Determining the effect of thermal loading on the remaining useful life of a power transformer from its impedance versus frequency characteristic

It is well known that cellulose materials used to insulate transformer windings gradually degrade during service due to a combination of thermal, mechanical, and electrical stresses. As a result, the mechanical characteristics of the paper change during use and may affect the useful life of the transformer. It has generally been assumed that the electrical characteristics remain relatively constant throughout the aging process. If, however, thermal aging changes the electrical characteristics, it may be possible to gauge the thermal age of a transformer by externally monitoring these electrical characteristics over time. In this work, samples of oil impregnated thermally upgraded Kraft insulating paper are subjected to accelerated thermal aging. The relative dielectric constant and loss tangent are investigated by monitoring changes in capacitance and conductance as a function of frequency and thermal aging. This paper presents data showing that the thermal aging process produces changes in the electrical characteristics of the insulating system paper. An example is presented comparing the admittance versus frequency for a model of a 765 kV 500 MVA auto transformer. This demonstrates that the effect of aging can theoretically be observed from the terminals of the transformer.