# (Download) UPSC IES Exam Paper - 2018 "Electrical Engineering Paper - 2"

## (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/s ^{2} (ts+1) , is
placed in cascade with a proportional and derivative controller K(s) = (1
+T_{d}s). 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, T_{d}
and t.**

**(c) Determine the mechanical time constant of rotor of an
electrical machine in terms of its moment of inertia J kg-m ^{2} 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 m _{a} = 0.8 and frequency modulation ratio m_{f}
= 21. Consider dominant harmonics to be frequency dominant and its nearby side
frequencies (both sides). Assume normalized Fourier coefficient for m_{a}
= 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 :**

**R _{1} = 0.2W R2 =
0.15W R_{sh} = 320W**

X_{1} =1.2W X_{2} = 1.29W X_{M} = 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 :**

**r _{1} = 0.4W; = r_{2 =}0.2W;
x_{1} = x'_{2} = 1.5W; X_{m}
= 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 < S _{m} (motor region) and s>s_{m} (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.**

## Study Material for IAS (UPSC) General Studies Pre. Cum Mains

## SECTION-B

**5. (a) A DC motor has an armature resistance of 0.5W
and Ko of 3 Vs. The motor is driven by a single-phase thyristorized full
converter. The input to the converter is an AC source of 230 V, 50 Hz. The motor
is used as a prime mover of a forklift. In the upward direction, the mechanical
load is 69 Nm and the triggering angle is a = 15°. In the downward direction,
the load torque is 180 Nm. Calculate the triggering angle required to keep the
downward speed equal in magnitude to upward speed. Assume continuous motor
current for all operation. Also calculate the triggering angle to keep the motor
at holding position while it was moving upward.**

**(b) The primary side of an ideal transformer (having 400
turns in primary winding and 720 turns in secondary winding) is excited by a
1000 V, 50 Hz AC source. The secondary of the transformer is connected to a
resistive load of 80 kW. There is one tapping in secondary winding at 480 turns
and this tapping is supplying a pure inductive load of 100 kVA. Determine the
primary current and its power factor.**

**
**

**(e)**

**
**

**6. (a) A 20 kW, 500 V DC shunt motor (having 90% full-load
efficiency) has 40% armature copper losses of its full-load losses. Calculate
the resistance values of a 4-section starter suitable for limiting starting
current between 120% to 200% of full-load current. Assume field resistance of
250W**

**(b) (i) Differentiate between characteristic impedance and
surge impedance of a line. What do you mean by surge impedance loading (SIL) of
a transmission line? **

**(ii) A three-phase, 50 Hz transmission line is 400 km long.
The voltage at the sending end is 220 kV. The line parameters are r = 0.125
ohm/km, x = 0.4 ohm/km and y=2.8x10- ^{6} mho/km. Find the sending-end
current and receiving-end voltage when there is no load on the line. Make a
comment on the value of receiving-end voltage.**

**(c) A boost converter is required to have an output voltage
of 48 V and supply a load current of 5 A. The input varies from 12 V-24 V. A
control circuit adjusts the duty ratio to keep the output voltage constant.
Select the switching frequency to be 200 kHz. Determine a value of inductor such
that the variation in inductor current is no more than 40% of average inductor
current for all operation. Prescribe a suitable value of capacitor such that
output ripple is no more than 2%.**

**
**

**(c) (i) In case of a circuit breaker, define the terms 'restriking
voltage and RRRV', and express their maximum values in terms of system voltage.**

**(ii) Which circuit breaker is preferred for voltages 132 kV
and above?**

**(iii) In a 132 kV system, the reactance per phase up to the
location of circuit breaker is 5W and capacitance to
earth is 0.03 uF. Calculate the maximum value of restriking voltage, the maximum
value of RRRV and frequency of transient oscillation.**

**
**

**(c)**

**
**