Importantly, all the above‐discussed processes are also involved in the generation of several gasses that are dissolved in the insulation oil, which further ruins the physicochemical parameters of the insulation system.
Figure 1.2 Conceptual illustration of degradation in oil filled apparatus.
Source: Rao et al. [24] / with permission of IEEE.
Understanding the degradation process and its consequences that influence the performance and life of the oil‐filled apparatus indicates the importance of condition monitoring. The current state of knowledge on condition monitoring revealed that degradation/aging is associated with numerous parameters. Henceforth, monitoring and analyzing one single parameter will not reveal the exact situations prevailing with the insulation system. Proper condition monitoring analysis of oil/paper insulation with scheduled maintenance activities enables effective asset management and risk analysis. This also acts in transforming the conditioning monitoring aspects from detective‐corrective mode to strategic‐preventive mode. ASTM standard test methods are available for monitoring all the oil/paper insulation parameters to assess the quality of oil and the contemporary status of the oil‐filled apparatus. Hence, periodical assessment of insulation parameters will lead to early detection of the degradation perspectives of oil/paper insulation. The same analysis will be apparently useful in planning appropriate diagnostics and prognostics.
1.4 Transformer Insulating Liquids
1.4.1 Conventional Liquid Dielectrics
There are many conventional liquid dielectrics, which are research outcomes of several innovative chemical modifications of one product over the other. This section highlights most popular insulation oils that have been survived for a long time as an insulating medium in transformers. The majority of these oils are not biodegradable and are hazardous to environment. The primary sources of these liquids are expected to reach depletion in the future and hence current situations demand perfect replicates for oil‐filled power transformers. Hence, the field of research entails vigorous search for alternatives to replace existing insulating oils is significant to strengthen, improve, and sustain oil‐filled transformer insulation technology.
1.4.1.1 Mineral Insulating Oils
Mineral oils are extracted from crude petroleum by refining and fractional distillation processes. These oils are complex composition of several hundreds of allied aromatics involving hydrogen and carbon molecules. Mineral insulating oil consists either saturated paraffin and naphthenes or unsaturated aromatics in appropriate compositions depending upon the requirements of manufacturers to satisfy clients and applications [19]. Mineral oil may be paraffinic base or naphthenic base depending on the ratio of their proportion exceeding one over the other. Suitable number of aromatics is also added to these oils for developing appropriate dielectric parameters.
1.4.1.2 Polychlorinated Biphenyl
Polychlorinated biphenyl (PCB) is produced by replacing two to five hydrogen atoms of a benzene ring with chlorine atoms. Based on the number of atoms replaced, they may be called as di, tri, tetra, and penta chlorinated biphenyl [27]. The generic name of these liquids is askarels, which means fire resistant. PCBs were known by their different commercial names in several countries across the world, e.g. Pyranol in the United States, Sowol in Russia, and Aroclors in many parts of the world.
1.4.1.3 High‐Temperature Hydrocarbons
High‐temperature hydrocarbons (HTHs) are also known as high molecular weight hydrocarbons (HMWH). HTHs are derived naturally as well as through synthesis. The former are paraffinic base hydrocarbons that are obtained from petroleum crude similar to mineral oils, except fractioned from higher levels of distillation. The latter are developed through polymerization of olefins and hence are called as polyalpha olefins (PAO) [27]. These liquids are also known as high fire point liquids.
1.4.2 Alternative Liquid Dielectrics
In this section, liquid dielectrics that are having high thermal performance (having high flash points and fire point) and high environmental performance (nontoxic and biodegradable) have been discussed. The dielectric liquids discussed in this section may be naturally produced, or they may be synthesized. The key advantage of using these insulating oils is their high fire points, readily biodegradable (ecofriendly), and are mostly extracted from sustainable and renewable sources [28]. These insulating oils are also known for their better emission profile and fire characteristics. Some important insulating oils of this category that can be further promoted for use in oil‐filled transformers as a replicate to mineral oils are discussed in the following sections.
1.4.2.1 Natural Ester Liquids
Basically, these are ester group compounds that are produced from glycerine and sebacic acids [29]. They are usually fatty acids having high fire point, high breakdown voltage, and good biodegradability. The natural esters are usually triglycerides and are found to have low oxidation stability. The general molecular structure of natural esters is similar to the molecular structure of vegetable oils [30]. These oils were restricted to distribution class transformers or breather‐free transformers with the concern toward their low oxidation stability. Nevertheless, attempts are being in pipeline for further improving the oxidation stability of natural esters.
1.4.2.2 Vegetable Oils
Vegetable oils are extracted from agro seeds that are accumulated as fatty acid within the seeds. These are also called as natural esters. They are available from renewable sources and are treated as eco‐friendly and biodegradable liquids [29]. Vegetable oils have higher viscosity and high pour point, which degrades the effectiveness to serve as a coolant [31]. At present, the research in the field of natural esters is on to emphasize reduction of viscosity and pour points with improved oxidation stability.
1.4.2.3 Synthetic Ester Liquids
Synthetic esters are such organic compounds that are being developed by synthesizing acids and alcohols. Synthetic esters are available in various chemical compositions depending on the acids and alcohols used for synthesis. Synthetic esters are biodegradable and are developed for high thermal performances with low viscosity and better oxidation stability. Moreover, ester group insulation oils participate actively in hydrolysis, which resists the rate of degradation of the oil.
The complete journey of the liquid dielectrics concerning the transformer insulation technology is outlined and discussed by I. Fofana [30] and U. Mohan Rao et al. [31, 32]. Given the fact that the high demand for alternative biodegradable insulating liquids allowed researchers focus on the performance analysis and behavior of ester liquids. Researchers across the globe have reported numerous studies; this book has been framed to give a complete overview of ester liquids' performance, behavior, and compatibility for current and foreseen transformer insulation technology. To accomplish the same, various topics have been organized in the subsequent chapters of this book. Authors of different chapters aimed to discuss particular topics in a detailed manner. Some fundamental concepts concerning the ester liquids have been spanned to multiple chapters. This is due to the fact that different chapters have been prepared by different research groups. However, due care is taken not to duplicate the sections and technical viewpoints. Therefore, this book editors and authors expect that this book would be useful for transformer owners, utility engineers, and researchers concerned with transformer insulation technology.
References
1 1 Tang, Y., Zhong, J., and Liu, J. (2016). A Generation adjustment methodology