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(2.12)
(2.15)
The degradation in natural esters occurs mainly because of oxidation and hydrolysis reaction. The thermal aging test setup is shown in Figure 2.9a and the change in color of the oil samples after laboratory aging is shown in Figure 2.9b. The oxidation reaction is a radical chain reaction involves three main phases: initiation, propagation, and termination. For oxidation, any conducting material like copper acts as catalyst. The strength of hydrogen–carbon bonds next to the carbon–carbon double bond sites in fatty acid groups is weaker, so a free radical is easily formed in natural esters by eliminating a hydrogen atom from the methylene group next to a double bond [69, 70]. This may be the result of high temperatures or an application of high voltage. Some have reported that natural esters having polyunsaturated fatty acid oxidize even at room temperature whereas the natural esters with mono‐unsaturated fatty acids oxidize only at higher temperatures. The degree of unsaturation and the temperature rise are the main reasons that determine the oxidation stability of natural esters.
As shown in Eqs. (2.10)–(2.13), triglyceride hydroperoxide is formed by the reaction of free radical, oxygen, and oil molecules [70, 71]. In the chain reaction, hydroperoxides break down to more free radicals. A variety of compounds like smaller oxygen‐containing by‐products such as alcohols, aldehydes, ketones, and high molecular acids are produced by decomposition of triglyceride hydroperoxides. In the termination reaction, stable high molecular compound ROOR is formed by combination of the peroxy radicals RO2. At the last stage of oxidation, secondary nonvolatile substances of oxidation are subjected to polymerization which leads to the formation of high molecular weight gelatinous compounds, which tend to stick to the walls of the transformer or bottom of the aging vessels in laboratory experiments. Hydrolysis is another phenomenon which occurs simultaneously with the oxidation process in the natural esters. This is an autocatalytic reaction, because FFA molecules themselves accelerate the hydrolysis reaction. The steps involved in hydrolysis reaction, which are reversible in nature, are shown in Eqs. (2.14)–(2.16) [72]. The hydrolytic degradation mainly increases the acidic level of NEOs in due course of time. Temperature contributes to a large increase in the acidity of NEOs under high heat due to pronounced hydrolytic degradation.
Figure 2.9 (a) Aging test setup. (b) Oil samples depicting change of color due to aging.
For carrying out the aging experiment to understand the degradation of the oils, the single temperature technique in sealed beaker or an open beaker technique may be used simulating the transformer interior as per the relevant standards. During this process, thermal stress is applied on the composite system of the oil and the corresponding quantity of solid insulation, at a fixed temperature. The duration of thermal stress is changed and samples are collected after the set time intervals for performing the experiments. Complete drying of the solid insulation is carried out to make them moisture‐free. The maximum temperature to carry out the aging process of oils should be as per IEEE C57.147 and preferably kept below 180 °C to avoid fluid scorching. The oil dilatation must be taken into consideration while filling the vessels for aging of oil samples as they expand on application of heat. The coefficient of thermal expansion of natural esters is approximately 0.0007/°C, so there is a 1% increase in volume for every 14 °C rise in temperature, by using the expression,
(2.17)
where ΔV is the change in volume, β is the thermal expansion coefficient, V is the initial volume, Δt is the temperature difference.
A few studies to ascertain as to whether the structure of the functional groups has changed in the oil samples are given below.
2.5.1 Fourier