parmar Q&A part 1 to 4

8/13/2019 parmar Q&A part 1 to 4

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Electrical Q&A Part-1

(1) Why ELCB cant work if Neutral input of ELCB does not connect to ground?

ELCB is used to detect earth leakage fault. Once the phase and neutral are connected inan ELCB, the current ill flo through phase and that !uch current ill ha"e to returnneutral so resultant current is #ero.

Once there is a ground fault in the load side, current fro! phase ill directl$ pass throughearth and it ill not return through neutral through ELCB. %hat !eans once side currentis going and not returning and hence ecause of this difference in current ELCB ill tripand it ill safe guard the other circuits fro! fault$ loads. 'f the neutral is not grounded,fault current ill definitel$ high and that full fault current ill co!e ack through ELCB,and there ill e no difference in current.

()* +h$ in a three pin plug the earth pin is thicker and longer than the other pins

't depends upon role/a here area(a* is in"ersel$ proportional to resistance (*, so if

(a* increases, decreases & if is less the leakage current ill take lo resistance pathso the earth pin should e thicker. 't is longer ecause the %he 0irst to !ake the connection and Last to disconnect should e

earth Pin. %his assures afet$ for the person ho uses the electrical instru!ent.

(2*Why elta !tar "ransfor#ers are used for Lighting Loads?

0or lighting loads, neutral conductor is !ust and hence the secondar$ !ust e starinding. and this lighting load is ala$s unalanced in all three phases.

%o !ini!i#e the current unalance in the pri!ar$ e use delta inding in the pri!ar$.o delta / star transfor!er is used for lighting loads.

(3*+hat are the ad"antages of star-delta starter ith induction !otor

(1*%he !ain ad"antage of using the star delta starter is reduction of current during thestarting of the !otor. tarting current is reduced to 2-3 ti!es Of current of 4irect onlinestarting.

()* 5ence the starting current is reduced6 the "oltage drops during the starting of !otor ins$ste!s are reduced.

(7*+hat is !eant $ regenerati"e raking

+hen the suppl$ is cut off for a running !otor, it still continue running due to inertia. 'norder to stop it 8uickl$ e place a load (resistor* across the ar!ature inding and the!otor should ha"e !aintained continuous field suppl$. so that ack e.!.f "oltage is !adeto appl$ across the resistor and due to load the !otor stops 8uickl$. %his t$pe of reakingis called as 9egenerati"e Breaking:.

(;*+hen "oltage increases then current also increases then hat is the need of o"er "oltage rela$and o"er current rela$ Can e !easure o"er "oltage and o"er current $ !easuring currentonl$

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ust !easuring the current onl$ ecause thecurrent increases not onl$ for o"er "oltages ut also for under "oltage(As !ost of theloads are non-linear in nature*.o, the o"er "oltage protection & o"er current protectionare co!pletel$ different.

O"er "oltage rela$ !eant for sensing o"er "oltages & protect the s$ste! fro! insulationreak don and firing. O"er current rela$ !eant for sensing an$ internal short circuit,

o"er load condition ,earth fault there$ reducing the s$ste! failure & risk of fire. o, fora etter protection of the s$ste!. 't should ha"e oth o"er "oltage & o"er current rela$.

(?*'f one la!p connects eteen to phases it ill glo or not'f the "oltage eteen the to phases is e8ual to the la!p "oltage then the la!p ill glo.

+hen the "oltage difference is ig it ill da!age the la!p and hen the difference iss!aller the la!p ill glo depending on the t$pe of la!p.

(@* +hat are 5C fuses and here it is used

5C stand for 9high rupturing capacit$: fuse and it is used in distriution s$ste! forelectrical transfor!ers

(*ention the !ethods for starting an induction !otor

%he different !ethods of starting an induction !otor 4OLdirect online starter tar delta starter Auto transfor!er starter esistance starter eries reactor starter

(1D*+hat is the difference eteen earth resistance and earth electrode resistance

Onl$ one of the ter!inals is e"ident in the earth resistance. 'n order to find the secondter!inal e should recourse to its definition

Earth esistance is the resistance eisting eteen the electricall$ accessile part of auried electrode and another point of the earth, hich is far aa$.

%he resistance of the electrode has the folloing co!ponents(A* the resistance of the !etal and that of the connection to it.(B* the contact resistance of the surrounding earth to the electrode.

(11*+hat is the poer factor of an alternator at no load

At no load $nchronous '!pedance of the alternator is responsile for creating angledifference. o it should e #ero lagging like inductor

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(1)*+h$ !ost of analog o/p de"ices ha"ing o/p range 3 to )D !A and not D to )D !A

3-)D !A is a standard range used to indicate !easured "alues for an$ process. %he reasonthat 3!a is chosen instead of D !A is for fail safe operation .

0or ea!ple- a pressure instru!ent gi"es output 3!A to indicate D psi, up to )D !A toindicate 1DD psi, or full scale. 4ue to an$ prole! in instru!ent (i.e* roken ire, itsoutput reduces to D !A. o if range is D-)D !A then e can differentiate hether it isdue to roken ire or due to D psi.

(12*%o uls of 1DD and 3D respecti"el$ connected in series across a )2D" suppl$ hichul ill glo right and h$

ince to uls are in series the$ ill get e8ual a!ount of electrical current ut as thesuppl$ "oltage is constant across the ul(PFG)/*.o the resistance of 3D+ ul isgreater and "oltage across 3D+ is !ore (F'* so 3D+ ul ill glo righter.

(13*+hat is !eant $ knee point "oltage

Hnee point "oltage is calculated for electrical Current transfor!ers and is "er$ i!portantfactor to choose a C%. 't is the "oltage at hich a C% gets saturated.(C%-currenttransfor!er*.

(17*+hat is re"erse poer rela$

1. e"erse Poer flo rela$ are used in generating stations= protection.). A generating stations is supposed to fed poer to the grid and in case generating units are

off, there is no generation in the plant then plant !a$ take poer fro! grid. %o stop theflo of poer fro! grid to generator e use re"erse poer rela$.

(1;*+hat ill happen if 4C suppl$ is gi"en on the pri!ar$ of a transfor!er

ainl$ transfor!er has high inductance and lo resistance. 'n case of 4C suppl$ there isno inductance, onl$ resistance ill act in the electrical circuit. o high electrical currentill flo through pri!ar$ side of the transfor!er. o for this reason coil and insulationill urn out

+hen AC current flo to pri!ar$ inding it induced alternating flu hich also link tosecondar$ inding so secondar$ current flo in secondar$ inding according to pri!ar$current.

econdar$ current also induced e!f (Back e!f* in secondar$ inding hich opposeinduced e!f of pri!ar$ inding and thus control pri!ar$ current also.

'f 4C current appl$ to Pri!ar$ inding than alternating flu is not produced so no

secondar$ e!f induced in secondar$ inding so pri!ar$ current !a$ goes high and urntransfor!er inding.

(1?*+hat are the ad"antage of free heeling diode in a 0ull +a"e rectifier

't reduces the har!onics and it also reduces sparking and arching across the !echanicalsitch so that it reduces the "oltage spike seen in a inducti"e load.

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(1@*4ifferent eteen !egger and contact resistance !eter

egger used to !easure cale resistance, conductor continuit$, phase identificationhere as contact resistance !eter used to !easure lo resistance like rela$s ,contactors.

(1*+hen e connect the capacitor ank in series

e connect capacitor ank in series to i!pro"e the "oltage profile at the load end intrans!ission line there is considerale "oltage drop along the trans!ission line due toi!pedance of the line. so in order to ring the "oltage at the load ter!inals ithin itsli!its i.e (I or J K; *of the rated ter!inal "oltage the capacitor ank is used in series

()D*+hat is 4i"ersit$ factor in electrical installations

4i"ersit$ factor is the ratio of the su! of the indi"idual !ai!u! de!ands of the "arioussudi"isions of a s$ste!, or part of a s$ste!, to the !ai!u! de!and of the holes$ste!, or part of the s$ste!, under consideration. 4i"ersit$ factor is usuall$ !ore than

one.

()1*+h$ hu!!ing sound occurred in 5% trans!ission line

%his sound is co!ing due to ioni#ation (reakdon of air into charged particles* of airaround trans!ission conductor. %his effect is called as Corona effect, and it is consideredas poer loss.

())*+hat is different eteen grounding and earthings

rounding !eans connecting the neutral point of the load to the ground to carr$ theresidual current in case of unalanced conditions through the neutral to the groundhereas earthing is done in an electric e8uip!ent in order to protect he e8uip!ent inoccurrence of fault in the s$ste!.

()2*+h$ should e the fre8uenc$ is 7D h# onl$ h$ not others like an$thing, h$ should e!aintain the fre8uenc$ constant if so h$ it is onl$ 7D

+e can ha"e the fre8uenc$ at an$ fre8uenc$ $ou like, ut than $ou !ust also !ake $ouron !otors, transfor!ers or an$ other e8uip!ent $ou ant to use.

+e !aintain the fre8uenc$ at 7Dh# or ;Dh# cos the orld !aintains a standard at 7D /;Dh# and the e8uip!ents are !ade to operate at these fre8uenc$.

()3*'f e gi"e )223 A, 73DF on Pri!ar$ side of 1.1)7 FA step up transfor!er, then hat ille the econdar$ Current, 'f econdar$ Foltage11 HF

As e kno the Foltage & current relation for transfor!er-F1/F) ')/'1+e Hno, F' 73D F6 F)11HF or 11DDD F6 '1 )223 A!ps.B$ putting these "alue on elation-

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73D/11DDD ')/)223o,') 113.7 A!ps

()7*hat are the points to e consider for CB(!iniature circuit reaker selection

'(L*M1.)7'(AN* !ai!u! current. c specification are done on !ai!u! currentflo in circuit.

();*hat is the full for! of HFA

+e kno there are three t$pes of poer in Electricals as Acti"e, apparent & reacti"e. oHFA is stand for 9Hilo Folt A!ps ith eacti"e co!ponent.

()?*'n three pin plug ; A!p. ))D" AC rating. h$ earth pin dia!eter is higher than ohter topin hat its purpose

Because Current flo in the conductor is in"ersel$ proportional to the conductor

dia!eter. o if an$ short circuits occur in the s$ste! first high currents $passed in theEarthling ter!inal.( Pl/a area of the conductor increases resistance"alue decreases*

()@*5o can $ou start-up the 3D tue lite ith )2D" AC/4C ithout using an$ choke/Coil

't=s possile $ !eans of Electronic choke. Otherise it=s not possile to ioni#e theparticles in tue. light, ith nor!al "oltage.

()*+hat is 9pu: in electrical engineering

Pu stands for per unit and this ill e used in poer s$ste! single line diagra! there it is

like a huge electrical circuit ith no of co!ponents (generators, transfor!ers, loads* ithdifferent ratings (in FA and HF*. %o ring all the ratings into co!!on platfor! e usepu concept in hich, in general largest FA and HF ratings of the co!ponent isconsidered as ase "alues, then all other co!ponent ratings ill get ack into this asis.%hose "alues are called as pu "alues. (p.uactual "alue/ase "alue*.

(2D*+h$ link is pro"ided in neutral of an ac circuit and fuse in phase of ac circuit

Link is pro"ided at a


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gi"e the poer needed for that particular speed ut in case of ordinar$ rheostat t$pe reg.the poer astage is sa!e for e"er$ speed and no poer is sa"ed.'n electronic regulatortriac is e!plo$ed for speed control.$ "ar$ing the firing angle speed is controlled ut inrheostatic control resistance is decreased $ steps to achie"e speed control.

(2)* +hat happen if e gi"e ))D "olts dc suppl$ to d ul r tue light

Buls de"ices for AC are designed to operate such that it offers high i!pedance to ACsuppl$.


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AF is an are"iation for Auto!atic Foltage egulator. 't is i!portant part in $nchronous enerators, it controls the output "oltage of the

generator $ controlling its ecitation current. %hus it can control the output eacti"ePoer of the enerator.

(2*4ifference eteen a four point starter and three point starter

%he shunt connection in four point starter is pro"ided separatel$ for! the line here as inthree point starter it is connected ith line hich is the draack in three point stater

(3D*+hat is the difference eteen surge arrestor and lightning arrestor

LA is installed outside and the effect of lightning is grounded, here as surge arrestorinstalled inside panels co!prising of resistors hich consu!es the energ$ and nullif$ theeffect of surge.

(31*+hat happens if i connect a capacitor to a generator load

Connecting a capacitor across a generator ala$s i!pro"es poerfactor,ut it ill helpdepends up on the engine capacit$ of the alternator, otherise the alternator ill e o"erloaded due to the etra atts consu!ed due to the i!pro"e!ent on pf.

econdl$, don=t connect a capacitor across an alternator hile it is picking up or ithoutan$ other load

(3)*+h$ the capacitors orks on ac onl$

enerall$ capacitor gi"es infinite resistance to dc co!ponents(i.e., lock the dcco!ponents*. it allos the ac co!ponents to pass through.

(32*5o !an$ t$pes of colling s$ste! it transfor!ers

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(3;*hat is ouchol# rela$ and the significance of it in to the transfor!er

Bouchol# rela$ is a de"ice hich is used for the protection of transfor!er fro! itsinternal faults,

it is a gas ased rela$. hene"er an$ internal fault occurs in a transfor!er, the ouchol#rela$ at once gi"es a horn for so!e ti!e, if the transfor!er is isolated fro! the circuitthen it stop its sound itself other ise it trips the circuit $ its on tripping !echanis!.

(3?*+h$ e do ) t$pes of earthing on transfor!er (ie*od$ earthing & neutral earthing , hat isfunction. i a! going to install a 7oo k"a transfor!er & 2@D k"a 4 set hat should the earthing"alue

%he to t$pes of earthing are 0a!iliar as E8uip!ent earthing and s$ste! earthing. 'n E8uip!ent earthing od$ ( non conducting part*of the e8uip!ent should e earthed to

safeguard the hu!an eings. $ste! Earthing 'n this neutral of the suppl$ source( %ransfor!er or enerator* should e grounded. +ith this, in case of unalanced loadingneutral ill not e shifted. o that unalanced "oltages ill not arise.

+e can protect the e8uip!ent also. +ith si#e of the e8uip!ent( transfor!er oralternator*and selection of rel$ing s$ste! earthing ill e further classified into directl$earthed, '!pedance earthing, resisti"e (


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0or )DD +att Bul )DD )2D/D.@;; );3.7 oh!s 0or 1DD +att Bul 1DD )2D/D.323@ 7)@.@ oh!s and 0or ;D +att Bul ;D )2D/D.);D@@1.; oh!s respecti"el$


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%he trans!ission line toers ould nor!all$ e higher than a sustation structure, unless$ou ha"e a !ulti-store$ structure at $our sustation.

Earth ats are essential in all sustation areas, along ith dri"en earth electrodes (unlessin a dr$ sand$ desert site*.

't is likeise nor!al to run catenaries= (aerial earth conductors* for at least 1k out fro!all sustation structures. %hose earth ires to e properl$ electricall$ to each supporting

trans!ission toer, and onded ack to the sustation earth s$ste!. 't is i!portant to ha"e the catenaries= earth conductors ao"e the poer conductor lines,

at a sufficient distance and position that a lightning strike ill not hit the poerconductors.

'n so!e cases it is thus an ad"antage to ha"e to catenar$ earth conductors, one eachside of the trans!ission toer as the$ protect the poer lines elo in a etter !anner.

'n lightning-prone areas it is often necessar$ to ha"e catenar$ earthing along the fulldistance of the trans!ission line.

+ithout specifics, (and $ou could not presentl$ gi"e toer pictures in a Post ecause of aC3 er"er graphics upload prole!*, specifics ould include

tructure Lightning Protection J eneral At the ustation, it is nor!al to ha"e "ertical electrodes onded to the structure, and

pro>ecting up fro! the highest points of the structure, ith the location and nu!er ofthose electrodes to e sufficient that if a lightning strike arri"ed, it ould ala$s e a"ertical earthed electrode hich ould e struck, rather than an$ electrical e8uip!ent.

'n so!e older outdoor sustation structures, air-reak isolator sitches are often at a "er$high point in the structure, and in those cases s!all structure etension toers areinstalled, ith electrodes at the tapered peak of those etension toers.

%he etension toers are nor!all$ ;DD!! s8uare approi!atel$ until the etensiontoer changes shape at the tapered peak, and in so!e cases pro>ect upards fro! thegeneral structure ) to ; !etres, ith the electrode so!e ) to 2 !etres pro>ecting upardsfro! the top of the etension toer.

%he sustation nor!all$ has a Lightning Counter J hich registers a strike on thestructure or connected catenar$ earth conductors, and the gathering of that infor!ation(Lightning 4a$s, nu!er per 4a$/onth/Sear, A!perage of each strike* gi"es theEngineering taff good statistics for future sustation design.

1. Location). Countr$2. ite soil t$pe and resisti"it$3.


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er"ice factor is the load that !a$ e applied to a !otor ithout eceeding alloedratings. 0or ea!ple, if a 1D-hp !otor has a 1.)7 ser"ice factor, it ill successfull$deli"er 1).7 hp (1D 1.)7* ithout eceeding specified te!perature rise. ects to electro static stresses. +hen the potential increases still further thenthe ato!s present around the conductor starts ioni#e. %hen the ions produced in thisprocess repel ith each other and attracts toards the conductor at high "elocit$ hichintern produces other ions $ collision.

%he ioni#ed air surrounding the conductor acts as a "irtual conductor and increases theeffecti"e dia!eter of the poer conductor. 0urther increase in the potential difference inthe trans!ission lines then a faint lu!inous glo of "iolet colour appears together alongith hissing noise. %his pheno!enon is called "irtual corona and folloed $ productionof o#one gas hich can e detected $ the odor. till further increase in the potential

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eteen the poer conductors !akes the insulating !ediu! present eteen the poerconductors to start conducting and reaches a "oltage (Critical Breakdon Foltage* herethe insulating air !ediu! acts as conducting !ediu! results in reakdon of theinsulating !ediu! and flash o"er is oser"ed. All this ao"e said pheno!enonconstitutes COO


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4ia!eter of the conductor can e increased to reduce the coronadischarge effect. B$ using hollo conductors corona discharge effect cane i!pro"ed.

B$ using Bundled Conductors B$ using Bundled Conductors also corona effect can e reduced

this is ecause undled conductors ill ha"e !uch higher

effecti"e dia!eter co!pared to the nor!al conductors. B$ sing Corona ings or rading ings

%his is of ha"ing no greater significance ut i presentedhere to understand the Corona ing in the Poer s$ste!.Corona ings or rading 'ngs are present on the surgearresters to e8uall$ distriute the potential along theurge Arresters or Lightning Arresters hich are presentnear the ustation and in the %rans!ission lines.

(;1* +hat is electrical corona

Corona is the ioni#ation of the nitrogen in the air, caused $ an intense electrical field. Electrical corona can e distinguished fro! arcing in that corona starts and stops at

essentiall$ the sa!e "oltage and is in"isile during the da$ and re8uires darkness to see atnight.

Arcing starts at a "oltage and stops at a "oltage aout 7DK loer and is "isile to thenaked e$e da$ or night if the gap is large enough (aout 7/@T at 27DD "olts*.

(;1*. +hat are the indications of electrical corona

A si##ling audile sound, o#one, nitric acid (in the presence of !oisture in the air* thataccu!ulates as a hite or dirt$ poder, light (strongest e!ission in ultra"iolet andeaker into "isile and near infrared* that can e seen ith the naked e$e in darkness,

ultra"iolet ca!eras, and da$light corona ca!eras using the solar-lind a"elengths onearth created $ the shielding o#one la$er surrounding the earth.

(;)* +hat da!age does corona do

%he accu!ulation of the nitric acid and !icro-arcing ithin it create caron tracks acrossinsulating !aterials. Corona can also contriute to the che!ical soup destruction ofinsulating ce!ents on insulators resulting in internal flash-o"er.

%he corona is the onl$ indication. 4efects in insulating !aterials that create an intenseelectrical field can o"er ti!e result in corona that creates punctures, caron tracks ando"ious discoloration of


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't "aries depending upon the configuration of the insulators and the t$pe of insulator,


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(2* 5igh esistance rounding (5* J 't is used here ser"ice continuit$ is "ital, suchas process plant !otors. +ith 5, the neutral is grounded through a high resistance sothat "er$ s!all current flos to the ground if ground fault occurs. 'n the case of groundfault of one phase, the fault$ phase goes to the ground potential ut the s$ste! doesn=ttrip. %his s$ste! !ust ha"e a ground fault !onitoring s$ste!. %he use of line to neutral(single phase* is prohiited (


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(?7* %he difference eteen round and


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(2* 4i"ide the F1 $ the rated pri!ar$ "oltage of the transfor!er and !ultipl$ $ 1DD.%his "alue is the percentage i!pedance of the transfor!er.

+hen e di"ide the pri!ar$ "oltage F1 ith the full load "oltage e ill get the shortcircuit i!pedance of the transfor!er ith refereed to pri!ar$ or WD1. 0or getting thepercentage i!pedance e need to use the for!ula WD1M%ransfor!er FA /(8uare ofPri!ar$ line "oltage*.

(?@* PCC 'nco!ers & Bus Couplers are nor!all$ 3-Pole./ +hen is


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ela$ co-ordination and fault clearing ti!e. tandard practice of li!iting earth fault current. 'n case no data or calculation is possile,

go for li!iting E/0 current to 2DDA or 7DDA, depending on sensi"it$ of rela$.

(@1* h$ a neutral grounding contactor is needed in diseal generator

%here ould not e an$ current flo in neutral if 4 is loaded e8uall$ in 2 phases , ifthere an$ fault(earth fault or o"er load* in an$ one of the phase ,then there ill e unalanced load in 4 . at that ti!e hea"$ current flo through the neutral ,it is sensed $C% and trips the 4. so neutral in grounded to gi"e lo resistance path to fault current.

An electrical s$ste! consisting of !ore than to lo "oltage 4iesel enerator setsintended for parallel operation shall !eet the folloing conditions

(i* neutral of onl$ one generator needs to e earthed to a"oid the flo of #ero se8uencecurrent.

(ii* during independent operation, neutrals of oth generators are re8uired in lo "oltagesitchoard to otain three phase, 3 ire s$ste! including phase to neutral "oltage.

(iii* re8uired to achie"e restricted earth fault protection (E0* for oth the generators

hilst in operation.olution

Considering the re8uire!ent of earthing neutral of onl$ one generator, a contactor ofsuitale rating shall e pro"ided in neutral to earth circuit of each generator. %hiscontactor can e ter!ed as 9neutral contactor:.


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are using one each on phase and one in neutral. 't is orking on the principle of alancedcurrents eteen phases and nuetral.nrestricted E/0 protection orking on the principleof co!paring the unalance on the phases onl$. 0or E0 protection PN class cts areusing ut for E0 7P)D Cts using.

0or 4ifferential Protection C%s using on oth side 5% & LF side each phase, andco!paring the unalance current for this protection also PN class C%s are using.

(@3* %ransfor!er tertiar$ inding

Pro"iding a tertiar$ inding for a transfor!er !a$ e a costl$ affair. 5oe"er, there arecertain constraints in a s$ste! hich calls for a tertiar$ transfor!er inding especiall$ inthe case of considerale har!onic le"els in the distriution s$ste!. 0olloing is anecerpt fro! the ook 9%he X&P %ransfor!er Book:.

%ertiar$ inding is !a$ e used for an$ of the folloing purposes (A*%o li!it the fault le"el on the LF s$ste! $ sudi"iding the infeed that is, doule

secondar$ transfor!ers. (B*%he interconnection of se"eral poer s$ste!s operating at different suppl$ "oltages. (C* %he regulation of s$ste! "oltage and of reacti"e poer $ !eans of a s$nchronous

capacitor connected to the ter!inals of one inding. 't is desirale that a three-phase transfor!er should ha"e one set of three-phase indings

connected in delta thus pro"iding a lo-i!pedance path for third-har!onic currents. %hepresence of a delta connected inding also allos current to circulate around the delta inthe e"ent of unalance in the loading eteen phases, so that this unalance is reducedand not so greatl$ fed ack through the s$ste!.

ince the third-order har!onic co!ponents in each phase of a three-phase s$ste! are inphase, there can e no third-order har!onic "oltages eteen lines. %he third-orderhar!onic co!ponent of the !agnetising current !ust thus flo through the neutral of astar-connected inding, here the neutral of the suppl$ and the star-connected indingare oth earthed, or around an$ delta-connected inding. 'f there is no delta inding on a

star/star transfor!er, or the neutral of the transfor!er and the suppl$ are not othconnected to earth, then line to earth capacitance currents in the suppl$ s$ste! lines cansuppl$ the necessar$ har!onic co!ponent. 'f the har!onics cannot flo in an$ of thesepaths then the output "oltage ill contain the har!onic distortion.

E"en if the neutral of the suppl$ and the star-connected inding are oth earthed, thenalthough the transfor!er output a"efor! ill e undistorted, the circulating third-orderhar!onic currents floing in the neutral can cause interference ith teleco!!unicationscircuits and other electronic e8uip!ent as ell as unacceptale heating in an$ li8uidneutral earthing resistors, so this pro"ides an added reason for the use of a deltaconnected tertiar$ inding.

'f the neutral of the star-connected inding is unearthed then, ithout the use of a deltatertiar$, this neutral point can oscillate ao"e and elo earth at a "oltage e8ual in

!agnitude to the third-order har!onic co!ponent. Because the use of a delta tertiar$pre"ents this it is so!eti!es referred to as a staili#ing inding. +hen specif$ing a transfor!er hich is to ha"e a tertiar$ the intending purchaser should

ideall$ pro"ide sufficient infor!ation to enale the transfor!er designer to deter!ine theorst possile eternal fault currents that !a$ flo in ser"ice. %his infor!ation (hichshould include the s$ste! characteristics and details of the earthing arrange!ents*together ith a knoledge of the i!pedance "alues eteen the "arious indings, illper!it an accurate assess!ent to e !ade of the fault currents and of the !agnitude of

)D


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currents that ill flo in the tertiar$ inding. %his is far preferale to the purchaseraritraril$ specif$ing a rating of, sa$, 22.2K, of that of the !ain indings.

(@7* +hat is a transfor!er and ho does it ork

A transfor!er is an electrical apparatus designed to con"ert alternating current fro! one"oltage to another. 't can e designed to 9step up: or 9step don: "oltages and orks onthe !agnetic induction principle. A transfor!er has no !o"ing parts and is a co!pletel$static solid state de"ice, hich insures under nor!al conditions, a long and troule-freelife. 't consists, in it=s si!plest for!, of to or !ore coils of insulated ire ound on ala!inated steel core. +hen "oltage is introduced to one coil, called the pri!ar$, it!agneti#es the iron core. A "oltage is then induced in the other coil, called the secondar$or output coil. %he change of "oltage (or "oltage ratio* eteen the pri!ar$ andsecondar$ depends on the turns ratio of the to coils.

(@;* +h$ do transfor!ers hu!

%ransfor!er noise is caused $ a pheno!enon hich causes a piece of !agnetic sheetsteel to etend itself hen !agneti#ed. +hen the !agneti#ation is taken aa$, it goesack to its original condition. %his pheno!enon is scientificall$ referred to as!agnetostriction. A transfor!er is !agneticall$ ecited $ an alternating "oltage andcurrent so that it eco!es etended and contracted tice during a full c$cle of!agneti#ation.

o %he !agneti#ation of an$ gi"en point on the sheet "aries, so the etension andcontraction is not unifor!. A transfor!er core is !ade fro! !an$ sheets ofspecial steel to reduce losses and !oderate the ensuing heating effect. %heetensions and contractions are taking place erraticall$ all o"er a sheet and eachsheet is eha"ing erraticall$ ith respect to its neighor, so $ou can see hat a!o"ing, rithing construction it is hen ecited. %hese etensions are !iniscule

proportionall$ and therefore not nor!all$ "isile to the naked e$e. 5oe"er,the$ are sufficient to cause a "iration, and conse8uentl$ noise. Appl$ing "oltageto a transfor!er produces a !agnetic flu, or !agnetic lines of force in the core.%he degree of flu deter!ines the a!ount of !agnetostriction and hence, thenoise le"el.

o +h$ not reduce the noise in the core $ reducing the a!ount of flu%ransfor!er "oltages are fied $ s$ste! re8uire!ents. %he ratio of these"oltages to the nu!er of turns in the inding deter!ines the a!ount of!agneti#ation. %his ratio of "oltage to turns is deter!ined !ainl$ for econo!icalsoundness. %herefore the a!ount of flu at the nor!al "oltage is fied. %his alsofies the le"el of noise and "iration. Also, increasing (or decreasing*!agneti#ation does not affect the !agnetostriction e8ui"alentl$. 'n technicalter!s the relationship is not linear.

(@?* 5o can ' reduce airorne noise

Belo is a list of $our !ost effecti"e options (1*Put the transfor!er in a roo! in hich the alls and floor are !assi"e enough to

reduce the noise to a person listening on the other side.


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()*Put the o>ect inside an enclosure hich uses a li!p all techni8ue. %his is a !ethodhich uses to thin plates separated $ "iscous (ruer$* !aterial. As the noise hits theinner sheet so!e of its energ$ is used up inside the "iscous !aterial. %he outer sheetshould not "irate.

(2*Build a screen all around the unit. %his is cheaper than a full roo!. 't ill reduce thenoise to those near the all, ut the noise ill get o"er the screen and fall elsehere (at a

loer le"el*. creens ha"e een !ade fro! ood, concrete, rick and ith dense ushes(although the latter eco!es ps$chological*

(3*4o not !ake an$ reflecting surface coincident ith half the a"e length of thefre8uenc$. +hat does this !ean +ell, e"er$ fre8uenc$ has a a"e length. %o find thea"e length in air, for instance, $ou di"ide the speed of sound, in air (generall$understood as 112D feet per second* $ the fre8uenc$. 'f a noise hits a reflecting surfaceat these di!ensions it ill produce hat is called a standing a"e. tanding a"es illcause re"ererations (echoes* and an increase in the sound le"el. 'f $ou hit thesedi!ensions and get echoes $ou should appl$ asorent !aterials to the offending alls(fierglass, ool, etc.*

(@@* +hat are taps and hen are the$ used

%aps are pro"ided on so!e transfor!ers on the high "oltage inding to correct for highor lo "oltage conditions, and still deli"er full rated output "oltages at the secondar$ter!inals. %aps are generall$ set at to and a half and fi"e percent ao"e and elo therated pri!ar$ "oltage.

(@* +hat is the difference eteen 9'nsulating:, 9'solating:, and 9hielded +inding:transfor!ers

'nsulating and isolating transfor!ers are identical. %hese ter!s are used to descrie theseparation of the pri!ar$ and secondar$ indings. A shielded transfor!er includes a

!etallic shield eteen the pri!ar$ and secondar$ indings to attenuate (lessen*transient noise.

(D* Can transfor!ers e operated at "oltages other than na!eplate "oltages

'n so!e cases, transfor!ers can e operated at "oltages elo the na!eplate rated"oltage. 'n


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(1DD* hat is te!perature rise in a transfor!er

%e!perature rise in a transfor!er is the a"erage te!perature of the indings andinsulation ao"e the eisting a!ient te!perature.

(1D1* hat is 9Class: in insulation

'nsulation class as the original !ethod used to distinguish insulating !aterials operatingat different te!perature le"els. Letters ere used for different designations. Letterclassifications ha"e een replaced $ insulation s$ste! te!peratures in degrees celsius.%he s$ste! te!perature is the !ai!u! te!perature at the hottest spot in the inding.

(1D)* Are te!perature rise and actual surface te!perature related


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polarit$. %his lea"es one high "oltage and one lo "oltage ter!inal unconnected. +henthe transfor!er is ecited, the resultant "oltage appearing across a "olt!eter ill e thesu! of the high and lo "oltage indings. %his is useful hen connecting single phasetransfor!ers in parallel for three phase operations. Polarit$ is a ter! used onl$ ithsingle phase transfor!ers.

(1D@* +hat is eciting current

Eciting current is the current or a!peres re8uired for ecitation. %he eciting current on!ost lighting and poer transfor!ers "aries fro! approi!atel$ 1DK on s!all si#es ofaout 1 HFA and less to approi!atel$ )K on larger si#es of ?7D HFA.

(1D* Can air cooled transfor!ers e applied to !otor loads

%his is an ecellent application for air cooled transfor!ers. E"en though the inrush orstarting current is aout 7 to ? ti!es nor!al running current, the resultant loer "oltagecaused $ this !o!entar$ o"erloading is actuall$ eneficial in that a cushioning effect on

!otor starting is the result.(11D* Can a three phase transfor!er e loaded as a single phase transfor!er

Ses, ut the load can not eceed the rating per phase and the load !ust e alanced.(HFA/2 per phase*

0or ea!ple A ?7 kFA 2 phase transfor!er can e loaded up to )7 kFA on eachsecondar$. 'f $ou need a 2D kFA load, 1D kFA of load should e supplied fro! eachsecondar$.

(111* 5o !an$ B%=s of heat does a transfor!er generate

%he heat a transfor!er generates is dependent upon the transfor!er losses. %o deter!ineair conditioning re8uire!ents !ultipl$ the su! of the full load losses (otained fro!factor$ or test report* of all transfor!ers in the roo! $ 2.31 to otain the B%s/hour.0or ea!ple A transfor!er ith losses of )DDD atts ill generate ;@)D B%s/hour.

Electrical Q&A Part-)

4onload in P40

(1* +hat=s the reason of grounding or earthing of e8uip!ent

ith a ground path, in case of short circuit the short circuit current goes to the od$ of thee8uip!ent & then to the ground through the ground ire.hence if at the !o!ent of faultif a person touches the e8uip!ent od$ he ill not get a shock cause his od$ resistance(in thousands of oh!s* ill offer a high resistance path in co!parison to the ground ire.5ence the fault current ill flo thru the ground ire & not thru hu!an od$.

Pro"iding a ground path helps in clearing the fault. A C% in the ground connectiondetects the high "alue fault current hence the rela$ connected to the C% gi"es reaker atrip co!!and.

)7
http://www.ziddu.com/download/14262448/ElectricalQAPart-2.pdf.htmlhttp://www.ziddu.com/download/14262448/ElectricalQAPart-2.pdf.html


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rounding helps in a"oiding arcing faults. '0 there ould ha"e een no ground then afault ith the outer od$ can cause a arcing to the ground $ reaking the air. %his isdangerous oth for the e8uip!ent & the hu!an eings.

()* A t$pe-C CB has ther!o !agnetic capailit$ 7'n to 1D'n that !eans a short circuit currentill e interrupted as the "alue ill reach eteen 7'n to 1D'n ut the CB reaking capacit$ is(for ea!ple* define as 1DkA.

7'n to 1D'n is the pickup threshold for the !agnetic trip ele!ent. %he CB ill tripinstantaneousl$ hen the current is eteen these li!its. 1DkA is the short circuitithstands capacit$ of the CB. nder nor!al condition, a current li!iting t$pe CBill trip on short circuit (!agnetic trip* and the current during circuit interruption ill e!uch less than the prospecti"e current. 5oe"er, the CBs ha"e to ha"e a short circuitcapacit$ !ore than or e8ual to the fault le"el at the location here it is installed.

(2* +hat is 0errari Effect

0erranti Effect is due to the rise in "oltage at the recei"ing end than that of the sendingend.%his occurs hen the load on the s$ste! reduces suddenl$ %rans!ission line usuall$ consists of line inductance ,line to earth capacitance and

resistance. esistance can e neglected ith respect to the line inductance .+hen the loadon the s$ste! falls the energ$ stored in the capacitance gets discharged. %he chargingcurrent causes inducti"e reactance "oltage drop. %his gets added "ector all$ to thesending end "oltage and hence causes the "oltage at the recei"ing end to raise

A Long trans!ission line dras significant a!ount of charging current. 'f such line isopen circuited or "er$ lightl$ loaded at the recei"ing end, the "oltage at the recei"ing end!a$ eco!e greater than sending end "oltage. %his effect is knon 0erranti effect and isdue to the "oltage drop across the line inductance (due to charging current* eing inphase ith the sending end "oltages. %herefore oth capacitance and inductance is

responsile to produce this pheno!enon. %he capacitance (charging current* is negligile in short lines, ut significant in !ediu!and long trans!ission line. 5ence, this pheno!enon is applicale for !ediu! and longtrans!ission line.%he !ain i!pact of this pheno!enon is on o"er "oltage protection s$ste!, surgeprotection s$ste!, insulation le"el etc.

C% ith D.)s class is !ore accurate than ith D.) class. ecause in D.) class C%, ratio &phase angle errors !ust e ithin the specified li!its at 7K, )DK, 1DDK & 1)DK ofrated secondar$ current. +hereas in D.)s class C%, ratio & phase angle errors !ust eithin the specified li!its at 1K, 7K, )DK, 1DDK & 1)DK of rated secondar$ current.Also in D.)s class, atio & Phase angle errors li!its are loer than D.) classes.

Earthing an e8uip!ent doesn=t !ean it re8uires a ground or actual earth U its >ust

enough to ha"e a sink hich can dissipate all the fault current. so in the case of aeroplane as ell as ship the od$ of the sa!e is enough to dissipate all the fault current.(ship can e grounded to ater also*. it !ean to sa$ the fault current ill e dissipatedthrough the od$.

A lightening strike ill opt for the shortest path to reach the earth.an aeroplane ill in noa$ connected to earth so a lightening strike ill ne"er take its path through aero plane.'t !ean to sa$ an aero plane ill ne"er see a lightening strike in its path.

);


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($) Why earth point in % pin socket is twice in si&e

h$ earth point in 2 pin socket is tice in si#e co!pared to phase & neutral hereasconductor e used for oth end of socket for earth point ha"ing sa!e cross-section as eused for phase & neutral

k M(L/A* kresisti"it$ AArea LLength so large is the area, the less ill e resistance. 5ence, if an$ e8uip!ent ha"e current in the od$/co"er, then it ill follo the less

resistance path. And, od$ current ill e earthed. %his is the hard a$ co# e ha"e to consider to the derating factor, current up rating

factor, effect of short circuit current , electrod$na!ics effect, strength of !aterial, But itill take ) da$s to calculate the si#e in such a !anner. o i usuall$ opt for the eas$ go

i#e of Copper us ar Continuous Current ating / D.@i#e of Alu!inu! us ar Continuous Current ating / 1.)eg if the continous current rating is 3DDA!peres , then if $ou choosing Al us ar ,si#e should e greater than 3DD/ 1.) 7DDs8!!si#e of Cu usar should e greater than 3DD/ D.@ 222s8!!

(@ *+here is Auto-recloser is used

en. protection / %rans. Protection / %/L / Bus ar protection Auto reclosure is generall$ used for %rans!ission lines here the general t$pe of faults

are transient in nature. 't can e three phase auto-reclosure or single pole auto-reclosure. %he single pole auto reclosures are generall$ for 3DDkF line. elo this if e!plo$ed are

three pole auto- reclosures. %he reason ' kno is for a line the single pole reclosure pro"ides a etter stailit$ of the

s$ste!.. since so!e part of poer is still transferred through the health$ phases. Also 3DDkF reaker till date has a independent dri"e/ trip/ close coils for the three poles,

elo that all reakers ha"e co!!on dri"e/ trip / closing coils for the three poles. %he usual si#e used of copper us ar is )7Y; Cu for panel.. %he si#e is calculated as per the fault current. refer 'EEE-@D for C a!pacit$

calculations. %he general thu! rule is )DDA/s8 !! for Cu and 1DDA/s8 !! for Al for 1 sec C

ratingU

(1D* +hat is difference eteen poer transfor!ers & distriution transfor!ers

4istriution %ransfor!ers are designed for a !ai!u! efficienc$ at 7DK of load.hereas poer transfor!ers are designed to deli"er !a efficienc$ a$ DK and ao"eloads.

%he distriutions transfor!ers ha"e lo i!pedance so as to ha"e a etter regulation U

poer transfor!ers ha"e higher so as to li!it the C current. Poer transfor!ers are used to step up "oltages fro! 11 k" hich is the generating

"oltage to 12) or hate"er ill e the trans!ission "oltage le"els. Poer transfor!ersare ha"ing 4EL-4EL connection. +ill e located at poer generating stations.

4istriution transfor!ers are used to step don "oltages fro! transfor!er le"els to 11k"/317 ". +ill e ha"ing 4EL-S. +ill e located in sustations near load centers.

%he !ain asic difference lies in the 4esign stage itself as poer transfor!er are tooperate at near full load so there sensing is such that the$ achie"e e8ual. of copper losses

)?


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& iron losses at full loads hereas this is achie"ed in the design itself at aout 7DKloading in dist transfor!er ut friends there is a dile!!a as our dist. transfor!er areal!ost full$ loaded & e$ond so the$ ne"er go operate at their full eff. & also poor"oltage regulation.

%he difference eteen poer and distriution transfor!ers refers to si#e & input"oltage. 4istriution transfor!ers "ar$ eteen )7 kFA and 1D FA, ith input "oltage

eteen 1 and 2; kF. Poer transfor!ers are t$picall$ units fro! 7 to 7DD FA, ithinput "oltage ao"e 2; kF. distriution transfor!er design to ha"e a !a efficienc$ at aload loer than full load.poer transfor!er design to ha"e a !a efficienc$ at full load.

(11*+h$ it that the "oltage le"els in 'ndia is is in ultiples of 11

Foltage le"els in 'ndia are the !ultiples of 1.1. co# ripple factor of a perfect sine a"e is1.11. 5ence to transfer an e8ui"alent of 1DD "olts, e need to transfer 1.11M1DD i.e. 11D"olts. But not in dc "oltage.

'n 'ndia, trans!ission is !ostl$ done at 22k". for longer transfer of poer, e use 33Dk"at the !ost. the highest "oltage at hich poer is transferred is ?;7k", that=s in northA!erica..

%here has een no change in ' and 'EC considering digital !eters, !oreo"er, urden ofinstru!ent transfor!er depends on the lead ire used in the secondar$ of C% or P%.

enerall$ highl$ accurate C% or P% of D.1 or D.) are used for !easure!ent purpose,e$ond this, accurac$ class of D.D7 or D.D1 is used $ standard laoratories like E4AO CP' for caliration purpose of ct or pt.

9election of urden depends co!pletel$ on the use of leads on secondar$ side of C% orP% .:

+hen C% or P% is to e calirated, it has to e calirated using the sa!e lead urden ithhich it has to e connected at site for connecting it to !eters.

C% Burden ' ' ( ct I )l I !*here ct C% 'nternal resistance

l esistance of lead/cale! esistance of !eter(s* connected in series P% Burden F F / (( pt I !*, for a conser"ati"e design $ou can ignore the lead

resistance since it ill reduce the urden on P%

(12*+hat ill e happen if the neutral isolator ill e open or close during the running conditionof poer s$ste!

4uring nor!al condition the neutral isolating sitch should e kept close. 'n case it iskept open, under alanced load conditions the current thru neutral ill not flo &nothing har!ful ill take place ut in case an earth fault takes place then there ill e noearth fault current floing thru the s$ste! & the generator ill run as a ungrounded

generator. %hus the earth fault ill not e cleared. 'n case of ) or !ore generators connected to a co!!on us ithout a transfor!er in

eteen, asicall$ in h$dro stations, one of the


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all the olted connections for >ust a s!all test like checking $our insulation ith a !eggaretc. for such things $ou need a


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%he LA is pro"ided ith a dedicated POPE earthing hich !a$ e in the for! of auried treated electrode net to it.LA connection is securel$ !ade ith the electrode "ia asurge counter. 'f $ou directl$ earth the LA through structure then the surge counter illnot e ale to !easure the no of surges. for lesser rating the counter is not pro"ided,hence $ou can $pass the insulated ase. But then proper earthing has to e assured.

's !otori#ed earth sitch alloed for isolator as per ' and 'EC

' think it is alloed cause e"en if it is !otori#ed the interlocking ill pre"ent fro!eathing during the closed condition

()D* +hat is !ain #one and check #one in case of )1 protections

ain #one is the #one in hich the distance rela$ ill !onitor the i!pedance & duringthe fault if the i!pedance falls under its characteristics (circle/8uadrilateral* then therela$ ill operate. Check #one is a ack up for the !ain #one.

()1* ' %5E +EL4'


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D.1R-D.1R-7D.)R-D.)R-1DD.7R-D.7R-)D1R-1R-3D2R-2R-to e specified.

i!ilarl$ indicating instru!ents shall ha"e accuracies & accordingl$ application as

depicted elofor testing the folloing "alues are generall$ usedfor routine tests accurac$ class 1for t$pe tests accurac$ class D.7 or etter.indicating !eters generall$ ill ha"e accurac$ of 1.

()7* 0irst pole to clear factor-Circuit reakers

%he first pole to clear factor (kpp*is depending on the earthing s$ste! of the netork.%he first pole to clear factor is used to calculating the transient reco"er$ "oltage for threephase faults. 'n general folloing cases appl$-1. kpp 1.2 corresponds to three phase faults in s$ste! ith an earthed neutral.). kpp 1.7 corresponds to three phase faults in isolated or resonant earthed s$ste!.2. kpp 1.D corresponds to special cases e.g. raila$ s$ste!s.A special case is hen there is a three phase fault ithout in"ol"ing earth in a s$ste!ith earthed neutral. %his case responds to kpp 1.7 . %his special case is hoe"er notnor!all$ considered in the standards.

();* +hat is the roll of ecti"esof-pre"enting the arcing,-li!iting the fault current-& isolating the faulted s$ste! are achie"ed

21


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Of course $ou can ha"e different t$pes of earthing s$ste!s ased on $our re8uire!ent &s$ste! configuration.i hope the eplanation as understandale.


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11kF is the phase to earth "oltage for ))DkF ))D/ (s8rt(2*M11*1)


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%he "oltage rating of LA is selected as-Line "oltage s8rt()*/ s8rt(2*so for 11kF line its kF

'n that case also the "alues ont differ !uch if u take the %OF factor as 1.3. 5oe"er,$ou can take the "alue of 1.7; as %OF to e !ore precise.

0rankl$ speaking one ill ha"e to do an intensi"e poer s$ste! stud$ after that one as to

calculate the o"er"oltage taking place at different locations and at the location of fault.Once that is done e need to otain the te!porar$ o"er "oltage factor hich is thencalculated fro! so!e 'EC cur"es. But such a stud$ is done onl$ hen an intestine poers$ste! stud$ is carried out.

%he selection criterion of the insulator discs depend on the installation !ethod & thes$ste! "oltage, rightl$ said e can ha"e 11 k" & 22 k" disc.

enerall$ 11k" discs are stacked together to pro"ide enough creep age so as to pre"entan$ electrical fault to ground & also !eans for suspension. 5ence s$ste! "oltage di"ided$ 11 shall gi"e $ou a rough idea of the nu!er of discs.

After this one needs to see the force that the stack has to ear. 'f $ou ha"e a strain t$pe offitting ie the stack has to ear hori#ontal conductor tension, eight load of the conductor,ind load, ice load etc then the nu!er of insulator discs re8uired !a$ e !ore. Sou illha"e to calculate the forces occurring & hat the discs can not ear.

But for a suspension t$pe s$ste! hich has to ear onl$ the eight then nu!er of discsre8uired !a$ e less than hat $ou get $ di"iding $ 11. %hat is the reason $ou ha"eseen onl$ )2/)3 discs in 3DD k" line cause in that case the creep age otained !ust ha"eeen enough & also the strain re8uire!ent.

22k" insulators is generall$ used in a "ertical installation & are not stacked togethercause that ill !ake the suspension "er$ rigid.

Lo fre8uenc$ (7D J ;D 5#* AC currents can e !ore dangerous than si!ilar le"els of4C current since the alternating fluctuations can cause the heart to lose coordination,inducing "entricular firillation, hich then rapidl$ leads to death.

5oe"er an$ practical distriution s$ste! ill use "oltage le"els 8uite sufficient toensure a dangerous a!ount of current ill flo, hether it uses alternating or directcurrent. ince the precautions against electrocution are si!ilar, ulti!atel$, the ad"antagesof AC poer trans!ission outeighed this theoretical risk, and it as e"entuall$ adoptedas the standard.

(%') What will happen if dc supply will gien to 1 w *ul*+

Of course the ul ill glo.


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(2@* +hat all are the applications here high speed grounding sitches are used.

enerator neutral is earthed directl$ or through distriution transfor!er. %his neutralearthing is through done through a sitch. %his is fine if there is onl$ one generator.

0or to generators in parallel to a us the neutral earthing differs. o!e sa$s oth theneutral are to earthed. o!e sa$ onl$ one neutral has to e earthed.

%he eplanation gi"en as, if oth the neutral earthing are closed the negati"e se8uencecurrent ill e floing though oth the generator taking earth as path. %his leads toincrease in loss and increase in te!perature ( %his !a$ leads to false tripping also *.5ence once the second generator is s$nchroni#ed ith the us or grid the neutral isisolated.

neutral grounding sitch e don=t need a high speed grounding sitch. a nor!al sitchith the correct rating capacit$ ould ork.

(2* +hat is kin Effect[[[ & !ore i!portantl$ 5o does it happen

e!e!er farada$s la of electro!agnetic induction a conductor placed in a

changing !agnetic field induces an e!f ell the sa!e is happening here %he effect of ack e!f is !a at the centre cause of !ai!u! lines of field there. 5encethe !ai!u! opposition nthere & !ini!u! opposition at the surface..hence the currenttries to follo at the surfaceU't is due to this reason that e take hollo tue conductorsin us duct.

%aking into account the inductance effect, its si!ple consider the 4C current..ince itsconstant & not "ar$ing hence no ack e!fUut if u graduall$ start increasing thefre8uenc$ then the flu cutting the conductor goes on increasingUhence greater thefre8uenc$ greater the alternating flu cutting the conductor & hence greater the ack e!f& therefore greater the skin effect[[[

(3D* +h$ there is a need to ground the sheath of single core poer cales. +h$ the grounding

a"oided at oth the ends

A single core cale ith a sheath is nothing ut a conductor carr$ing current surrounded$ another conductor(sheath*.

5ence the AL%E


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%here is another s$ste! called the cross onding s$ste! in hich the sheath aresectionaliosed & cross connected so that the circulating currents are !ini!i#ed. Althoughso!e potential ill also eist eteen sheath & ground, the sa!e eing !ai!u! at thelink oes here onding is done. %his !ethod pro"ides !ai!u! possile currentcarr$ing capacit$ ith the !ai!u! possile lengths.

(31* +hat is E4O & 4O t$pe reaker

'n the Breakers for the operation spring charging is !ust. 'n E4O reaker the spring charging is done ith a !otor and dra out !anuall$ $ hand.

so E4O !eans Electricall$ spring charged 4ra Out reaker 'n 4O reaker the spring charging is also done $ hand !anuall$ and dra out aout

also $ hand onl$. so 4O !eans anual spring charge 4ra Out reaker

(3)* +h$ transfor!er rating is in HFA or FA

Because poer factor of the load is not defined in case of transfor!erUthats h$ it isnot possile to rate transfor!er in H+.

%he losses (cu loss and iron loss* of the transfor!er depends on current and "oltagepurel$, not on load i.e, phase angle eteen the current and "oltage i.e. h$ transfor!errated in kFA

%ransfor!er is not a load and ha"ingno effect on P.0 (that=s h$ no change in its poer factor* and it onl$ transfer theconstant poerfro! one "oltage le"el to another "oltage le"el ithout changing fre8uenc$. since oththe losses"i# copper loss(depends on current* and iron loss(depends on "oltage* are independent of

poer factor, that is h$ a %ransfor!ers rating is not on k+, ut on HFA

(32* Co!plete use & operation on auto-reclosure

an$ faults on o"erhead trans!ission lines are transient in nature,DK of faults are used$ irds,tree ranches.these condition results in arching faults and the arc in the fault cane etinguished $ de-energi#ing the lines $ si!o opening of CB on the oth ends of thelines.O-D.2-CO-2!in-COthis is the se8uence of A.i.e.-OPE


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single phase a/r- onl$ the fault$ phase is reclosed ithout causing interruption ininterchange of s$n.poer eteen to s$s through other to health$ phases.

%SPE O0 A/ (ased on atte!pts of reclosure*

single shot J in this sche!e c is reclosed onl$ once on a gi"en fault efore lockout of coccurs .high speed a/r for E5% s$ste! is in"arial$ single shot.

ulti shot J in this sche!e !ore than one reclosing atte!pt is !ade for a gi"en faultefore lockout of c occurs.epeated closure atte!pts ith high fault le"el ould seriousl$ affect c.

%he factor that !ust taken into account.a* c li!itation.* s$ste! condition.

%SPE O0 A/ (ased speed*

high speed

lo/dela$ed a/r

(33* +h$ the secondar$ of C% ne"er open hen urden is connected on the C%.

secondar$ of a ct is ne"er opened as ecause ct is ala$s connected to the line so openingthe secondar$ ill !ean there ill e no counter !!f to alance the pri!ar$ current as aresult of hich a "er$ high induced e!f !a$ appear in the secondar$ as flu is "er$ highand no counter !!f and this ill e dangerous for the personnel in the secondar$ sideand for pt if it is shorted then ith full "oltage applied to the pri!ar$.

if e short the secondar$ then !uch high current ill circulate in the secondar$ due tohigh induced e!f !uch higher than the actual full load current as a result of hich thetransfor!er=s secondar$ inding !a$ urn out.

(37* 4istance rela$ setting

tep1 et the conductor 4etails (i.e Positi"e e8uence '!pedance (W*, Wero e8eunce '!pedance(WD**hich is in Pri!ar$ "alue. Con"ert in ter!s of secondar$ "alues.

tep ) Based upon the calculated "alue di"ide into "arious #ones

i.e.Wone 1 (0orard* !eans @DK of $our protected line length.Wone ) (0orard* !eans 1DDK of Protected line length I )DK Ad>acent hortest lineWone 2 (0orard* !eans 1DDK of Protected line length I 7DK Ad>acent Longest line.Wone 3 (e"erse* !eans 1DK of protected line.

'f $ou need !ore kindl$ earch 5and ook of Poer $ste!( ABB, AEFA anafacctures*.

(3;* 4ifference eteen C% class D.) and D.)

2?


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D.) is pecial class for !etering. it is !ore accurate than D.) class. generall$ if u useD.)s class ct than ur FA urden of core is also co!e don. But, PC'L &


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'nsulating and isolating transfor!ers are identical. %hese ter!s are used to descrie theisolation of the pri!ar$ and secondar$ indings, or insulation eteen the to.

A shielded transfor!er is designed ith a !etallic shield eteen the pri!ar$ andsecondar$ indings to attenuate transient noise.

%his is especiall$ i!portant in critical applications such as co!puters, process controllersand !an$ other !icroprocessor controlled de"ices.

All to, three and four inding transfor!ers are of the insulating or isolating t$pes.Onl$ autotransfor!ers, hose pri!ar$ and secondar$ are connected to each otherelectricall$, are not of the insulating or isolating "ariet$.

3. Can transfor!ers e operated at "oltages other than na!eplate "oltages

'n so!e cases, transfor!ers can e operated at "oltages elo the na!eplate rated"oltage.

'n


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%he reason for this li!itation in HFA si#e is, the turns ratio is the sa!e as the "oltageratio.

Ea!ple A transfor!er ith a 3@D "olt input, )3D "olt outputR can ha"e the outputconnected to a )3D "olt source and there$ eco!e the pri!ar$ or input to thetransfor!er, then the original 3@D "olt pri!ar$ inding ill eco!e the output or 3@D"olt secondar$.

On transfor!ers rated elo 1 HFA single phase, there is a turns ratio co!pensation onthe lo "oltage inding. %his !eans the lo "oltage inding has a greater "oltage thanthe na!eplate "oltage indicates at no load.

0or ea!ple, a s!all single phase transfor!er ha"ing a na!eplate "oltage of 3@D "oltspri!ar$ and )3D "olts secondar$, ould actuall$ ha"e a no load "oltage of approi!atel$)7D "olts, and a full load "oltage of )3D "olts.

'f the )3D "olt inding ere connected to a )3D "olt source, then the output "oltageould conse8uentl$ e approi!atel$ 3;D "olts at no load and approi!atel$ 33) "oltsat full load. As the HFA eco!es s!aller, the co!pensation is greaterRresulting inloer output "oltages.

+hen one atte!pts to use these transfor!ers in re"erse, the transfor!er ill not ehar!ed6 hoe"er, the output "oltage ill e loer than is indicated $ the na!eplate.

@. Can a ingle Phase %ransfor!er e used on a %hree Phase source

Ses. An$ single phase transfor!er can e used on a three phase source $ connecting thepri!ar$ leads to an$ to ires of a three phase s$ste!, regardless of hether the sourceis three phase 2-ire or three phase 3-ire. %he transfor!er output ill e single phase.

. Can %ransfor!ers de"elop %hree Phase poer fro! a ingle Phase source


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D-)7 HFA single phase D-17 HFA three phase Bus-ar ter!inations(drilled to


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'ndustrial control e8uip!ent de!ands a !o!entar$ o"erload capacit$ of three to eightti!es nor!al capacit$. %his is !ost pre"alent in solenoid or !agnetic contactorapplications here inrush currents can e three to eight ti!es as high as nor!al sealed orholding currents ut still !aintain nor!al "oltage at this !o!entar$ o"erloadedcondition.

4istriution transfor!ers are designed for good regulation up to 1DD percent loading, ut

their output "oltage ill drop rapidl$ on !o!entar$ o"erloads of this t$pe !aking the!unsuitale for high inrush applications.

'ndustrial control transfor!ers are designed especiall$ for !aintaining a high degree ofregulation e"en at eight ti!es nor!al load. %his results in a larger and generall$ !oreepensi"e transfor!er.

Electrical Q&A Part-3

Electrical ,uestions and -nswers .art/0

(1* +hen


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'n solid earthing, $ou do not ha"e this prole!, as at the instant of an earth fault, thefaulted phase goes to neutral potential and the high fault current ould in"arial$ causethe O"er current or short circuit protection de"ice to operate in sufficientl$ 8uick ti!eefore an$ har! could e done.

(2* %ransfor!er tertiar$ inding

Pro"iding a tertiar$ inding for a transfor!er !a$ e a costl$ affair. 5oe"er, there arecertain constraints in a s$ste! hich calls for a tertiar$ transfor!er inding especiall$ inthe case of considerale har!onic le"els in the distriution s$ste!. %ertiar$ inding is!a$ e used for an$ of the folloing purposes

1* %o li!it the fault le"el on the LF s$ste! $ sudi"iding the in feed that is, doulesecondar$ transfor!ers.

)* %he interconnection of se"eral poer s$ste!s operating at different suppl$ "oltages. 2* %he regulation of s$ste! "oltage and of reacti"e poer $ !eans of a s$nchronous

capacitor connected to the ter!inals of one inding.

(3* %he trans!ission toer construction iddle ar! is longer than the upper and loer ar! %o pre"ent a roken upper line fro! falling on one or !ore of the phase lines elo. %he clearance fro! other phase.

o utual inductance !ini!i#ation.o Pre"enting droplet of ater/ice to fall on otto! conductor.o %o pre"ent ig irds fro! u!ping their heads against the o"erhead line hen

the$ sit on the ire elo.

(7* +hat is the difference eteen 9'nsulating:, 9'solating:, and 9hielded +inding:transfor!ers

'nsulating and isolating transfor!ers are identical. %hese ter!s are used to descrie the separation of the pri!ar$ and secondar$ indings. A shielded transfor!er includes a !etallic shield eteen the pri!ar$ and secondar$

indings to attenuate (lessen* transient noise.

(;* +h$ 4on=t +e Break


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1. Location). Countr$2. ite soil t$pe and resisti"it$3.


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%he !agnitude of earth fault current depends on (a* %he eternal earth loop i!pedance of the installation (i.e. e$ond the suppl$

ter!inals* (* %he i!pedance of the acti"e conductor in fault (c* %he i!pedance of the earth cale. i.e. 0ault current "oltage / a I I c


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ore the KW of transfor!er, !ore copper used for inding, increasing cost of the unit.But short circuit le"els ill reduce6 !echanical da!ages to indings during short circuitshall also reduce. 5oe"er, cost increases significantl$ ith increase in KW.

Loer KW !eans econo!ical designs. But short circuit fault le"els shall increasetre!endousl$, da!aging the inding & core.

%he high "alue of KW helps to reduce short circuit current ut it causes !ore "oltage dip

for !otor starting and !ore "oltage regulation (K change of "oltage "ariation* fro! noload to full load.

0ollo the steps elo (1* hort the secondar$ side of the transfor!er ith current !easuring de"ices

(A!!eter* ()* Appl$ lo "oltage in pri!ar$ side and increase the "oltage so that the secondar$

current is the rated secondar$ current of the transfor!er. easure the pri!ar$ "oltage(F1*.

(2* 4i"ide the F1 $ the rated pri!ar$ "oltage of the transfor!er and !ultipl$ $ 1DD.%his "alue is the percentage i!pedance of the transfor!er.

+hen e di"ide the pri!ar$ "oltage F1 ith the full load "oltage e ill get the shortcircuit i!pedance of the transfor!er ith refereed to pri!ar$ or WD1. 0or getting thepercentage i!pedance e need to use the for!ula WD1M%ransfor!er FA / (8uare ofPri!ar$ line "oltage*.

(17* +h$ a neutral grounding contactor is needed in diesel generator

%here on=t e an$ current flo in neutral if 4 is loaded e8uall$ in 2 phases, if therean$ fault (earth fault or o"er load* in an$ one of the phase, then there ill e

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