On-load tap-changer influence on frequency response analysis of transformer: A case study

Frequency Response Analysis (FRA) has been used as a comparative diagnosis method to explore mechanical defects in transformer. FRA is quite sensitive method and it can be influenced by different factors in transformer simply; however, this sensitivity will make it superior rather than other transformer winding deformation diagnosis techniques. This study has concentrated on tap-changer position influence on FRA signature. Practical study is performed on a 33 kV, 40 MVA three-phase core type transformer to examine on-load tap-changer (OLTC) position on frequency response spectrum in different frequency bands in Bode diagram. Oscillations in different frequency bands are discussed and their fluctuations are interpreted. Low-frequency band oscillations of FRA spectrum due to tap position changes are analyzed and discussed in detail. Distinguishing the tap-changer influence on FRA spectrum from winding deformation is quite crucial for FRA data interpretation.