improving its aging characteristics. The water saturation limit of MO is approximately 60 mg/kg [57] whereas vegetable ester oil has 1000 mg/kg [58]. When the moisture content is increased, the dielectric strength of the liquid insulation decreases. When the MO ages, the moisture transfers to the solid insulation like kraft paper and pressboard and degrades its dielectric properties, and thereby decreasing the performance and efficiency of the power transformer. During the operating conditions of a transformer, temperature of the oil increases and the paper then releases water to the transformer oil by undergoing oxidation reaction. This further augments the neutralization value and degrades the oil quality.
2.4.2.2 Sulphur Content
The production of corrosive sulphur is a major problem in transformers filled with MO. However, with natural esters, this is not a problem as they are bio‐based and extracted from plant seeds. Hence, using natural esters for insulation will not cause corrosion problems in any equipment. The corrosive sulphur is not present in NEOs, hence performing corrosive tests can confirm if there are any kind of additives added to the NEOs on purchase. During the operational lifetime, corrosive sulphur may be present in the transformer oil, even after using NEOs. This may be due to improper handling of oil during filling, reaction of the conducting parts, and solid insulation with the oil [59]. Most of the sulphur compounds are in stable form, but under certain circumstances, these stable compounds can be transformed into reactive compounds. In a study, comparison is carried out among the properties of transformer filled with MO and some NEOs [60].
2.4.2.3 Total Acid Number (TAN)
Total acid number (TAN) or neutralization value gives the measure of acidity in the sample. It indicates the number of milligrams of KOH required to neutralize the H+ ions present in 1 g of oil. The total acid number provides the purity of the dielectric oil. Stability toward oxidation of a liquid insulation can be analyzed by determining the neutralization value, DDF, and specific resistance. Natural esters undergo oxidation and decompose by the process of hydrolysis and generate a variety of by‐products like acids and alcohols. Some reports showed that the acid number of natural esters increased correspondingly with passing time [61]. The acidity for some of the oils ranged from 1 to 3 mg of KOH/g when stored for a period of three months [62]. According to ASTM D4625 (30 °C/50 weeks), the total acidity of Pongamia biodiesel rose up to 6 mg KOH/g. Studies have shown that Low Molecular Weight Acids (LMA) is higher than High Molecular Weight Acids (HMA) in MO, while HMA is higher than LMA in NEOs. Also, the total acid is much greater in NEO and hence the absolute quantity of LMA in a natural ester is higher than MO. This might be because that natural ester is much more polar and hence the interface between solid insulation and natural ester favors the stay of LMA in natural ester.
2.4.2.4 Oxidation Stability
The liquid insulation that is used in power transformer must have higher stability toward oxidation. In terms of oxidation stability, natural esters are not known to perform very well as they tend to oxidize very easily and thus they are not suitable for free‐breathing transformers. When these oils come in contact with oxygen, they produce sludge and also the viscosity changes. The addition of antioxidants, however, can slow down the oxidation process. The degradation reaction occurring in the MO leads to the breakage of bonds between two carbon atoms and the formation of alkenes. This happens mainly because of oxidation, dehydrogenation, and cracking. In natural esters, oxidation and hydrolysis occur forming CO, CO2, and other by‐products. The method of oxidation varies between natural esters and MO as the by‐products formed in NEOs do not solidify and settle down, but it increases the viscosity.
Though antioxidants can be added to NEOs, they still tend to show lower oxidation stability due to higher biodegradability than MO, and so it is essential to use natural esters in sealed transformers [63]. It is always essential to choose a NEO with the optimum combination of saturated and unsaturated triglycerides. It is to be noted that ester oils with higher content of saturated triglycerides exhibit higher pour points and viscosity, whereas the ones with higher unsaturated triglycerides are more prone to oxidation. Stability toward oxidation of natural esters depends on the distribution of fatty acids, refining process, 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
