Application of relaxation current measurements to on-site diagnosis of power transformers

Results of relaxation current measurements on impregnated pressboard samples and on multi-layer arrangements of pressboards in series with oil ducts are presented. The measurements have been performed dependent on the material properties of oil and pressboard and the volume ratio of these materials. Calculated values of related dielectric quantities such as tan/spl delta/ versus frequency, based on equivalent circuits obtained from relaxation currents are also shown. The relaxation currents of multi-layer test objects are calculated from the dielectric properties of the individual components and their geometrical capacitances. Finally, it is shown that relaxation currents of power transformers can easily be measured and their evolution can be described well by basic relations of linear dielectric theory.