The relevance of the study. The phenomenon of internal resonance was not taken into consideration by any executive directive in the World when reasoning the necessary level of longitudinal insulation. Studying the phenomenon and stating recommendations will help to increase the working reliability of transformers. The main aim of the study is to state the basics of modeling, which guarantee the adequacy of simulating electromagnetic processes in transformer windings, under the influence of network overvoltage. The methods used in the study: frequency methods of natural experiment and electrical circuit analysis. The results. The main requirements to the models are stated: 1. A design model has to include elements, which simulate magnetic core, insulation, windings, windings' parts and their interaction with the main magnetic flux. 2. A design model has to reflect electromagnetic processes in a transformer, influenced by voltages of free form and frequency. 3. As the components of the common magnetic flux (magnetic flux inside a magnetic core and magnetic flux outside a magnetic core) influence the parameters of electromagnetic processes in a transformer in a different way, these components have to be modelled separately. Conclusions. Voltage distribution along a winding and between windings is nonlinear during resonance between a transformer with linear parameters and capacitance of the network (including), where the transformer transfers energy. The task of simulation is to define overvoltage ration on the most dangerous section of a winding. The influence of magnetic flux results in developing of resonance processes of a frequency depended on parameters of windings and their parts, in disabled windings. This issue should be taken into account during modeling by creating corresponding mutual inductance connections on the magnetic flux return path, outside a magnetic core. The authors developed the method of simulating mutual inductance connections between windings and their parts. The method takes into account the violation of reciprocity principle of mutual inductance in real transformers. The authors developed the method of accounting magnetic core nonlinearity, when a transformer is influenced by network overvoltage, and the method of experimental definition of components of complex impedance of windings and their parts for a given mode of operation, and the method of realization of frequency characteristics of complex impedance components. Standard software packages, which implements the developed models, including the ones, which take into account the geometry of a transformer, are not adapted for implementing the approaches, given here. The problem of increasing transformer reliability in cases of resonance occurrence is not the problem of CIGRE, but IEC.