and the presence of natural antioxidants. The refining process of natural esters also affects the oxidation stability as there are many intermediate processes that lead to decrease in amounts of natural antioxidants, if present.
2.4.3 Physical Properties
2.4.3.1 Pour Point
Pour point is defined as the temperature at which a liquid just starts to flow under certain conditions. Its value should be minimum in order to use the fluid at a very low temperature as per ASTM D97. Natural esters have pour point in the range of −6 to −25 °C which is higher than that of MO. This is because of the presence of triglyceride structure in the natural esters. When crude oil is trans‐esterified, these triglycerides form mono‐glycerides and support the flowability of the liquid and make it comparable to that of MO [54, 64]. Some comparison of the pour point values of different oils can be seen in Figure 2.7a.
Figure 2.7 Comparison of (a) pour point values of different oil samples and (b) flash point values of different oil samples.
The problem of higher pour point can also be improved with the addition of pour point suppressants, winterization, or blending with other fluids having lower pour points. In case of natural esters, operating at low temperatures becomes difficult as they have high pour points and tend to solidify faster than MO. So, the aspect of pour point must be taken into consideration when designing a transformer or other apparatus for operation in the colder regions. Blending of natural ester with some other compatible fluids may show lower pour points. Many transformer specifications require lower ambient temperatures of less than −20 to −25 °C. In general, the temperature of a running transformer is sufficient to keep the insulating liquid flowing. However, for outdoor installations, especially in colder regions, maintaining a free flow of the liquid becomes a concern as the temperature of the atmosphere drops below 0 °C. There are certain properties of the NEO, which determine the pour point like the acid chain length, level of unsaturation, and type of branching in the oil. Unsaturated fatty acids help in reducing the pour point and also the existence of aromatic groups in natural esters aids in maintaining a lower pour point value.
2.4.3.2 Flash and Fire Point
The growing demand of increased fire safety requires a fluid with high flash and fire points. Flash point can be defined as the temperature at which a combustible liquid can be heated to give off sufficient vapor to momentarily form a flammable mixture with air when a small flame is applied under specified conditions. This is determined by employing a Pensky–Martens closed cup apparatus as per ASTM D93. The NEOs tend to have higher flash points when compared with the MO as seen in Figure 2.7b. The fire point is defined as the temperature at which the liquid itself catches fire. The primary benefits of NEOs are their higher flash and fire points than the conventional MO. Fire point plays a key role during shipment of oils and installation of transformers in indoor as well as outdoors, with lesser concern about the fire safety protocols. Natural esters have fire points of above 300 °C and meet the requirements of “K” class insulators. These oils are widely used in many practical installations and have regulatory advantages in many sites [65].
2.4.3.3 Interfacial 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)