Overpressure phenomena in distribution transformers with low impedance faults: Experiment and theory

While the problem of disruptive failure of distribution transformers has received much recent study, uncertainty still remains as to the precise phenomena which cause cover blowing or tank distortion. This paper presents some new insights into the mechanisms of cover blowing based on a comprehensive and highly instrumented test program. Unique contributions from this study should help clarify many points which have been the subject of much discussion during the last few years. Besides observing and correlating the effects of various arcs drawn inside several different transformers, some actual failed transformers were tested and the effects of the failures observed and measured, giving a better understanding of what occurs while a transformer is in the process of failing. Special tests using high speed photography have confirmed the existence of failure modes which up to now have been only suspected. In addition, measurements from these tests have verified theoretical studies and so have helped provide a more complete picture of the diverse phenomena involved. Practical solutions to the cover blowing phenomena are discussed.