Capacitive humidity sensors for transformer ageing monitoring

Ageing of kraft paper winding insulation is predominantly governed by temperature, and water and oxygen content in the paper. Today, water in a transformer is normally measured in the oil once a year under varying loading conditions giving uncertain estimates of the water content in the paper. This water content will also vary with the local temperatures, adding to the uncertainty of the estimates. New sensors, continuously monitoring e.g. temperature and water content in oil, combined with validated ageing models offer possibilities for improved control of the thermal ageing and risks from water migration in the insulation system.