JNTU B.Tech II Semester Examinations, SWITCHGEAR AND PROTECTION, Apr/May 2008
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
1. A generator connected through a 3-cycle Circuit Breaker to a transformer is rated
10MVA, 13.8kV with reactances of X??d = 10%, X?d =15% and Xd =100%. It is
operating at no load and rated voltage when a 3-phase short circuit occurs between the breaker and the transformer. Determine
(a) the sustained short circuit current in breaker;
(b) the initial symmetrical rms current in the breaker;
(c) the maximum possible d.c component of the short circuit current in the breaker;
(d) the momentary current rating of the breaker;
(e) the current to be interrupted by the breaker and
(f) the interrupting kVA. 
2. Explain the principle of arc extinction and What are the different methods of arc extinction. 
3. Define the following terms and explain their significance in distance protection
(a) Reach of a distance relay.
(b) Under reach. [8+8]
4. Explain with a diagram, the application of the Merz-Price circulating current system to the protection of alternators. What precautions must be taken in installing this system? [8+8]
5. (a) A 3-phase, 66/11kV star-delta connected transformer is protected by Merzprice protection system. The CTs on the LT side have a ratio of 420/5 ampr.
Show that the CTs on the HT side will have a ratio of 70: 5?3 .
(b) Explain with reasons the connections of C.T.s for protecting a delta/star trans-
former. Write the scheme of protection for [10+6]
i. Internal fault and
ii. External fault.
6. (a) Explain over current protection of feeder.
(b) Explain a scheme of protection for a ring mains. [8+8]
7. (a) What are the reasons leading to the general practice of earthing the neutral point of a power system? Explain.
(b) Explain the phenomenon of arcing grounds and discuss the method to minimize the effect of this phenomenon. [10+6]
8. (a) How over head transmission lines are protected from lightning strokes.
(b) Why ground wire is provided as the top lost conductor in high voltage trans-
mission lines. [6+10]