Development of Ester Oil Based Nanofluid as Liquid Insulation for Power Transformers

Abstract

The conventional mineral oil is the most widely used insulating oil in transformers. The reason for its wide usage is its easy availability and good insulating and cooling properties. However, with these superior qualities, the usage of mineral oil poses some serious threats like the depletion of its resources, its non bio-degradibilty and high inflammability. Due to these issues, there has been a continuous increase in research for finding suitable alternative insulating fluids which can possess comparable insulating and cooling properties and can also eliminate the risk of threats posed by the usage of mineral oil. Vegetable oils are gaining popularity nowadays as alternatives to conventional mineral oil for transformers. Also known as natural ester oils, the dielectric properties of these oils are very much comparable to the mineral oil and in many cases better than the conventional mineral oil. For example, the dielectric strength of vegetable oils is generally better than the mineral oil. However, in a few qualities like total acidity and oxidation stability, these vegetable oils lack behind the conventional mineral oil. The latest trend in research these days is to further enhance these qualities of vegetable oils by dispersing suitable nanomaterials in them in order to enable the researchers to synthesize 3rd generation insulating fluids for transformers and to increase the transformer voltage ratings. In the field of nanomaterials, a recent in-trend nanomaterial, i.e., graphene oxide, has created a boom in research. Graphene oxide is constantly emerging as an attractive alternative to various other materials due to its low cost and large-scale production. Graphene oxide is currently becoming a basis for exploring innovative opportunities in the fields of emerging trends of research. While being extensively researched in the fields of medicine, chemistry and physics, the role of graphene oxide in insulation systems still remains unexplored. Moreover, till date, almost all of the research in the field of developing 3rd generation insulating fluids is based on dispersing conducting and /or semi-conducting nanomaterials in either mineral oil or in vegetable oil and to study the dielectric properties of synthesized nanofluids. The 3rd generation transformer oils refer to nanoparticles dispersed insulating fluids which have improved dielectric as well as cooling properties. However, for this research, a natural ester oil based blend having vegetable oil as a primary oil and mineral oil as a secondary oil has been optimized on the basis of its dielectric strength and the optimized blend has been chosen as the base fluid for dispersing the nanomaterials. The selected oil blend has been dispersed with three different types of nanomaterials on the basis of their conductivity – non-conductive graphene oxide (GO) nanoparticles, semi-conductive titanium di-oxide (TiO2) nanoparticles and conductive zinc oxide (ZnO) nanoparticles. The main focus is kept on the graphene oxide dispersed nanofluid and its dielectric as well as physio-thermal properties are extensively studied and compared with TiO2 and ZnO dispersed nanofluids. The effects of these nanofluids on transformer solid insulation before and after subjecting the liquid-solid insulation to extreme thermal stress conditions are also studied. The insulation design of transformers is analyzed when proposed to be filled with the synthesized nanofluids and the proposed changes in transformer design have been formulated. Results indicate that dispersion of nanomaterials results in an overall enhancement in the dielectric properties of nanofluids. The results have shown that with the addition of GO nanoparticles, the breakdown voltage of nanofluids increases up to 42% as compared to pure mineral oil, 17% as compared to pure ester oil and 15% as compared to the base oil-blend. The breakdown voltage of TiO2 dispersed nanofluid is reported to be about 30% greater than pure mineral oil, 24% greater than pure ester oil, and 17% greater than the oil-blend. Likewise, the breakdown voltage of ZnO dispersed nanofluid is observed to be around 28% greater than pure mineral oil, 19% greater than pure ester oil, and 16% greater than the oil-blend. The other dielectric properties like relative permittivity as well as the dissipation factor also improve significantly. In terms of physio-thermal properties, the viscosities of GO and TiO2 dispersed nanofluids show no significant change whereas the viscosity of ZnO dispersed nanofluid keeps on increasing with the increase in concentration level, thus making it a ‘bad’ coolant. Results also show that at peak values, the ac breakdown voltage of nanofluids decreases by just 2.6% after ageing when compared to the base oil for which the ac breakdown voltage decreases by 8.4% after ageing. Similarly, at peak values, the impulse breakdown voltage of nanofluid decreases by just 2.9% after ageing as compared to the base oil for which the impulse breakdown voltage decreases by 7.3%. The dispersion of GO also leads to the possibility of reduction in core size by 14.3% when compared to the base oil. Thus, the developed nanofluids can be used in transformers with successful operation.