AIEE Proposed Guide for Transformer Impulse Tests
Abstract
The Standards of AIEE, NEMA, and ASA plus the purchaser's specifications determine the specific requirements for impulse tests. This Guide will not change the Standards in any way, but adds background information which will aid in the interpretation and application of these Standards. These Standards now provide for some alternate ways of conducting some tests or parts of tests. These alternates have been developed by different testing laboratories with consideration for their individual problems of transformer design, test facilities, etc. It is the object of this Guide to discuss these differences and to show how effective failure detection can be achieved with the testing techniques employed. Although the Guide is written primarily for power transformers, it is applicable generally to distribution and instrument transformers.Scope
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 failure of the transformer. A measure of the effectiveness with which insulation performs 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 phenomena, and as impulse testing apparatus was developed, it became apparent that the distribution of impulse voltage stress through the transformer winding may be very different from the low-frequency voltage distribution. 2. Low-frequency voltage distributes itself throughout the winding on a uniform volts-per-turn basis. Impulse voltages are initially distributed on the basis of winding capacitances. If this initial distribution differs from the final low-frequency inductance distribution, 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.
Topic
Power, Energy and Industry Applications, Components, Circuits, Devices and Systems