SWITCH GEAR AND PROTECTION DEVICES
(ii) Induction type directional power relay
10. Describe the construction and principle of operation of an induction type directional
overcurrent relay.
11. Explain the working principle of distance relays.
12. Write a detailed note on differential relays.
13. Describe the Translay scheme of protection.
14. Write short notes on the following :
(i) Percentage differential relays (ii) Definite distance relays (iii) Time-distance relays
CHAPTER QUESTIONS
1. What is the difference between a fuse and a relay ?
2. What is the difference between an overcurrent relay and current differential relay ?
3. Back-up protection is not a good substitute for better maintenance. Discuss.
4. Why are differential relays more sensitive than overcurrent relays ?
5. What is the difference between a balanced voltage relay and a Translay relay ?
CH-7 PROTECTION AGAINST OVER VOLTAGE AND LIGHTING
ASSIGNMENT
1. Fill in the blanks by inserting appropriate words/figures :
(i) The most severe surges on the line are produced by ...............
(ii) Lightning produces a ............... fronted wave.
(iii) Transients on the power system due to current chopping are taken care of by ...............
(iv) Arcing ground can be prevented by ...............
(v) The lightning currents range from 10 kA to about ............... kA.
(vi) For successful working of ground wire, the footing resistance of tower should be ...............
(vii) A surge diverter should be located ............... to the apparatus to be protected.
2. Fill in the blanks by picking correct words/figures from brackets :
(i) A 1/50 μs surge is ............... harmful than 3/50 μs surge. Assume the same peak value.
(more, less)
(ii) The ............... lightning strokes are very rare on the power system. (direct, indirect)
(iii) Most of the lightning strokes are due to ............... charged clouds. (negatively, positively)
(iv) The stroke A will always occur on ............... (tallest object, earth)
(v) ............... cannot protect the equipment from the travelling waves reaching the equipment.
(ground wires, lightning arrester)
(vi) In sub-stations, the most commonly used type of arrester is ............... arrester.
(Thyrite, horn gap, rod gap)