Distribution Transformer Frequency Response Analysis: Behavior of Different Statistical Indices during Inter-disk Fault

Transformer Frequency Response Analysis (FRA) is one of the most efficient methods to examine the mechanical condition of the transformer without opening the transformer tank and extracting the active part. FRA is a comparative method, where the measured response curve is compared to the reference trace. In this study, various statistical indices for frequency response results interpretation are introduced and the influence of inter-disk fault on each method is evaluated via practical study. Performance of each indicator is discussed and the most reliable index for FRA results interpretation in inter-disk fault suggested. Practical study of inter-disk short-circuit is performed by the rheostat connected in parallel with the winding of distribution transformer. The voltage taps of the transformers are used in order to have the access to different percentage of the transformer winding. Thus by regulating transformer output voltage and emulating short-circuit, there is possibility to distinguish the critical values for "green", "yellow" and "red" zones for transformer FRA and condition evaluation. These critical zone values are advised to be used for assessing the transformer and FRA data via statistical indicators.