-
www.examrace.com
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 i~"!/:.::th the input voltage.
Two incandescent light bulbs of 40 W an~~g 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
-
www.examrace.com
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) ~maximum supply voltage
1.7 It is desirable to eliminate s'h harmonic voltage fi'om the phase vol~tan 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 alternatorCr~d armature current, unity power factor and also at no load. The magnetiz~ve for rated armature current, 0.8 power factor leading is given b"r> +
Noload ~=~
(a) curve A (d) curveD
1.9 The core flux of apr. (a) is strictly ~~a~n!l)lt.l""=lroad changes (b) increases I h I oad (c) increases a quare root of the I oad (d) de re es ~h 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'
-
www.examrace.com
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 a~i n s (a) m agn eti zing inrush current (b) u nb al anced operation (c) lightning (d) switching over-val~
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 ~s~ong it (d) has sending end voltage higher than receivr~age and unity power
factor at sending end ~
The polar plot of a type-1, 3-pole, op7_ -IIU~Je em is shown in Fig.P1.14. the closed loop system is ~-v
(a) (b)
~~~ wi~ one pole on the right half s-plane n!l~-m~ith 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) X55 =[~] [-10] (c) Xss = 10 (d) X55 =[:]
-
www.examrace.com
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
~ ~ X y
(b) x+ y (c) x + y (d) xy+x
-
www.examrace.com
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 o he followm g relationship between their triggering angles (txt and tx0i) (a) txt+ tx2 = 180 (b) txt+ ~ (c) txt- !Xz = 180 (d) r rJ.: 0 In case of an armature controlled separa~~ motor drive with closed-1 oop speed control, an inner current I o~ m ecause it (a) limits the speed of the motor to a ~ v -(b) he Ips in i m proving the drive e r !V~i ency (c) (d)
the perm issi bl e value
tl&'-.~~II:ICi ~eld 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./S1rs0
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
-
www.examrace.com
(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.,__/
c---------.~~
(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)
~- ,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) ~;(i-j+ 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
-
www.examrace.com
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 sh~on 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 00.. 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 ~( ~ !QM\ p. ~ t -jO.lpu I=lput V. = 1puLo' pf=l -I {
(a) Pt (b) Pz (c) p3 (d) P4
-
www.examrace.com
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,--...L25
-- rad/s
For extending its range to measure 500~A, the shunt requ1red 1s ofres1stance (1n 0.) (a) 20.0 (b) 22.22 (c) 25.0 (d) 50.0
-
www.examrace.com
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
(c)~ln2 (d) 2~ 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 p~,ven below. MVIA, 10H "'V
BACK: NOP ADD B
R~r:lf Tho oom b" of 0~.l/!, '''"''" "OP will bo '"=~d ;, ''"''
-
www.examrace.com
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 -~'::-Hz. If the average load currends t is the time duration of condu o c feedback diode in a cyde? (a) 5 msec (b)rA.~ (c) 20 msec ~-(Tc
SECTION- B This sect1on cons1sts~ 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
answers~ 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 +
-
www.examrace.com
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'--::---'
-
www.examrace.com
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 u~ and of the second genera tor increased by 5%, find the sharing of ad and MVAR) between the generators. Assume Xd = Xq = 0.8 p.u,Ce~ turation and rated voltage across load. Reasonable approximations m -o-e. A 132 kV transm1ss1on lme AB 1s connected to a ~C+The charactenst1c 1m pedan ces of the overhead line and the cable are ~~ j nd 80 0. respect1 vely. Assume that these are purely res1st1ve. A 250 kQ1t~surge 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.
-
www.examrace.com
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 ~ G(s) = 10000
2 ~
s(s+10) ~ .. + (a) Determine the magnitude of G(joo) in dB au~r 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? ~ Given the characteristic equati~
s3+2s2 +Ks+K=O.Qf.~~- _ 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 ~ is 1110 (i.e., Q3.Q2,Q1,Qo= 1110). Determine the MOD numbe r ~~he counter. ~... rl'
--~' ~ D Q, D Qz D Q, D Qo
1\ /' 1\
Clo ck
-
www.examrace.com
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.
"-=~ 4kn akn .. z r,_
The transistor 1n the am pllfier c1rcu1t shown 1n F1g .P19 1s b1as~ = 1m A. Use V7 = (= kT /q) = 26mV, Po= 200,rb = 0, and r 0 --+>. ~ .. + (a) D eterm in e the ac small signal midband val t~~ 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. ~
;f 20. A simple
trans~ ~ 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 ~"' ~ R, R1 C Yo -yVV'-----j 1;-----'---J
-
www.examrace.com
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. ~ Cal cui ate the max 1m um mstan tan eou s output val tage of the chop per.
lh, I =20A 1~ D!' D!'" ,.~ c;O v.,
Load lOOV
It constant
