Correlation between hot-spot temperature and aging factor of oil-immersed current transformers

Effect of a variable and continuous load on top-oil (TOT) and hot-spot (HST) temperature rise and in turn, on aging of mineral-oil-immersed transformers has been well studied. Detailed loading procedures covered in relevant IEEE Standard serve utilities manage critical assets in the form of power and distribution transformers. Current transformers (CT) have neither been specifically addressed nor independently studied. Due to a significantly different electromechanical design and in particular lower volume of oil, it is necessary to check the thermal characteristics of a CT against existing general guidelines. A state-of-the-art technology using optical fibers was employed for direct measurement of HST at various load profiles. Results indicate a significant difference between HST reference value in standards and the experimentally obtained value for CTs. Moreover, accelerated aging factors for CTs appear to be different from those calculated by using the standards for power and distribution transformers.