Traditionally, power transformers use an insulation system that comprises cellulose insulation immersed in mineral oil, which limits the maximum hot spot temperature to 98 °C [1]. IEC 60076-14 [2] classified new insulation systems into different categories and specified the temperature rise limitations of each category. These are referred to as hybrid insulation systems.
A hybrid insulation system allows the loading capacity to increase substantially while reducing the long-term ageing of the insulation. Hybrid insulation systems with ester fluids are also becoming common.
This type of system is a response to mitigate fire risk and resulting environmental damage in case of fire. One of the types of hybrid insulation systems is the semi-hybrid system, where the conductor insulation may see temperatures higher than 98 °C but the other parts remain below 98 °C since they are cooled by the bulk oil flow that will not see a significant temperature increase due to the higher thermal time constant of the oil compared to the copper.
