Impact of other windings on frequency response of power transformers

The diagnostics of power transformers is a very fast developing branch. Due to increasing average age of assets and changes in asset management strategies, nowadays companies introduce asset management based on technical condition. One of important methods used for diagnostics of a transformer’s active part is Frequency Response Analysis (FRA). It allows determination of mechanical condition of windings, their displacements, deformations and electric faults, as well as some problems with internal leads and connections, core and bushings. In the FRA measurements of many power transformers there can be observed a first resonance, which position on frequency axis and damping factor seems to be similar for primary and secondary windings. The authors are going to explain this strange phenomenon, not depending on inductance of the winding being measured.