IEEE C57.98-2011 pdf download IEEE Guide for Transformer Impulse . Tests
1.Overview
1.1 Scope
To aid in the interpretation and application of the impulse testing requirements of the IEEE Standard TestCodes for Transformers.
1.2 Purpose
This guide is written primarily for power transformers, but it is also generally applicable to distribution andinstrument transformers.Other lEEE standards, plus the purchaser’s specifications determine the specificrequirements for impulse tests. The purpose of this guide is not to change those standards in any way, but toadd background information that will aid in the interpretation and application of those standards. Theinformation contained in this guide is a compendium of technical information provided by engineers andtechnicians well versed in the art of transformer impulse testing It is hoped that this guide will provide abasis for a better understanding of impulse test technigues and troubleshooting procedures
2. Normative references
The following referenced documents are indispensable for the application of this document (i.e., thcy mustbe understood and used, so each referenced document is cited in text and its relationship to this document isexplained). For dated references, only the edition cited applies. For undated references, the latest edition ofthe referenced document (including any amendments or corrigenda) applies.
IEEE Std 4r, IEEE Standard Techniques for High-Voltage Testing.!, 2
IEEE Std C57.12.00,IEEE Standard General Requirements for Liquid-Immersed Distribution, Power,and Regulating Transformers.
IEEE Std C57.12.01,IEEE Standard General Requirements for Dry-Type Distribution and PowerTransformers Including Those With Solid Cast and/or Resin-Encapsulated Windings.
EEE Std C57.12.90,IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and RegulatingIransformers.
EEE Std C57.12.91,IEEE Standard Test Code for Dry-Type Distribution and Power Transformers.
EEE Std 1122,IEEE Standard for Digital Recorders for Measurements in High-Voltage Impulse Tests
3. Impulse Testing
3.1 General
Insulation is recognized as one of the most important constructional elements of a transformer. Its chief functionis to confine the current to uscful paths. preventing its flow into harmful channcls. Any weakncss of insulationmay result in failure of the transformer. A measure of the effectiveness with which insulation performs is thedielectric strength. lt was once accepted that low-frequency tests alone were adequate to demonstrate thedielectric strength of transformers. As more became known about lighting and switching phenomena, and asimpulse testing apparatus was developed, it became apparent that the distribution of impulse-voltage stressthrough the transformer winding was very different from the low-frequency voltage distribution.
Low-frequency voltage distributes itself throughout the winding on a uniform volts-per-turn basis.lmpulscvoltages 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 twodistributions until the energy is dissipated and the inductance distribution is reached. In severe cases, thescinternal oscillations can produce voltages to ground that approach twice the applied voltage.