Transmission Line Constants Interview Questions and Answers:
1. Distinguish between ac and dc resistances of a conductor. Why the two differ ?
Ans. The resistance offered to dc, by a conductor is known as dc resistance while the resistance offered to ac is known as ac resistance or effective resistance. For a given conductor ac resistance is more than its dc resistance. This is because an alternating current flowing through a conductor does not distribute uniformly but tends to concentrate near the surface of the conductor. The result is that the effective area of the conductor is reduced causing an increase in ac resistance.
2. What is skin effect ?
Ans. The phenomenon of concentration of an electric current near the surface of the conductor is known as skin effect.
3. On what factors does the skin effect depend ?
Ans. The skin effect depends upon the type of material, frequency of current, diameter of conductor and shape of conductor. It increases with the increase of cross section, permeability and supply frequency.
4. For determination of inductance of a circuit, determination of flux linkages is essential. Why ?
Ans. It is well known fact that a current carrying conductor is surrounded by concentric circles of magnetic lines. In case of ac system this field set up around the conductor is not constant but changing and links with the same conductor as well as with other conductors. Due to these flux linkages the line possesses inductance, defined as the flux linkages per unit current. Thus for determination of inductance of a circuit, determination of flux linkages is essential.
5. What is meant by GMR of a conductor ?
Ans. The product r e-1/4 is known as GMR of a conductor and is equal to 0.7788 times the conductor radius.
6. What is a bundle conductor and why is it used ?
Ans. A bundled conductor is a conductor made up of two or more conductors, called the sub-conductors, per phase in close proximity compared with the spacing of phases.
The use of bundled conductors per phase reduces the voltage gradient in the vicinity of the line and thus, reduces the possibilities of the corona discharge.
Although the bundled conductors are used on EHV transmission lines primarily to reduce corona loss and radio interference, but they have several other advantages over single conductors such as given below :
- The bundle conductor lines transmit bulk power with reduced losses, thereby giving increased transmission efficiency.
- Since the bundle conductor lines have a higher capacitance to neutral in comparison with single conductor lines, therefore, they have higher charging current, which helps in improving the power factor.
- Since by bundling, the self GMD or GMR is increased, the inductance per phase, in comparison with single conductor lines, is reduced. As a result reactance per phase is reduced.
- Since surge impedance of a line is given by Z0 = √L/C and the bundle conductor lines have higher capacitance and lower inductance in comparison with single conductor lines, therefore, bundle conductor lines have comparatively lower surge impedance with a corresponding increase in the maximum power transfer capability. The following table represents relative power transfer with number of sub-conductors forming a bundle conductor.
|Number of sub-conductors
|Relative power transfer
7. What is bundling of conductors ?
Ans. The use of more than one conductor per phase is called the bundling of conductors.
8. Differentiate between bundled and composite conductors.
Ans. The basic difference between a composite conductor and a bundled conductor is that the sub-conductors of a bundled conductor are separated from each other by a constant distance varying from 0.2 m to 0.6 m depending upon designed voltage and surrounding conditions throughout the length of the line with the help of spacers whereas the wires of a composite conductor touch each other. The bundled conductors have filter material or air space inside so that the overall diameter is increased.
9. For what purposes bundled conductors are primarily used in transmission lines ?
Ans. The bundled conductors are used in EHV transmission lines primarily to reduce corona loss and radio interference.
10. What are the advantages of bundled conductors.
Ans. The advantages of using bundled conductors in EHT transmission lines are : Reduced possibilities of the corona discharge; reduced corona loss; reduced radio interference; reduced losses resulting in increased transmission efficiency; higher charging current resulting in improved pf; reduced inductive reactance per phase; lower surge impedance resulting in increased power transfer capability.
11. What is the effect of ground on the capacitance of a transmission line ?
Ans. The presence of ground increases the capacitance of transmission line by a small amount.
12. Why the effect of ground on the line capacitance can be neglected ?
Ans. The effect of ground on the line capacitance is very-very small because the height of conductors is large as compared to the distance between them.
13. What is meant by transposition of line conductors ?
Ans. Transposition of line conductors means changing the positions of the three phases on the line supports twice over the total length of the line. The line conductors in practice, are so transposed that each of the three possible arrangements of conductors exist for one-third of the total length of the line.
14. Why transposition of conductors in a three-phase transmission lines is essential ?
Ans. The inductance and capacitance of each phase will be different in case of conductors of three-phase line being spaced irregularly. The apparent resistance of the conductors is also affected on account of transfer of power between the phases, which occur due to mutual inductance. Thus all the three line constants are affected by irregular spacing of the conductors in a 3-phase line.
Also, due to unsymmetrical spacing, the magnetic field external to the conductors is not zero, thereby causing induced voltages in adjacent electrical circuits, particularly the telephone lines, that may cause disturbances in the telephone lines.
Transposition of line conductors in transmission lines is essential to counter the unbalancing effect, mentioned above, of irregular spacing of line conductors.