Direct determination of moisture in solid oil-paper insulation

The life of oil-paper insulation depends critically on the moisture content in paper. At present no method is available for direct measurement of moisture in paper insulation in transformers. This paper summarizes the results from a project for development of a method for direct measurement of moisture in paper in oil-paper insulated electrical equipment. Several techniques were investigated for their suitability for this application. The infrared absorption was found to be the most promising technique. The effectiveness of the infrared technique was examined on model transformer coils and on a power transformer, using optical fibre bundles for directing infrared radiation. The ratio of intensity of radiation back scattered from paper at a reference wavelength to that at a water absorption wavelength (Br/Bw) was used as a measure of the moisture content. From dry to ambient paper condition, the parameter Br/Bw changed by over 60%, and these readings were reproducible to better than 1%. Thus the infrared technique is sensitive enough to detect changes of the order of 0.1% in moisture content (eg, 1.9% to 2.0%) of paper insulation