Question

Following are the methods to increase the surge impedance loading of a transmission line:

i. Increase in voltage level.

ii. Reducing length of the line.

iii. Addition of lumped inductance in parallel.

Out of the given. the correct one(s) is/are?

1. Only iii
2. Both ii and iii
3. Only i
4. Both i and ii

Answer 1

Correct Answer - Option 3 : Only i

Concept:

The surge impedance or characteristic impedance of a long transmission line is given by,

\({Z_C} = \sqrt {\frac{Z}{Y}} \)

Z is series impedance per unit length per phase

Y is shunt admittance per unit length per phase

Surge Impedance for the transmission line is about 400 ohms it is around 40 ohms for underground cables.

Surge impedance loading (SIL):

Surge impedance loading is defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.

SIL \( = \frac{{{V^2}}}{{{Z_C}}}\;MW\)

SIL is directly proportional to voltage so statement i is correct.

Z_C is independent to length of the line, so reducing the lenth will not increase the SIL, so statement ii is wrong.

Addition of lumped inductance in parallel will not alter the SIL, so statement iii is wrong.

∴ Option 3 is correct.