The Use of Thermal and Load Data to Identify Large Autotransformers that Have Aged and Degraded Electrical Insulation

This paper reports on the use of thermal and load data to identify large autotransformers that have aged and degraded electrical insulation. The top oil temperature rise and the oil time constant at rated load are used to guarantee that the transformer temperature will not exceed the thermal limits at the rated capacity. The thermal parameters from a heat run test are typically supposed to be unchanged over the lifetime of the transformer. Nevertheless, the thermal behaviors could be affected by the degradation of electrical insulation. Those parameters are recalculated by fitting operational measurements, which comprised of load profiles, winding temperature indicator (WTI) measurements and ambient temperature, to the IEC 60076-7 model. Significant increases in the thermal parameters indicate that there is a possibility that the autotransformer has aged and degraded electrical insulation.