Evaluation of Power Transformers´ Effective Hot-Spot Factors by Thermal Modeling of Scrapped Units

The ultimate life of a power transformer is critically governed by the condition of its insulation paper which undergoes thermal degradation during the operation. A thermal model has been published in the IEC transformer loading guide 60076-7 and can be used to calculate the windings´ hot-spot temperature based on operating conditions and thermal parameters, and to assess the insulating paper´s aging rate. However, the accuracy of hot-spot temperature is greatly affected by the generally unknown hot-spot factor. In this paper, utilizing the paper samples of scrapped transformers and the degree of polymerization (DP) predicted thermal lives as benchmarks are proposed and through the IEC thermal model to reversely derive the transformers´ effective hot-spot factors (EHSFs) while the paper´s aging mechanisms and moisture accumulation effect over lifetime have been taken into consideration. As a result, the median of 35 scrapped units´ EHSF is 2.95. These EHSFs are assigned to the field sister transformers to give a practical assessment of transformer cooling efficiency and loadability.