Measurement of the frequency response, from a few Hz to a few MHz, is now commonly used in the transformer industry for the condition assessment of transformer windings and has demonstrated its sensitivity for detecting various mechanical and electrical failure modes. The present generally applied practice for interpretation is visual comparison of frequency responses, either with a previous measurement on the same or an identical unit, or between the phases of a three-phase transformer. Examples of curve comparison for typical mechanical and electrical failure modes have previously been published in CIGRE and IEEE guides. Over the last 15 years, numerous technical papers have been published regarding the interpretation of the results in an aim to make it more objective and quantitative. In 2016, CIGRE initiated a new working group titled "Objective interpretation methodology for the condition assessment of transformer windings using Frequency Response Analysis (FRA)". This paper, written on behalf of the new working group, reviews the basics of FRA interpretation and summarizes the state-of-the-art regarding the potential methods that can be applied to achieve a more objective and quantitative interpretation of the results.
Picher, P., Tenbohlen, S., Lachman, M., Scardazzi, A., & Patel, P. (2017). Current state of transformer FRA interpretation: On behalf of CIGRE WG A2.53. In Procedia Engineering (Vol. 202, pp. 3–12). Elsevier Ltd. https://doi.org/10.1016/j.proeng.2017.09.689