CORONA DISCHARGE EFFECTS ON HIGH VOLTAGE POWER TRANSMISSION LINES

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.

 Audible Noise
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).

The octave-band measurements show a sharp drop at frequencies over 20 kHz, due principally to the limited frequency response of the microphone and associated sound-level meter. Similar to the case of electromagnetic interference, the ability of the line conductors to produce audible noise is characterized by the generated acoustic power density A, defined as the acoustic power produced per unit length of the line conductor under specific operating conditions.

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