(Download) UPSC IES Exam Paper - 2018 "Electrical Engineering
Paper - II"
Exam Name: Engineering Services Exam (IES)
Paper : ELECTRICAL ENGINEERING Paper - II
Year: 2018
File Type: PDF
ELECTRICAL ENGINEERING
Paper - II
Time Allowed : Three Hours
Maximum Marks : 300
QUESTION PAPER SPECIFIC INSTRUCTIONS
Please read each of the following instructions carefully before attempting
questions.
There are EIGHT questions divided in TWO Sections.
Candidate has to attempt FIVE questions in all.
Question Nos. 1 and 5 are compulsory and out of the
remaining, THREE are to be attempted choosing at least ONE
question from each Section.
The number of marks carried by a question / part is indicated against it.
Assume suitable data, if considered necessary and indicate the same clearly.
Unless otherwise mentioned, symbols and notations carry their usual standard
meanings.
Attempts of questions shall be counted in sequential order. Unless struck
off, attempt of a question shall be counted even if attempted partly.
Any page
or portion of the page left blank in the Question-cum-Answer Booklet must be
clearly struck off.
Answers must be written in ENGLISH ONLY.
SECTION ‘A’
1. (a) A single-phase AC voltage controller is feeding a
resistive load of 26.45 2 from an AC source of 230 V, 50 Hz. Compute the firing
angle to deliver 1000 W to the load. Also compute the p.f. at which this power
is delivered. Draw a neat circuit diagram and waveforms of voltage at load
terminals with current flowing in the load.
(b) An open-loop system G(s)=1/s2 (ts+1) , is
placed in cascade with a proportional and derivative controller K(s) = (1
+Tds). If their unity feedback closed-loop system oscillates at a
frequency of Ö2 rad/second, find the ranges/ values
of the system and controller parameters, i.e., ranges/values of K, Td
and t.
(c) Determine the mechanical time constant of rotor of an
electrical machine in terms of its moment of inertia J kg-m2 and
windage cum friction coefficient fN-m/rad/s. Also explain the method to
determine mechanical time constant experimentally in laboratory.
(d) An electric train running between two stations A and B,
10 km apart and maintained at voltages 550 V and 500 V respectively, draws a
constant current of 600 A. The resistance for both go and return conductors is
0.04W/km. Find the point of minimum potential between
the stations, the voltage at that point and currents drawn from both the
stations at that point.
(e)
2. (a) A single-phase full bridge inverter is used to produce
a 50 Hz voltage across a series R-L load (R = 10 W
and L = 20 mH) using bipolar PWM. The DC input to the bridge is 380 V, the
amplitude modulation ratio ma = 0.8 and frequency modulation ratio mf
= 21. Consider dominant harmonics to be frequency dominant and its nearby side
frequencies (both sides). Assume normalized Fourier coefficient for ma
= 0.8 to be 82% for dominant harmonic frequency and 22% for the nearby side
frequencies.
Determine
(i) amplitude of 50 Hz component of output voltage and
current;
(ii) power absorbed by the load resistor;
(iii) THD of the load current.
Also compare the amplitude of 50 Hz component of output
voltage with square wave and quasi-square wave output.
(b) A 3-phase, 6-pole, 460 V, 50 Hz induction generator
operates at 480 V. The generator has its rated output power of 20 kW. It is
driven by a turbine at a speed of 1015 r.p.m. The generator has the following
electrical parameters :
R1 = 0.2W R2 =
0.15W Rsh = 320W
X1 =1.2W X2 = 1.29W
XM = 42W
Find the active power delivered by the generator and reactive
power it requires from the system to operate.
(c) (i) Under what condition a single line-to-ground fault at
the terminals of a generator can be more severe than a 3-phase symmetrical fault
at the same location?
(ii) A 3-phase power system is represented by one-line
diagram as shown in the figure below :
3.(a)
4. (a) (i) What do you mean by grading of cables? What are
the methods of grading?
(ii) Derive the condition for minimum value of gradient at
the surface of the conductor.
(iii) Determine the economic overall diameter of a
single-core cable metal sheathed for a working voltage of 75 kV, if the
dielectric strength of the insulating material is 60 kV/cm.
(b) A 400 V, 50 Hz, 6-pole, 960 r.p.m., Y-connected induction
motor has the following parameters per phase referred to stator :
r1 = 0.4W; = r2 =0.2W;
x1 = x'2 = 1.5W; Xm
= 30W
The motor is controlled by a variable frequency inverter at a
constant flux of rated value for operation below synchronous speed,
super-synchronous operation region flux is weakened by keeping voltage constant
at rated value. Assume straight line for torque vs. slip characteristics for
slip s < Sm (motor region) and s>sm (generator region).
The connected load on the shaft is constant torque type.
Calculate the inverter frequency and current drawn by the
stator when torque on the shaft is half-rated while motoring at 500 r.p.m.
(c) Why is the waveshape of magnetizing current of a
transformer non-linear? Explain the phenomenon of in-rush magnetizing current
and derive its expression in terms of a, the angle of the voltage sinusoid at t
= 0 and or, the residual core flux at t = 0.
Use the graph sheet to show non-linearity of current from the
assumed 0-i diagram of magnetic core of the transformer.