Corona is the result of transient gaseous ionization in an insulation system when the voltage stress exceeds a critical point. When an alternating potential difference is applied across two conductors whose spacing is large compared to their diameter, there is no apparent change in the condition of the atmospheric air surrounding the wires if voltage is low. However, when the p.d. is increased, then a point is reached when a faint luminous glow of bluish colour appears along the lengths of conductors and at the same time a hissing sound is heard. This bluish discharge is known as corona. Corona is always accompanied by the production of ozone which is readily detected because of its characteristic odour. If the p.d is further increased, then the glow and hissing both increases in intensity till a spark–over between the conductors takes place due to the break–down of air insulation. If the conductors are smooth and polished, the corona glow is uniform along their length but if there is any roughness, they will be picked up by relatively brighter illumination. In the case of conductors with spacing shorter as compared to their diameters, sparking may take place without any visible glow. If the p.d. between wires is direct instead of alternating, there is a difference in the appearance of the two wires. The positive wire has a smooth glow about it whereas the glow about the negative wire is spotty.
Corona and arcing activity which may occur at numerous points at the transmission lines, substation and distribution power system may result in audio noise (AN). Information suggests that corona is only a problem on extra – high voltage transmission systems usually operating at or above 345kV. At night corona effect also presents luminous blow, which manifest in the form of blue coloured light, radio interference complaints or indicate the effective components that may be close to failure. Ability to identify the location of such activity and pinpoint the offending component so that it may be replaced has long been a utility goal. Corona as it applies to high voltage transmission lines, is the ionization of the air surrounding the conductor. Corona occurs when the electrostatic stress in the air around the conductors exceeds 30kVA (maximum)/cm or 21.1kV (r. m. s)/cm. Corona activity generates light (mainly in the air spectrum), sound wave, electromagnetic radiation, Ozone and other by products. One example is dry band arcing on contaminated insulators. Since both corona and arcing may result in customer complaints or identify a defective component, It is important to identify the precise location of the source so that appropriate action may be taken. Corona occurs on all types of transmission lines but it becomes more noticeable at higher voltages (345kV and higher). Under fair weather conditions, the audible noise from corona is minor and rarely noticed. During wet and humid conditions, water drops collect on the conductors and increase corona activity. Under these conditions, a crackling or humming sound may be heard in the immediate vicinity of the line confirming the presence of corona.
(1) B.L Theraja, A.K. Theraja, “A text book of electrical
technology, multicolor edition, pp. 1642–1645, S. Chand $ company Ltd, Ran Nagar, New Delhi–110055, 2008.
(2) ASTM D 1711–83, “Standard definition of terms relating to electrical insulation”.