Earthing-Transformers.pdf

8/14/2019 Earthing-Transformers.pdf

1/12

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.com

EARTHING TRANS!R"ERS

!R



2/12

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.comA#STRA$T%&

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



3/12


-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


4/12

www.1000projects.comwww.fullinterview.comwww.chetanasprojects.com"a,netiin, current 9er' less Normal

-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


5/12


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


6/12


#$ %!&'*+!!'*&', 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.


7/12


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.



8/12


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


Three phase inter connected star neutral earthing

transformer

v/3

v/3

v/3


9/12


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.


Schematic winding arrangement ofinterconnected star earthing transformer


10/12


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



11/12


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.



12/12


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.

http://www.google.com/http://www.google.com/http://www.google.com/

Date post:	04-Jun-2018
Category:	Documents
Upload:	sorry2qaz
View:	220 times
Download:	0 times

Earthing-Transformers.pdf

Documents