Oil Additives for the Evaluation of Hot Spot Temperature in Power Transformers During Overload Tests

An oil additive of the blowing-agent type, which decomposes rapidly above 132°C with accelerated gas evolution at a concentration of 1000 ppm in oil, has been used to determine the hot-spot temperature during overload tests on a 16 MVA power transformer. The formation of gas in the Buchholz relay indicates that this e ottest-oil temperature of 132 ± 2°C has just been reached somewhere in the transformer during the overload test. The method is simple and does not require the hot-spot location to be known in advance as in the case of acoustic or pressure transducer devices. Used during heat-run tests, when voltage is relatively low and gas formation not harmful to the transformer, it can bring useful information about the actual overload ecapability of the transformer. The hot-spot temperature obtained by this method with the 16 MVA power transformer was substantially higher than that deduced from the ANSI and IEC loading guides.