Factors influencing the insulation coordination of transformers

IN THE early days of the electrical industry, surges were relatively mysterious as to their origin, magnitude, and behavior. The ordinary factory tests now required by the A.I.E.E. rules were the reflection of experience and judgment rather than the analysis which has been made possible in the last few years. From experience, it was found on the one hand that it was necessary to increase the insulation of some parts of the transformers far above the requirements of the A.I.E.E. rules (i. e., turn and coil insulation bushings) and, on the other hand, to limit the voltages transmitted to the transformer from the line, by the specification of line insulation and protective gaps. In the meanwhile, progress has been made in the laboratory study of the surge characteristics of the insulators, gaps, and insulation structures commonly used in transformers.