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Insulation system is one of the most important parts of power transformer. Theoretical background for insulation design is given, which includes explanation of dielectric properties of main insulation materials, their withstand to breakdown voltage, their verification by dielectric tests, and statistical data based theory for breakdown probability. Two known numerical methods (BEM and FEM) for calculation of electric field are used and for each numerical method an appropriate model of transformer is made. Safety factors for main insulation between HV and LV winding are calculated after which the oil gap width is reduced. Additional analysis of safety factors and field nonuniformity along windings is performed.Index Terms-numerical calculation, boundary element method, finite element method, electric field, power transformer