A Bootstrapping Solution for Effective Interpretation of Transformer Winding Frequency Response

The current interpretation procedure for the transformer Frequency Response Analysis (FRA) is practically based-on either subjective visual inspection or an observed value of a Statistical Indicator (SIn) programmed in a commercial software. However, both solutions are unable to provide very detailed information on transformer condition in industry. The former approach highly depends on the experience of the technical personnel, while the latter suffers from uncertainties of SIn-based decision boundaries for classification. This study has specifically focused and introduced a new comprehensive FRA data interpretation and classification method. Evaluation of the proposed technique is performed by emulation of winding short-circuit faults in transformers via a parallel connected rheostat. The method is able to categorize the operational mode of transformer into the normal, suspicious, and critical classes based on the cumulative effect of 12 frequently used SIns, and then provide a confidence level (CL) associated with classification to support operators for making a precise decision. Bootstrap sampling technique is introduced for the first time in FRA data evaluation to simulate the effect of many experiments in silico and overcome the difficulty of limited available measured data. Particularly, in a novel approach, a large set of SIn-specific decision boundaries is established by bootstrap sampling of the real experimentally obtained boundaries. In addition, a new transformer condition assessment method that employs cumulative effect of individual SIns is developed and examined. Classification results and the confidence levels in FRA evaluation have a substantial practical significance and will support on-site utility managers or engineers for further diagnosis and maintenance.