Mixed Insulating Liquids with Mineral Oil for High Voltage Transformer Applications: A Review

Due to the growing interest in environmental concerns, synthetic and natural esters have become the key focus of the picture in the last two decades as insulating fluids for high-voltage equipment. This is because, unlike mineral oil, ester liquids are biodegradable, nontoxic, and safe for the environment and human health. These fluids are derived from renewable sources and have high fire resistance. However, synthetic and natural ester fluids are still used in a rather limited number of electrical equipment for several reasons (high cost, high viscosity and density, increased tendency toward electrostatic charge, faster propagation of streamers in an inhomogeneous electric field, etc.). Besides, a huge amount of equipment is filled with mineral oils. The massive replacement of oil-filled equipment with natural or synthetic esters may be extremely expensive to utilities and transformer owners. In addition, in the event of partial or complete retrofilling of mineral oil with an ester fluid, the equipment may evidently contain a mixture of two insulating fluids, and therefore, demanding the scope for research on mixed insulating liquids. The intent of this article is to present a comprehensive review of the literature on the blend of mineral oil and other alternative dielectric fluids. The critical research progresses, highlights, and challenges related to mixed liquids along with significant tutorial elements as well as some analyses are discussed. This review should be useful for researchers, utilities, and transformer owners concerned with ester liquids and retrofilling aspects.