Diagnosis of thermal degradation for thermally upgraded paper in mineral oil

The life of a transformer is limited to the deterioration of its solid insulation. Winding conductors and other solid insulation materials in oil-immersed transformers have been insulated using cellulose products. For many years, manufacturers have met the needs of special applications by designing transformers using thermally upgraded materials to achieve lighter weight, higher power density and increased life. Recently, the effect of thermally upgraded insulation on diagnostic techniques such as gas-in oil analysis, and their indication of insulation degradation have been reviewed. This report describes an evaluation of thermal degradation on oil-impregnated thermally upgraded paper ("Thermo-ZA"), carried out under conditions directly relevant to practical situations. Our results confirm that an index of aging in the thermally upgraded paper can use both CO₂ and CO contents in oil, although the paper does not produce as much 2-Furfural as Kraft paper.