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The possibility of using coconut oil in a distribution transformer has been recently reviewed based on the literature on coconut oil. From that, it seems that much of the data available regarding coconut oil for this purpose is a result of some preliminary studies only. It has been observed that it is hard to make a reliable conclusion whether the coconut oil completely fulfils the breakdown strength recommended by ASTM D6871/IEC 62770 for new insulating oil. Two reasons were attributed to this. First, the AC breakdown voltages were obtained using different test techniques, and the second is the quality of coconut oil, i.e., moisture content or solid particles, which were not entirely known. It is, thus, essential to assess the dielectric breakdown performance of coconut oil by testing it with other standard natural esters using the same test environments and methods.
This research has been featured in the ‘Electric Power Systems Research,’ an international journal dedicated to presenting original papers focused on the generation, transmission, distribution, and utilization of electrical energy. The journal is specifically designed to showcase significant outcomes in this field, encompassing applied research, the development of novel procedures or components, the original application of existing knowledge, or innovative design approaches.
Until now, some specific oils extracted from natural oil seeds could be processed efficiently and commercially established as an alternative insulating fluid, such as BIOTEMP, FR3, and MIDEL eN 1204, to name a few. For instance, soybean seeds’ vegetable oil typically contains about 18% oleic acid, 55% linoleic acid, and about 13% linolenic acid. It is found that the higher the oleic acid content and the lower the linolenic acid content, the better. Through hydrogenation, the oleic acid content can be raised to 40%, while the linolenic content can be reduced to 3%. Overall, the electrical insulating fluid derived from soybean comprises soybean oil up to 98.5% by weight of the fluid. Therefore, such fluids contain about 90% of the fatty acids which are unsaturated (like triolein), and the remaining 10% are saturated fatty acids, which is precisely the reverse in the case of coconut oil, where 90% of the oil has saturated fatty acids which comprise trimyristin, trilaurin, tripalmitin, and tristearin.
Although in the recent past, ester fluids have been applied to power-level transformers, they are (especially natural esters) mostly limited to distribution-level transformers where these fluids are exposed to AC voltage stress at 50/60 Hz. Therefore, extensive studies have been conducted to characterize the breakdown properties of ester fluids subjected to AC electric field stress at power frequency. However, recent advancements in HVDC technologies, and greater demand to use eco-friendly fluids in pulsed electric field equipment, require that a complete understanding of the breakdown mechanism of ester fluids be gained, not only under AC field stress but also when exposed to high voltage lightning impulses. Impulse breakdown testing is thus required to fully understand the electrical breakdown property of an insulating oil subjected to those transient voltage conditions; however, the impulse test results strongly depend on several factors, such as electrode configuration and surface condition, the level of excitation potential and its polarity, and rate of rise of potential excitation. Therefore, the straight comparison between results available in the literature is not always feasible.
Previously, equivalent lightning impulse breakdown (LI BDV) performance had been revealed between mineral oil and natural ester when tested with electrodes satisfying industry design criteria. Additionally, by insertion of pressboard spacers between the conductors, it was observed that although average impulse breakdown strength lowers, the creep strength of pressboard spacers remained equivalent in mineral oil and natural esters. It was found that although the LI BDVs for natural (FR3 fluid) and synthetic ester (MIDEL 7131) are identical, regardless of the polarity of excitation; the LI BDVs of ester fluids were about 26% inferior to mineral oil with negative excitation whereas there is no difference in the LI BDVs between ester fluids and mineral oil with positive excitation.
The impulse breakdown voltage of coconut oil is shown to be almost similar to mineral oil under a quasi-uniform electric field at a short gap of ≤ 3.8 mm. However, as far as the impulse breakdown voltage of coconut oil in a strong divergent field is concerned, this information is still lacking. It becomes essential then to verify whether the coconut oil subjected to impulse voltage in a divergent field also displays similar performance characteristics to the other natural esters developed for transformers. With these facts into consideration, various sets of data were obtained on the lightning impulse breakdown voltage of coconut oil and standard natural ester at both polarities of the point electrode.
Measurement results presented in this work are limited to dielectric breakdown subjected to 50 Hz AC applied voltage and lightning impulse voltage wave. The effect of thermal aging (at 130°C) of coconut oil on its dielectric breakdown has also been tested and analysed. While these tests are by no means exhaustive, the measurement results discussed in this work clearly helped in analysing whether coconut oil can withstand thermal aging and how it compares with natural commercial ester available for transformers.
It is concluded that the dielectric performance (AC or impulse) of RBD grade coconut oil was affected the least, whether it was the increase in oil moisture or aging time. The fundamental reason why coconut oil seems to have a superior dielectric breakdown performance as compared to MIDEL eN1215 is due to the molecular diversity of the two types of natural esters. The fact that the concentration of low ionization potential triacylglycerol molecules in unsaturated natural ester (i.e., MIDEL eN1215) is always greater than those in saturated natural ester (like coconut oil) may have caused the differences in their dielectric breakdown performance, which is closely linked to the molecular structure in natural esters.