Aging of transformer insulation of experimental transformers and laboratory models with different moisture contents: Part II — moisture distribution and aging kinetics

Part II of the paper presents the results of the experimental data processing presented in Part I dedicated to distribution of water content in the standard Kraft paper along the height of HV windings during aging of experimental transformers and comparisons of paper aging rates of experimental transformers and laboratory models. The dependence of aging constant (pre-exponential factor of Arrhenius equation) on the moisture content of cellulose insulation is introduced. The developed functional dependence is obtained by detailed processing of the data - hot-spot temperature, moisture and degree of polymerization of cellulose insulation during 460 days of aging experiment of two transformers on load, with conductor insulation moisture of ca. 1 % and ca. 4 % and 300 days of aging experiment on corresponding laboratory models. Based on the results shown in this paper, aging curve, i.e. change of degree of polymerization during transformer operation insulated with Kraft paper and inhibited mineral oil was proposed, including varying water contents during aging. Using this equation it is possible to evaluate aging rate of solid insulation, starting from known hot-spot temperature and moisture content in cellulose insulation. Good comparison of aging curves of experimental transformers and laboratory models, i.e. measured and calculated values of degree of polymerization of paper to existing literature findings was obtained.