Influence of a tap changer position on the transformer's frequency response

The paper discusses the problem of tap changer position influence on frequency response of transformer windings. Frequency Response Analysis (FRA) is a standard test used for diagnostics of mechanical condition of transformer's active part. By changing the tap number some part of the regulation winding is added or subtracted from the main winding. This leads to changes in the shape of measured FRA curves. If the test is taken on the other tap than the described one in a report, it may lead to serious misinterpretations. The problem is how to detect such mistake and distinguish it from the influence of real faults, e.g. short circuit or core magnetization. Examples of measurements conducted for various positions of tap changers for several units are given in the paper and criteria allowing to determine if the visible change is the result of wrong tap changer position are described.