High temperature reinforced cellulose insulation for use in electrical applications

Electrical grade kraft paper insulation is widely used in oil-filled electrical equipment because of its good insulating properties and economy. Its primary disadvantage is that the cellulose polymer is susceptible to hydrolysis when exposed to high operating temperatures. Thermally upgraded kraft was introduced during the late 1950s, which retarded the hydrolysis process and raised the standard continuous hot spot operating temperature of a transformer from 95/spl deg/C to 110/spl deg/C. Additional thermal benefits have been introduced by reinforcing the cellulose matrix with high temperature synthetic fibers. Preliminary testing indicates that the addition of high temperature fibers would allow raising the hot spot temperature of a transformer from 110/spl deg/C to at least 120/spl deg/C. The high temperature fibers form a web-like backbone that gives the paper mechanical integrity even after the cellulose component has weakened. This paper presents test results that demonstrate the superior thermal endurance of one such high temperature reinforced cellulose insulation compared with commercially available electrical kraft.