Corona discharges and their effects on insulation

During the last two decades, in particular, there has been an increasing realization of the importance of the effect of corona discharges on the practical life of electrical insulation under electric stress. During this time there have been numerous papers published particularly in Great Britain and the United States, as well as some significant contributions from other countries, all discussing the decline in a-c electric strength with time of voltage application and its relation to Corona Discharges. In spite of this large volume of literature on the subject, there still seems to be some confusion among insulation application engineers as to what corona is and what it means. Also there has been a notable lack of unanimity, even among the experts in corona studies, regarding definition of terms and methods of measurements. There is progress in resolving this problem in AIEE and ASTM Committees.This paper will present a simple description of what corona discharges are and where they occur. It will outline the electrical quantities characterizing the corona pulses and their measurement. Finally, it will describe how the corona discharges attack electrical insulation and cause, in the case of organic plastics in particular, a large decrease in electric strength with time of voltage application.