Any two conductors separated by an insulating medium
constitutes a condenser or capacitor.In case of overhead transmission lines,
two conductors form the two plates of the capacitor and the air between the
conductors behaves as dielectric medium. Thus an overhead transmission line can
be assumed to have capacitance between the conductors throughout the length of
the line. The capacitance is uniformly distributed over the length of the line
and may be considered as uniform series of condensers connected between the
conductors.
When an alternating voltage is applied across the
transmission line it draws the leading current even when supplying no load.
This leading current will be in quadrature with the applied voltage and is
termed as charging current. It must be noted that charging current is due to
the capacitive effect between the conductors of the line and does not depend on
the load. The strength of the charging currents depends on the voltage of
transmission, the capacitance of the line and frequency of the ac supply.
If the capacitance of the overhead line is high, the line
draws more charging currents which cancels out the lagging component of the
load current (normally load is inductive in nature). Hence the resultant
current flowing in the line is reduced. The reduction in the resultant current
flowing through the transmission line for given load current results in:
- Reduction of the line losses and so increase of transmission efficiency.
- Reduction in the voltage drop in the system or improvement of the voltage regulation.
- Increased load capacity and improved power factor
Significance of Charging currents:
Capacitance effect (responsible for charging currents) of
the short transmission lines are negligible. However they are significant in
medium and long distance transmission lines.
In long distance transmission lines, during light loaded
conditions receiving end voltage will be higher than sending end voltage. This
is because of the charging currents and capacitive effect of the line.