IEEE C57.98-2011 pdf free.IEEE Guide for Transformer Impulse Tests.
To aid in the interpretation and application of the impulse testing requirements of the IEEE Standard Test Codes for Transformcrs.
1.2 Purpose
This guide is written primarily for power transformers. but it is also generally applicable to distribution and instrument transformers. Othcr IEEE standards, plus the purchascrs specifications determine the specific requirements for impulse tests. The purpose of this guide is not to change those standards in any way, but to add background information that will aid in the interpretation and application of those standards. The information contained in this guide is a compendium of technical information provided by engineers and technicians well versed in the art of transformer impulse testing. It is hoped that this guide will provide a basis for a better understanding of impulse test techniques and troubleshooting procedures.
2. Normative references
The following referenced documents are indispensable for the application of this document (i.e, they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies.
3.1 General
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 how into harmful channels. Any weakness of insulation may result in failure of the transformer. A measure of the eufectivcncss with which insulation pcrfomis is the dielectric strength. It was once accepted that low-frequency tests alone were adequate to demonstrate the dielectric strength of transformers. As more became known about lightning and sitching phenomena, and as impulse testing apparatus was developed, it became apparent that the distribution of impulse-voltage stress through the transftwmer winding was very ditTerent from the low-frequency voltage distribution.
Low-frcqucncy voltage distributes itself throughout thc winding on a uniform volts-pci-turn basis Impulse voltages arc initially distributed on the basis of winding capacitances. If this initial distribution differs from the tinal low-t’rcquency inductance distnbution. the impulse energy will oscillate between these 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.
As circuit voltages became standardized, impulse levels corresponding to the respective voltage classes were also standardized Impulse levels, now referred to as basic insulation levels (BIL), were established in 1937 by an AIEE-EEI-NEMA Committee on Insulation Coordination. This committee was formed to consider laboratory technique and data, to determine the insulation levels in common use, to establish the insulation strength of all classes of equipment. and to establish insulation levels for various voltage classifications. Through the use of these Hl1.s, apparatus can he specified on the basis of demonstrating that the insulation strength of the equipment will be equal to or greater than the selected basic level, and protective equipment can be selected to provide adequate protection. The BILs and other insulation-test voltages are listed in IEEE Std C57.l2.OO and C57.1201.IEEE C57.98 pdf download.