Corona

A corona is a type of plasma “atmosphere” of the sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The Latin root of the word corona means crown. High electrical stresses applied to the surface of high voltage A.C transmission lines cause corona discharge near the surfaces. These partial discharges induce fast transient current pulses in the conductors. The induced pulses propagate in both directions along the multi–conductor transmission lines. The corona pulse sources are uncorrelated and uniformly distributed along the line. Transmission line conductor corona is responsible for many macroscopic phenomena on A. C transmission systems- foul weather (rain, fog, and snow and hoar- frost) can lead to discharge on high voltage transmission lines. These corona discharges result in a broadband crackling noise. In addition, in these situations with a low time at twice the mains frequency there often occurs a further noise component. Such audible noise from high voltage overhead transmission lines can reach locally undesirable levels. Both kinds of noise are associated with blemishes on conductors and other hardware surface and most importantly – water drops.

References:

(1)    Sarma Maruvada, P., “Coronaperformance of high voltage   transmission lines”, Research studies press Ltd,England, 2002.

(2) Comber, M. G. and Nigbor, R.J, “Transmission line reference book – 345kV and above”, chapter 5, Radio Noise, EPRI, 2^nd Edition, Revised 1989, pp. 205 –266.

(3) J.J. Clade, C.H. Gary, C.I.A. Lefevre, “Calculation of Corona losses beyond the critical gradient in alternative voltage”, IEEE Trans. PAS, Vol. 88, no5, pp. 695 – 703, 1969.

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Corona

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.

 

References:

(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”.