JNTU Previous Papers-B-Tech-Switch-gear And Protection-Apr/May 2008

JNTU  B.Tech  Year-III Examinations, SWITCH-GEAR AND PROTECTION, Apr/May 2008

(Electrical & Electronic Engineering)

Time: 3 hours Max Marks: 80


Answer any FIVE Questions

All Questions carry equal marks

1. Calculate the RRRV of a 220kV circuit breaker with earthed neutral. The short

circuit test data obtained is as follows:

The current breaker is symmetrical and the restriking voltage has an oscillatory frequency of 15 kHz. The power factor of the fault is 0.2. Assume the short circuit to be an earthed fault. [16]


2. (a) Compare the arc rupture in oil and air blast circuit breakers and summarize the relative advantages and disadvantages of these types of switch gear.

(b) Explain the operating duty of a circuit breaker. [8+8]


3. Describe the various types of construction of attracted armature type relay. Why can they operate in a.c and d.c?. State its salient features. [8+8]


4. An 11kV,100MVA generator is provided with differential scheme of protection. The

percentage of the generator winding to be protected against phase to ground fault is 80%. The relay is said to operate when there is 15% out of balance current. Determine the value of resistance to be placed in the neutral to ground connection. [16]


5. Write short notes on the following:

(a) Different transformer faults

(b) Biased differential protection for transformer

(c) Buchholtz Relay. [6+6+4]


6. (a) Explain bus bar protection need special attention. Why?

(b) What is back up protection of bus bars? [10+6]


7. (a) Discuss the advantages of neutral grounding.

(b) What is tower-footing resistance. [8+8]


8. (a) What are various methods of over voltage protection of overhead transmission


(b) Explain clearly how the rating of a lightning arrester is selected. What is the best location of a lightning arrester and why? [8+8]

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