FRA interpretation using numerical indices: State-of-the-art

Frequency Response Analysis (FRA) is an effective diagnostic technique for detection of mechanical deformations inside a power transformer. The results of previous studies have led to the standardization of the FRA measurement procedure, but the interpretation process is still a subject of study. The numerical indices are one of the algorithms proposed for the interpretation, where the amount of an index extracted from two FRA traces can indicate a mechanical fault. Different indices are proposed in the literature, and this paper tries to survey and to compare these indices from different aspects to expand the knowledge about their characteristics. First, the numerical indices proposed in the literature are formulized, and proper references are introduced for each index. Then, the indices are categorized into two main groups. The first group of the indices is derived using exact calculations whereas the second group is calculated based on the estimation of the transfer function. The advantages and disadvantages of each group are addressed based on previous studies. Afterwards, the experiences of working with indices are reported and, finally, the vital steps which are needed to standardize the indices are discussed. The survey provided here is beneficial for the standardization process of these indices.