Abstract. The work focuses on the service life of the insulation of the windings of a power dry transformer operating at a certain load and in a given temperature range. Based on the passport data, a mathematical model has been developed, the application of which allows assessing the process of consumption of the working resource of the insulation under the influence of ambient temperature and at a certain transformer load. In accordance with the mathematical model, a device structure has been developed that allows assessing the state of the residual operating life of the insulation of the windings of a power dry transformer both in the low-load mode and in the overload mode under certain environmental temperature influences.