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This report presents an algorithm that takes into account power transformer (PT) technical conditions and their potential for failure risk assessment to help determine maintenance precedence. The technical condition integrated index is assessed through the weighted average of condition indexes determined by the results of PT traditional and special control methods. Each control type index is the three-dimensional space vector where the coordinates are next: X-damage seriousness, Y-damage development speed, Z-damage development duration. The traditional control method defect identification and coordinate determination are carried out by an expert system without any specialist participation. Risk failure assessment includes: damages of suppliers and to consumers of electricity; transformer design features; lifetime; ES transformer unit (system) failure potential. The PT unit failure potentials were calculated using the flows of failures and damage statistics of 350 reliable facts of PT damage accumulated by the ES for 20 years of operation. Firstly, priority ranking for serviceable transformer operational activities are excluded from the sample. Then there are calculated coordinates on each of the remaining transformers. It coordinates the points, which are marked by the PT plane. The plane includes three zones: additional scheduled inspections and emergency repairs. The ES ranks PT by the distance from the origin within each zone.

The technical condition of the transformer oil affects the condition of the solid insulation, and therefore the residual life of electrical equipment, so it is important to study the aging mechanisms of the oil. The mechanism of transformer oil aging is studied using factor analysis of oil quality indicators, such as moisture content, acid number and dissipation factor tangent. Factors that have the greatest influence on the rate of oil aging have been found. In accordance with the found influence factors the permissible values of oil quality parameters of 110 kV power transformers with division by equipment tightness, oil brands and operating time have been determined. Oil aging models, taking into account the mutual influence of oil quality parameters, have been developed for transformers with different oil brands, type of oil protection and service life. An integral index, characterizing different aging processes of the oil, has been proposed. The criterion for evaluating the integral index of oil aging rates is calculated.

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