Moisture content assessment of transformer solid insulation using return voltage spectrum

Dielectric properties of oil-paper insulation transformer are influenced intensively by moisture; therefore, it is imperative to assess integral condition of oil-paper insulation for assuring normal operation of power transformers. As a non-destructive diagnostic technique, return voltage measurements have been introduced for the assessment of water content in transformer pressboard. This paper further investigates the possibilities of assessing moisture contents of power transformer by return voltage method. Field test results performed on several transformers and its interpretation are given. The derived integral moisture and ageing assessment procedures using the return voltage method are discussed. Moreover, the influence of on-the-spot factors on the test result are also detailed and analyzed in this paper, and the approaches to eliminating them were given accordingly. Finally, reliability of the method was assessed by comparing derived moisture contents based on return voltage measurements with dielectric loss. The test results show that this method is reliable and practical.