Temperature Rise and Dielectric Characteristics of Distribution Transformers with Over-Loading Operation

Temperature rise characteristics of medium voltage class transformers and dielectric characteristics of its insulation oil according to the loading rate were investigated using the transformer that has thermocouples embedded in the windings. The hottest-spot of the transformer locates at about 2/3 height of the windings where they adjoin each other among the 3 phase windings. The temperature difference between the hottest-spot and top-oil is about and the value keeps uniformly in short time after changing the loading rate. Capacitance of the oil varies in inverse proportion to temperature and the inclination rate is about -0.0106 that is not dependent on the aging rate. On the other hand the capacitance is increased depending on the aging rate, so the capacitance characteristics of the oil could be applied to the diagnostic technique.