Exploring Statistical Index Criteria for Transformer Frequency Response Interpretation

Abstract

Large power transformers are considered as the most expensive assets in power system network after hydro generators. Therefore, monitoring of such equipment needs to take special attention. Frequency Response Analysis (FRA) is one of the efficient methods to examine the mechanical condition of the transformer without opening the transformer tank. FRA is a comparative method, where the measured response is compared to the reference fingerprints. Therefore, interpretation of the FRA results needs to be done by an expert in the field. To overcome this problem, so that untrained personnel would be able to use FRA for transformer condition monitoring the interpretation of the frequency response should be based on standard or on some criteria. In this study, various statistical indices for frequency response results interpretation will be introduced and evaluated. Frequency responses of single-phase 0.4-1kVA transformers and three-phase transformers up to 40kVA are interpreted by statistical indices. Outcome of each indicator is discussed and the most reliable ones for FR interpretation are suggested. The simulation of inter-disk short circuit was performed by the rheostat connected in parallel with the winding of transformer. The voltage taps of the transformers were used in order to have the access to different percentage of the transformer winding. With the help of different voltage taps and different resistances in parallel, the two levels of critical values were found and advised to use.