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Foreword

This guide covers general recommendations for measuring voltage transients in dry-type distribution and power transformers.

Insulation is recognized as one of the most important constructional elements of a transformer. Its chief function is to confine the current to useful paths, preventing its flow into harmful channels. Any weakness of insulation may result in the failure of the transformer. Dielectric strength is a measure of the effectiveness with which insulation performs. It was once accepted that low-frequency tests alone were adequate to demonstrate the dielectric strength of transformers. As more became known about lightning phenomena, and as impulse-testing apparatus was developed, it became apparent that the distribution of impulse voltage stress through the transformer winding varies with the configuration of the windings.

Impulse voltages are distributed initially on the basis of winding capacitances. If this initial distribution differs from the final low-frequency inductance distribution, the impulse energy will oscillate between the two distributions until the energy is dissipated and the inductance distribution is reached. In severe cases, these internal oscillations can produce voltages to ground that approach twice the applied voltage. Along with the variation in size of transformer windings and the physical configuration of the windings, the impulse voltage distribution when chopping the applied wave was considered by the task force that developed this guide. Since there was insufficient information on how to interpret the short-time oscillations on the insulation system, the inclusion of the chopped wave was deferred until a later date.

The Dry-Type Dielectric Working Group was formed by the Transformers Committee of the IEEE Power Engineering Society to determine standard methods for examining the impulse voltage distribution within dry-type transformer windings; to establish a means for defining the location and magnitude of maximum voltage stress in a dry-type transformer coil; and to support other committee activities, such as the Thermal Evaluation Working Group (C57.12.56).