75.3
a Low erucic acid variety of rapeseed oil; more recently canola oil containing over 75% monounsaturate content has been developed.
The bubble inception temperature in the insulating oil is dependent on the moisture content of the paper insulation. As the moisture content of paper increases, the bubble inception temperature decreases. The bubble inception temperature also depends on the type of paper (kraft paper/thermally upgraded paper), gas content in oil, age of oil, rate of temperature rise, apart from the moisture content [18]. Any air bubble entrapped due to higher viscosity of the NEOs may reduce the impulse BDV of the NEO [19].
The NEOs are thermally more stable than the MO and the dissolved gas content is higher in the MO. The heat transfer characteristics of the oils can be improved by adding the nanoparticles (NPs). The NPs also help in increasing the BDV of the oils because of the electron‐scavenging nature. As the NEOs have already enhanced BDV as compared with MO, their BDV increases further with the stable dispersion of the NPs [4, 14, 20]. The moisture equilibrium is also an important factor when considering the use of NEOs for power applications. It is the balance between the amount of moisture quantity in paper and the moisture quantity in the oil and is temperature dependent. The shape variation of moisture equilibrium curve for paper–NEO is similar to the paper–MO while the moisture content in NEO is considerably higher than that of MO for same moisture content of paper [21]. The NEOs mostly comprise of triglycerides, and these triglycerides convert to long chain fatty acids on water ingression from paper to oil. These long chain fatty acids in the presence of cellulose, main composition of solid insulation, get transesterified. This leads to the lower moisture levels in the transformer, and thereby the degradation of insulation is slower. Another explanation for the lower degradation is the formation of the gel around the solid insulation, which avoids further degradation of the solid insulation [3, 5]. But if such gel formation happens, it reduces the thermal conductivity in the transformer and the hotspot temperature increases leading to winding damage.
The NEOs are reported to have higher breakdown strengths compared with the conventional MO. The stopping length of the streamers propagating in NEOs is known to be longer than that of streamers in MO when compared at the same value of testing voltage. The pre‐breakdown phenomenon in NEO needs to be studied extensively for condition monitoring and maintenance of the transformer liquid insulation. Pongamia oil methyl ester (POME) derived from karanji seeds is produced by transesterification of crude karanji seed extract and the POME shows enhanced properties when compared with MO on oxidative aging [16]. However, interfacial tension (IFT) and pour point are not superior for POME when compared with MO.
Dissolved gas analysis (DGA) is an important tool in diagnosis of oil‐filled transformers. The main objectives of DGA is to monitor the condition of equipment, operating conditions, and possible localization of faults [22, 23]. For low energy faults, same ratio of hydrogen and acetylene are released for NEO and MO, whereas for the low thermal faults, ethane is the main constituent [22]. The DGA response is the same in NEOs and MOs, but the percentage or gas ratios needs to be redefined because the rate of gas generation is lower for NEOs, which means the NEO is thermally more stable than MO [23, 24]. Stray gassing is observed in FR3 fluid at temperatures between 40 and 50 °C [25]. The use of antioxidants decelerates the aging of NEOs and kraft paper [12].
2.2 Significant Natural Ester Liquids
The liquid insulation in a power/distribution transformer is used for various purposes:
1 A cooling agent – During the operational time, the core and windings get heated, so the liquid insulation helps to transfer the heat outside.
2 An insulating agent – The liquid insulation helps to insulate the different conducting parts of the transformer.
Owing to the concerns regarding environment and sustainability, researchers started looking for other alternate options to the conventional MO [27]. NEOs are a perfect substitute to the conventional MO and other synthetic liquids. These oils are completely biodegradable and nontoxic in nature. However, the downside of using these oils is that they have high pour point, higher viscosity, and oxidize easily. Some of these oils have greater content of unsaturated fatty acids which lowers the viscosity and aids in faster oxidation. Whereas, some others consist of high percentage of saturated acids, which improves oxidation stability. Thus, the oil chosen should be such that it provides a finest balance for smooth operation of an apparatus. Pongamia and jatropha oils are chosen for discussion because of their nonedible nature and they serve as potential substitutes for the conventional MO. Along with them, some other NEOs are also briefly described below.
2.2.1 Soybean Oil
Soybean oil is one of the most prominent NEO, which is being used as an insulating agent in transformers [28, 29]. It is extracted from soybean seeds and mainly consists of unsaturated fatty acids like the polyunsaturated α‐linolenic acid, linoleic acid, and monounsaturated oleic acid [30]. This oil has lower oxidation stability and its viscosity is also on the higher side. This oil polymerizes easily contributing to even higher levels of viscosity. With the addition of antioxidants, it shows relatively better oxidation stability and lower viscosity [31]. Efforts are carried out to obtain a better soybean oil with enhanced properties. This oil shows better oxidation stability when used with antioxidants like citric acid and the tert‐butyl hydroquinone [31]. The oil is also winterized to lower the pour point to be suitable for use in colder climates. Studies have shown that some of the properties of soybean oil are superior to that of MO [32].
2.2.2 Pongamia Pinnata Oil
Pongamia pinnata oil or commonly known
