Abstract
In the recent past, Frequency Response Analysis (FRA) has been used as a kind of effective method to determine mechanical defects inside the transformer. FRA provides an opportunity to detect mechanical defects without disassembling the transformer active part, whereas visual inspection to identify internal defects is difficult and time consuming. In this study, main statistical indices for FRA result interpretation are introduced and discussed. A failed 400 MVA step-up three-phase transformer is taken as a test object and FRA responses on failed winding are examined through different statistical indices. The outcome and advice of each indicator is also discussed in detail. Finally, the most reliable statistical indices for FRA interpretation are.