61. Whenever the conductors are dead-ended or there is a change in the direction of transmission line, the insulators used are of the
(a) Pin type.
(b) Suspension type
(c) Strain type.
(d) Shackle type.
62. Post type insulators are generally used in lines operating
(a) above 100 kV.
(b) below 33 kV.
(c) at any voltage level, HV or EHV.
63. The number of discs in a string of insulators for 400 kV AC overhead transmission line lies in the range of
(a) 32 to 33
(b) 22 to 23
(c) 15 to 16
(d) 9 to 10
64. The non-uniform distribution of voltage across the units in a string of suspension type insulators is due to
(a) unequal self-capacitance of the units.
(b) non-uniform distance of separation of the units from the tower body.
(c) the existence of stray capacitance between the metallic junctions of the units and the tower body.
(d) non-uniform distance between the cross-arms and the units.
65. The voltages across the various discs of a string of suspension insulators having identical discs is different due to
(a) surface leakage currents.
(b) series capacitance.
(c) shunt capacitance to ground.
(d) series and shunt capacitances.
66. The string efficiency of a string of suspension insulators is dependent on
(a) size of the insulators.
(b) number of discs in the string.
(c) size of tower.
67. 100 per cent string efficiency means
(a) one of the insulator discs shorted.
(b) zero potential across each disc.
(c) equal potential across each insulator disc.
(d) none of the above.
68. In a suspension type insulator the potential drop is
(a) maximum across the lowest disc.
(b) maximum across the topmost disc.
(c) uniformly distributed over the discs.
69. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency
(a) will increase.
(b) will decrease.
(c) remain unchanged.
(d) may increase or decrease depending on the line parameters.
70. The string efficiency of a high-voltage line is around
71. In three-unit insulator string, voltage across the lowest unit is 17.5 kV and string efficiency is 84.28%. The total voltage across the string will be equal to
(a) 8.285 kV
(b) 44.25 kV
(c) 88.25 kV
(d) 442.5 kV [U.P.S.C. I.E.S.]
72. Two-insulator discs of identical capacitance value C makes up a string for a 22 kV, 50 Hz, single-phase overhead line insulation system. If the pin to earth capacitance is also C, then the string efficiency is
(d) 86% [ I.E.S. 2003]
73. In a cable the sheath radius is R and conductor radius is r. As r changes from 0.5R to 0.25R the maximum voltage gradient in the dielectric
(a) decreases by about 6%.
(b) increases by about 6%.
(c) increases by about 15%.
(d) decreases by about 15%
74. Consider the following statements : In the case of suspension type insulators, the string efficiency can be improved by
- using a longer cross arm.
- using a guard ring.
- grading the insulator discs.
- reducing the cross-arm length.
Of these statements
(a) 1, 2, and 3 are correct.
(b) 2, 3 and 4 are correct.
(c) 2 and 4 are correct.
(d) 1 and 3 are correct.
75. The ratio of puncture voltage to the flash-over voltage of a line insulator is
(a) equal to 1
(b) lower than 1
(c) much greater than 1
76. The insulators may fail due to
(a) flash over.
(c) deposition of dust.
(d) any of the above.
77. The purpose of guard ring in transmission lines is to
(a) reduce the earth capacitance of the lowest unit.
(b) increase the earth capacitance of the lowest unit.
(c) reduce the transmission line losses.
(d) none of the above.
78. The use of a guard ring
(a) equalizes the voltage division between insulator discs.
(b) is unnecessary complication.
(c) decreases string efficiency.
79. The component inductance due to the internal flux-linkage of anon-magnetic straight solid circular conductor per meter length, has a constant value, and is independent of the conductor diameter, because
(a) All the internal flux due to a current remains concentrated on the peripheral region of the conductor.
(b) The internal magnetic flux density along the radial distance from the centre of the conductor increases proportionately to the current enclosed.
(c) The entire current is assumed to flow along the conductor axis and the internal flux is distributed uniformly and concentrically.
(d) The current in the conductor is assumed to be uniformly distributed throughout the conductor cross section.
80. Consider a long, two-wire line composed of solid round conductors. The radius of both conductors is 0.25 cm and the distance between their centers is 1 in. If this distance is doubled, then the inductance per unit length
(c) increases but does not double.
(d) decreases but does not halve.