On-site diagnosis of power transformers by means of relaxation current measurements

The purpose of this paper is to show how the relaxation currents of a power transformer can be calculated, when the geometrical configuration of the main insulation and the dielectric properties of its components, oil and pressboard, are known. To model this insulation a simple equivalent circuit is presented, which is derived from basic relations of linear dielectric theory. The dielectric characteristics of oil and pressboard necessary for modelling are determined from dielectric response measurements on samples. Therefore some results of relaxation current measurements of pressboard samples in dependence on moisture content and temperature are presented. Finally, some examples of on-site measurements of relaxation currents on hitherto unused power transformers are shown which are used here to demonstrate how the moisture content of pressboard of these transformers can be quantified from evaluation of their relaxation currents.