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 of solid insulation. 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.
