Moisture measurement in insulating fluids and transformer insulation – an evaluation of solid state sensors and chemical methods

This tutorial deals with possibilities and restrictions for convertibility between absolute water content determined by means of Karl Fischer titration method and relative saturation measured using capacitive sensor for different insulating liquids, as well as with the uncertainties of such conversions. It is shown that it is possible to derive evaluation criteria for on-line monitoring of moisture in electrical equipment based on the correlation between dielectric strength and relative moisture saturation in a dielectric liquid. In addition, the relative saturation/temperature hysteresis plot is found to be an effective way to indicate the wetness of transformer insulation based on practical examples in service.