Switchgear & Protection | Nagpur University | Winter-17

B.E. (Electrical Engineering (Electronics & Power) / Power Engineering)
Eighth Semester (C.B.S.)

Switchgear & Protection

Time : Three Hours
Max. Marks : 80
Notes : 1. All questions carry marks as indicated.
2. Solve Question 1 OR Questions No. 2.
3. Solve Question 3 OR Questions No. 4.
4. Solve Question 5 OR Questions No. 6.
5. Solve Question 7 OR Questions No. 8.
6. Solve Question 9 OR Questions No. 10.
7. Solve Question 11 OR Questions No. 12.
8. Due credit will be given to neatness and adequate dimensions.
9. Assume suitable data whenever necessary.
10. Illustrate your answers whenever necessary with the help of neat sketches.
11. Use of non programmable calculator is permitted.

1. a) What is meant by primary and back up protection. What are the causes of failure of primary protection. Why usually a back-up protection is time delayed.  [07 M] 

b) Explain the terms. [06 M]
i) Selectivity                                                       ii) Sensitivity
iii) Discrimination stability


2. a) Describe with a single line diagram overlapping of zones of protection. What purpose is achieved by such overlapping.  [06 M] 

b) What are the principle types of faults in a power system. In what way it is harmful to the power system.  [07 M] 

3. a) Discuss a suitable protection scheme for ring mains feeder with 4 sections. Give the selection of relays with Justification.  [05 M]

b) A 20 MVA transformer which is used to operate at 30% overload feeds an 11kV busbar through a circuit breaker. The transformer circuit breaker is equipped with a 1000/5 current transformer and feeder circuit breaker with 400/5 current transformer and both the C.T. feed
IDMT relays having the following characteristics.

PSM 2 3 5 8 15 20
Time in seconds for TMS = 1 10 6 4.1 3 2.5 2.2

The relay on feeder circuit breaker has 125% plug setting and 0.3 time multiplier setting. If a fault current of 5000A flows from the transformer to the feeder. Determine –  [08 M] 
i) Operating time of feeder relay.
ii) Suggest suitable PSM and TSM of the transformer relay to ensure adequate
discrimination of 0.5sec between the transformer relay and feeder relay.


4. a) What is meant by directional features of directional over current relay. Describe the
construction, principle of operation and application of directional overcurrent relay.  [07 M]

b) What is directional relay connection angle. Explain the quadrature connections of directional relay with the help of phaser diagram and show CT and PT connections with the relay to implement it.  [06 M]

5. a) Suggest the suitable distance relays for protecting the following with justification. [06 M]

i) Short transmission line
ii) Medium transmission line
iii) Long transmission line

b) Draw impedance, reactance and Mho characteristics to protect the 100% of the line having
(3.6 + j6)Ω impedance. A fault may occur at any point on the line through arc resistance of 2.2Ω. Determine the maximum % age of line section which can be protected by each type of relay.  [08 M] 


6. a) Explain ‘Carrier Blocking scheme’. Discuss its merits and demerits over other type of carrier aided distance protection.  [06 M]

b) Explain phase comparison method of carrier current protection.  [08 M]

7. a) Explain in brief problems arising in differential protection of transformer.  [04 M]

b) A 3 phase 33000/6600 volt transformer is connected in  ϒ/Δ   and the protecting CT on the low voltage side have a ratio of 300/5. What will be the ratio of the current transformer on the high voltage side. Draw complete protective scheme.  [06 M]

c) Explain in detail protection of generator against loss of excitation.  [07 M] 


8. a) Explain different types of faults & protection used in Induction motor.  [06 M]

b) Explain the need for bus bar protection. With neat sketch, explain the differential scheme
for bus bar protection.  [07 M] 

9. a) Compare static and electromagnetic relays. [06 M]

b) Explain the working of static Mho relay using amplitude comparator.  [07 M] 


10. a) Explain duality between the comparators giving suitable examples. [07 M] 

b) Explain any one scheme for generating sine and cosine comparator. [06 M] 

11. a) Explain the terms. [10 M]

i) Current chopping              ii) Restriking voltage
iii) Recovery voltage             iv) RRRV
v) Making cap of C. B.

b) A circuit breaker is rated at 1kA, 1200MVA, 33kV, 3 second, 3 phase Determine.   [04 M] 
i) Breaking capacity ii) Symmetrical breaking capacity
iii) Making capacity iv) Short time rating.


12. Write short note on.

i) Minimum Oil circuit Breaker. [07 M]

ii) SF6 circuit breaker. [07 M] 

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