In Latvian Power Company (Latvenergo) almost half from power high-voltage transformers fleet works 25-40 and more years. Mass ageing of the transformer equipment is a serious problem in the Latvian power supply system demanding the cardinal decision. The impossibility to provide investments in necessary sizes carries on to some changes of the purposes and tasks of maintenance of the transformer equipment. The prolongation of real resource of transformers over normative terms (till 35-40 years and more) through diagnostics, modernization, repair and other measures becomes as one of tasks. In article measures on replacement and modernizations of high-voltage transformers are considered. The generalized technical and economic model on the basis of the total discounted costs at the user by the transformer for all period of operation (life cycle) is developed. The minimum of total costs is accepted as criterion of an optimality of a variant. On the basis of the generalized mathematical model particular criterion function is resulted. Practical estimated calculations of measure on modernization of unit of regulation of a voltage for 110 кV the transformer by capacity 63 МVА and its replacement by different years of operation are executed.