# Power Systems Questions and Answers – Underground Cables – 1

This set of Power Systems Multiple Choice Questions & Answers (MCQs) focuses on “Underground Cables – 1”.

1. Single core cables for AC systems are not provided with armouring.
a) True
b) False

Explanation: Because of the armouring of the cables leads to eddy current in the power cables adding more to the losses.

2. Single core cables for ac systems are provided with armouring.
a) True
b) False

Explanation: This is so because the eddy currents induced in the steel armour cause the additional power losses.

3. Potential gradient of single core cable is ________________

Explanation: The electric field intensity,

4. For a transmission system of voltage ‘V’ volts having the core radius of ‘r’ m and outer radius of ‘R’ m. The maximum dielectric stress on single core cable is ___________

Explanation: The maximum stress will occur at the surface of the conductor such that x(min) = radius of core.

5. For a transmission system of voltage ‘V’ volts having the core radius of ‘r’ m and outer radius of ‘R’ m. The minimum dielectric stress on single core cable is ______________

Explanation: The minimum stress will occur at the outermost surface of the conductor such that x(max) = outer radius of core.

6. For a transmission system of voltage ‘765 kV’ volts having the core radius of ‘5’ cm and outer radius of ‘0.40’ m. The maximum dielectric stress on single core cable is _________
a) 7357.7 kV
b) 919.7 kV
c) 4247 kV
d) 530 kV

Explanation: The maximum stress will occur at the surface of the conductor such that x(min) = radius of core.

7. The ratio of dielectric stress of the on the cable corresponding to the maximum and minimum is ____________
a) R/r
b) r/R
c) rR
d) (R/r)2

8. The condition at which the minimized dielectric stress condition (R/r) will take place is ______________
a) 2.7188
b) 0.7188
c) 1/2.7188
d) 2.71882

9. Grading of the cables is performed in order to achieve ____________
(i) uniform stress
(ii) reduction in quantity of insulation
(iii) reduction in quality of insulation
a) (i), (ii)
b) (ii)
c) (i),(iii)
d) (ii), (iii)

Explanation: Grading is performed to reduce the stress at one cable and to make it uniform throughout. Also it reduces the insulation material to be used and thus the cost saving.

10. The resistance of the insulation used for the cables is 40 ohms for a length of ‘x’ km. If the proposed plan needs to extend the transmission line cable to extra ‘2x’ m then the resistance of the cable offered will be _____________ (in ohms).
a) 13.33
b) 20
c) 120
d) 40

Explanation: The resistance of the cable is inversely proportional to the length of it.

11. Consider a system having a cable of capacitance 5 nF, with 765 kV star connected at 50 Hz having a power loss in the cable as 82.1 kW. Then the dielectric loss angle is _________(in degrees).
a) 15
b) 7.5
c) 20
d) 4.9

Explanation: Dielectric loss = ωCV2 tanδ

12. Dielectric power loss of a transmission line cable operating for ‘V’ volts, ‘f’ Hz having a capacitance of ‘C’ Farads with the tangent loss angle of δis ____________
a) ωCV2 tanδ
b) ωCV2/tanδ
c) (tanδ/ωCV)2
d) ωCV2 tanδ2

Explanation: The Dielectric loss = ωCV2 tanδ for a cable.

13. The power losses occurring in the transmission cable has the relation with loss tangent angle as _____
a)
b)
c)
d)

Explanation: The Dielectric loss = ωCV2 tanδ
So, Pd α tanδ.

14. In a transmission tower having ‘n’ insulators with equivalent pin-to-earth capacitance of ‘C’ farads. For the static shielding of the conductors, the capacitance from the shield to the kth link from the top is ______

Explanation: The generalized expression for the capacitance of the kth metal ring is,
C(k) = k*pin-to-earth/(n-k).

Sanfoundry Global Education & Learning Series – Power Systems.

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