Winding movement in power transformers: a comparison of FRA measurement connection methods

Frequency response analysis (FRA) is an effective diagnostic tool for detecting transformer winding movements. Various FRA traces can be measured from a set of transformer winding terminals each of which relate to a different test connection scheme. Practical considerations of test and analysis time dictate that only some of the connections are used, and currently there is no standard test connection. This paper presents a comparison of three FRA measurement connections widely employed in the industry today, namely: end-to-end voltage ratio, input admittance and transfer voltage ratio measurements. Using a simulation model of a 132/11 kV, 30 MVA transformer, FRA traces were generated under these connection schemes and their sensitivity towards three types of winding movement, namely: axial displacement, forced buckling and axial bending was studied. A correlation exists between the FRA measurement results of end-to-end voltage ratio, input admittance and transfer voltage ratio connection methods, provided that the HV neutral is grounded. Among the three connection methods assessed, the transfer voltage ratio connection method has the best sensitivity to axial displacement and forced buckling, whereas the end-to-end voltage ratio method has the best sensitivity towards axial bending