Aging of high temperature insulation systems with alternative fluids

For many years traditional design of liquid-immersed transformers has been based on the combination of cellulose solid insulation and mineral oil. As a result, average temperature rise for windings is normally limited to 65 K, and top oil rise to 60 K as given in IEC 60076-2. Sometimes space or weight limitations or high ambient temperatures force designers to look for new solutions such as those with higher temperature rises using high temperature materials. Using aramid paper with a much higher thermal capability than Kraft cellulose paper can allow operating the equipment at a higher winding temperature rise without any negative impact on the insulation life. Aramid solid insulation can be combined with either mineral oil or alternative fluids, like silicones or esters. Silicone and ester fluids are also materials with higher temperature class than mineral oil. Therefore combining them with aramid solid insulation gives a chance of not only operating windings at higher temperature but entire insulation systems to be more effective in terms of thermal properties. Using high temperature insulation materials allows for significant reduction of cooling system. Allowing higher temperatures within windings results in the possibility of applying higher current densities in winding conductors. This leads straight to significant savings in raw materials. Smaller cooling system, less conductor, and use of natural ester fluids could mean less environmental footprint of the equipment built. In previous papers we presented thermal evaluation results of two alternative fluids, silicone and synthetic ester. In this paper we will also present findings of a natural ester fluid and the behavior of aramid solid insulation in these fluids at a range of temperatures.