industry and researchers. The critical concepts with the significant research findings over the years and the topics that will need to be further emphasized will be digested in the chapters of this book. The knowledge embodied in this book will also act as a quick reference for utility engineers, transformer owners, academia, and researchers those interested in alternative and biodegradable insulating fluids for liquid‐filled transformers. The scope of this book will aid to the focus of the IEEE DEIS technical committee on liquid dielectrics and lies within the interests of the IEEE DEIS community and transformer industry across the world. This book will be a unique sample of its kind and will act as a reference pertaining to the biodegradable insulating liquids. The contents of this book are framed in a way to cover the pertinent orbits of ester liquids, key aspects that need to be emphasized to further expand the existing knowledge, and act as a quick guide to the utility engineers. This book also covers the discussions on critical challenges that will be helpful for the engineers in successful operation of the ester‐filled transformers. Importantly, reputed and experienced researchers who are working on the individual subtopics for a long time contribute the chapters of this book. This will allow widening the scope and technical orbits of the book in an effective and unique manner. This book will also act in bridging the gap between the researchers and utility engineers concerning the transformer insulation systems.
1.2 Insulation System in Liquid‐Filled Transformers
Liquid insulation in transformers plays a critical role in assessing the lifetime of a transformer. Useful life of a transformer relies on the electrical health and effectiveness of insulating oil. Owing to prevalent thermal conditions within an in‐service oil‐filled transformer, the performance of solid insulation paper, pressboard, etc., and the effectiveness of the insulating oil to serve as an insulator and as a coolant, reduces significantly over a period. High thermal excursions in the transformer tend to accelerate the aging process of the oil–paper insulation. Eventually, insulation paper degrades by releasing certain gases, moisture, furan‐based compounds, and suspended particles percolating into the insulation oil; thus, enhances the deterioration of oil. As per ASTM D117‐18 [20] standard, commonly adapted parameters of insulation oil to monitor the transformers are classified as electrical, physical, and chemical.
Hence, degradation of insulation paper can be estimated by performing diagnostic and prognostic tests on the insulation oil. Simply, characterization of insulation oil facilitates prediction of performance aspects of insulation and helps estimate the prevailing conditions within the in‐service oil‐filled transformers. It has been a past practice of the power industry to perform diagnosis tests on the insulation oil in view of condition monitoring of an oil‐filled transformer. Insulation system being the heart of the transformer comprises the liquid insulation (typically oils) and solid insulation system (typically cellulose). Traditionally, mineral insulating oils are put up in practice for the transformer insulation technology. Due to the fact that mineral oils are expected to reach depletion, alternative candidates for these oils is a challenge. Further, mineral oils are toxic and nonbiodegradable. Thus, demanding an alternative candidate, which is biodegradable and nontoxic while meeting the technical requirements that an insulating liquid should exhibit. It is desired that a transformer insulating liquid has good dielectric strength, high thermal performance (high fire point), and has good compatibility with the transformer solid insulation. Classification of several insulation parameters that are used by the industry to monitor the performance of transformer insulation is shown in Figure 1.1.
Parameters highlighted in Figure 1.1 are fundamental parameters that majority of the utilities monitor to access the aging level of insulation in the transformers. All the abovementioned parameters of in‐service insulation oil are classified based on the nature of liquid insulating medium (property) in oil‐filled transformers. However, the aging aspects of transformers are highly interrelated and measuring oil–paper insulation aging has become an established interdisciplinary engineering application field of materials science and chemical engineering through latest instrumentation and aging measurements by various techniques and processes.
Figure 1.1 Basic insulation parameters for aging assessment of oil–paper insulation.
1.3 Insulation Aging Phenomena in Transformers
The service life of the oil‐filled apparatus is mainly attributable to the rate of degradation of the oil/paper insulation system. The preliminary causes for the degradation of the oil/paper insulation system are its decomposition aspects. The degradation process of the insulation system in oil‐filled apparatus is governed by the: (i) heat (dissipated through the core and winding assembly), (ii) moisture (ingressed from the air and liberated from cellulose), and (iii) oxygen (ingressed from air and liberated from oil/paper system). This operating heat will adversely affect the longevity and performance of the insulation system and the life of the apparatus if not dissipated properly in time [21]. As per IEC 60076‐2 standard (thermal layout), insulation paper will serve for up to 55 years and more, subject to the absence of dielectric defects in the insulation system. Albeit due care is taken by the manufacturers in designing, the generation of heat from coil–core assembly and subsequent degradation of the insulation system is unavoidable. The degradation of oil/paper insulation generates soluble and colloidal particles in the oil. This increases the oil viscosity, which hampers the flow rate in cooling tubes and affects the heat dissipation efficiency. This excess heat is responsible for the accumulation of hot spots within the transformer, which further intensifies the degradation mechanisms. The degradation mechanisms of an oil/paper insulation system include oxidation, hydrolysis, thermal stressing, and electrical stressing [22, 23]. These mechanisms introduce different by‐products that further act in degrading the oil/paper insulation system. The complete picture associated with the degradation process of oil/paper insulation has been illustrated in Figure 1.2.
Moisture and oxygen in the transformer may be ingressed from the external environment (for breathing transformers) or evolved from cellulose fibers of solid insulation. Moisture, oxygen, metals (components of the transformer), and heat (liberated from coil–core assembly) act as catalysts for deterioration of insulation systems and provide a scope for decomposing by‐products and acids. These catalysts and by‐products further expedite oxidation, hydrolysis, thermal, and electrical decomposition reactions with respect to operating times. All these degradation reactions are highly interrelated and the by‐products of one reaction act in catalyzing the other reactions. For analyzing the degradation perceptions of oil/paper insulation, these reactions may be approached thermally, electrically, chemically, and physically.
Thermally, oil gets oxidized and initiates sludging within the insulation oil while reducing the tensile strength of paper by paralyzing the cellulose fibers. Electrically, oil deteriorates and generates acids and free radicals with decomposition on a large surface of solid insulation. Chemically, the neutralization number of the oil gets adversely affected, and witnessing the evidence of furfural in oil. Physically, the osmotic behavior of moisture migrations through oil and paper with a change in temperature hampers the paper dryness and reduces the degree of polymerization of solid insulation by adding colloidal particles to oil [25, 26]. Ultimately, these degradation mechanisms are all about reducing the integral qualities of the insulation system with operating time. It is to be observed that, these aging mechanisms contribute to sludging, acidity, and decay contents, which reduce dielectric strength and increase the viscosity of the oil. These factors hamper the insulation properties and heat‐dissipating nature of oil, which are the primary objectives of the oil. It is to be understood that, these aging mechanisms contribute to the generation of furan contents, reduce the paper degree of polymerization and tensile strength, thus ruining the dielectric and mechanical integrity