Insulating Paper

The importance of moisture content of Manila paper on the life of insulation, when immersed in transformer oil, is shown by the work of F. M. Clark (1). The mechanical strength of cellulose insulation is reduced by half for each 100% increase in water content up to 0.5% of its dry weight. This rapid decrease in mechanical strength is attributed to aqueous ionization of acidic substances resulting from oxidation or the transformer oil and cellulose insulation. The instability of cellulose in the presence of strong mineral acids is well known. Organic acids such as acetic or formic, which could result from oxidation or cellulose or transformer oil, would also cause degradation at high temperatures. Qualitatively, the effect of moisture may be observed in the darkened appearance, acidity and caramel odor of cellulose insulation which has been aged without prior vacuum drying. Moisture is most effective after initial decomposition or the insulation has occurred thus substantiating Clark´s theory that accumulation or decomposition products and their subsequent ionization accelerates deterioration or cellulose insulation. The effects of intermittent heating at 150 °C are additive although in the presence of oxygen this relationship holds only until the tensile strength has been reduced to 50% or its original value. In general, exclusion of oxygen results in increased life, the effect of oxidation being greatest below 120 °C. These oxidative reactions are accentuated by the presence of moisture. A new method for determining moisture content or paper insulation, immersed in mineral oils, is described.