Corona in air spaces in a dielectric

The consideration of the extreme care necessary in preparing samples of a dielectric for test for electrical properties led to the investigation of the effect of air spaces, purposely formed of definite thickness and location, upon the power factor. This work in a way is an extension of some work done by Clark and Shanklin and Shanklin and Matson several years ago on air spaces in high voltage cables and wrapped armature coils. In their investigation the effect of assumed air spaces of indefinite thickness, extent, pressure and location was shown by plotting effective resistance from the formula against potential gradient. A characteristic curve was obtained, a sharp inflection point in the curve being interpreted as indicating the starting point of corona. In the work by the writer, various materials were investigated both with air spaces excluded as much as possible, and with air spaces of definite thickness, extent, and location at atmospheric pressure. The results were plotted showing variation of power factor with potential gradient. A definite increase in power factor with potential gradient indicated the starting of corona. The thicker air space with a given thickness of dielectric showed the more abrupt change in power factor, and this took place at a lower potential gradient. By plotting power factor against voltage, a maximum was shown indicating that a saturation of ionization was approached which resulted in a decrease in power factor. By plotting effective resistance assuming the resistance to be in series with the dielectric and to be given by the formula a very definite change in R as well as in power factor indicated the beginning of corona. The curve obtained in this way did not possess superior advantages over the plot for power factor for indicating corona and had the disadvantage that the points were somewhat scattered on the upper range of potential gradient. The results of the i- vestigation show that: 1. It is extremely difficult to exclude air spaces from a dielectric so that it does not result in corona formation. Corona is shown by a more or less abrupt change in power factor with potential gradient. 2. The abruptness of the change in power factor with potential gradient depends upon the thickness of the air space, the thicker the air space the more abrupt the change. 3. The thicker the air space for any given thickness of dielectric the lower the potential gradient to produce corona. 4. The potential gradient to produce corona not only depends upon the thickness of the air space but also upon the extent of the air space as shown by observations on different areas of air space of the same thickness. 5. A maximum of power factor is reached with potential gradient indicating that saturation of ionization is approached after which the value of power factor decreases with potential gradient.