IEEE C57.98-1993-R1999.IEEE Guide for Transformer lmpulse Tests.
IEEE C57.98 This guide is written primarily for power transformers, but it is also generally applicable to distribution andinstrument transformers. Other standards, plus the purchaser’s specifications, already determine the specificrequirements for impulse tests. The purpose of this guide is not to change these standards in anyway, but toadd background information that will aid in the interpretation and application of these standards. Thesealternates have been developed by different testing laboratories with consideration for their individual prob-lems of transformer design, test facilities, etc. It is the objective of this guide to discuss these differences andto show how effective failure detection can be achieved with the testing techniques employed.
Insulation is recognized as one of the most important constructional elements of a transformer. Its chieffunction is to confine the current to useful paths, preventing its flow into harmful channels. Any weakness ofinsulation may result in failure of the transformer. A measure of the effectiveness with which insulation per-forms is the dielectric strength. It was once accepted that low-frequency tests alone were adequate to demon-strate the dielectric strength of transformers. As more became known about lightning and switchingphenomena, and as impulse testing apparatus was developed, it became apparent that the distribution ofimpulse-voltage stress through the transformer winding may be very different from the low-frequency volt-age distribution.
Low-frequency voltage distributes itself throughout the winding on a uniform volts-per-turn basis. Impulsevoltages are initially distributed on the basis of winding capacitances. If this initial distribution differs fromthe final low-frequency inductance distribution, the impulse energy will oscillate between these two distribu-tions until the energy is dissipated and the inductance distribution is reached.In severe cases, these internaloscillations can produce voltages to ground that approach twice the applied voltage.
As circuit voltages became standardized, impulse levels corresponding to the respective voltage classes werealso standardized. Impulse levels,now referred to as basic lightning impulse insulation levels (BIL), wereestablished in 1937 by an AIEE-EEI NEMA Committee on Insulation Coordination.This committee wasformed to consider laboratory technique and data， to determine the insulation levels in common use, toestablish the insulation strength of all classes of equipment, and to establish insulation levels for variousvoltage classifications.IEEE C57.98 pdf download.