Many power companies of the developed countries last years are compelled to solve a problem of mass ageing of the transformer equipment. This situation influences reliability of power supply of consumers of the electric power. The obvious decision of this problem is replacement morally and physically out-of-date equipment new with modern characteristics. This way considerably improves the general technical condition of transformer park, but demands for the realization of large investments. In conditions of deficiency of financial assets in many power systems search of variants for prolongation of real service life of transformers over normative is conducted. This way requires rather small investments into the initial moment of time, allows to continue operation of transformers and to remove for some time the decision of a question on replacement of the equipment. The opportunity of term prolongation of a life of transformers is based that high voltage power transformers are one of the most reliable and economic types of the power equipment. Long-term operating experience of transformers in electric networks of the developed countries confirms that real term of a life of 30 % - 60 % of maintained transformers is higher than normative. Besides during term of operation power transformers work, as a rule, with real loadings below nominal that allows to keep good technical parameters of the equipment. Nevertheless, the decision concerning prolongation of operation of transformers over normative term can be accepted only after carrying out of their diagnostics. By results of inspection modernization of separate elements of transformers, if necessary, is spent. One from technical measures on prolongation of service life of the transformer equipment is replacement or regeneration of oil.. The malfunctions connected with deterioration of properties of oil, are easily removed at the initial stage of development, in the further conduct to severe to damages. Regeneration of oil is one of ways of a delay or the full discontinuance of development defects of winding isolation. The technical and economic analysis of measures on modernization and increase of reliability of functioning of the transformer equipment, probably to execute on the basis of the generalized mathematical model developed in Institute of Power of the Riga Technical University. As criterion function the total discounted costs of the user for realization of measure and the subsequent period of operation of the transformer after realization of measures are accepted. It allows estimate efficiency of concrete measures on increase of reliability of functioning of the transformer equipment. On the basis of the specified mathematical model the estimation of measure on regeneration of transformer oil for 110 кV the transformer by capacity 63 МВА is executed. On the basis of calculation it is drawn a conclusion on economic feasibility of carrying out of measure on regeneration of oil at maintained transformers.