Transformer frequency response analysis: mathematical and practical approach to interpret mid-frequency oscillations

Frequency Response Analysis (FRA) has been utilized as an off-line diagnosis test since last decade to investigate mechanical integrity of transformer. FRA data typically reports as a spectrum in Bode diagram over determined frequency band. To evaluate FRA data, determined frequency band may be conveniently divided into three bands, namely low-, medium- and high- frequency bands. It is well-known low-frequency band dominated by core, mid-frequency region dominated by winding structure and high-frequency band influenced by measurement connection leads. This study has concentrated on midfrequency oscillations of transformer winding frequency response spectrum presented in Bode diagram. Mathematical approach using travelling wave theory is employed to explore frequency response trace behavior. Practical studies on a prepared glassy transformer as well as two 66 kV, 25 MVA continuous and interleaved disc windings have been performed to validate mathematical calculation. In addition, two 245 kV, 45 MVA and 66 MVA power transformers have been examined to study mid-frequency oscillations and compare the result with mathematical evaluation.