Protection of Feeders and Transmission Lines Interview Questions and Answers:

1. What is unit type protection ?

Ans. Unit system of protection is one in which the protection responds to faults in the protected zone alone and it does not respond to through faults (faults beyond the protected zone). Non-unit systems do not have exact zone boundary.

2. What are unit systems and non-unit systems of protection ?

Ans. Selectivity is absolute if the protection responds only to the faults within its own zone, and relative if it is obtained by grading the settings of the protective relays of several zones all of which may respond to a given fault. Systems of protection which in principle are absolutely selective are called the unit systems. Systems in which selectivity is relative are non-unit systems. Differential protection and frame leakage protection fall in the former category while the current time graded protection and distance protection fall in the latter category.

3. Give the examples of unit/and non-unit systems of protection.

Ans. Unit type protection includes pilot wire differential protection, carrier current protection based on phase comparison etc. while non-unit type protection includes time graded overcurrent protection, current graded overcurrent protection, and distance protection.

4. What is meant by time graded protection ?

Ans. Time graded protection is a scheme of overcurrent protection, in which discrimination is incorporated i.e., the time setting of relays is so graded that in the event of fault, the smallest possible part of the system is disconnected.

5. What is the purpose of time grading of protection system and where is it employed ?

Ans. The purpose of time grading of protection system is to incorporate time discrimination. It is employed for the protection of radial feeders, parallel feeders and interconnected systems.

6. What type of relay is suitable for radial feeders ?

Ans. Induction type inverse definite minimum time (IDMT) relays are most suitable for radial feeders.

7. What is the minimum time interval which can be permitted for the two adjacent circuit breakers to operate in radial feeders ?

Ans. 0.4 second.

8. Why induction type IDMT relays are most suitable for protection of radial feeders ?

Ans. Induction type IDMT relays are most suitable for protection of radial feeders because their time-current characteristics are similar in shape and in no case they cross each other at any point.

9. On what fact is the current graded protection based ?

Ans. Current graded protection is based on the fact that the short-circuit current along the length of the protected circuit decreases with the increase in distance between the supply end and the fault point.

10. What type of relay is suitable for current graded protection ?

Ans. High-speed high-set overcurrent relays.

11. Can Merz-Price system be employed for protection of ring mains and duplicate feeders ?

Ans. Yes.

12. Does Merz-Price system provide backup protection or overload protection ?

Ans. No.

13. Why Merz-Price protection is not suitable beyond 33 kV ?

Ans. Difficulties are experienced in balancing the secondaries of the two CTs and that is why Merz-Price protection system cannot be used beyond 33 kV.

14. Is the operation of relays affected by capacitive currents in Translay system of protection of feeders?

Ans. No.

15. Why is use of wire pilot relaying scheme restricted to protection of short transmission lines ?

Ans. In case of medium and long transmission lines the pilot wire protection scheme becomes expensive because of cost of pilot wires.

16. What are the main elements of current carrier protection ?

Ans. The main elements of the current carrier protection scheme are: (i) transmitter (ii) receiver (iii) coupling equipment and (iv) line trap.

17. What purposes are served by line trap and coupling capacitors in carrier channels ?

Ans. Line trap is an LC network inserted between the bus-bar and connection of coupling capacitor to the line and tuned to resonance at the high frequency and are used to confine the carrier currents to the protected section so as to avoid interference with or from adjacent carrier current channels and also to avoid loss of the carrier current signal in adjoining power circuits for any reason whatsoever.

The coupling capacitor is used to connect the high frequency (carrier) equipment to one of the line conductors and simultaneously serves to isolate the carrier equipment from the high power line voltage.