Limitations to the determination of water content in transformers solid insulation from measurement of water in oil

Determination of the water content in the winding insulation of aging transformers is an essential part of their condition assessment. Since this insulation cannot be accessed directly on a transformer in service, the moisture content has to be inferred from other available measurements. One method frequently applied is to measure the water content in the insulating oil and use a set of physical relationships to determine the water content in the winding insulation. This method is now being deployed on a larger scale as the availability of on-line monitoring devices grows. This paper reviews the limitations intrinsic to this method and assesses the overall accuracy that can be expected for a transformer in service under varying operating conditions. It is concluded that for most cases of interest the accuracy is about ±20% but, for very dry transformers, the uncertainty may rise above ±40%.