Effect of core magnetization on frequency response analysis (FRA) of power transformers

This paper presents how the frequency response analysis (FRA) measurements on a transformer can be influenced by magnetization condition of the core. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-phi supply, of relaxation demagnetization and of dc flux on the frequency response of winding impedance. The most important observations were that any sudden change of excitation field (applying or removing magnetization current) yields a slowly varying magnetic relaxation, which causes the impedance to change with time due to magnetic viscosity. Results of diagnostic measurements of transformer winding impedance can therefore depend on the level of remanent magnetic flux appearing in the core limbs after switching-off a transformer. These observations lead to a conclusion that, for avoiding the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from network.