The relevance of the study. Electrical energy is one of the most important resources on the planet, which is used everywhere and lets other industries work. Electrical-power transmission from the place, where electrical energy is produced, to the place, where it is consumed, as one of the fundamental principle of the world power supply process, is possible because of its parameters transformation, done by a transformer. The phenomenon of internal resonance has not been taken into consideration by any executive directive in the World during 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 research is to study the features of internal resonance phenomenon appearance with a group of transformer windings influenced by network overvoltage simultaneously. The methods used in the study: frequency methods of natural experiment and electrical circuit analysis. The results. The important unknown before feature of the processes during internal resonance is inequality of voltage distribution along windings. Resonance frequencies of the processes, which are observed in transformer elements, depend not only on parameters of a transformer, but also on network parameters, which the transformer gives energy into. Resonance frequencies of the processes in low voltage windings differ from resonance frequencies of the processes between high voltage windings and within the high voltage winding. The intensity of resonance processes is also different, which results in significantly different overvoltage ratios.