Thermal rating of transformers. Introduction of multiflow principle

Abstract

The thermal rating of power transformers has hitherto been based on the average temperature rise of the windings as determined from the increase in resistance, because that has been the best practical measurement. Unfortunately, for a given and guaranteed temperature rise by resistance, the hottest-spot temperature can vary over quite a wide range depending on individual design characteristics. The introduction of the multiflow principle allows the hottest-spot temperature to be determined from measurements made during a single temperature-rise test. Using this method, it is suggested that the temperature rise by resistance should be abandoned as a standard guarantee, and replaced by the hottest-spot temperature, this being the fundamental basis of the true thermal rating. The designer will then have wider latitude in choosing his parameters, and freedom to exploit more intensive cooling systems such as increased oil circultation and directed flow. The resulting reductions in both winding- and oil-heat-exchanger capacities are substantial. As a broad comparison with existing practice, the principle of multiflow in forced-oil-cooled transformers permits an increase in the average winding-temperature rise from the present international guaranteed limit of 65°C to 75°C, or more, without any increase in the hottest-spot temperature.