The main features of combined transformers are considered, and their technical and economical advantages singled out. It is pointed in particular to the advantages of Konar’s own concept of combined transformers with a integral insulation system. The methodology of calculation of combined transformer insulation system includes an investigation of partial discharges on models of oil-paper insulation with a view to assessing insulation ageing, and its results have been statistically processed applying Weibull distribution. A mathematical model of the active part of combined transformers has been defined for calculation of voltage stress in both low-frequency and high frequency ranges. Several modes of determination of equivalent input parameters of the model have been evaluated. For that purpose measurement methods, calculation methods based on analytical expressions, and finally numerical 2D and 3D computations based on finite elements method were used. For the adopted model, computations of the distribution of power-frequency voltage, and of lightning and switching impulse voltages along the highvoltage winding of the combined transformer are described. On the example of a combined transformer for the highest voltage for equipment of 420 kV, the procedure for finding a satisfactory design solution is described in detail, comparing computation and measurement results. It is shown that these results are in exceptionally good agreement. A prototype of the transformer made on the basis of investigations described in this thesis has passed the prescribed routine and type tests. A design solution for a combined transformer for the highest voltage for equipment of 550 kV is given as well.