Transformer Frequency Response Analysis: A mathematical approach to interpret mid-frequency oscillations

Diagnosis methods have been employed to recognize transformer internal defects since many years ago. Frequency Response Analysis (FRA) has been utilized as an offline diagnosis test since last decade to investigate mechanical integrity of transformer while this technique is quite capable to provide worthy information about electromagnetic characteristics of the windings. This study has concentrated on transformer winding frequency response trace oscillations presented in Bode diagram. Mathematical approach using travelling wave theory is employed to explore frequency response trace behavior. The correlation between winding behavior in frequency domain and time domain is highlighted. Eventually, practical study has been performed on continuous and interleaved 66 kV disc windings to validate mathematical calculation.