Diagnosis of the state of power transformer windings by on-line measurement of stray reactance

A strategy for online diagnosis of the state of the windings in power transformers is discussed. Earlier work showed that it is possible to separately determine core and copper losses of transformers in real operating conditions. A similar measurement is suitable for the online determination, with good precision, of the stray reactance variations in transformers. Since the stray reactance is indicative of windings geometry, this technique enables a diagnostic system to be set up that provides indications of conductor insulation reliability, recommending shutdown when necessary. The method was tested on a 25-MVA-150-kV/16.2-kV three-phase transformer, showing the possibility of performing this online determination with an accuracy at least on the same order as that obtained in the usual methods with a transformer not in service