Parameters of Transmission Line:
A transmission line has four
parameters, namely resistance, inductance, capacitance and conductance. The resistance
‘R’ of a line is because of conductor
resistance, series inductance ‘L’ is
due to the magnetic field surrounding the conductors, shunt capacitance ‘C’ is due to the electric field between conductors,
and shunt conductance, ‘G’ is because
of the leakage current between phases and ground.
What is Transposition of Conductors?
The interchange of conductor positions of a transmission line at regular intervals along the route is known as Transposition of Conductors.Why transposition is needed?
In the power transmission line
when the line conductors are asymmetrically spaced i.e. not equally spaced, the
inductance of each phase is different causing voltage drops of different
magnitudes in the three phases even if the system is operating under balanced
condition (load currents are balanced in the three phases). Also the
magnetic field external to the conductors is not zero thereby inducing voltages
in adjacent communication lines and causing what is known as “telecommunication
interference”. This can be overcome by the interchange of conductor positions
at regular intervals along the route and this practice is known as “transposition
of conductors”.
How transposition is done?
In a transposed transmission line
each of the three conductors occupies all the three positions relative to other
conductors (position 1, position 2, and position 3) for one-third of the total length of the transmission line. Transposition
also balances out the line capacitance so that electro-statically induced
voltages are also balanced. Figure shows the transposition of conductors over a
complete cycle.
A complete cycle of transposition of line conductors.
Complications of Conductor Transposition:
Frequent transposition usually leads to complication of support structures (as can be seen by the picture below), increase the cost because of increased number of insulator strings and total weight of supports.
Transposition on 400 kV, double circuit transmission line, near Bhopal, M.P.
