Electrical Engineering ⇒ Topic : Capacitance between Two Parallel Wires

CAPACITANCE BETWEEN TWO PARALLEL WIRES

The practical importance of this case is in overhead lines as shown in Figure (a). A two wire system is shown in Figure (a) (either dc or ac). In ac system, if the length of the transmission line is too long and voltage is too high, the charging current drawn by the line due to capacitance between two wires has a considerable value and it affects the performance of the line. Let the radius of each wire be r metre and the two wires be separated by a distance d metre. Let a charge of +Q coulomb be given to A. The charge of B will be -Q coulomb.

We will consider electric intensity at C which is in between A and B as shown in Figure (a).Electric intensity at C due to charge +Q coulomb/metre on A is:

                                                               figure (a)

                                                                       ........... (1)

Therefore, the capacitance for a length of l metre is given by

                                                                                                      .................  (2)

Therefore, the capacitance per kilometre is given by

                                                                                                        ............. (3)

This case is of practical importance in overhead transmission lines. The simplest system for power transmission is 2-wire d.c. or -a.c. system. Consider 2-wire transmission line consisting of two parallel conductors A and B spaced d metres apart in air. Suppose that radius of each conductor is r metres. Let their respective charges be + Q and - Q coulombs per metre length [See Figure (a)].

                                   FIGURE  (A)

The total p.d. between conductor A and neutral "infinite" plane is

Similarly, p.d. between conductor B and neutral "infinite" plane is

 Both these potentials are w.rt. the same neutral plane. Smce the unlike charges attract each other, the potential difference between the conductors is

The capacitance for a length l is given by

                                                   C_AB           ... in air

                                                              .       .. in a medium