Tension (IFT)
The IFT value of insulating liquids provides important information for detection of impurities or polar contaminations in the oil. The poor value of the IFT leads to drop in the quality of the oil, which is caused by the generation of oxides and peroxides in the insulating oil during its service. The value of IFT is measured in accordance with the standard ASTM D971. IFT is determined by the differences of the interactions between the molecules of one fluid to another fluid [66]. It is also observed that the IFT of NEO is lower than that of MO because of the molecular structure of NEO, which contains unsaturated fatty acid chains and the moisture content present in the oil. The NEOs have variances in the fatty acid structure which differ in their carbon chain lengths and in the number of double bonds or unsaturation. IFT is a physical attribute that is closely associated with the molecular configuration. The number of unsaturated fatty acids and the length of the fatty acid hydrocarbon chain affect the IFT value and the tension also increases when the chain length increases.
2.4.3.4 Thermal Conductivity
One of the most significant characteristics to understand the heat transfer property of any insulating oil is thermal conductivity. In order to understand this property of cooling in the transformer, thermal conductivity is measured by using the device KD2 pro at room temperature. Single probe transient hot‐wire method is used to observe the thermal conductivity. The governing equation for thermal conductivity is
(2.9)
Figure 2.8 Comparison of viscosity values of different oil samples.
where “k” is the thermal conductivity, “q” is the heat flow per unit length of the source, and “T1” and “T2” are the temperatures of the heat source at times “t1” and “t2”, respectively [67]. The thermal conductivity of natural ester is higher than MO because it contains the triglyceride molecular structure.
2.4.3.5 Viscosity
Viscosity of an oil is defined as the measure of the internal friction of the flowing liquid. The viscosity of an insulating fluid influences its ability to transfer heat from the interior of the transformer to the environment. With a higher viscosity, it becomes difficult for the liquid to flow and transfer heat quickly, which results in rise of the hot‐spot temperatures within the transformer. Generally, at operating temperature of a power transformer, NEOs show lesser viscosity value than silicone oils but higher than MOs. The natural esters having higher viscosity may indicate that their cooling capability is lesser than MOs. But this is not the case, as it is observed from many studies, that a high specific heat capacity and high thermal conductivity upgrade natural esters to exhibit better cooling performance [68]. As seen from Figure 2.8, the viscosity values of pongamia, jatropha, and palm oil are comparable to MO. However, more research is required when using high‐viscosity fluids in power transformers designed for MO insulation.
2.5 Degradation of Different Vegetable Oils
(2.11)
(2.12)
(2.15)
