Factors influencing the mechanical strength of cellulose insulation

CELLULOSE insulation, like mineral oil with which it is usually associated in high-voltage electrical apparatus, is subject to deterioration in an increasing degree as the temperature of exposure is raised. This deterioration is a manifestation of the chemical instability of the cellulose. For a proper understanding of this change, it must be recognized that the fundamental chemical reactions involved are of two types. There are those changes which result from the oxidation of the cellulose and those which arise from the decomposition of its molecule. These changes are invariably intermingled in the normal commercial use of cellulose dielectric materials. To prevent dangerous deterioration and to secure the maximum service life of cellulose insulation, the effect produced by each type of reaction, the probability of its occurrence, and the means for its control must be carefully evaluated. Summary 1. Cellulose insulation will deteriorate in mechanical strength at a rate which increases with the increasing temperature of exposure. Deterioration will occur in the absence of an oxidizing atmosphere. 2. The rate of cellulose deterioration at a fixed temperature varies as the heat treatment is continued. 3. The mechanical deterioration of cellulose insulation at elevated temperature is the result of oxidation and pyrochemical decomposition. 4. When heated, cellulose passes through a “stable period” during which the mechanical properties are maintained. The presence of oxygen is chiefly effective in reducing the duration of this “stable period.” 5. The single effect of oxidation is restricted to the initial periods of treatment at temperatures lower than approximately 120 degrees centigrade. At higher temperatures, pyrochemical decomposition occurs. Even in the most favored temperature range, the effect of cellulose oxidation merges into the effects produced by pyrochemical decomposition as the deterioration progresses. 6. Heated under conditions which favor oxidation or pyrochemical change, the un-impregnated cellulose insulation deteriorates more rapidly than the same insulation after oil treatment and immersion. 7. The effect of oxidation is accelerated when pure oxygen gas is in contact with the insulation. No substantial difference in the rate of mechanical deterioration is observed when the oil-treat ed and oil-immersed insulation is aged in contact with nitrogen gas as compared to aging in contact with air.