Experimental Investigation of the Overvoltage Steepness Effects on Corona Inception Characteristics

Abstract

The overvoltage surges propagating along overhead line conductors are gradually distorted and attenuated because of impulse corona discharge. The extent of these effects is dependent mainly on the associated corona energy losses. The inception characteristics of impulse corona are most commonly assessed based on corona inception under steady or slowly varying electric fields. This, however, may result in inaccurate assessment of the impulse corona effects, since impulse corona characteristics are greatly affected by the overvoltage steepness. This study presents an experimental investigation on corona inception characteristics under lightning impulse voltages in the coaxial configuration. The steepness and polarity of the applied impulse voltage are considered as influencing parameters. The corona inception voltage and the total charge injected in the gap increase markedly as the overvoltage steepness increases. This effect is more pronounced for impulse voltages with shorter wavefronts and for positive than negative corona. There is a critical overvoltage steepness, depending on impulse voltage wavefront, above which the increase of corona inception voltage, as well as of the total corona charge injected in the gap, with overvoltage steepness is less pronounced; this is associated with the transition from the glow to streamer mode of corona inception. At transition between corona modes, the total charge injected in the gap decreases with increasing wavefront duration and is markedly lower for negative than positive corona. In an effort to model corona losses for studies on attenuation and distortion of lightning overvoltages, a procedure is introduced that considers the corona inception U-t characteristic and a working relationship for the estimation of the total corona charge injected in the gap.