Capacitance on transmission line

1. Topic: Capacitance on transmission line

2. 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.

3. Capacitance on transmission line
I. Capacitance of three phase line with
equilateral spacing.
II. Capacitance of three phase line with
unsymmetrical spacing.

4. 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

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

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

8. 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.

9. • 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.

10. 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.

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

12. Similarly average voltage drop between a
and c is given by,
We have, Vab + Vac = 3 Van

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