21. Define working voltage, flashover voltage and puncture voltage.
Ans: Working voltage:
It is the voltage at which an insulator is designed to bear the steady state voltage stress.
Flashover voltage:
It is the voltage at which flashover occurs through air surrounding the insulator
Puncture voltage:
It is the voltage at which the insulator breaks through between conductor and pin. It destroys the insulator.
22. What are the characteristics of an ideal insulator?
Ans: i) There should not be any pores or air spaces
ii) There should not be any impurities
iii) There should be perfectly homogeneous material
iv) Leakage current through insulators should be minimum
v) Insulators should be able to withstand over voltage and normal working voltage. It should be mechanically strong to bear
23. Define safety factor.
Ans: A safety factor is defined relating the flashover and working voltages.
Mathematically,
Safety factor = Flashover voltage / Working voltage
24. What are the types of insulators used in overhead transmission lines.
Ans: i. Sackle type (230-440 V lines)
ii. Pin type
iii. Suspension type
iv. Strain type
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25. What are the advantages of suspension type insulators?
Ans: i. Economical for voltage above 33 kV
ii. Each insulator is designed for 11 kV and hence for any operating voltage, a string can be made.
iii. Failure of any unit can be replaced without changing the whole string
iv. Since it is allowed to swing in the air, mechanical stress at a point of attachment is reduced
v. Flexible in extension of voltage rating by adding more units
vi. Since the conductors lay below the cross arm, the line outages due to lightening strokes are reduced.
26. Give various insulating materials used for cable.
Ans: i. Vulcanized rubber
ii. Varnished cambric
iii. Polyvinyl chloride
iv. Impregnated paper
v. Polythene
27. What do you mean by grading of cables?
Ans: Grading means distribution of dielectric material such that the difference between Emax and Emin is reduced. Thereby a cable of same size can be operated
for high voltage or for the same operation voltage, the size can be reduced.
28. Give the reasons of high capacitance in cables than the overhead transmission lines?
Ans: i) High value of permittivity of insulating materials
ii) Distance between the core and the earthed sheath is small
iii) Small distances between the cores (phases) itself
29. What are the grading methods available for a cable?
Ans: i) Capacitance grading(more than one dielectric materials are used)
ii) Inner sheath grading( the same dielectric material is used but potentials at certain radius are held constant by using sheaths)
30. What are the difficulties in practical system grading of cables?
Ans: i) Non availability of varying permittivity of insulating materials
ii) Change in the permittivity with time, which changes the distribution of stress that lead to rapture of insulating material at normal working voltage.
iii) Damage of Inner sheath during laying or due to aging may lead to severe stress
iv) Charging current flows through the Inner sheath which may damage the cable due to overheating
v) There may be resonance problem in Inner sheath grading due to inductance of transformer and capacitance of cable
vi) Grading may not be economical in low voltage cables
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