Abridgment of the law of corona and dielectric strength of air — IV: The mechanism of corona formation and loss

Abstract

The mechanisms of corona and corona loss have been studied with the cathode-ray oscillograph. High voltage power of the order of 0.1 watt can be measured with an accuracy of 1 per cent with this instrument. The measurements show that the loss follows the quadratic law above the visual critical voltage. On polished wires there is no loss until the visual critical voltage is reached. The loss then starts quite suddenly and takes a finite value on the quadratic curve. On cables and imperfect conductors there is a loss below the visual critical voltage on brushes at local “rough” spots. The loss due to these irregularities can be represented by the probability law. This is quite in accord with former work. In practise it is important not to mutilate the conductors in stringing. The really important factor in design is the irregularity factor, mo, for weathered conductors. No line should be operated with a corona loss under fair weather conditions. It is not necessary from the economic standpoint since large diameters can be obtained with special types of conductors. The visual critical corona voltage can be calculated with great accuracy. As the applied a-c. voltage is increased above the visual critical value, the instantaneous critical voltage becomes lower and lower until finally corona starts at the zero point of the wave. This occurs when the applied voltage is twice the visual critical voltage. At still higher voltage, corona starts below zero or on the falling wave. The effect is as if the instantaneous critical voltage is reduced by an amount approximately equal to the excess of the applied voltage above the visual critical voltage. Thus when the excess is equal to the visual critical voltage the instantaneous voltage is zero. This occurs when the applied voltage is twice the visual critical voltage. The reason for this is clearly shown as well as many other interesting facts. Artificial corona was readily produced with all of the characteristics of real corona after the mechanism was determined. The quadratic law seems to be the rational expression for the loss. Details of measurements are given in the supplemental paper on measurements by Starr and Lloyd, “Methods Used in an Investigation of Corona Loss by Means of the Cathode Ray Oscillograph.”