Numerical Investigation of the Effects of Temperature-Dependent Permittivity and Space-Charge Transport on the Electric-Field Distribution in the Oil–Paper Insulation of an Oil-Immersed Transformer

To investigate the effects of the thermal state under rated operation and space-charge transport on the electric-field distribution in oil–paper insulation, a sequentially coupled multiphysics model is developed for a 30 kVA single-phase three-limb oil-immersed transformer. The non-uniform temperature field is mapped onto transformer oil and insulating paper, and electric-field distributions with constant and temperature-dependent dielectric parameters are compared. Short-term charge-transport calculations and initial space-charge sensitivity analysis are also performed under fixed peak-potential boundary conditions. The results show that temperature-dependent dielectric parameters increase the maximum electric-field strength in transformer oil from 0.358 to 0.374 kV/mm and decrease that in insulating paper from 0.568 to 0.541 kV/mm, without shifting the major high-field regions. In Case 3 (under initially charge-neutral conditions), positive and negative space charges accumulate in the oil regions adjacent to the low- and high-voltage winding ends, respectively, while the space-charge density in the middle of the main insulation oil duct remains near zero. Compared with Case 2 (under temperature-dependent dielectric conditions), the maximum field strengths in oil and paper decrease by 2.14% and 24.58%, respectively. For Cases 3–6, from initially charge-neutral to moderate space-charge conditions, maximum-field variations remain within approximately 5%, whereas Case 7 (highest initial positive-charge condition) shows pronounced field enhancement.