Impact of corona discharges on the design of high-voltage
lines has been recognized since the early days of electric power transmission
when the corona losses were the limiting factor. Even today, corona losses
remain critical for HV lines below 300 kV.
With the development of EHV lines operating at voltages
between 300 and 800 kV, electromagnetic interferences become the designing
parameters. For UHV lines operating at voltages above 800 kV, the audible noise
appears to gain in importance over the other two parameters.
The physical mechanisms of these effects—corona losses,
electromagnetic interference, and audible noise—and their current evaluation
methods are discussed below.
Corona Losses
The movement of ions of both polarities generated by corona
discharges, and subjected to the applied field around the line conductors, is
the main source of energy loss. For AC lines, the movement of the ion space
charges is limited to the immediate vicinity of the line conductors,
corresponding to their maximum displacement during one half-cycle, typically a
few tens of centimeters, before the voltage
changes polarity and reverses the ionic movement.
For direct current (DC) lines, the ion displacement covers
the whole distance separating the line conductors, and between the conductors
and the ground. Corona losses are generally described in terms of the energy
losses per kilometer of the line.
They are generally negligible under fair-weather conditions
but can reach values of several hundreds of kilowatts per kilometer of line
during foul weather. Direct measurement of corona losses is relatively complex,
but foul-weather losses can be readily evaluated in test cages under artificial
rain conditions, which yield the highest energy loss.
The results are expressed in terms of the generated loss W,
a characteristic of the conductor to produce corona losses under given
operating conditions.
Electromagnetic Interference
Electromagnetic interference is associated with streamer
discharges that inject current pulses into the conductor. These pulses of steep
front and short duration have a high harmonic content, reaching the
tens of megahertz range. A tremendous research effort was
devoted to the subject during the years 1950–1980 in an effort to evaluate the
electromagnetic interference from HV lines.
The most comprehensive contributions were made by Moreau and
Gary (1972a,b) of E ´ lectricite´ de France, who introduced the concept of the
excitation function, G(v), which characterizes the ability of a line conductor
to generate electromagnetic interference under the given operating conditions.
The high temperature in the discharge channel produced by
the streamer creates a corresponding increase in the local air pressure.
Consequently, a pulsating sound wave is generated from the discharge site,
propagates through the surrounding ambient air, and is perfectly audible in the
immediate vicinity of the HV lines.
The typical octave-band frequency spectra of line corona
contain discrete components corresponding to the second and higher harmonics of
the line voltage superimposed on a relatively broadband noise, extending well
into the ultrasonic range (Ianna et al., 1974).
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