Water Absorption of Cellulosic Insulating Materials used in Power Transformers

The combination of cellulosic paper and pressboard and mineral oil is still the most efficient and economic way to insulate power transformers. One of the drawbacks of cellulose is its highly hygroscopic nature. The hygroscopicity is caused by the cellulose molecule itself, whereas the rate of water absorption is influenced by many parameters. Thin, low-density papers exposed to a warm and humid climate show a much higher rate of water absorption than thick high-density pressboards exposed to a cold and dry environment. Impregnating cellulosic insulation materials with oil slows down the rate of water absorption, yet the moisture content at equilibrium is the same as for non-oiled materials. Tests for the determination of the sorption isotherms showed that data published in 1960 could be matched very well. But the tests also showed that the raw materials have a significant impact on the sorption curves.