The relevance of the study: At the present stage of CIS countries network development the problem of increase the operation reliability of transformers, which main part has been already out of date is of the first-priority. The problem is also up to date for designing, testing and producing new transformers. The main aim of the study is to state, using the experimental research, the terms, when the existing levels of turn insulation are not sufficient for reliable transformer operation in the electrical network. The methods used in the study: Frequency methods of natural experiment and electrical circuit analysis. The results: For the first time it has been stated that resonance overvoltage in winding parts causes turn faults in transformer windings, while they are influenced by power system inner overvoltage. Making the longitudinal insulation of a winding stronger gives a possibility to increase operation reliability of the transformer. As an alternative, there is a possibility to develop actions for network, which eliminate dangerous impacts on specific frequencies. Conclusions: Besides the classical type of internal voltage resonance between winding inductance and insulation capacitance, resonances between constructively-identical parts of high voltage winding were established. They exist in a wide frequency range in terms of current resonance and in a few discrete frequency ranges, in terms of voltage resonance. For the first time the mechanism of overvoltage appearance between parts of transformer windings has been stated - the resonance between mutual induction voltages of winding parts is caused by energy exchange between the winding parts on the ways of magnetic flux beyond the magnetic core. Overvoltage, which values are fundamentally higher, than the applied ones, occurs inside a transformer as any network overvoltage appear.