(AUTONOMOUS)
B. Tech VII Semester Supplementary Examinations, May 2018
(Regulations: VCE-R14)
POWER SYSTEM SWITCHGEAR AND PROTECTION
(Electrical and Electronics Engineering)
Date: 10 May, 2018 AN Time: 3 hours Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
Unit I
1. Explain the construction, principle of operation and advantages and disadvantages of an
air break circuit breaker.
7M
In a 220kV, 50Hz system, the reactance and capacitance up to the circuit breaker are
10Ω and 0.02μF respectively. A resistance of 750Ω is connected across the terminals of
the CB. Determine:
i. The undamped natural frequency of oscillations
ii. Damped frequency of oscillations
iii. Critical value of resistance
iv. Value of resistance which will give a damped frequency one fourth the undamped
frequency
8M
2. Discuss the recovery rate theory and energy balance theory of arc interruption in a
circuit breaker.
8M
Draw the equivalent circuit for a fault on a feeder occurring near the circuit breaker. For
such a fault, obtain the expression for the restriking voltage, rate of rise of re-striking
voltage, and the maximum rate of rise of re-striking voltage. Neglect the resistance of
the circuit.
7M
Unit II
3. Define the following terms with respect to over current relay:
i. Pickup current
ii. Drop off current
iii. Plug setting multiplier
iv. Time setting multiplier
v. Current setting
5M
With a neat sketch, explain the construction and working of directional over current
relay.
10M
4. What are the essential qualities of a protective relaying? Explain each one of them. 8M
List the merits and demerits of static relays over electromagnetic relays.
7M
Unit III
5. Describe with a circuit diagram, how inter-turn protection of a generator is achieved
using zero sequence voltage. State where such a scheme is used.
8M
An 11kV, 100MVA generator is provided with differential scheme of protection. The
percentage of generator winding to be protected against phase to ground fault is 75%.
The relay is set to operate when there is 15% out of balance current. Determine the
value of resistance to be placed in the neutral to ground path.
7M
6. With diagrams of relay location and operation, explain how the Buchholz relay is used in
transformer protection. How does the relay help in determining the type of fault?
7M
Explain how a three-phase feeder can be protected by translay protection. 8M
Cont…2
2
Unit IV
7. Discuss the phenomenon of wave reflection and refraction. 5M
An overhead transmission line having a surge impedance of 500Ω and branches into
two lines having a surge impedance of 40Ω and 60Ω respectively. If a travelling wave
of vertical front and magnitude 100kV travels along the overhead line, calculate the
magnitude of voltage and current in the overhead line and in the two branches
immediately after the travelling wave has reached the fork.
10M
8. Explain reflections in a transmission line when terminated in open circuit and short
circuit.
8M
Two stations are connected by an underground cable having a surge impedance of
60Ω and a transmission line having a surge impedance of 400Ω. If a surge having a
maximum value of 100kV travels along the cable towards the junction with the
overhead line, determine the value of the reflected and transmitted wave of voltage
and current at the junction.
7M
Unit V
9. Explain the effect of having an ungrounded neutral on system performance. 7M
Describe solid and reactance grounding. State their advantages and disadvantages.
8M
10. Explain the schemes adopted to protect a transmission line against direct lightning
strikes.
8M
Describe with a neat diagram, the construction and operation of a valve type lightning
arrestor.
7M