# Objective Questions on Electrical Transmission | 7

1. The charging electric current in a cable

In cables there will be more capacitance effect than inductor effect. Whenever two cables are placed nearer then two conductors act as conducting plate and insulation acts as dielectric medium between them. Hence capacitance effect will result in more. Hence the charging electric current in a cable leads the voltage by 90 °.

2. The voltage of super tension cables is up to

According to voltage the classification of cables can be done:
Low tension cables up to 1 kv
High tension cables up to 1 kv-11 kv
Super tension cables up to 11kv-33 kv
Extra tension cables up to 33kv-66 kv
Extra super high tension cables is above 132 kv.

3. If the length of cable decreases then insulation resistance

The insulation resistance of a cable is inversely proportional to the length of cable. As the surface of the insulation through which leakage electric current will pass to the ground will be reduced if cable length is reduced.

4. Insulation resistance of a cable is

If the length of the cable is increased, then the leakage area is also increased. Hence more electric current can leak. In other words insulation resistance is decreased.

5. The bridge used for measurement of insulation cables

SCHERING BRIDGE is used to measure insulation of cables. Since insulation of cable has a capacitive effect.

6. For voltage level beyond 66 kV, 3 phase system employs

3 single core cables are used for voltage levels > 66KV

7. If cable of homogenous insulation has maximum stress of 5KV/mm , then the dielectric strength of insulation should be

dielectric strength of insulation is same as the maximum stress for a cable of homogenous insulation.

8. How does the depth of laying affect the electric current carrying capacity of cables?

As depth of laying increases, heat dissipation becomes more difficult and accordingly the electric current carrying capacity decreases. Current carrying capacity of any conductor majorly depends upon temperature rising limit of the conductor.

9. Function of bituminous compound in joints of cable is

bituminous compounds provide protection to cables against moisture penetration .

10. Insulated conductor is termed as cable

bare conductor is called wire and insulated conductor is called cable.there are different layers of insulations.cables carry much higher voltage.

11. What are the types of EHT cables?

In oil filled EHT cable low viscosity oil is used and in case of gas filled pure nitrogen is used which is maintained at pressure of 14kg/sq cm.

12. What type of cable are used in hill /mines?

MIND (mass impregnated non draining) type cable is used in mines/hills . These cables are available for all voltage including 33kv. It is suitable for vertical run particularly on hills/mines.

13. To increases the visual critical voltage of corona voltage of corona for an overhead line one solid phase conductor is replaced by a bundle of four smaller conductor. The area of bundle conductor is equal to that of the solid conductor. The radius of solid conductor is 40 mm then the radius of each of the bundle conductors would be

As the area of solid and bundle conductor are equal, π X R2 = n X π X r2
r = R / √n = 40 / √4 = 20 mm.

14. Corona effect can be minimized

If the spacing between conductors is made large, then reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.

15. Equal area criterion gives the information regarding

Equal area criterion is a simple graphical method of predicting the transient stability of two machine systems or a single machine against infinite bus. The criterion does not require solution of swing
equation [Md2δ⁄dt2 = PA] for the determination of absolute stability conditions. The stability conditions are determined by equating the areas of segments on the power angle diagram between the P-curve and the new power transfer line for the given condition.

16. Steady state stability of a power system is the ability of the power system to

steady state stability of a power system is defined as the ability of the system withstand even after a disturbance occur. Or Steady state stability of a power system is the ability of a power system to maintain synchronism between machines and on the external tie lines.

17. Overhead voltage distribution system is applied to city A & cable voltage distribution system is applied to city B . Voltage variation at consumer terminal is less in which city?

Voltage variation at consumer terminal is less in city B because cables have more capacitance & less inductor than overhead lines.

18. As per IE rules the maximum allowable variation between declared and actual voltage at consumer premises should be

According to IE rules, the voltage at the point of commencement of supply shall not be vary more than ±6% in the case of low voltage(250 V-650 V), ±7% to 9% in the case of high voltage (upto 33 kV). So that the second option is correct.

19. Bundled conductor are mainly used in high voltage overhead lines to

corona proportional to (V-Vo)^2 Vo proportional to conductor size ,large size means large critical disruptive voltage and smaller will be the difference (v-vo)^2 hence smaller corona loss.

20. Localization of cable faults done by

There are several methods for locating the faults in underground cables.However, two popular methods known as loop tests known as loop tests are:- 1.Murray loop test, 2. Varley loop test
These simple tests can be used to locate the earth fault or short-circuited fault in underground cables provided that a sound cable runs along the faulty cable.Both these tests employ the principle of Wheatstone bridge for fault location.
The Murray loop test is the most common & accurate method of locating earth fault or short circuit fault in underground cables. The Varley loop test is also used to locate earth fault or short-circuit fault in underground cables. This test also employs Wheatstone bridge principal.