Corona voltages of typical transformer insulations under oil

IN RECENT years, the importance of designing oil-immersed transformers to avoid deterioration during insulation tests has been emphasized. This is indeed proper, since it is very desirable to prevent deterioration in any form, especially when it may result in future trouble to the user. It has been found that insulation deterioration is caused by corona or local discharges in the oil and over solid insulation surfaces. It is therefore necessary that the designer know the laws of corona in oil, as determined by the configuration of the parts and the strength of materials.