Seat No.: ________ Enrolment No.
___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER–VII (NEW) EXAMINATION – WINTER 2021
Subject Code:3170908 Date:10/12/2021
Subject Name:Switchgear And Protection
Time:10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
4. Simple and non-programmable scientific calculators are allowed.
Q.1 (a) What do you understand by 30˚ and 60˚ connections with reference to 03
directional overcurrent relays?
(b) The percentage differential protection of transformer will not operate during 04
an external fault. Explain and justify with suitable figure.
(c) What do you understand by a zone of protection? Discuss various zones of 07
protection for a modern power system. Why do adjacent protective zones
overlap?
Q.2 (a) Explain the effect of arc resistance on Mho relay reach and the directional 03
property of Mho relay with the help of characteristics on R-X plane.
(b) Explain capacitor voltage transformer. Give its comparison with 04
electromagnetic potential transformer.
(c) 07
Figure 1
Two relays R1 and R2 are connected in two sections of a feeder, with CT ratios
of 1000/5 A as shown in Figure 1 above. The plug settings of relay R1 and R2
are 150% and 200% respectively. Operating time characteristic of both relays
at TMS=1 is given in Table 1 below.
Table 1
PSM 2 4 5 6 8 10 12 15 20
Operating
10 5 4 3.6 3 2.8 2.6 2.2 1.6
Time (s)
The time multiplier setting of relay R1 is 0.3. The time grading scheme has a
discriminative time margin of 0.5 seconds between the relays. A three-phase
short circuit at F results in a fault current as shown in figure.
Find the actual operating times of R1 and R2.
What is the time multiplier setting of R2?
OR
(c) With suitable waveforms, explain the problem of magnetizing inrush in case 07
of transformer protection. Explain how percentage differential relay with
harmonic restraint can overcome this problem.
1
Q.3 (a) Briefly explain how the CT ratio should be selected for differential protection 03
of bus-bars.
(b) Explain in brief selectivity, sensitivity, discrimination and speed with 04
reference to protective relaying.
(c) In relation to circuit breaker arc interruption, explain the recovery rate theory 07
and energy balance theory. Also with suitable figure, define restriking voltage
and recovery voltage.
OR
Q.3 (a) What is back up protection and why is it needed? 03
(b) Explain the necessity of directional overcurrent relay in case of single end fed 04
parallel feeders and radial feeders.
(c) Compare Air Blast Circuit Breaker, SF6 Circuit Breaker and Vacuum Circuit 07
Breaker with reference to their arc quenching mechanisms.
Q.4 (a) Briefly explain various types of faults and abnormal conditions in 03
transformers.
(b) Explain unbalanced loading and loss of excitation as abnormal operating 04
condition of alternator.
(c) With the help of characteristics on R-X plane, compare between performance 07
of impedance and reactance relays in terms of: i) Directional property
exhibited by the relay; ii) Performance of relay during power swing.
OR
Q.4 (a) Explain the difference between CT cores used for measurement and those used 03
for protective relays.
(b) Explain three stepped distance protection of transmission line. 04
(c) Explain the basic schematic of differential protection of bus-bars. Explain its 07
behavior with reference to internal and external faults, clearly explaining the
reason of maloperation during external faults.
Q.5 (a) Explain the concept of adaptive relaying. 03
(b) Explain how to select short-time current rating of CT and accuracy class of 04
CT while selecting a CT.
(c) With schematic, explain protection from phase fault and ground fault inside 07
the three phase induction motor.
OR
Q.5 (a) Explain the problem associated with unbalanced supply voltage and the 03
method of protection from the same for a three phase induction motor.
(b) Draw and explain the general block diagram of numerical relay. 04
(c) List out various stator and rotor faults of generator. Explain transverse 07
differential protection of generator winding.
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