Electrical Power Systems

366 Electrical Power Systems
(e) Effect of Temperature Rise of Conductor by Load Current
Conductor current raises the conductor temperature and hence leading to an indirect
reduction in corona loss. Corona loss is larger if the conductor temperature is low and
this is due to the fact that at low temperature, dew drops collect on the conductor surface
during fog and humid weather. High conductor current prevents such condensation and
reduces corona loss.
(f) Effect of Snow or rost Layer
Snow or frost layer on transmission line conductor causes very high corona losses and
radio interference. This layer is found if temperature is 0ºC and remain for longer
duration if temperatures are lower.
Corona discharges with conductor surface covered with snow and electric field intensity
not exceeding 30 KV/cm conssists mainly of pulses in negative half-cycles and pulseless
steady corona at positive half-cycles. Corona discharges themselves affect the form of
snow layers and often blow off the snow in the neighbouring region of corona.
(g) Corona Loss of New and Old Conductors
On new conductors, corona loss is more due to scratches, burrs etc. As the line ages,
corona loss decreases. The old conductor is called weathered conductor.
(h) Effect of Supply Voltage
If the supply voltage is high, corona loss will be high. In low-voltage transmission lines,
corona is negligible due to insufficient electric field to maintain self-sustained ionization.
(i) Effect of Dust and Dirt
In the presence of dust and dirt, required voltage gradient is less for maintaining
sustained discharge. Disruptive critical voltage is reduced due to dust and dirt and hence
corona loss is more.
(j) Effect of Conductor Configurations
Conductors of three phase overhead transmission lines can be placed in either horizontal
or vertical configuration. The electric field intensity at the surface of middle conductor is
higher than the outer conductors. Therefore, the disruptive critical voltage for middle
conductor will be less than the two other outer conductors and hence there will be more
corona loss in middle conductor. If conductors are placed equilaterally, the average
electric field intensity at each conductor will be same. Since the ground is an equipotential
surface, the electric field distribution is affected by the presence of earth. Corona loss will
be less, if conductors are placed at more height.
14.11 EECT O CORONA ON LINE DESIGN
Transmission lines are designed in such a fashion that the corona loss is small enough in fair
weather condition because corona loss reduces the efficiency of the lines. If disruptive critical
voltage of about 10% more than the operating voltage, then it is acceptable even though some
corona loss will take place under foul weather condition. Generally, corona loss under foul
weather condition will be 10 times higher than the fair weather condition. An increase in D eq
and r increase the disruptive critical voltage, thus reduces the corona loss but increases the
cost.
Example 14.1: Conductors of a three phase transmission line are equilaterally spaced 6 m
apart. The radius of each conductor is 1 cm. The air temperature is 30ºC and pressure is 740 mm
of Hg. If surface factor is 0.83 and irregularity factor is 0.92, calculate the disruptive critical
voltage and visual critical voltage.