Insulation Coordination of Distribution Transformers

In this study of insulation coordination of distribution transformers, the operating conditions are reviewed and the requirements of transformers to be operated without protection are outlined. Test results on transformers designed to meet these requirements are given, showing that by the use of shields for the high-voltage windings and thyrite resistors for the low-voltage windings, transformers can be made in which the internal insulation will be protected by flashover of the bushings. The level of insulation chosen and the amount of system protection desired by the use of lightning arresters will determine the economic value of such transformers.