Crown effect on overhead power lines

The corona effect, now a well-known phenomenon, takes the form of a bluish sheath of light that appears around a thin wire when the latter is raised to a sufficiently high potential. It is reminiscent of the luminous halo visible at the periphery of the sun during eclipses, which gave it its name. From a physical and electrical point of view, this phenomenon is due to the ionization of air, as soon as the electric field in the immediate vicinity of the conductor becomes sufficient.

When using larger-diameter conductors, such as those used on overhead lines, the luminous sheath evolves into discrete discharges that specialists are accustomed to calling "egrets" or "effluvia".

It's worth noting that this phenomenon also occurs in natural conditions, particularly when a thunderstorm is approaching: under the effect of the intense electric field generated by the electrical charges of the storm cloud, effluvia or egrets form at the top of all points or asperities (masts, lightning rods, mountain peaks, etc.), accompanied by a characteristic crackling sound. Mountaineers are familiar with this sound, which they call "bee noise". The fires of Saint-Elme, described by ancient navigators, have no other origin.

During the corona effect, the dissipated energy causes electrical losses, and the electrical pulses associated with the egrets lead to radio interference.

By analyzing the results of previous measurements on numerous types of conductor, we have demonstrated the influence of the main parameters governing the amplitude of corona phenomena. In order of importance, these are: the conductor's surface electric field, its diameter, its surface condition, and the density of the surrounding air.

For 380 kV network operation, the corona effect becomes a nuisance, and the choice of bundled conductors limits losses and the disturbance field to reasonably acceptable values.

The surface condition of conductors is an important parameter, since the more it deteriorates, the greater the losses and the higher the disturbance field. Plant and industrial pollution, and even surface seepage of stranding grease, play an important role in degradation, but are unfortunately difficult to control. What's more, rain, due to the asperities created by water droplets suspended on conductors, multiplies the number of aigrettes generating the corona effect.

The EFCOR (EFfet CORona) calculation code, developed by EDF to analyze the formation of losses, can be considered a remarkable tool, as it enables the loss mechanism to be followed physically: among other performances, this code enables the movement of space charges to be visualized, and it is fascinating for the mind to...