Topic: Capacitance on transmission line

INTRODUCTION• Capacitance of a transmission line is the result of the potential

difference between the conductors; it causes them to be charged in the same manner as the plates of a capacitor when there is a potential difference between them.

• The capacitance between the conductor is the charge per unit of potential difference.

• capacitance between parallel conductors is a constant depending on the size and spacing of the conductors.

• For power lines less than about 80 km long the effect of capacitance is slight and is usually neglected.

• For longer lines of higher voltage, capacitance becomes increasingly important.

Capacitance on transmission line

I. Capacitance of three phase line with equilateral spacing.

II. Capacitance of three phase line with unsymmetrical spacing.

CAPACITANCE OF THREE PHASE LINE WITH EQUILATERAL SPACING

Figure show the three phase line composed of three identical conductors of radius r placed in equilateral configuration

We shall find the capacitance from line conductor to neutral in this symmetrically spaced line.

• Since there are no vicinity ,the sum of charges on the three conductor is zero .

CAPACITANCE OF A THREE PHASE LINE WITH UNSYMMETRICAL SPACING

The calculation of capacitance in case of conductors in three phase system which are not equally spaced is difficult.

If the line is untransposed the capacitances of each phase to neutral is not same.

• In case of transposed line the average capacitance of each line to neutral over a complete transposition cycle is same as the average capacitance to neutral of any other phase.

• Each conductor occupies the same position of every other conductor after equal distance.

• The effect of unsymmetry between the lines is small and calculations are carried out by considering transposition of lines.

When phase 'a' is in position 1, 'b' in position 2 and 'c' is in position 3

When phase 'a' is in position 2, 'b' in position 3 and 'c' is in position 1.

When phase 'a' is in position 3, 'b' in position 1 and 'c' is in position 2.

Average voltage between conductors 'a' and 'b' is given by

Similarly average voltage drop between a and c is given by,

We have, Vab + Vac = 3 Van

For balanced circuit (qa + qb + qc ) = 0