-

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NGr-U-\2 \01.5.: M1:.'L- .0;)!'-1

,

.'

,"'"

r '~

,.

"

-,"'"

,",!!!III,.,:~~'i'. !.,..'.tn~"c,

"';);~

(TRA'N.SPO,IIM,E:R'§)

This FreeQuaJity Manuatis Found Only at POWERLINE

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POWERLINE ~VIEWG~NTER . ,

,

'

"',,{.

.

MEZZANINE FLOOR. DONA AMPJ;\RO BUILDING C9,RNER

ESPANA

& G. M: TOLENTiNO ,STREETS

SAMPALOC; MANILA TEL. NOS. 735-73-02

"

'

& 733-2:1.-is

CEERS~ POWERLINE REVIEWCENTER TEL NOS. (032)26:1.-2244

& (032)26:1-8452

.::I

,--

I,

,

According to the core construction: a. Core type b. SheJl type

c.

Berry type

.

.' 2.

AccO1:t{ingtq t!!e purpose for which used: a. Transiillssion tr~sfonner b.. Disnibution transfonner c. Generator transformer d. Testing transformer e. Instrument transfonner 1. CLUTent ( CT ) .2. potential ( PT ) . 3. According to the, method of cooling: a. Self-cooled (AA) b. Forced air cooled (FA) c. Oil-self~cooled (OA) d. Forced. oil cooled with airor water (OA/F A)

IDEAL TRANSFORMER

...

.

.

A transfonner is consideredicteal if there is 110voltage drop ac,ross its windings, ifresistance and leakage flux are neglected, in which losses are neglect;d; if losses are neglected, power input is the same as power output. PRACTICAL TRAl'i'SFORMER \VTTHOUT LOAD & VECTOR DIAGRAM

rs

~


J(s

IN

15=0

lex

..

\P I I I n

Es =Vs [hte

~.

Ih+e

Notations: Np,Ns Vp, Vs Ep, Es rp, rs Xp,xs
= number oftums of primary and secondary winding respectively. =. primary impressed and secondary tenninal voltages respectively. = primary.and secondary induced voltage respectively. ' ~ .primary and secondary winding resistance respectively.

= primary and secondary = mutual flux, useful flux

leakage

reactance

respectively.

which links the primary

and secondary.

2


=

IN

= no-load primary current =

Ih+e lex

leakage flux, wasted flux J e.,+ Ih+c

.

= hysteresis and eddy current component = . = exciting or magnetizing current

Core loss ~

Ep

.

The total induced emfin each winding of the transformer is proportional to the number of turns of the winding i.e. Np

a"" -

Ep

Ns

TRANSFORMER

= --

Es

= ratio of transformation .

GENERATED

VOLTAGE EOUATION

Ep =4.44 f rp", Np in rms Volts Es

= 4.44

f fPmNs in rms Volts

Where: f
= frequency in HZ = mufual flux in Wb = number turns for every windings = a cOnS!anlequal to 4 (1.11) where 1.11 is the form factor of sinusoidal wave

Note:

K r = ~.I'v/Svalue = 1/../2 max. value A VE value (2/ 7r)max. value

PRACTICAL

LOADED TRANSFORMER

rs

1.11

& YECTOR DIAGRAM

Xs

Is ~ I' I: I,. i.'

Rc

~~,

lh+e

!;,{

Let ~., ;,

Ip Is Ip

I;'

.f~

= primary load current = secondary load current = I' p + 7 N

Assuming:

Inductive

"p

= load component of primary current its magnitude is such that. mmf primary = mmf secondary I~ Np=IsNs Np Is -=-=a Ns Ip' since IN «I;,

thus I p '" I~

Np Is a -- -=Ns- Ip

foad

3

TRANSFORMER a. ,

EOUIVALENT

CIRCUITS:

Approximate equivalent circuit referred to the primalY R~

~. I

+

+ I I I

Iii/a

Ip

I I 1

Xejj

aVs I

Rc

Vp I I .I.

I I

Where: Rep

= total

X,p

= total equivalent leakage reactance referred to .

equivalent resistance primalY = rp + a 2 rs

primary ZOP

= R op +

referred

= Xp+ a2 Xs

. X J " 'p ~

.r::-2:2

VR K ep

to

.

v

2

+ /\.e p

Re, XII] = core loss equivalent resistance (accounts for eddy current loss and hysteresis loss) & equivalent magnetizing reactance respectively.

-Vp = aV, + Ip (R.p +jXep) neglecting IN b. Approximate equivalent circuit referred to the secondary

. Res

+

alp

I I I I

I I I

= total equivalent resi~tance referred to .

r

Secondary = rs + X"

+

Vs

a Lh+e

-~ Res

1 I I

Xm Q2

'02

I I I

Where:

15

Rc

VB-6

Xes

~a-

= total equivalent leakage reactance referred to X

p :Second ary = Xs + ? a~

z" IR.ANSEORMER

=R" + j Xc, =~Re,2 + Xes 2

LOSSES ANDJ<;FFICIEN£Y 7J =

-Output Input

xl 0 007'0 = .'

Olltput Olltpill + Losses

)/

xlO 0°10=

input

- Losses 10 x 0%

Input

!!

TRANSFORl\1ER

1.

LOSSES

c.QRIL~-

arc IlXed losses result of hysteresis and cddy cilITent losses in the magnetic

circuit ( core) of the transformer

.

'/1.6 HysteresL,

Loss,

Ph =!CJ, f 13m 1..6 = .

n. PI"'.

Kh ~ f06 :::::>if

}\ a: f ; Ph = Kh f

E '" \<.Z.j

~,m & 0n, is constant

Eddy Curreri/toss. P, = Kef' Pm2 = KY'; P, oc f 2; P, = K, f 2 :::::,if CPm& p", is constant Note: Ph & P, can be detennmed by open circuit test.

£QPPER

2.

J~

- varf with the load and result ofMttting of transformer windings. :::::>based from exact equivalent circuit

Feu

= 1p'

p

= I/R,p = Is "R"

eu

rp + Is'rs

=:> based from approximate

Note: P,~ Ci'l be ddennine

equivaJem circuit.

by short circllit test.

LABOR,\, TQ.1lY TF.ST 0" TRANSFOR~I!E~';;_:

11. Q/h"E.sJr.r;uiuw ~_.,-

-

r-0

tA)~

:I

~

j:

\ .

I! '-t

I'

;

I

~ 'S'

i

I,jl

;',...d

voltage at the}nput .

Millrokj'"","'fln .

iI.

!:""':;

.;,,\l~ >"4\I_.ur.":\ ' ..,..""'.l~ n'l. q0 L.I. ~\\ ., a"

f I

«

Hi

t. I'

0

Xi

~

. I ,

j. I.

.--

Q

I somefi.'7JGs I omit1,;;d,

d\!)

::

~1Qrted L.V, side

~

H,T; . I

~\\! 10 l,T.

1j1\~ .

.

.

HZ

Wattmeter reading (W) Let

Open H,V. side

.

L

ws

~

Xz r""LcL,

\ ~v.

Source o. reauc-

--~

.1 ~ 'e 1a"sv V,'atlIHcte~ (!.. reac1mg "" Gul'

---,

rI

HI

r--

",l

I.

...

. . ,

.

I

j"

<:>tl(Ficircall1ii.1c ,

I

VI \1

\II

{0

o. "'..

Xl

.

I r\tV\.!;r I

SuppJy of n:~rmal V o!t~ge & ;:~equency

'

r

V ollmetcr reading Ammeter reftding

1

X.

'" fuli - load

copper loss

(V) = V in (A) = 1«

7 = Yi" ,Rc . L.,C f,e

~ ~0

f- 'c'

- \./~Z" &. .," e --

1?2 ",

-

r

5

I ,

I

CONDITIONS

OF MAXIMUM EFFICIENCY: POUT

VI cos ()

Tf=

xIOO%

POUT + PLOSSES

to have max. efficiency:

2

VI cas () + I

xlOO%

Re -1-PK

takedTf '7 0 dI

dTf dI

=

(Vi cos() + 12Re + PK)(V cos(}) - Vi cose(v case + 2IRe)

=

(vicase

=0

+ /2 Re+ PK)'

dividing by Vcas e & simplify:

I'~ = PK Variable loss = Constant loss Also;

-

,

kV A M".Efl"y- .,.. kV AFL ~V

T{

max. eff',..

I ,.,----'~ . I I I :

I

~ I I

PK

Pc"FL

.

I

I

17 (

I

V-

- PCUJul/ ~ad

J--~=:.PCUI I

Ful/load-

0

% loSs

~.n." 0

kVA

ALL- DA Y EFFICIENCY: It is the ratio of the energy delivered with in 24 hr period to 'the energy input in the same period of time. For distribution transfOlmers whose primary energized for 24 hrsa day, thus the core loss of this transfonner must be designed to be low as possible. It is always less than the commercial efficiency. . ENERGYlJUTPUT / day 0 = xlOOYo Tfall-da\' ENERGYOUTPUT / day + ENERGYwss

.

i day

at max. all day efficiency: Energy Coppcr Loss = Encrgy Core Loss TRANSFORMER C~1Jl1ectiol1

POLARITY

:

H1.

XI

Input Test Voltage ( V; ) HI

X2

.:1

COllditiolls: (!) When voltmeter reading is greater than the input test voltage tlie polarity is V' ADDITIVE. V = Vp' + 2, a E> When voltmcter reading is less than the input test voltage the polarity is .Vp' SJJBTRACTIVE. V = Vp -- a

1

Note: I3y industry standar,ds "lll
AUTOTRANSFORMER: The autotransfomler is one having a single continuous winding, part of which is common to both primary and sec:,ndary circuits. It can be used for either step-up or step-down application with a relatively small change in voltage between the input and output voltages, and should not used when there. is a large change in voltage because the circuits are connected, electrically, ' Step-down Autotransformer ( a> 1 )

a

,~

Jp b

Vp I I I

15

a =

i Icb c

N ac Ncb

= E'K =!.£ = VpEcb

II'

Vs

This autotranSf0ll11Cr is equivalent, in its transformation, to an ordinary two-wind.ing h'ansformer if winding ab is regarded as primary winding, with winding Qs as the secondary winding as N

Such: --'l...b

.'

Nhc

El

I -- { !. 1:. = .2. -1 = a-I

= ...".!z.""-<:2.= "s, Ebc

II'

{1'

{1'

Hence: Using the approximate equivalent circuit: Referred to f)rimanJ -I I I I

Ip

~

Rep

I I I (o-/)

Vp I

Referred

I

I I I I

Vs

Is I I I Vpj(o -I) I I I

tosecolldm)'

Res

---L Where:

Where:

rp Rep =1'p +(a-l)2r,

-r+~ Res - S

X,p = xp + (a - I)"xs

Xp- X +- 2 Xcs- ~$ (a-I)

Step - Up A utotTansformer 0

( a < I)

1s I I Vp I

b

Ip

N E. {. Herc: a = --k... = ~ =-2N nc Ene {l' Nbc = Ebe =!...£ = ~ Nab E nb 1be !P- 1s

Ibcl .

(a-I)'

C

7

.

.

Ai; an eqUiviilerit ordiriary two winding is Nbc =~ I-a Nob

-I

Power inductively

transformed

Power cot!ducdveJy

(thru ab)

( )

OPERATION

Pcon = Pin

if two or more

1. Y ohage ratings of both primaries and secondaries cunent will circulate through

to the lossc;sof the transformer). 2.

() .

~

a,

OF'rRANSFORMERS;.;.

'Important conditions' that must be fulfilled successfully in parallel to deliver a.common load:

the same. (otherwise,

transfonners

are to be' operak

must be identical, or the transformation rauo are the transformer windings which wi!! contribUte

.

.

The transformer must be propefly cOnnected with regards to polarity. (otherwise, circulate through the transfQrmer winding which could damage the windings ). a.

(tbr" bcJ

1'

a-I

Pw=(Yr- Ys)Ip =Pm -;;PARAU,EL

Tramiformed

a large CliITent will

The equivalent impedances should be inversely proportional to the respective kY A ratings. ( otherwise, the fransfonner will not share the load in proportional to their ratings).

Z a -1 b.

Ze, -=-52

e S Ze2 5', "" The ratio of the equivalelit resistance to the equivalent reactance of the an ti-ansfouner should

be the same.( Otherwise, each transformer will not takes its correct fraction of the load)

ReI

~.

XeJ

Xe2

Also the condition for two tcansformers cormected For unequal transformation

ratio (a",zatJthe

in parallel to operate at the same p_f.

sec()ndary currents wil! be

.-

1~1- (al -aj)VsJ. +ulZes2f--- Lcdd UtZ«s,+a2Zes2 For the circulating Ie, =

.'

1S2-- a, Zes,! LOad-(a2 -al)Vs, -.at Zes, +a2Zes2

current (set fUnd = 0 )

(i12 -a, )VSt alZesl +a2Zes2

MOST

.

Re2

CO!'vIMON

or

3(j) TRANSFORMER

Icz=- -(aZC-l/IWS1 aJZeSj + a2 Zes 2

CO~'ECJJONS:

1- Y C"_ywith connected neutrals has the advantage the windings are subjected to the line voltage clivi~by .J3. This coIDlection without neutrals. are rarely used since high lUI.equa!voltage may result trom an unbalanced load.

8 ,

, ...'.

. , "'2.

11-

11 connection

" .' . .

,.,'0

-

.'

'f

suitable for moderatevoltagt, largt cuiTbh opet'ation. 3. !l"'- Y cOr\nection

Primarily'used.to~tep-up the yoltage.It is also used instep - do"Yllapplications where a conventional distrilmtion voltage is reduced t? supply twovolt~~es,fo~:.lighting and power loads'

-

.'

4. Y -,6 connection use for stepping dowii voltages 5. Ibcott)

connection

.

used to convert 3cpto 2
PARALLEL OPERA nON OF 3
Ii.

i

.' y-y ,6-y Y-Il y-y ,6-11

y -¥ ,6 - y y -,6 ,6-,6 y -y

i; Ii; I~

~ ~

,6-,6 y-y y-y

Utility

Y-,6 .6,- y, Y-Il

!~

i~ ft W

Bonk A

I~

Bonk B

r~

"~ ~: Ii.

.

.

~~-

Notc: In dealing with 3cptransformer, calculations are made [or one phase only.

INSTRUMENTIRANSFORlVIERS

:

Instrument transformers are of two kinds: 1. Current transformer (CT) 2. Potential transformer (PT) These are used to supply power to ammeters, .voltmeters, wattmeters, relays and so on, Which is called the "Burden of Instrument Tral15former". .

-

.

.

~\

.POWERLINE REVIEW CENTERInc.

i.:ri\ ~':;I$%,~

The Ultimate B.E. Review Center

.

DOW.RLI..

AC MACHINES

(TRANSFORMER)

REVIEW

PRbBLEMS

1. What is the purpose ot air blowers in'substation transformer? (BP April 2006) CQ.) Increase Capacity A. Prevent dust accumulation B. Cooling the casing D. Prevent explosion 2. What would happen ifyou connected a transformer to a dc circuit with a voltage of 20% of the namePlate ratings after steady state condition is reached?(BP Apr 06) @ No voltage is registered at the secondary,

B. Rated no load current flowsto the secondary

.

C.Primary current is voltage over secondary primary impedance D. Voltage is established at the secondary 3. What is the maximum flux in a transformer

supplied by 60 hertz, 200 volts source? . (Ad 0.0037 .' .

'S: 0.0375

with 200 turns at the primary when .

C.

0.375

D. 3.75

4. An ideal transformer has a voltage rating of 2400/240 volts, a motor is connected to the secondary 'draws 50 A at 200 volts. what resistance must be connected to maintain this voltage? A.O.6 . C. 0.5 B. 0.3 \ D. 0.8 5. Device use by transformer to adjust load voltage if line supply is above or below

nominalvoltage. ( BP Sept 2003) /2. Thermal trip .~}

Tap changer

.

.

C. Differential Relay

D. Buchholz relay

6. A transformer has primary and secondary turns of 120 and 240 respectively. If the 120 turn winding has a 2:.2.5% tap, calculate the primary voltage ,applied to 97.5 percent of the winding that will permit the rated output voltage of 480 Volts? A. 492 Volts ~ 468 Volts B. 117 Volts e.g.) 234 Volts ,

.

,

7. A1,100 kVA, 2400/240 Volt, 60 Hz transformer has a primary resistance a~d secondary winding resistance of 0.828 ohms and 0.009 ohm respectively. Calculare the equivalent resistance referred to primary. A:' .1.728 ohms C. 1.6 ohms B. 0.837 ohms D. 1.46 ohms 8. A transformer rated 2000 kva, 34,500/240 volts has 5.75% impedance. What is the per unitimpedance (ap April2004) A. 0.0635 C. 0.0656 B. 0.0575 D. 34.2

[\1" ~. II

..

9. A 500 kva, 13200/2400 volts 60Hz single phase transformer hass 4% reactance and 1% resistance. The leakage reactance and resistance of the lovyvoltage winding are 0.25 and 0.055 ohm, respectively. Calculate the leakage reactance . ,and resistance in ohms of the high voltage winding. ,.' ";d ,

(lY 6.38 & 1.82'

C. 6.53 & 1.87

B. 4.86 & 1. 56

0.6.23

& 1.85

10.A 1<1> -250 kVA, 7967/240 Volt, 60 Hz transformer has 1.5% resistance. What is . the full-load copper 19S5? . A. 626.25 watis C. 2505 watts B. 1252.5 watts 19) 3750 watts 11.The followi'ng information is given in connection with test performed ,on a 50 kva,

4600/230 volts 60Hz.Transformer:

.

Short Circuit test: Esc~ 150 volts Psc= 615 wats Isc=1O.87 amperes What is the percent resistance & percent impedance? A. 1.26 & 3.62 C~ 1.16 & 3,58 ('S; 1.23 & 3.26 D. 1.32 & 5.8 ',-J' 12.A 500 kva, 11,000/2300 volts 60Hz. Transformer has been tested and gives the

following data:

"'.

':",

.

Short Circuit test: Esc= 600 volts Psc= 6000 wats

Isc=45A amperes

What is the percent regulation at full load 89:{'o power factor lagging? A. 3.26 B. 6.2'3"-0.

(c;J 4.21

5.28

13.A 1~ transformer is rated 4,160 Volts primary and 250 Volts secondary. It has 1,500 turns in the primary and a voltage regulation of 8%. What should be the number of turns in the secondary windings UBP Apr. '97) . A. 88 turns . .. LC) 97 turns B. 90 turns . D. 86.turns 14.Calculate the percent voltage regulation for a transformer with a percent resistance of 2 % and a reactance of 4 % of rating 550 kVA, when delivering 450 kVA at 0.6 p.f.lagging.; . ,

A. 5.5 %

B. 4.2 %

C. 4.4 % CQ)

3.6 %

.

15.A small single phase transformer has 10.2 watts no-load loss. The core has a volume of 750 cu em. The maximum flux density is 10,000 gauss and the hysteresis constal1t of the core is 5 x 10.4. Using the Steinmetz law, determine the eddy current loss. (BP Oct. '97) . . . ~. tf:!;1 4.55 Watts C. 5.55 Watts B. 3:55 Watts D. 2.55 Watts

-

16.Acertain !ransformer has a cqreloss of 100 watts at 30 Hz and core loss of 300 watts at 60 Hz. What is the hysteresis loss at 60 Hz? (J3P Ap(i/200;3) (.A'J)100

'

Er:l50'

\'

','

','

C.

'200'

D.

250

"

'

17. The eddy current loss in a ,2300vblts' 60Hz transforrheris 280 watts: What is the eddy currentloss at 2400 :' L ," " .' volts,50 Hz source?> '~ A. 280 Watts '(Q)305 Watts B. 300 Watts D. 257 Watts "

18. The hysteresis loss in a 6,600 Volts, 60 Hz transformer is 4'80 Watts. What will be the loss when the tranSformeris connected to a 6,900 Volts, 40 Hz source? A. 558 Watts C. 520 Watts

@) 657

Watts

D. 417 Watts

~,.

",

:~, 19.A50 KVA,5000/500V, 50 Hz, 1-phase transforriierhasthe high voltage winding with a 'resistance bf 8 ohms and a low viJltageWfnding resisfance of 0.06 ohm. The no load losses of the transformer is 1000 watts. Calculate theeffic1ency of tbe transforr;ryer,ifthe output drops to 80% of the rated value at 80% power factor lagging. (ep April 20,06) , A. 90.20% B. 91.05% ! ! I

I

i I.

1

C. 92.35%

@

94.39% ,

20.A 37.5 kva, 20,000/240 volts, single phase transformer has rated core loss of 350 watts and full load copper loss of 500 Watts. Calculate its load to achieve ,

.

maximum

efficiency.'

i i I

"

A. 32.5 kva B. 37.5 kva

@) 31.4 kva D. 26..25 kva

I

I, I

i I I

! I

r

21. The resistance of.a 500 kva, 11000/2300 volts 60 Hz transformer are respectively 0.8 and 0.04 ohm for high and low voltage sides. The iron loss is 3,000 watts. This transformer1s loaded daily as follows: 2 hrs at full load, 3 hrs at three quarter load, 'and 6 hrs at one-quarter.fulllqad. Calculate the all day efficiency. A. 94.7 C. 97.1 B. 96';1 D. 98.7

! !

l w

'

t,

t:

"f ,

I

! I

22.A 20kv/7.97kv auto-transformer draws 100A at its secondary. What is the current at th&common winding? (ap Sept. 20,0,2) . A. 139.85A C. 60.15A B. 100A D. 39.85A 23.A 2200/220 volts transformer has two 110-volt coils connected in series. Across the first of these two coils is connected a resistive load drawing 15 ampere. Across the second coil is connected a pure capacitor drawing 10 amperes. Across the two 110 volt coil in series is an 80% lagging power factor circuit drawing 20 amperes. Calculate the current to the 2200 volts coil. A. B.

-

4.50' 3.45

. , '

C. 8045 D. 2.45

f; t

r

i'" !' fI" i.

24.A 5 kva 60 Hz single phase transformer has a full load efficiency,,0f:~6'%'and an iron loss of 40 wattsuhderrated condition. If the transformer is'cohn~cted as an autotransformer aperating frOm a 240 volts mains and deliver"iilg 5 kw at unity power factor to a '120 volts circuit, calculate the percerlt efficiel)cy, A. 95.6C.

98.7"

B.9Q.8,

D. 98.4

", i.,,"

25. What is the typical.useof autotransformer? A. .control transformer C. isolating transformer B. variable transformer D. toy tra.nsformer 26.A 1500 Kva , 3% impedance transformer is to be operated in parallel with a '1250kva, 3% impedance transformer, the total load is 2500 kva, What kva load carried by each? (ap Sept 2002) A. 750 & 1750 C. 1200& 1300 D. 909 & 1591 B.1363 & 1,137 27. Two (2) 1~ transformers having equal turns, ratio and ratings are operated in parallel to supply a load of 280kW at 80 % pJ. lagging~ Transformer A has 2 % resistance and 8 % reactance, while transformer B has 1% resistance and 6 % reactance. What is the power delivered by each of the transformers to the load'? A. 123and 157 kW . C. 130 and 150 kW B. 142 and 138 kW D. 127 and 153 kW .' 28.Two (2)-100 kVA,1~ transformers are connected open-delta, calculate the maximum kva of a 3-phase balanced load without overloading any of the transformers. C.300 A. 200 0.173 B.150

29.An induction motor rated 500 hp 2400 volts 80% power factor, 95 percent efficiency is working at rated load when supplied from 3-250 kvEisingle phase transformerconnecteddelta secondary.If one of the transformer is brought for. repair, determine the overload kva of the remaining transformers connected opendelta. (ap Sept 2'004) C. 58 A. 73. . D. 64 B. 3~., 30, Three transformers are delta connected, one connection has been cut, what is the percent reduced? (ap April 2004) A. 33%' C. 67% B. 57.7 % D. 42%

i

j, l .§!

.,

-

---

---

-

31.A 2000 KW,2400 volt, 75 percent power factor load is to be supplied from a 34500 V,3 phase line through a single bankof transformers. Give the primary and secondary voltages in volts of the transformer . wY~Solln~pt.ipn,,(f3P\Ap~iI 200!j) , A..

.

~P.o.opZ13aO'.

'.' "

"

B.18590/13500 .

for the following cases

wye-

C. 1869011,300 D. 19000/13'~0

,

,

, ,

kv at the seconaary,it is desiredto connect a 115volt voltmeter

32. To measure 230

through a three phase potential transformer connected delta-delta, What is the required voltage transformer ratio? (ap April ~OO3) 5 .J3 115

A. 230;~00:~ ~3 ,. ,

B.

C.

230,000: 115.J3 '

D.

230,000:115

.

230,000:.J3

. '. 33.A2pOO kW,.2400 V, 75% pJ.loqd is to be supplied'from 34,500 V, 3"'phase line thru a single bank transformers.. Give the primqry ao.dsecondary line current in amperes for the conditions wye-wye. (ap April 2006) A. 50/700 C. 60/800 B. 48/650 D. 45/642

34. A 750 hp, 2400 V, 3-phase motor of 90 percent power factor and 95 percent efficiency is to be operated from a 13.2 kVutility service through a wye-delta c;;onnectedtransformer. What is the KVA rating Of each transformers? (BP ZOO5) A. 218 B.250

C.175 D.150

35. The pot~'ntial transfomier of a line have a ratio of 132,000/66. 'These are connected wye-delta. The voltmeter connected at the secondary reads 64 Volts. What is the voltage of the line? (BP Oct. 1!J98) .

A. 221.7 kV B. 112.5 kV

C. 135.2 kV D. 132 kV

36.ln a Scott-connection, calculate the line ,current in Amperes on the 3(pside if the load()n the 2(p side both draws 5000 A at 100 Volt and the 3~ line voltage is

3,300 Volt,Negleqttra(1sformerloss. .

A~'195 A

B.175A

.

C. 152 A D..140A

Sept. 2007

"."

"Problems

Supplementary

-Transformers

"' " 1. For three singlephase transformers,a 600hp 2,400 volts three phase 60 Hz motor

with

<:Inefficiency

of 90% and power factor of 90% supplied from 34,500 volts utility:

For d,elta-delta connection, What is the primary and secOndary transformer? (BP'sept 2004) A. 32.500/2500 C. 34,000/2500 B. 35,000/2500 D. 34.500/2400

voltage

rating of

each'

2. A 2000 kV,a,34,500/240volts, 60 Hz, 3-phase oil filled transformer hc:lsan impetJance of5.75%. Determine the secondary full load current. (BP Sept 2004)

A. 5102' B. 5280

C. 5350 D. 4811

3. Transformer connection to convert two phase to three phase (BP Sept 2003) ,~ Open delta C. Scott. connection B. Wye-delta D. Delta

4. Two 230 hp 24QO-volt 3-phase motor of 90% power factor 93% efficiency are suppliedfrqm 34500 volt transmission line by 3-single phase transformer connected delta-delta. Specify the bank KVA. (BP Sept 2003) A. 50.0., " C.400

I

I

B.446

D.550

5. A 13.8/480 v, 1b MV A three phase

transformer

has

5%

impedance.

What

is the

-

ohmic impedancereferred to the primary?(BP April 2003) A. 0.952 C. 2.1 B. 5 -' D. 0.05 . 6. I

A 50 MVA 23Qkv/34.5kv wye grounded autotransformer carries a 3-phase load. If the wimary ground current is 10 cis 30 deg, what is the secondary

current? (BPApril

2003)

"

A. 56.7 cis 30 deg. B. 66.7 cis 30 deg.

Ii

unbalance ground

C. 10 cis 30 deg. D. 66.7 cis (-30 deg.)

7. Which of the fo~lowing is common to insulator, bushing, potential transformer and transformer winding? (BP Sept. 2001)

A. Noise level B. In~~lator Oil

"

,C. current rating D. BIL Rating

8. A three phase customer has a maximum demand of 85 kVA. What transformer rating shall be used if connected open-wye -open delta? (BP April 2001) A. 45 kVA .C. 33 kVA B.50 kVA 0.100 kVA

I

9. A 100 kva transformer with 4% impedance is connected in parallel with a 75 kva with 3% impedance. The secondary voltage is 220 volts. What is the division of load when the total load is 125 kva, 80% power factor? Both transformer have the same voltage ratio. The resistance of each transformer can be neglected. (BP Oct.2000) A. 71.43kva/53.57kva C. 62.5 kva/62.5 kva B. 82.5kva/42.5 kva D. 75 kva/50kva

-

. 10. A 69/13.8

KV, 10MVA t~ree phase transformer is connected wye-grounded-delta.

What is the KV rating of the s~90ndarywinding?(BP OcL 19!#8} ,

A.

7.97

,.

',.'

,"

B. 15

'(;:23';9"

",L

D. 13.8

11. Which of the following given below is the eddy current-loss not proportional' to? (BP Oct: 1998)" . .' . ' . A. square of the thickness C. square Of the freqUency B. square of the load current D. square of the flux density' : 12.A12~' KVA distributiontransformer with 4% impendence is conneqted in parallel with another transformer rated 75KV A and with 3% impedance: Both'havesame voltage ratio. (Neglett'resistance of each transformer) The total load is 140 KVA at 85% power factor. How much load does each carry? (BP March. 1998) A. 80.5 KVA /59.5 KVA C. 80 KVA /60 KVA B. 77.78 KVA/ 62.22 KVA D. 87.50 KVA /52.5 KVA . 13. A 13.8 kv/480 V, '10MVA three phase transformer has 5% impedance. impedance in ohm referred to the primary? (BP March. 1998) A. 0.9520hms C.5.1250hms B. 0.03 ohms D. 9.01 ohms

What is the

14. A 3cp transformer is rated 30 MVA, 115i\'/13.8Y kV, and 2 % impedance, the resistance is negligible compared with the reactance. Determine the percerit (%) voltage drop at 'the secondary at 80 % load and at 75 % power factor. (BP Oct '97) A. 4.1 % . C. 2.26 % B: 1.05.% 0.3.15 % 15. A power transformer rated 5Q,00a kva, 34.5 kv/13.8 kv is connected wye-grounded at the primary andde,lta on the secondary. Dete,mine the full load phase currents on the secondary side. ( BP Oct. '97) A. 2092 amperes C. 1449 amperes D. 1208 amperes B.1725 amperes . 16. A 1rjJtransformer is rated 4,160 Volts primary and 250 Volts secondary. It.has 1,500 turns in the primary and a voltage regulation of 8%. What should be the number of turns in the secondary windings? fBP Apr. '97) A. 88 turns C. 97 turns B. 90 turns D. 86tUrns 17. A 3-phase transformer tw0"-2.5 % taps' above ,

has a primary

voltage

rating

of 34.5 kV, wye-connected

with

and two-2.5 % taps below the rated voltage. The secondary winding israted460 Volts and is delta-connected. If it has 100 turns, what will be tJ:1e effectiv~ number of turns in the primary winding if the voltage tap -is set at minus 2.5%

? (Bt/ Oct 1996) A. 7,133turns B. 4,222 turns

,.' C.7,312turns D. 4,333 turns

18. A 1 rjJ,440/110 Volt tra[lsformer takes a no-load current of 5 ~mpat 9.25 pJ. lagging. If the secondary supplies a ,current of 100 Amp pta.s p.t: lagginfJ . Estimate the current taken by the primary winding. . A.. 32 Amp C. 25 Amp B. 22 Amp D. 29 Amp

.

19. A transformer has primary ane:!secoridary turns of 60~nd 240 respectively. Ifthe 60 turn winding has a 5 % tap, calculate the primary voltage applied to 105 percent of the wihding that willpermit the rated output voltage of 480 Volts? A. 504 Volts C. 456 Volts B.126Volts D.114Volts 20. A 1~, 30 kVp." 2400/240 Volt, 60 Hz transformer has 2% resistance. If the low voltage winding resistance is 0.015 ohm, what is the high voltage winding resistance? A. 2.43 ohms C. 2.34 ohms B. 3.24 ohms D. 3.34 ohms 21. A 230/460 V transformer has a primary resistemceof 0.2 ohm and reactance of 0.5 ohm and the corresponding values for the secondary are .075 ohm and 1.8 ohm respectiyely. What is the secondary terminal voltage when supplying 10 A at 460V, 0.80 p.f. lagging? A. 455.~ V C. 450.6 V . B. 425.3 V D. 430.8 V 22. A 3,300/230 V, 50 kva, single phase transformer is found to have impedance of 4% and a copper loss of1.8% at full load. What is the percent reactance? A. 3.74% C. 3.83% B. 3.57% D. 3.79% 23. A single phase 60 Hz transformer has maximum flux density is 1.0 tesla and the find the hysteresis loss. A. 755 Watts .' B. 675 Watts

its core volume of 1.5x10.3 cu m. The hysteresis constant of the core is 7500. C. 355 Watts D. 255 Watts

24. In a transformer, the iron loss is found to be 42 W at 30 hZand 90 W at 60 hi: measurf?d at the same flux density. What is the hysteresis and eddy current loss at 60 hi? . A. 60 W & 30 W B. 78W&12W

C. 30 W & 68 W D. 12W&78W

25.A 4,400 Volt, 60 Hz transformer has a core loss bf 840 Watts, of which one-third is eddy-current loss. Determine the core loss of this tn3nsformer when it is connected to a 4,600 Volts, 50 Hz. C. 879 Watts A. 779 Watts D. 977 Watts B.. 679 Watts 26. A 25 kva, single phase transformer the core loss and full load copper loss are 320 and"380 watts respectively. What is the efficiency at half load and 0.866 power fu~~ .

A. 97.8% B 95.3%

.

C. 94.7%

D. 96.3%

27. What is the ratio of capacity of open delta bank to the capacity of close delta bank? A. 72'% C. 57.7% B.63.2% 0.66.7%

-

28 In a SCQtt-conhection, calculate the values of the line currents in Amperes on the 3~00 vO,lts3~ sid.~ if the,lqSld On the 2~ side are both 300 kW i?t 100 Volt and 70. r% 'laggT@pJ:"

,

,

'

,

G. .145.2 D'.85.7

A, 157.2 B. 148.5

29. A 1~, 5 kVA transformer has 35 Watts iron loss and 40 Watts copper loss at full-load. It operates at full-load, 0.80 p.f. lagging for. 6 hours, half-load ,0.75 p.f. lagging for 12 hours and ql.larter~load at 'unity pf for 6 hours. Its all-day efficiency in percent is nearest to .. . A. 98.9% C. 97.8% B. 95.2% D. 96.5% :30.A 120/180 Volt auto-transformer draw,? power at 12'0 Volt and supplies power to a 2 kyVload at 180 Volts with a p.f. of 0.80 lagging: An additional, load of 1 kW is supplied at unity p.f. from the 60 Volt Winding. What is the current drawn from the 120 Volt supply line? . . A. 26 C.28 B.30 D.32 ""

.'

31. Three-1

~ transformers,

rated at 50 kV A, 2,400/2,400

Volts {1: 1ratio) each, are lobe

connected as autotransformer to deliver 4800 V from a 2400 V line. What is the maximum kVA that can be delivered by these auto-transformers if they are connected . in L\? A. 300 kVP, C. 275 kVA B. 250 kVA D. 150 kVA 32. Three (3)-1~,75 kVAtransformers indeita-delta bank supplying a balanced load 160 kVA (3~) at 0.80 p.L lagging. If one (1) transformer is removed for-repair. The V-V bank was overloaded, then to prevent overloading of the 'remaining units, capacitor bank is connected across the 3
C.96,8

'

0.97.6

34. A power transformer rated 50,000 kVA, 34.5/13.8 kV is connected wye-wye, What are

thEJline currr;mtsin Amp atfulH()ad ? ( BP Oct. '97) A. 525.2 & 1725 . . B. 483.1 & 1207.7

,

.

C. 836.7& 2092 D. 1449.3 & 3623.2'

35. A 750 hp, 2400 voits, 3-phase motor 90% power factor and 95% efficiency is to be operated from 13.2 kv utility service through a wye-delta connected transformer bank. Whatjs the,t,otalthe kva bank rating of the transformer? (BP Sept 2004)

A.750

B.575

.

C:595 D.615

'

"-"-~-'-~~

- 36. A 750 hp, 2400V, 3-phase.motorof 90 percent factor and 95 percent efficie;ncy is tq be operated from a 13,2 kV utility 'service throughawye~deltaconnected , transformers. Compute the secondary or load current in ampere of the transformer. (BP April 2005) A. 180.6 C. 157.3 B. 125.6' D.. 175.4 ,

'

37. The.9wrent at the common winding of a 20kv/7.97kv auto-transformer is 200A. What is the current at the,~econdary? (BP Sept. 2002) A. 200A C.142.22A B. 332.45A D. 132.45A / !

Ii

r 1 j

11

!'

i

38. Name the transformer that provides an almost continuously variable stepless output from zero rated line voltage. (BP April 2005) A. Booster C. Variac B. AVR D. UPS 39. What is the normal secondary current rating of current transformer in amperes? (BP April 2005) A.20 C.5 B.10 D.15 40. Tertiary winding is provided in transformers that have A. delta/delta winding C. wye/wye winding, - B. delta/wye winding D. wye-grounded/delta winding ,. '

,

41. The chemical used in a bre~er for transformer should have the quality of A. ionizing air .' C. absorbing moisture B, cleansing the transformer oil D. cooling the transfo.rmer oil 42. The chemical used in breather is A. asbestos

fiber

B. sodium chloride

'

C. silica sand D. silica gel

43. The following are protection device built-infor safe operation .ofa power transformer, EXCEPT: . ,

A. breather

C.

conservator

B. bushing insulator

D. pothead

44. Which of the foilowing is NOT TRUE about "Buchholz Relay" ? A. it can detect faults above oil level in'transformers B, it is installed between main tank and conservator C. it'is a gas actuated relay D. it can detect faults below oil level in transformers 45. A common method of cooling a power transformer is A. natural air cooling C. air blast cooling B, Oil cooling' D. any of the above PRACTICE Makes Perlect!!!

-