Degradation Assessment of Ester Liquids

Liquid dielectrics are used as insulating medium and coolant in many power system components such as transformers and cables, and as arc quencher in high‐voltage circuit breakers. In transformers, they form the bulk insulation medium, with cellulosic paper as the insulation for the winding conductors. The dielectric properties and other physical and chemical properties of liquid dielectrics have been the focus of research studies since the early use of AC systems for transmission and distribution of electrical power. Traditionally, in the early 1980s, researchers were focused on studying mineral oils and other hydrocarbon‐based liquids for transformer insulation and cooling applications. These liquids offer superior dielectric properties compared to other synthetic liquids such as polychlorinated biphenyl (PCBs), which have been shown to release various toxic substances. These properties associated with low cost of production have kept mineral‐oil‐filled transformers still widely in use in the industry nowadays. However, the depletion of petroleum‐based resources and their nonbiodegradability have brought sustainability and environmental concerns. In addition, the effectiveness of oil reclamation processes reduces after a period of repeated treatment. This has recently led to a switch of focus toward alternative dielectric liquids such as agricultural‐based oils and ester fluids. Ester‐based fluids provide a high benefit‐to‐cost ratio, with increased flash point and fire point. Besides, they are easily biodegradable in nature, making them a feasible alternative for use in transformer insulation. This chapter first gives an overview of the types and chemical structure of ester fluids and their applications. This is followed by a description of their dielectric properties and the various mechanisms responsible for their degradation. Lastly, it illustrates and discusses different techniques for condition monitoring of liquid dielectrics and paper insulation.