Investigation of IEC Thermal Models of Power Transformers

The power transformer is a crucial component of electric power system for continuity of electricity. The reliability of power system depends greatly on the operational performance of the power transformers under different operating conditions. The most important indicator of the operating performance of oil-immersed transformers is the top-oil and the hottest-spot temperature values. Therefore,different thermal models have been developed in the literature to estimate temperature changes of power transformers. In this study, thermal models developed by IEC Loading guide for oil and winding temperatures of power transformers are studied. To the best of our knowledge,description of IEC models in the literature have some ambiguous points. Thus, ambiguous points has been clarified and some modification to IEC models has been introduced to have better estimates of top-oil and hottest-spot temperatures of power transformers. With these modifications, top-oil and hottest-spot temperatures, which are given for 250MVA ONAF transformer unit in IEC Annex-b, are presented during different load variations as an example.