Important Questions of Underground Cables:

1. Compare the merits and demerits of underground system versus overhead system.

2. With a neat diagram, show the various parts of a high voltage single-core cable.

3. What should be the desirable characteristics of insulating materials used in cables ?

4. Describe briefly some commonly used insulating materials for cables.

5. What is the most general criterion for the classification of cables ? Draw the sketch of a single-core low tension cable and label the various parts.

6. Draw a neat sketch of the cross-section of the following :

(i) 3-core belted cable

(ii) H-type cable

(iii) S.L. type cable

7. What are the limitations of solid type cables ? How are these overcome in pressure cables ?

8. Write a brief note on oil-filled cables.

9. Describe the various methods of laying underground cables. What are the relative advantages and disadvantages of each method ?

10. Derive an expression for the insulation resistance of a single-core cable.

11. Deduce an expression for the capacitance of a single-core cable.

12. Find an expression for the most economical conductor size of a single core cable.

13. Explain the following methods of cable grading :

(i) Capacitance grading

(ii) Intersheath grading

14. Write short notes on the following :

(i) Laying of 11 kV underground power cable

(ii) Capacitance grading in cables

(iii) Capacitance of 3-core belted cables

15. Derive an expression for the thermal resistance of dielectric of a single-core cable.

16. What do you mean by permissible current loading of an underground cable ?

17. With a neat diagram, describe Murray loop test for the location of (i) earth fault (ii) short-circuit fault in an underground cable.

18. Describe Valley loop test for the location of earth fault and short-circuit fault in an underground cable.

19. Overhead system can be operated at 400 kV or above but underground system offers problems at such voltages. Why ?

20. Why are VIR cables preferred to paper insulated cables for smaller installations ?

21. Why do we use 3 single-core cables and not 3-core cables for voltages beyond 66 kV ?

22. What is the mechanism of breakdown of an underground cable ?

23. How do voids in the insulation cause breakdown of the cable ?

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