where k = 1, 2, 3 as per the three gases mentioned above.
The quantity of cases with NEO‐filled transformers is less, so to assess the state of transformer with evolution of the various gases, creating the conditions replicating the inside of the transformer offers fruitful results. The different combinations of gases evolve depending on the temperatures created by the faults. PD is indicated by the formation of H2 and CH4 at low temperatures, whereas arcing faults are indicated by the formation of C2H2 at high temperatures. When the solid insulation deteriorates and reacts with the insulating oil, the gases like CO and CO2 are formed. A gas chromatograph (GC) is used to measure the extracted gases in the oil. Some studies show the results of DGA of two types of NEOs, namely FR3 and JAT, in Figure 2.12a and b [73]. The total gas content (TGC) is calculated using the formula:
(2.19)
The H2 production is not very significant in both oils, but it is higher in the case of FR3. The insignificant concentration of H2 gas even after 2000 hours of aging makes the JAT less flammable compared with FR3. A few works show that the higher concentration of C2H6 may be due to faults at low temperatures below 150 °C. C2H4, which is the chief indicator for high‐energy thermal faults, is comparable for both oil types. The presence of solid insulation on reaction with the oil produces CO and CO2 in significant amounts, which indicates cellulose degradation. This happens because paper starts degrading at temperatures above 105 °C and the aging atmosphere influences the deterioration of oil [73]. The gas levels are quite substantial in the case of jatropha oil which might indicate that solid insulation is better safeguarded by FR3. Also, in general, the concentrations of all the gases in this oil increased with the aging duration. The small variations in gas concentrations can be ascribed to the different origination of these oils. The Duval Triangle 3 (recommended for non‐mineral oils) is used for both NEOs, as formulated by Michel Duval. The two NEOs are based on the different compositions of saturated and unsaturated oleic and fatty acids, which result in different patterns of gas generation. In the Duval Triangle 3, CH4, C2H4, and C2H2 are selected as the sides 3. The fault gas data are replaced into it, and it is observed from Figure 2.12c, that all of the data points for both oil types fall in corresponding thermal faults region. In some circumstances, the data points lie in the boundary regions and it becomes challenging to identify the actual faults. Also, many times stray gases are produced at temperatures below 200 °C in the PD, T1, or T2 zones, and consequently may affect the correct detection of these faults [22]. For analysis purpose of low‐temperature faults, Duval Triangle 6 uses the low‐energy gases (H2, CH4, and C2H6) as represented in Figure 2.12d. It is seen that stray gassing occurs in FR3 for all aging durations, however, for JAT, overheating is observed.
2.7 Challenges in Using Natural Esters as Insulating Liquid
The challenges faced by natural esters to be used as transformer insulating liquids are mainly in the area of oxidation stability, viscosity, acidity, and pour points. They have higher pour points, lower oxidation stability, higher acidic nature, and high viscosity. Generally, higher viscosity results in lower heat transfer capability. The lower the heat dissipation, the higher is the hotspot temperature. However, some literatures have suggested that having a high specific heat capacity and high thermal conductivity upgrades natural esters to exhibit better cooling performance [68]. The higher hotspot temperature leads to faster degradation of paper and oil. The cost involved in processing the NEO is higher than that of MO; therefore, the final price of NEO is higher than the MO. However, the growing needs for an efficient and eco‐friendly insulating liquid for transformers have been on the rise, so the requirements for the natural esters are increasing. Additives are used for lowering pour points and increasing the oxidation stability; however, these additives lead to the nonuniform electric field in the liquid insulation. The adding of additives also leads to increase in viscosity. The production of natural esters from edible sources will also lead to concern regarding the food economy. The NEO must therefore be processed from the nonedible seeds to avoid the food crisis. The time required for growing nonedible vegetable seeds from the plants is a long process, whereas MO can be quickly extracted from the crude oil.
Figure 2.12 (a) DGA analysis of aged FR3 and JAT – ethane, ethylene, and hydrogen, (b) carbon monoxide and carbon dioxide, (c) Duval Triangle 3 for the aged FR3 and JAT oil samples, and (d) Duval Triangle 6 for the aged FR3 and JAT oil samples.
Source: Baruah et al. [73] / with permission of IEEE.
The chemical structure of natural esters and MOs are not the same, but they comprise analogous bonds, like C–H bonds, so the fault gasses are similar for both the fluids. DGA helps in analyzing the incipient faults in MO, which too can be used for fault detection in natural esters, under thermal and electrical fault conditions. The formation of fault gases takes place when the chemical bonds are broken. The formation of gases in natural esters is greater during PD and lower in arcing, when compared with MO. As the standard for gas formation is mainly concentrated toward MO, it becomes essential to critically analyze the gases generated in natural esters. The key fault gases are significant for interpretation of faults in the transformer. Thus, it is critical to recognize the key gases of natural esters under diverse types of faults so that accurate identification
