Evaluation of the Thermal Stability of Electrical Insulating Paper

Transformers with a variety of chemically treated insulating papers are being offered for 65°C operation. However, there are no industry standards for qualifying treated insulation for the new rating and it is impossible to compare the performance of different brands of insulation on the basis of published literature because of differences in the test conditions. A laboratory method, for evaluating the thermal stability of insulating paper, is presented that measures the performance at various stages of deterioration, makes direct numerical comparisons, and can be used to establish specifications for the treated paper used to build 65°C-rise transformers. Insulating paper is aged in oil in a sealed cylinder in the presence of copper, iron, and carefully controlled amounts of oxygen and water. Deterioration, as measured by loss in tensile and bursting strength, is plotted against time for each temperature. Insulation life is then plotted as a function of temperature. Evolution of gases is also measured. Tests indicate there are significant differences in the performance of the brands of treated insulation tested. The effects of water, oxygen, and competitive brands of transformer oil on insulation life can be studied independently.