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EARTHING TRANS!R"ERS
!R
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Normall' power s'stems an( net wor)s are operate( un(er varia*le comple+ stresses.
In power s'stems the faults are not avo(a*le even after ta)in, utmost care at ever'
sta,e&from plannin, to maintainance. The ,roun(in, of a circuit re(uces potentialstresses un(er fault con(ition. -ower fee(in, from (elta (elta or if there is no
accessi*ilit' for star connecte( transformers occasionall' shorte( to ,roun( is ver'
common&un&intentional ,roun(in, occurs an' where from the fee(in, s'stem to
utiliation e/uipment The main o*jective of ,roun(in, neutral is to ma)e a short circuit
current sufficient in ma,nitu(e for the rela' action. This article restricte( to i,&a, t'pe
with oil fille( transformers. The neutral point is usuall' availa*le at ever' volta,e level
from ,enerator to transformers. In the a*sence of a power transformer of suita*le
capacit' connection an( (esi,n a separate ,roun(in, transformer can *e use( .
The' are in(uctive (evices inten(e( primaril' to provi(e a neutral point for ,roun(in,
purpose. The main purpose of this paper is to (iscuss a*out the (ifference *etween
Earthin, transformer an( -ower transformer an( to (iscuss a*out the (esi,nin, of earth
transformers.
INTR!2$TI!N%&
A transformer is a (evice which electroma,neticall' transfers A.$. volta,e from one
level to another level. An earthin, transformer is a transformer primaril' to provi(e a
neutral point for ,roun(in, purpose. The sole (ut' of the ,roun(in, transformer is to pass
,roun( current (urin, an earth fault. an( it carries no useful loa(.
The (esira*le /uantities of an earthin, transformer are low ero
impe(ance an( low losses 3no loa( losses4.
2N$TI!N ! EARTHING TRANS!R"ERS%&
Apart from provi(in, an eas' path to ,roun( current (urin, an earth fault the
followin, a((itional functions are also to *e achieve(
Hol(in, the neutral shift with in the limits
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-ermittin, the circulation of un*alance( loa( current in the neutral
To limit the current (urin, line to earth faults
To earth the s'stem
To provi(e sin,le phase line to neutral loa( $an *e use( with resistance 5reactance5arc suppression coil
$!"-ARISI!N #ET6EEN EARTHING AN -!6ER
TRANS!R"ERS%&
The selection an' transformer mainl' *' two factors&namel' primar' volta,e
an( loa( .or an earthin, transformer loa( is short time or short (ut'
3 onl' at the time of fault4 normall' from a few secon(s to one minute
COMPARISION CHARACTERISTICS
2.29MVA (S.C) 6.6KV 600 Amps(ET@) 2MVA11/0.433KV D!" 1 (PT@)
ET7 -T7
-urpose To provi(e a neutral point
for earthin,
Rai(in, or lowerin,
volta,es89A ratin, Short time $ontinuous
9olta,es in ,eneral 2p to ::)v Hi,her volta,es3even;;089 an( a*ove4
No of win(in,s !ne5two !ne5two5three
irection of power flow 2ni (irection #I&(irectional
S.$ with stan( (uration 10sec.to 1 min ; sec
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-riorit' for impe(ance =ero se/uence impe(ance Short circuit impe(ance
Short circuit current epen(s on s'stem fromfew hun(re( amperes to fewthousan( amperes
2p to ;> times normalcurrent
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Significance of zero sequence parameters
In s'mmetrical components the positive se/uence is one havin, normal phase
se/uence? the secon( is of ne,ative se/uence with reverse( phase se/uence? an( the
thir( that has no se/uence an( is calle( ero se/uence @ are important parameters. An
earth fault is one that part of the current in the fault' phase returns to the suppl'
throu,h the earth. This is (ue to a s'mmetrical component consistin, of three sin,le
phase currents & one in each line an( all *ein, in phase.
In ,eneral the impe(ance of the transformer is the main factor which influences
applications. The ero se/uence impe(ance of a stan(ar( earthin, transformer can *e
calculate( from the followin, formula%
:E
=s BCCCC
In
6here =sBImpe(ance in ohms 5 phase
EB
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#$ %!&'*+!!'*&', s&- %!,%! %s p''',
The purpose to esta*lish a suita*le ,roun( path can *e achieve( with the help ofthree pairs of concentric coils connecte( to oppose ampere turns. As the flu+es
oppose the transformer ta)es a ver' small ma,netiin, current (urin, normal
con(ition.
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R-&%! -!, %&s %!&''"-&', p--m'&'s + -! '-&$%! &-!s+m'
The earthin, transformer is of short time ratin, 310 secon(s to 1 minute4. The ratin,
of an earthin, transformer is entirel' (ifferent from that of a power transformer.
-ower transformers are (esi,ne( to carr' total loa( continuousl' whilst an earthin,
transformer carries no loa( an( supplies current onl' if one of the lines *ecomes
,roun(e(. It is usual to specif' the sin,le phase earth fault current that the earthin,transformer must carr' for sufficient time. Since it is almost wor)in, on no&loa(
(ictates to have low iron losses. #ecause of it *ein, a short time (evice its sie an(
cost are less than that of a continuous (ut' transformer of e/ual 89A ratin,. The
89A ratin, of a three phase earthin, transformer or a *an) is the pro(uct of normal
line to neutral volta,e 3894 an( the neutral or ,roun( amperes that the transformer is
(esi,ne( to carr' un(er fault con(itions for a specifie( time. The total earth fault
current an( 9 the line volta,e the earthin, transformer short time ratin, is e/ual to
:9I.
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6hen specif'in, ratin, of the earthin, transformer the important parameters are%
1. V+"&-' The line&to&line volta,e of the s'stem.;. C'!&The ma+imum neutral current to carr' for a specifie( (uration. In a
,roun(e( s'stem it is *ase( on the t'pe of ,roun(in,. epen(in, on their
(uration several rates of short
:. T%m'esi,ne( to carr' rate( current for a short time (uration i.e. 10
secon(s to F0 secon(s .epen(in, upon the time settin, of the protective ,ear
on the s'stem An( the location of the transformer .Earthin, transformer time
is 10 sec for protection an( for fee(er it I F0 sec.
. R'-*&-!*'this /uantit' is a function of the initial s'mmetrical three phase
short circuit 89A. It is also *ase( on the t'pe of ,roun(in, an( t'pe of
application of li,htnin, arrester an( transient over volta,es.
M-+ ,'s%!5 *+!s&*&%+!-" -!, m-!-*&%! '-&'s
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Three phase inter connected star neutral earthing
transformer
v/3
v/3
v/3
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The (esire( electrical parameters of the earthin, transformer are to *e achieve(
*' verif'in, the calculations *ase( on electrical mechanical an( thermal
computations. Such as the re/uire( electrical stren,th mechanical ru,,e(ness
('namic an( thermal resistance of the win(in,s in the event of short circuit are to *eSolve( carefull' at the (esi,ne( sta,e. when (esi,ne( as an earthin, transformer it is
2suall' manufacture( as an auto transformer 3inter&start4.
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Schematic winding arrangement ofinterconnected star earthing transformer
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The followin, *rief (etails refer to a core t'pe oil fille( transformer with c'lin(rical
win(in,s for i,&a,&with or without au+iliar' win(in,&which are popular all over the
worl(. The core of such a transformer is *uilt in the same manner as that of atransformer @the onl' (ifference is that instea( of two win(in,s per lim* which is
(ivi(e( into two e/ual portions an( connecte( as shown in fi,ure. The currents
flowin, are in opposition there *' un(esira*le harmonics are avoi(e(. The choice of
the win(in, confi,urations are (ictate( *' ero se/uence impe(ance in a sin,le
win(in, .with an au+iliar' *' *oth ero se/uence an( short circuit impe(ance.
Au+iliar' win(in, ma' *e use( continuousl' to suppl' station au+ilaries. The t'pe of
win(in,s ma' *e either multila'er helical or (isc win(in,s mainl' *ase( oncurrent an( volta,es .,enerall' volta,es of these transformers are up to :: 89 an(
currents up to several )ilo amps *ase( on s'stem re/uirements .figure-3
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shows (ifferent confi,urations .fi,ure&:*:care re/uire( to meet *oth ero se/uenceimpe(ance an( short circuit impe(ance .ma,netic an( electric loa(in,s are similar to that
of a power transformer .the *asis for intimate temperature rise calculation is that all the
heat is store( in the copper without (issipation to insulatin, me(ia or surroun(in,s
.accor(in, to stan(ar(s the en( temperature rise (epen(s on the (uration of the fault the
ultimate *ein, ;>0 c for a copper win(in, in oil .for continuous ratin, the calculation is
similar to that of a normal power transformer .insulation (esi,n is mainl' *ase( on (&
electric constants of paper an( oil .the i(eal stress (istri*ution can *e o*taine( throu,h
fiel( plottin, *' the finite element metho( .*ecause of the fre/uent short circuit
capa*ilit' reme(ial measures li)e stren,thenin, of the coils proper materials an(
avoi(ance of ma,netic as'mmetr' are to *e o*serve(. Since onl' iron losses are to *e
(issipate( the tan) (issipation is a(e/uate in a majorit' of the cases with au+iliar'
loa(in, proper care is necessar' in respect of heat (issipation for the relia*ilit' of the
transformer. In case of earthin, transformers for in(ustrial applications an( for heavil'
pollute( atmosphere the (eman( is for *ushin,s of hi,her creep a,e (istances to avoi(
fre/uent flashovers. urthers the e+ternal surfaces are to *e painte( with epo+' *ase
paint .testin, of these transformers are in accor(ance with national an( international
stan(ar(s to verif' various parameters. An( transportation of these transformers are
similar to small power transformers.
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C+!*"s%+!
Throu,h the earthin, transformers are of smaller ratin,s compare( to normal poweran( EH9 transformers these are ver' critical e/uipments in utilit' an( in(ustrial
applications. Their fault currents are (ifferent for (ifferent installations. $omparison of
these in(ustrial transformers are to *e carefull' (ifferentiate(. A (eep stu(' is re/uire(
for (ifferent earthin, metho(s an( for coor(ination of the s'stem e/uipments for relia*le
operation. urther research stu(' on a (istri*ution transformer with out major
mo(ification of their electrical parameters is (esire(. 6hen ,eoma,netic fiel(
(istur*ances cause( *' $ in(uce( currents enter power s'stem at ,roun(e( neutral
these are to *e carefull' evaluate( in comparison with line to earth fault currents.
R'''!*'
1. Electrical In(ia journal
;. www.,oo,le.com.
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