Calculation of the transfer function of a power transformer with online measuring data

Mechanical deformation in transformer windings can be detected with the transfer function (TF). Normally the transformer will be disconnected from the public power supply to record the TF (offline measuring). Another possibility is to calculate the TF from the transients overvoltages generated from switching operations or lightning strikes. With adequate sensors these overvoltages can be recorded during operation of the transformer (online-measuring). On the basis of these online-datas diverse problems like separationof excitation- and response signal, too low signal-to-noise ratio, etc. and their solution will be shown in this paper.