What is the function of a transformer oil dryer and how does it remove moisture?

The function is to maintain low moisture content to preserve dielectric strength and reduce insulation aging. It removes moisture by lowering water’s partial pressure and separating it from oil, most effectively via vacuum dehydration. Oil is warmed and dispersed to increase surface area, then vacuum causes dissolved water to evaporate and be extracted. Alternative designs use desiccant adsorption. Drying is validated with moisture ppm tests and BDV improvement, and it is often paired with filtration to remove particles that can trigger partial discharge.