %ero se&uence impedance constant even w!en auxiliary windin#
under load.
/rom t!e above( it is very clear t!at t!e inter-connected star windin# can be utili%ed eit!er as an eart!in# trans"ormer or power trans"ormer( or in combination @ dependin# upon t!e re&uirement.
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T!e eart!in# trans"ormer is o" s!ort time ratin# ; seconds to ; minute. T!e ratin# o" an eart!in# trans"ormer is entirely di""erent "rom t!at o" a power trans"ormer. $ower trans"ormers are desi#ned to carry total load continuously( w!ilst an eart!in# trans"ormer carries no load( and supplies current only i" one o" t!e lines becomes #rounded. t is usual to speci"y t!e sin#le p!ase eart! "ault current( t!at t!e eart!in# trans"ormer must carry "or su""icient time. Since it is almost workin# on no-load( dictates to !ave low iron losses. Because o" it bein# a s!ort time device( its si%e and cost are less t!an t!at o" a continuous duty trans"ormer o" e&ual 67A ratin#. T!e 67A ratin# o" a t!ree p!ase eart!in# trans"ormer or a bank is t!e product o" normal line to neutral volta#e 67 and t!e neutral or #round amperes t!at t!e trans"ormer is desi#ned to carry under "ault conditions "or a speci"ied time. T!e total eart! "ault current and 7 t!e line volta#e( t!e eart!in# trans"ormer s!ort time ratin# is e&ual to 87.
Three phase inter connected star neutral earthing transformer
v/√3
v/√3
4!en speci"yin# ratin# o" t!e eart!in# trans"ormer t!e important parameters are:
1. V&*5,T!e line-to-line volta#e o" t!e system.
2. C++*%5,T!e maximum neutral current to carry "or a speci"ied duration. n a
#rounded system it is based on t!e type o" #roundin#. +ependin# on t!eir duration( several rates o" s!ort
". Tm*5,+esi#ned to carry rated current "or a s!ort time duration i.e.( ;
seconds to F seconds .+ependin# upon t!e time settin# o" t!e protective #ear on t!e system( And t!e location o" t!e trans"ormer .)art!in# trans"ormer time is ; sec "or protection( and "or "eeder it F sec.
!. R*-%-*5,t!is &uantity is a "unction o" t!e initial symmetrical t!ree p!ase
s!ort circuit 67A. t is also based on t!e type o" #roundin#( and type o" application o" li#!tnin# arrester and transient over volta#es.
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T!e desired electrical parameters o" t!e eart!in# trans"ormer are to be ac!ieved by veri"yin# t!e calculations based on electrical( mec!anical and t!ermal computations. Suc! as t!e re&uired electrical stren#t!( mec!anical ru##edness( dynamic and t!ermal resistance o" t!e windin#s in t!e event o" s!ort circuit( are to be Solved care"ully at t!e desi#ned sta#e. w!en desi#ned as an eart!in# trans"ormer it is ,sually manu"actured as an auto trans"ormer inter-start.
T!e "ollowin# brie" details re"er to a core type oil "illed trans"ormer wit! cylindrical windin#s "or %i#-%a#-wit! or wit!out auxiliary windin#-w!ic! are popular all over t!e world. T!e core o" suc! a trans"ormer is built in t!e same manner as t!at o" a
Schematic winding arrangement of interconnected star earthing
trans"ormer @t!e only di""erence is t!at instead o" two windin#s per limb w!ic! is divided into two e&ual portions( and connected as s!own in "i#ure. T!e currents "lowin# are in opposition( t!ere by undesirable !armonics are avoided. T!e c!oice o" t!e windin# con"i#urations are dictated by %ero se&uence impedance in a sin#le windin# .wit! an auxiliary by bot! %ero se&uence and s!ort circuit impedance. Auxiliary windin# may be used continuously to supply station auxilaries. T!e type o" windin#s may be eit!er multilayer !elical or disc windin#s mainly based on current and volta#es .#enerally( volta#es o" t!ese trans"ormers are up to 88 67 and currents up to several kilo amps based on system re&uirements . figure-3
s!ows di""erent con"i#urations ."i#ure-8bG8care re&uired to meet bot! %ero se&uence impedance and s!ort circuit impedance .ma#netic and electric loadin#s are similar to t!at o" a power trans"ormer .t!e basis "or intimate temperature rise calculation is t!at all t!e !eat is stored in t!e copper wit!out dissipation to insulatin# media or surroundin#s .accordin# to standards (t!e end temperature rise depends on t!e duration o" t!e "ault t!e ultimate bein# 9> c "or a copper windin# in oil ."or continuous ratin# t!e calculation is similar to t!at o" a normal power trans"ormer .insulation desi#n is mainly based on d-electric constants o" paper and oil .t!e ideal stress distribution can be obtained t!rou#! "ield plottin# by t!e "inite element met!od .because o" t!e "re&uent s!ort circuit capability( remedial measures like stren#t!enin# o" t!e coils( proper materials( and
avoidance o" ma#netic asymmetry are to be observed. Since only iron losses are to be dissipated( t!e tank dissipation is ade&uate in a ma'ority o" t!e cases wit! auxiliary loadin# proper care is necessary in respect o" !eat dissipation "or t!e reliability o" t!e trans"ormer. n case o" eart!in# trans"ormers "or industrial applications and "or !eavily polluted atmosp!ere t!e demand is "or bus!in#s o" !i#!er creep a#e distances to avoid "re&uent "las!overs. /urt!ers( t!e external sur"aces are to be painted wit! epoxy base paint .testin# o" t!ese trans"ormers are in accordance wit! national and international standards to veri"y various parameters. And( transportation o" t!ese trans"ormers are similar to small power trans"ormers.
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T!rou#! t!e eart!in# trans"ormers are o" smaller ratin#s compared to normal power and )07 trans"ormers t!ese are very critical e&uipments in utility and industrial applications. T!eir "ault currents are di""erent "or di""erent installations. Comparison o" t!ese industrial trans"ormers are to be care"ully di""erentiated. A deep study is re&uired "or di""erent eart!in# met!ods( and "or coordination o" t!e system e&uipments "or reliable operation. /urt!er researc! study on a distribution trans"ormer wit! out ma'or modi"ication o" t!eir electrical parameters is desired. 4!en #eoma#netic "ield disturbances caused by +C induced currents enter power system at #rounded neutral( t!ese are to be care"ully evaluated in comparison wit! line to eart! "ault currents.
R*3*+*%-*5
