Thermoset stator bar insulation systems

Generator stator bar insulation systems have experienced a steady evolution since the beginning of central power plants in the early 1900s. Initially (1900-1915), natural products were used, e.g., shellac, mica flake, paper. These materials apparently worked well on small, low-duty units, but were inadequate beyond perhaps 10 or 15 MW. As units grew in size problems developed, and during the period 1915-1950, asphalt-mica was the common system. These systems used mica, with paper or cloth carrier, and vacuum-pressure impregnation with carefully selected asphalt compounds. These systems initially performed well, with minor problems associated with puffing and asphalt bleeding if operated at too high a copper temperature, above roughly 95°C. But as generator size increased, the “tape migration” issue became so destructive that use of asphalt-mica systems was discontinued in the 1950s for generators rated higher than about 25 MW 1.