1.

1.1

1.2

1.3

SECTION -A

This question consists of TWENTY-FIVE sub-questions (1.1 - 1.25) of ONE mark each. For each of these sub-questions, four possible alternatives (A, B, C and D) are given, out of which 0 N L Y 0 NE is correct. Indicate the correct answer by darkening the app rap ri ate bubble against the question number on the I eft hand side of the Objective Response Sheet (ORS). You may use the answer ok provided for any rough work, if needed.

In a series R LC circuit at resonance, the m agn i tude of the val t5 .I oped across the capacitor

(a) is always zero a (b) can never be greater than the input val tage

(c) can be greater than the input val tag e, however, it is 9 0° of phase with the input voltage r:t._ +

(d) can be greater than the input voltage, and ｩ ｾ ＢＡＯＺＮＺＺ ｴ ｨ＠ the input voltage.

Two incandescent light bulbs of 40 W ｡ｮ ｾｾ ｧ＠ are connected in series across the mains. Then

(a) the bu I bs together consume 10 0 VoJl. .. (b) the bulbs together consume 50 .'-.,

(c) the 60 w bulb glows bright

(d)

(a)

(b)

RL c1rcu1t with zero initial

A passive 2-port network is in a steady-state. Com pared to its input, the steady state output can never offer

(a) higher val tage (b) I ower impedance

(c) greater power (d) better regulation

1. 6 A si ngl e-p hase transformer is to be switched to the supply to have minimum inrush current. The switch shou I d be closed at

(a) maximum supply voltage (b) zero supply val tage

(c) .k maximum supply voltage (d) ｾｭ｡ｸｩｭｵｭ＠ supply voltage

1.7 It is desirable to eliminate s'h harmonic voltage fi'om the phase ｶｯｬ ｾｴ｡ｮ＠alternator. The coi Is should be short-pitched by an electrical angle of

(a) 300 (b) 36° (c) 72° (dO 1.8 Fig.P1.8 shows the magnetization curves of an ｡ｬｴ･ｲｮ｡ｴｯｲ ｃ ｲ ｾ ､＠ armature

current, unity power factor and also at no load. The ｭ｡ｧｮ･ｴｩｺ ｾ ｶ･＠ for rated armature current, 0.8 power factor leading is given b"r> • +

Noload ｾ］ﾷ ｾ＠

(a) curve A (d) curveD

1.9 The core flux of apr.

(a) is strictly ｾｾ ｡ ｾ ｮ ＡｬＩｬ ｴ Ｎ ｬ ＢＢ］ｬｲ ｯ｡､＠ changes

(b) increases I h I oad

(c) increases a • quare root of the I oad

(d) de re es ｾ ｨ＠ 1n creased I oad

nee and su b-tr ans1 ent d-ax 1 s reactance of a synchronous mach me 1vely. Wh1ch of the followmg statements 1s true?

xd > Xd> xd (b) xd > Xd>Xd (c) xd > xd > xd (d) xd > xd > xd

1.11 A 50 Hz balanced three-phase, Y-connected supply is connected to a balanced three phase Y-connected load. If the instantaneous phase-tx of the supply voltage is Vcos(oot) and the phase-tx of the load current is Icos(oot-+), the instantaneous three-phase power is

(a) a constant with a m agn i tude of VI cos'

1.12

1.13

1.14

(b) a constant with a magnitude of (3/2)VIcos+

(c) time-varying with an average value of (3/2) Vlcos+ and a fi"equency of 100 Hz.

(d) tim e-v aryi ng with an aver age value of Vi cos+ and a fi"equen cy of 50 Hz.

In the protection of transformers, harm on ic restraint is used to guard ｡ ｾ ｩ＠ n s

(a) m agn eti zing inrush current (b) u nb al anced operation

(c) lightning (d) switching ｯｶ･ｲＭｶ｡ｬ ｾ＠

A lossless radial transmission line with surge impedance loaditt\J (a) takes negative VAR at sending end and zero VAR at recei g e

(b) takes positive VAR at sending end and zero v AR at recei vi n t1

(c) has flat voltage profile and unity power factor at ｾ ｳ ｾ ｯｮｧ＠ it

(d) has sending end voltage higher than ｲ･｣･ｩｶ ｲ ｾ ｡ｧ･＠ and unity power factor at sending end ｾ＠

The polar plot of a type-1, 3-pole, op7_ Ｍｉｉ ｕｾ ｊ ･＠ em is shown in Fig.P1.14. the closed loop system is ｾ Ｍ ｶ＠

(a)

(b)

ｾ ｾ ｾ＠ ｷｩ ｾ＠ one pole on the right half s-plane

ｮ Ａｬｾ Ｍｭ ｾ ｩｴｨ＠ two poles on the right half s-plane

. [-3 1] X= X 0 -2

the steady state value X 5 5 =lim x (t), given the initial state value of x(O) = [10 -, .... *

101": is

(a) ｘ ＵＵ ］｛ｾ｝＠ [-10] (c) Xss = 10 (d) X55 =[:]

1.16 If an energy meter disc makes 10 revolutions in 100 seconds when a load of 450 w is connected to it, the meter constant (in rev/kWh) is

1.17 (a) 1000 (b) 500 (c) 1600 (d) 800 The minimum number of wattmeter balanced or unbalanced power is

(a) 1 (b) 2

( s) required to measure 3 -phase, 3 -wire

(c) 3 (d) 4

1.18 In the single-stage tran si star am pi i tier d rcui t shown in Fig. P 1.18, the c is removed. Then, the ac small-signal midband voltage gain of the

1.19.

1.20.

(a) xy+x

.-------.::----40 +12V

+--t f-_....--+-1.. Input •

(d) counting type

its inputs are at logic "0". The gate is

(b) a NOR or an EX-OR gate

(d) a NOR or an EX-NOR gate

f of the 4-to-1 MUX shown in Fig.P1.21 is

2 Va:; MUX

'---- 1

.-- 0 s, So

ｾ＠ ｾ＠X y

(b) x+ y (c) x + y (d) xy+x

1.22.

1.23.

1.24.

1.25

2.

2.1

An op -amp has an open -1 oop gain of 10 s and an open-loop upper ou toff frequency of 10 Hz. If this op-am p is connected as an am pi i fier with a d osed-1 oop gain of 10 0, then the new upper cutoff frequency is

(a) 10Hz (b) 100Hz (c) 10kHz (d) 100kHz

The main reason for connecting a pulse transformer at the output stage of a thyristor triggering drcuit is to

(a) am pi i fyi n g the power of the triggering pulse

(b) provide electrical isolation

(c) reduce the turn on time of the thyristor

(d) avo1d spun ous tn g gen n g of the th yn star due to no1 se

AC -ta-D C ci rcul ati n g current dual converters are opera ted ohe followm g relationship between their triggering angles (txt and tx

0i)

(a) txt+ tx2 = 180° (b) txt+ ｾ＠(c) txt- !Xz = 180° (d) r rJ.: 0

In case of an armature controlled ｳ･ｰ｡ｲ｡ ｾｾ＠ motor drive with closed-1 oop speed control, an inner current I ｯ ｾ＠ m ecause it

(a) limits the speed of the motor to ｡ ｾ＠ v -

(b) he Ips in i m proving the drive ··e r Ａｖｾ ｩ＠ ency

(c)

(d)

the perm issi bl e value

ｴｬＦＧ ＭＮ ｾ ｾｉｉＺ ｉｃｩ＠ ｾ ﾷ･ｬ､＠ E(in volts/metre) at the point (1,1,0) due to a point charge of ed at ( -1, 1,1) (coordinates in m etres)is

10-6 (2' k) 20./S1rs0

1 -

-1o-6 (2' k) 20./S1rs

0

1 -

(b)

(d)

10-6

(2i-k) 201rs0

-1o-6(2i-k) 207r80

2. 2. A connected network of N > 2 nodes has at most one branch di recti y connecting any pair of nodes. The graph of the network (:. \ 1"1"'1 1 1ct h :.Ho :.t I o:.ct f\1 hr :.nrh oc fnr nno nr 1"1"'1 nro rl ncori n :.th c tn ov ict

(b) can have an un I im i ted number of branches

(c) can on I y have at most N branches

(d) can have a minimum number of branches not decided by N

2. 3. Consider the star network shown in Fig. P2. 3. The resistance between term ina s A

2.4.

2.5.

and B with C open is 60., between terminals B and C with A open is 11 between terminals C and A with B open is 90.. Then

A

Ro

B.,__/

｣ＭＭＭＭＭＭＭＭ ＭＮ ｾｾ＠

(a) RA=40.,R8 =20.,Rc=50.

(c) RA=30.,R8 =30.,Rc=40.

Given the potential function in li'ee

the mag ni tude (in volts/metre). (1,-1, 1), where the dimension

(a)

(c)

ｾ Ｍ ,R8 =40.,Rc =70.

•"U =50.,R8 =10.,Rc = 100.

l:ie V(x) = (5ox2 +50y2 +50z2) volts,

t rection of the electric field at a point etres, are

(b) ｾ［ＨｩＭｪＫ＠ k)

100 1::: (d) ./3 ; [(1- J - k)/v3]

output power' wi II have a torque speed

(d) rectangular h yperb ol a

2.7. A 3-phase transformer has rating of20 MVA, 220 kV (star)- 33 kV (delta) with I eak age reactance of 12%. The transformer reactance (in ohms) referred to each phase of the L. v. delta-connected side is

(a) 23.5 (b) 19.6 (c) 18.5 (d) 8.7

2.8.

2.9.

A 75 NVA, 10 kV synchronous generator has Xd = 0.4 p.u. The Xd value (in p.u.) is a base of 100 MVA, 11 kV is

(a) 0.578 (b) 0.279 (c) 0.412 (d) 0.44

A star-connected 440 V, 50 Hz alternators has per phase synchronous reactance of 100.. It supplies a balanced capaat1ve load current of 20 A, as ｳｨ ｾｯｮ＠ e per phase equ1valent c1rcu1t of Flg.2.9. It 1s desirable to have zero ta reg ul at1 on. The I oad power factor should be

JlOn 20A 0

(a) 0.82 (b) 0.47

CJ (l)•

2.10. A 240 V single-phase ac sour8e is 10L6 0°0.. A capacitor is connect

(a) 3600 W

2.11.

• model shown in Fig.P2.12 of a transmission line with a series s mid-point. The maximum voltage on the line is at the location

jO.lpu jO.lpu

ｾＨ＠ ｾﾷ＠ !QM\ p. ｾ＠ • t -jO.lpu I=lput

V. = 1puLo' pf=l -

I {

(a) Pt (b) Pz (c) p3 (d) P4

2.13. A power system has two synchronous generators. The Governor-turbine characteristics corresponding to the generators are

2.14.

2.15.

2.16.

P1 = 50( 50 - f), P2 = 100(51 - f)

Where f denotes the system fi'equency in Hz, and P1 and P2 are, respectively, the power outputs (in MW) of turbines 1 and 2. assuming the genera tors and tr ansm i ssi on network to be I ossl ess, the system fi'equen cy for a total I oad o 0 MW is

(a) 47.5 Hz

2e-2s (a) s+3

(b) 48.0 Hz (c) 48.5 Hz

-40dB/dec

(d) 49.

(d) 2s+ 72

(s+3)

-40dB/dec /

-60dB/dec ./

-+"o.""l __ ..J.2-...!

5,--...L

25-- rad/s

For extending its range to measure ＵＰＰｾａＬ＠ the shunt requ1red 1s ofres1stance (1n 0.)

(a) 20.0 (b) 22.22 (c) 25.0 (d) 50.0

2.18. Resistances R1 and R2 have, respectively, nominal values of 100. and 50., and taler ances of± 5% and ± 10% . The range of values for the parallel comb in ati on of R1 and R2 is

2.19.

2.20.

2.21.

(a) 3.077 n to 3.636 n (c) 3.237 n to 3.678 n

Ｈ｣ＩｾｬｮＲ＠

(d) Ｒｾ＠ In 2

(b) 2.805 n to 3.371 n (d) 3.192 n to 3.435 n

An Intel 8085 processor 1s executmg the ｰ ｾＬ ｶ･ｮ＠ below.

MVIA, 10H "'V BACK: NOP

ADD B

ｒｾ ｲＺｬｦ＠

Tho oom b" of ＰｾＮｬＯＡ Ｌ＠ '''"''" "OP will bo ＧＢ］ｾ､＠ ;, ''"'' <o (a) 1 ( 2 (c) 3 (d) 4

A sam e- ､Ｍ ｾ ､＠ (S/H) circuit, having a holding capacitor of 0.1 nF, is used at the in n ADC (analog-to-digital converter). The conversion time of the ADC

ｾｩｬＡｬｩｬ ｾＺＮ ｾ ｡ｮ＠ during this time, the capacitor shou I d not I ose more than 0. 5% of ha ｧｯ ｾ＠ put across it during the sam piing tim e. The maximum value of the

i g nal to the S/H circuit is 5 v. The I eak age current of the S/H circuit shou I d le s than

(b) 0.25 mA (c) 25.0 ｾａ＠ (d) 2.5 ｾａ＠

(b) 6.28 MHZ (c) 10.0 MHZ (d)62.8MHz

2.23. An n-channel JFET, having a pinch-off voltage (Vp) of -5 V, shows a trans-cend uctan ce ( gm) of 1 m A/V when the applied gate-to-source val tag e (V Gs) is -3V. Its maximum transconductance (in m A/V) is

2.24.

2.25.

(a) 1.5 (b) 2.0 (c) 2.5 (d) 3.0

180°

(c) 200°

(d) 120°

A sing I e-p hase full-bridge val tage source inverter feeds a purely inductive load, a shown in Fig.P2.25, where T1, T:a T3, T• ar power tr ansi stars and D 1, D 2, D 3, D • a feedback diodes. The inverter is operag square-wave mode with a frequen c 50 Ｍ ｾ ＧＺＺＭ

Hz. If the average load currends t is the time duration of condu · o c feedback diode in a cyde?

(a) 5 msec Ｈ｢Ｉ ｲａＮｾ＠(c) 20 msec ｾＭ Ｈｔ ｣＠

SECTION- B This sect1on ｣ｯｮｳＱｳｴｳ ｾ＠ questions of FIVE marks each. ANY FIFTEEN out of them have to be ans ed If more number of questions are attempted, score off the ｡ｮｳｷ･ｲｳ ｾ＠nt b eval , else, only the first fifteen unscored answers will be considered. + 3. D et rm e resonance fi'equen cy and the Q-factor of the circuit shown in

c +

4. An ideal transformer has a linear B/H characteristic with a finite slope and a turns ratio of 1: 1. The primary of the transformer is energized with an ideal current source, produdng the signal i as shown in Fig.P4. Sketch the shape (neglecting the seale factor) of the foil owing signals, I abel i ng the time axis clearly:

5.

6.

7.

(a)

(b)

(c)

(d)

...,.,f--,---\-.:-f'--::---'<--.-• t( s)

the core flux '•c With the secondary of the transformer op(J the open-circuited secondary terminal voltage vi,

the short-circuited secondary current i2(t), and

the core flux 'sc with the secondary of the tr

• Consider the val tage waveform v, shown in Fig .PS. Find.

(a) the de component of v, .

component of v, and t(ms)

(c) the rms value of ｴｨ･ ｾ＠ v. -lV

In a de motor ru nn in ｡ ｾｾ＠ m, the hysteresis and eddy current I asses are so ow and 200W r •· . If the flux remains constant, calculate the speed at which the ｴｯｴ｡ｬ ｾ ＧＺ＠ .s are halved.

A de serie m ｯ ｾ ｴ･､＠ 230V, 1000 rpm, 80 A (refer to Fig.P7). the series field ·s O• Hl., and the armature resistance is 0.140.. If the flux at an

ren"'!: of 20A is 0.4 times of that under rated condition, calculate the educed armature current of 2 OA.

r-----e•+

j Rt=O.lln l 230V

8.

9.

10.

11.

A 50 kW synchronous motor is tested by driving it by another motor. when the excitation is not switched on, the driving motor takes 800W. When the armature is short-circuited and the rated armature current of 10 A is passed through it, the driving motor requires 25 DO W. On open-d rcui tin g the armature with rated excitation, the driving motor takes 1800W. Calculate the efficiency of the synchronous motor at 50% load. Neglect the losses in the driving motor.

Two identical synchronous generators, each of 100 MVA, are working · supplying 100 MVA at 0.8 lagging p. f. at rated voltage. Initially the m a es e sharing load equally. If the fie I d current of first genera tor is red ｵ ｾ＠ and of the second genera tor increased by 5%, find the sharing of ad and MVAR) between the generators. Assume Xd = Xq = 0.8 ｰＮｵ Ｌｃ･ｾ＠ turation and rated voltage across load. Reasonable approximations m -o-e.

A 132 kV transm1ss1on lme AB 1s connected to a ｾ ｃ Ｋ ｔ ｨ･＠ charactenst1c 1m pedan ces of the overhead line and the cable are ｾ ｾ＠ j nd 80 0. respect1 vely. Assume that these are purely res1st1ve. A 250 ｫ ｑ Ｑ ｴ ｾ ｳｵｲｧ･＠ travels from A to B.

(a) Cal cui ate the value of th 1 s val tage sur st reaches C.

(b) Cal cui ate the value of the refl ecte t of this surge when the first reflection reaches A.

(c) Cal cui ate the surge curren t'i n

For the V-bus matrix given in fourth row refers to bus 1

u values, where the first, second, third and 4 resp ecti vel y, draw the reactance diagram.

12. us ljen erator is connected to an infinite bus through a I ossl ess double ci rc it issi on I i ne. The genera tor is delivering 1. 0 per unit power at a I oad

3 ° when a sudden fau It reduces the peak power that can be 1 to 0.5 per unit. After clearance of fault, the peak power that can be i tted becomes 1. 5 per unit. Find the critical clearing angle.

A single I i ne-to-groun d fau It occurs on an unloaded generator in phase a positive, negative, and zero sequence impedances of the generator are j 0. 2 5 p. u., j 0. 25 p. u., and j 0 .15 p. u. respectively. The generator neutral is grounded through a reactance of j 0. 0 5 p. u. The prefau It genera tor terminal voltage is 1. 0 p .u.

(a) Draw the positive, negative, and zero sequence networks for the fault given.

(b) Draw the interconnection of the sequence networks for the fault analysis.

(c) Determine the fault current.

14.

15.

16.

17.

A power system has two generators with the foil owing cost curves:

Generator 1: C1(PG1) = 0.006P2Gt + BPGt + 350 (Thousand Rupees/Hour)

Generator 2: C2(PG2) = 0.009P2Gz + 7PGt + 400 (Thousand Rupees/Hour)

The generator I im its are

100 MW s PGt s 650 MW

50 MW s PG2s 500 MW

A load demand of 600 MW is supplied by the generators 1n an opt1m a r.

of each generator.

A unity feedback system has an open -1 oop transfer fi.J nct1 on or ｾ＠G(s) = 10000

2 ｾ＠

s(s+10) ｾ＠ .. + (a) Determine the magnitude of G(joo) in dB ｡ ｵｾ ｲ＠ frequency of oo=20

rad/sec.

(b) Determine the phase margin in degreeYlf

(c) Determine the gain margin in dB.

(d) Is the system stab I e or u nstab I e? ｾ＠

Given the characteristic ･ｱｵ｡ｴｩ ｾ＠ｳ Ｓ ＫＲｳ Ｒ ＫｋｳＫｋ］ｏＮ ｑｦＮｾｾ Ｍ _

Sketch the root I ocus as from zero to infinity. Find the angle and real axis intercept ofth;:Ea mp t , break-away/break-in points, and imaginary axis CrOSSing pOintS, If

For the ring coO ttt. n in Fig.P17, find the steady state sequence if the initial state of the ｾ＠ is 1110 (i.e., Q3.Q2,Q1,Qo= 1110). Determine the MOD numbe r ｾ ｾ ｨ･＠ counter.

ｾＮＮＮ＠ • rl'

ＭＭｾＧ＠ ｾ＠D Q, D Qz D Q, D Qo

1\ /' 1\

Clo ck

18.

19.

For the op -amp circuit shown in Fig. P 18, determine the output val tage v0.

Assume that the op-am ps are ideal.

ＢＭ］ｾ＠4kn akn

.. z r,_ The transistor 1n the am pllfier c1rcu1t shown 1n F1g .P19 1s ｢Ｑ｡ｳ ｾ＠ = 1m A. Use V7 = (= kT /q) = 26mV, Po= 200,rb = 0, and r 0 --+><>. ｾ＠ .. + (a) D eterm in e the ac small signal midband val ｴ ｾｾ＠ of the circuit.

(b) D eterm in e the required value of CE for t cir · t to have a I ower cutoff fi'equ ency of 10 Hz.

ｾ＠

;f 20. A simple

ｴｲ｡ｮｳｾ＠ ｾ＠

pas I

ctiv0 is shown in Fig.P20. Assume ideal op-amp. Derive the tio+ vo'v1 of the circuit, and state the type of the filter (i.e., high-s, band-pass, or band-reject). Determine the required values of R 1,

i order for the filter to have a 3-dB frequency of 1 kHz, a high-input resistance of 10 0 k 0., and a high fi'equ en cy gain m agn i tude of

ｾ ＢＧ＠ｾ＠

R,

R1 C Yo -yVV'-----j 1;-----'---J

21. A voltage com mutated th yri star chopper circuit is shown in Fig .P21. The chopper is operated at 50 0 Hz with 50% duty ratio. The I oad takes a constant current of 20A.

22.

(a)

(b)

(c)

Evaluate the circuit turn off time for the main thyri star Th ;.

Calculate the value of inductor L, if the peak current through the main thynstor Th,1s llm1ted to 180% of the load current. ｾ＠

Cal cui ate the max 1m um mstan tan eou s output val tage of the chop per.

lh, I =20A

Ｑｾ＠ D!' D!'"· ＬＮｾ＠ c;O v.,

Load

lOOV •

It constant