Vacuum process in drying of power transformer insulation

Abstract:  Abstract The drying of transformer insulation to remove absorbed water spans the spectrum of vacuum pumping systems from mechanical rotary piston pumps and mechanical boosters, to liquid ring pumps down to vapor booster and vapor diffusion pumps. Power transformers can weigh 500?000 lb and contain 30?000 lb of insulation. A primary dry of the cellulosic insulation using narrow boiling hydrocarbons as a boiling heat transfer medium occurs in large-volume tanks of varying capacity from 7000-21?000 ft3. Internal assemblies containing the insulation are first dried to water contents of 0.1% by weight. The pressure and temperature in these tanks range from 25 to 120?°C with pressure levels as low as 0.3 Torr at cycle completion. Secondary transformer drying takes place after internal assemblies are installed within steel transformer tanks. These tanks range in free volume to 3500 ft3. Drying may take place at ambient or elevated temperatures (depending on atmospheric exposure history) at pressures approaching 0.001 Torr.