JNTU B.Tech II Semester Examinations, SWITCHGEAR AND PROTECTION, Apr/May 2008

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. [16]


2. Explain the principle of arc extinction and What are the different methods of arc extinction. [16]


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]

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