# JNTU B-Tech II Semester Examinations,Switchgear And Protection, Apr/May 2008

JNTU PAPERS

JNTU B.Tech II Semester Examinations,

Â Switchgear And Protection

Apr/May 2008

(Electrical & Electronic Engineering)

Time: 3 hours Max Marks: 80

All Questions carry equal marks

SET-IV

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

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

3. (a) Describe the construction of an induction disc relay. State its principle of operation. What are the advantages to induction relays. How is the current setting and time setting obtained?

(b) With a neat sketch, describe the difference between definite characteristic and

inverse characteristic of relays. [8+8]

4. (a) Explain how the inclusion of a resistance in the neutral earthing circuit of an

alternator affects the performance of the differential protection of the three-phase stator.

(b) Describe how protection is provided in large turbo-alternators against earth fault in the rotor [8+8]

5. Explain with a neat circuit diagram of the percentage differential protection scheme

to protect Y – _ transformer. [16]

6. (a) Explain over current protection of feeder.

(b) Explain a scheme of protection for a ring mains. [8+8]

7. (a) A 50Hz over head line has the line to ground capacitance of 1.2?F. It is decided to use a ground fault neutralizer. Determine the reactance to neutralize the capacitance of

(i)100% of the length of the wire, and (ii) 80% of the length of the wire

(b) Write short notes on biased differential protection for transformer. [10+6]

8. Write short notes of the following:

(a) Causes of over voltages in a power system.

(b) Switching surges.

(c) Protection against over voltages. [16]