Experience with diagnostic tools for condition assessment of large power transformers

The application of advanced off-line methods and procedures for condition assessment of new and service-aged power transformers are discussed. The selection of the diagnostic methods is based on the main operational stresses (electrical, mechanical, thermal) and on typical failure modes: (1) detection of polar components in the paper/oil insulating system using frequency domain spectroscopy (FDS) or using measurements of polarisation/depolarisation currents (PDC) in the time domain, (2) localization of inhomogeneities in the insulating system using sensitive partial discharge detection (PD) and (3) detection of deformations and movement of windings in the active part using measurement of the transfer function (FRA).