Performance of Hybrid Nanofluids of Mineral Oil and Vegetable oil based Ester

The prosperity and economic development of a nation depend on its capability to maintain reliable and high-quality electric power management. It is achieved through the availability of extremely reliable ancillary electrical equipment. A power transformer is one such piece of crucial equipment that transfers electricity from one circuit to another without altering the frequency. Mineral oil has been conventionally used as a liquid dielectric in transformers since the last century. However, in recent days, vegetable oil-based esters have emerged as a favourable alternative to mineral oils due to their biodegradability and sustainability. The introduction of nanotechnology led to massive studies to enhance the quality of liquid dielectrics. Further, the addition of more than one nanoparticle to the base fluid has broadened the scope of nanofluids’ property enhancement. This study focused on the assessment of the dielectric properties of hybrid nanofluids consisting of ZnO and reduced graphene. Mineral oil and vegetable-based transformer oil were chosen as base fluids to prepare the nanofluids. The mass of the added nanoparticles was 0.01%. The ratio between the nanoparticles was selected as 1:1. The hybrid nanofluids were characterized for breakdown voltage viscosity at temperatures ranging from 40 °C to 100 °C, interfacial tension, tan delta, and resistivity. The overall properties of hybrid nanofluids were found to be improved due to the inclusion of ZnO and reduced graphene.


Introduction
It is essential to improve the performance of power transformers for the upgrade and advancement of electrical power systems.One of the most common varieties of power transformers is the oil-filled transformer.Transformer oil serves as both an electrical insulator and a thermal coolant.Therefore, enhancing the power transformer's thermal and dielectric qualities has a positive impact on its overall performance.Mineral oils (MO), being inexpensive, having a high thermal capacity, and having a good pour point at low temperatures, have been used in power transformers for a long time.MOs are less sustainable for extended periods due to a number of drawbacks.For instance, these materials are derived from a non-renewable source, are low-eco-friendly and have a significant fire hazard [1].It is reported that the biodegradability of MO is less than 30 % [2].A significant increase in the demand for petroleum products is also a result of society's increased industry and motorization.The aforementioned issues have drawn more attention to the need to find alternatives that can be prepared from resources that are abundant in nature and may provide the best long-term chances.1300 (2024) 012031 IOP Publishing doi:10.1088/1757-899X/1300/1/012031 2 Vegetable oil esters (VO) are regarded as a good replacement for mineral oil.These insulating liquids are derived from both seeds and flowers.VOs are the subject of several studies in an effort to use them as insulating oils in transformers and create eco-friendly environments [3].VOs are thought to be more nature-friendly liquids due to their high biodegradability (>95 %), high flash and fire points (both >300 °C), and lesser flammability [4].In addition, compared to MOs, these VOs absorb more moisture [5].The viscosity of MO is lower than that of VO.Moreover, the VOs show lower oxidation stability.Therefore, blending VO and MO is another area of research interest [6].
Numerous researchers have focused on the enhancement of the properties of liquid dielectrics in light of the technological qualities outlined above and ecological protection considerations [7].Nanofluid (NF) is a colloidal liquid containing suspended nano-scale particles in a base liquid such as water, ethylene glycol, or oil [8].Recent literature makes it clear that many scientists throughout the world have been interested in the addition of nanoscale particles to VOs.Some of the nanoparticles, according to reports and consensus, are able to change the characteristics of VOs to improve their performance in transformers.Therefore, the significance of nanoparticles in modifying the characteristics of VOs has received particular attention.It should be noted that numerous assessment reports are available in the literature focused on the use of nanoparticles to enhance the characteristics of insulating oil.
The nanoparticles are classified depending on their chemical, physical, and electrical properties, such as metal oxide, nitrides,carbides, non-conducting, semiconducting, conducting, spherical, tube, layer, etc. [9].Even though there has been extensive study on single-particle-based NFs, enhancing their thermophysical characteristics and stability remains a significant problem.Utilising the hybridization process to create hybrid nanofluids is one way to get around this constraint.
Hybrid NFs are prepared by suspending two or more different types of nanoparticles in a base fluid.Hybrid NFs based on water or ethylene glycol are the most commonly reported [10].However, much attention has not been given to studies of VO-based hybrid NFs.A large number of metal oxides, such as Al2O3, SiO2, TiO2, Fe2O3, and CuO are investigated for their performance in VOs as compared to ZnO.Reduced graphene-based bio-nanofluids appear to be one of the potential liquid dielectrics.Therefore, adequate research is needed for the development of this material [11][12].A lower viscosity is preferred for a good liquid insulator.It is reported that the addition of NPs resulted in escalating viscosity as compared to the base fluid [1].So, the effect of the mixing of nanoparticles with an ester on its viscosity property is required for detailed studies.
Therefore, it is proposed to include blended VO and MO in this study.This study focused on the physicochemical and electrical characterization of ZnO-reduced graphene-based blended NFs of VO and MO.The mass of the added nanoparticles was 0.01%.The ratio between the nanoparticles was selected as 1:1.The performance of hybrid NFs was assessed by estimating their breakdown voltage, viscosity, interfacial tension, tan delta, and volume resistivity.

Results Materials and methods
The base fluids selected for the study were mineral-oil based transformer oil (MO) and vegetable oilbased ester oil (VO).The VO used in this work was obtained from M/s. Savita Oil Technology, India.This VO is commercially known as Biotransol-HF, and it conforms to IEC 62770.Table 1 describes the characteristics of the base fluids VO and MO.The nanoparticles picked for the work were ZnO of size less than 50 nm and reduced graphene (RG).The mass of the added nanoparticles was 0.01%.The ratio between the nanoparticles was selected as 1:1.The details of the composition of the samples prepared for the work are shown in table 2. Nanofluids for this study were prepared following a two-step method.The required amount of nanoparticles was added to the base fluids.The first step for the proper dispersion of the mano particles was performed through magnetic stirring of the composite fluid for 2 hours.The second step of the process of further homogenization was conducted using an ultrasonic bath for 1 hour.The prepared samples were kept in an air oven for 48 hours for drying.The hybrid NFs were characterized for breakdown voltage viscosity at temperatures ranging from 40°C to 100°C, interfacial tension, tan delta, and resistivity.

Results and discussions
The inner machinery of a transformer becomes hot during its operation as a result of energy losses in the form of hysteresis loss, eddy current loss, and copper loss.The insulating oil acts as a coolant to dissipate the excess heat.The viscosity of oil indicates its resistance to flow.A dielectric fluid with low viscosity is a preferred material as it provides higher heat dissipation through an enhanced rate of circulation.Therefore, assessing liquid insulators for their viscosity is essential.Figure 1 shows the viscosity of the samples measured at various temperatures, such as 40 °C, 60 °C, 80 °C, and 100 °C.It is observed from figure 1 that at 40 °C, the reduction in viscosity was significant in VO-ZnO nanofluids, with a decrease of about 4*E-6 m 2 /s.The decrease in viscosity for VO-RG nanofluid and VO-ZnO-RG hybrid nanofluid was about 2*E-6 m 2 /s.However, there was an insignificant effect on MO-based nanofluids and hybrid nanofluids.The viscosity of VO and its nanofluids appeared to have higher values as compared to MO and its nanofluids.Among the VO-based nanofluids, the ZnO-RG hybrid nanofluid showed a lower viscosity up to about 75 °C.The viscosity of blended and hybrid nanofluids (S9) was in between the values of VO-based and MO-based nanofluids, indicating the possibility of the usefulness of this nanofluid.
The specific resistance is the difference in DC resistance between two sides of an oil block that is one cm 3 in size.A substance with higher resistance has better insulation properties.The nanoparticles that are capable of tapping larger numbers of electrons are better for the improvement of the nanofluid.Figure 2(a) shows the specific resistance (SR) of VO and its nanofluids, whereas figure 2(b) shows the SR for MO and its nanofluids at 90℃.The resistivity of MO and its nanofluids was higher as compared to VO and its nanofluids.It has been observed that the resistivity of VO-RG was much better than that of the other VO-based nanofluids, though it was still higher in the base fluid.On the other hand, a significant improvement in SR was seen in MO-based nanofluids.The SR value was almost double in MO-RG (S8) nanofluid as compared to MO.The dielectric dissipation factor (DDF) and dielectric constant of the specimen are shown in table 3. DDF measures the dielectric losses in insulating materials.A good liquid insulator should have low dielectric losses.The DDF of a sample increases due to the presence of polar materials.An insulating liquid with a DDF of <0.1 is preferred for use in power transformers.The DDF values of VO-based nanofluids did not exhibit any significant improvement.However, a noticeable reduction in DDF was seen in MO-ZnO nanofluid.This observation indicates that MO-ZnO nanofluid is capable of controlling dielectric losses.The capacity of a material to transport electrical potential energy is determined by its dielectric constant.A dielectric substance is one that, although having poor conductivity, can store a charge when an electric field is applied.The dielectric constant is defined by the ratio of the capacitance formed by two plates with a material kept between them to the capacitance of the same plates placed in a vacuum.As the additives themselves may have different dielectric constants than oil molecules, adding additives should influence the resulting dielectric constant of the oil.The dielectric constant value of VO was reduced from 3.039 to 2.895 and 2.971 by the addition of 0.01 % ZnO and reduced graphene, respectively, indicating no significant effect of nanoparticle addition in VO.A similar inference is drawn for MO-based nanofluids.

Conclusions
This study focused on the characterization of ZnO and RG-based blended NFs of MO and VO.The mass of the added nanoparticles was 0.01%.The ratio between the nanoparticles was selected as 1:1.The hybrid nanofluids were characterized for breakdown voltage viscosity at temperatures ranging from 40 °C to 100 °C, tan delta, and resistivity.The findings of the investigation are: A reduction in viscosity was significant in VO-ZnO nanofluids, with a decrease of about 4 cSt at 40 o C. The decrease in viscosity for VO-RG nanofluid and VO-ZnO-RG hybrid nanofluid was about 2 cSt.Among the VO-based nanofluids, the ZnO-RG hybrid nanofluid showed a lower viscosity up to about 75℃.The viscosity of blended and hybrid nanofluis (S9) was in between the values of VO-based and MO-based nanofluids, indicating the possibility of the usefulness of this nanofluid.
The resistivity of VO-RG was much better than that of the other VO-based nanofluids, though it was still higher in the base fluid.A substantial improvement in SR was seen in MO-based nanofluids.The SR value was almost double in MO-RG (S8) nanofluid as compared to MO. MO-ZnO nanofluid exhibited a noticeable reduction in DDF.This observation indicates that MO-ZnO nanofluid is capable of controlling dielectric losses.The Weibull probability distribution for the dielectric breakdown strength showed a major improvement in shape values for hybrid nanofluids.

Figure 1 .Figure 2 (
Figure 1.Viscosity of the samples at the temperature range from 40℃ to 100℃.

Figure 2 (
Figure 2(b).Specific resistance (SR) of MO and its nanofluids at 90 o C.

Figures 3 (
Figures 3(a), 3(b), and 3(c) depict the graphical presentation of the Weibull probability distribution for the dielectric breakdown strength the oil samples.The shape and scale values of the Weibull probability distribution for BDV of oil samples are shown in table 4. The shape values of the plot show the linear fit features' slope.The scale values demonstrate the degree of breakdown.Figure 3(a) describes the Weibull probability distribution curve of the base fluids VO and MO.The shape and scale values of VO were 4.71578 and 62.13511, whereas for MO, they were 6.89011 and 56.3964, respectively.The Weibull probability distribution curves of the ZnO and reduced graphene-based nanofluids VO and MO are shown in figure 3(b).Figure 3(c) illustrates the Weibull probability distribution curve of the hybrid nanofluids VO and MO.There was a significant improvement in shape values for hybrid nanofluids.
Figures 3(a), 3(b), and 3(c) depict the graphical presentation of the Weibull probability distribution for the dielectric breakdown strength the oil samples.The shape and scale values of the Weibull probability distribution for BDV of oil samples are shown in table 4. The shape values of the plot show the linear fit features' slope.The scale values demonstrate the degree of breakdown.Figure 3(a) describes the Weibull probability distribution curve of the base fluids VO and MO.The shape and scale values of VO were 4.71578 and 62.13511, whereas for MO, they were 6.89011 and 56.3964, respectively.The Weibull probability distribution curves of the ZnO and reduced graphene-based nanofluids VO and MO are shown in figure 3(b).Figure 3(c) illustrates the Weibull probability distribution curve of the hybrid nanofluids VO and MO.There was a significant improvement in shape values for hybrid nanofluids.

Figure 3 (Figure 3 (
Figure 3(a).Weibull probability distribution curve of the base fluids VO and MO.

Table 3 .
Dielectric dissipation factor and dielectric constant of oil samples at 90℃.

Table 4 .
Dielectric Shape and scale values of Weibull probability distribution for ACBDV of oils.