The Fourier Transform Infrared Spectroscopy (FTIR) analysis is performed to identify the existence of the diverse functional groups in the fresh and aged oil samples. This technique aids in analyzing the uniformity of the oil after degradation in other properties because of temperature and duration. In this process, the infrared (IR) radiation is passed through the oil, out of which some percentage is absorbed and some percentage is transmitted. There are different values of frequencies which indicate different functional groups and thus the resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. When the IR radiation is absorbed, the molecules of the oil vibrate and it leads to bending or stretching of the bonds. It is observed from studies that the transmittance of the a few natural esters like FR3 and Jatropha (JAT) remain unaffected after aging for 2000 hours at 150 °C [73]. As seen in Figure 2.10a and b, the existence of alkanes is denoted by peaks near to 3000 cm−1 wavenumbers for C–H stretching. For both oils, the existence of esters is indicated by peaks near to 1740 cm−1 for C=O stretching. The CH2 bending is witnessed at wavenumbers around 1460 cm−1 for both the oil samples. The peaks at 1000–1260 cm−1 for both oil samples for C–O stretching indicate the presence of alcohol. Aging in oil leads to the formation of carbonaceous particles. Nonetheless, the integrity of the oil is intact as all the functional groups are present even after long hours of aging. There is no significant difference observed for a particular oil in various aging times considered. Thus, the FTIR study might provide evidence if the aging is performed for a longer duration. The chemical structure of the oils is not affected but the performance of the oil degrades after aging, as the other thermophysical and electrical properties deteriorate.
Figure 2.10 FTIR spectrum of (a) FR3 aged at 1000 and 2000 hours along with fresh FR3 and (b) Jatropha aged at 1000 and 2000 hours along with fresh JAT.
Source: Baruah et al. [73] / with permission of IEEE.
2.5.2 Nuclear Magnetic Resonance (NMR) Study
The alteration of the chemical assembly of the oil can be studied using the nuclear magnetic resonance (NMR) study. In the NMR spectrum, multiplets are observed for CH3, CH2, and allylic protons of the fatty acid fragments in the section of 0.87–2.78 ppm, for both the oil samples, as seen in Figure 2.11. In the range of 4.12–5.35 ppm, two multiplets are identified meant for the CH2 and CH protons of the glycerol moiety. The existence of methyl ester group is established at 2.3 ppm. The peak that represents the chloroform solvent for both the oils is observed at 7.28 ppm, and this is considered the reference peak [74]. The NMR spectroscopy shows if there are any changes in the structure of the samples over a long duration. However, no significant difference is observed for the oil spectra of NEOs after accelerated aging hours.
Figure 2.11 NMR analysis of (a) new FR3, (b) aged FR3 for 2000 hours, (c) new JAT, and (d) aged JAT for 2000 hours.
Source: Baruah et al. [73] / with permission of IEEE.
2.6 Dissolved Gas Analysis in Natural Esters
Occurrence of faults in the electrical network is very much likely, but proper monitoring can help avert any catastrophic incidence. The two main categories of faults arising in the transformers are thermal and electrical. These faults lead to breakdown of insulation, and release gases in the interior of the transformer, which are detrimental to the overall functioning of the apparatus. Thus, it is very much important to measure the health of the transformer at fixed durations. DGA is an indispensable method to assess the condition of a transformer to gauge the severity of the incipient faults. The key gases developing in the transformer as a result of fault and aging are: (i) hydrogen and hydrocarbons – hydrogen (H2), ethane (C2H6), methane (CH4), ethylene (C2H4), acetylene (C2H2), (ii) Carbon oxides – carbon monoxide (CO) and carbon dioxide (CO2), and (iii) propylene (C3H6) and propane (C3H8). The increase in temperature leads to the evolution of these gases and with progressing time, the concentration of these gases change. Detecting the individual concentration and applying the various standard methods can help identify the incipient faults that might occur inside the transformer.
The standard IEEE C57‐104 is dedicated to MO whereas the standard IEEE C57‐155 is about Interpretation of Gases Generated in Natural Ester and Synthetic Ester‐Immersed Transformers. As all the gas ratio techniques are elaborated in IEEE C57‐104, it is used for analysis. The Duval Triangle method is mentioned as per IEEE C57‐155 and it is used with regards to the stray gassing phenomenon in natural esters under the effect of the thermal stress.
2.6.1 Standard Gas Ratios
There are several methods to infer the DGA results of a transformer, with some of them listed below. The most important hydrocarbon gases are methane (CH4), ethane (C2H6), hydrogen (H2), ethylene (C2H4), and acetylene (C2H2). These gases are taken into consideration when analyzing the gas ratios in IEC, Rogers, Doernenburg, and Duval’s triangle methods. All these gas ratio methods indicate the types of faults likely to occur in a transformer after the oils are subjected to thermal or electrical stress. The five gas ratios according to standard are: Ratio 1 (R1) = CH4/H2, Ratio 2 (R2) = C2H2/C2H4, Ratio 3 (R3) = C2H2/CH4, Ratio 4 (R4) = C2H6/C2H2, and Ratio 5 (R5) = C2H4/C2H6.
2.6.1.1 IEC Gas Ratios
The IEC 60599 standard is one of the prevalent approaches for an elucidation of the faults occurring in a transformer, which is based on ratios of five key gases: CH4, H2, C2H4, C2H6, and C2H2. In this method, the ratios R1, R2, and R5 are measured to make the interpretation of the faults as per Tables 2.5 and 2.6. A grouping of the individual codes of R1, R2, and R5 indicates the type of incipient fault. However, this method does not give very accurate results for all fault types.
2.6.1.2 Doernenburg Ratio Method
This technique uses the four ratios R1, R2, R3, and R4 for diagnostics of faults in the transformer. In this method, it is primarily determined whether a fault exists in the transformer by examining the quantity of each gas associated to a minimum concentration limit L1 as given in Table 2.7. There is a fault in the transformer if any of the gases from H2, CH4, C2H2, and C2H4 exceeds double the recommended limit L1 and the quantity (ppm) of any one of the other two gases (C2H6 and CO) exceeds this limit L1. This procedure is reliable only if the quantity of at least one of the gases in each ratio exceeds the limit value, otherwise oil samples may be again collected for repeated analysis. If the ratio analysis is