Dielectric and Thermal Performance Up-Gradation of Transformer Oil Using Valuable Nano-Particles

Long-term transformer oil usage deteriorates the thermal and dielectric properties of the oil. The main aim of our work is to find a suitable replacement for mineral oil with superior characteristics. Titanium Oxide (TiO2), Zinc Oxide (ZnO), and Aluminum Oxide (Al2O3) Nano Particles (NPs) are used inside the virgin mineral oil to form Nano Fluids (NFs). NFs are subjected to accelerated multi-aging (thermal and electrical) for 1000 hours. The special aging chamber is designed to apply electrical and thermal stresses to the oil under consideration. Thermal and dielectric characteristics of the NFs are determined using breakdown voltage, water content, tan delta, flash point, viscosity, and pour point tests. The aforementioned tests are conducted before and after the accelerated aging of the samples. Thermal and dielectric characteristics of ZnO based NFs are found superior to other NPs. ZnO NPs show the highest breakdown voltage and minimum values of water content, tan delta, pour point, and viscosity. The highest value of flash point confirmed the superiority of ZnO NF over other NFs even after accelerated multi-aging. Therefore, the experimental analysis concluded that ZnO based NFs possess remarkably good thermal and dielectric properties as a replacement of mineral oil in the transformer operation.

SECTION I. Introduction

Future development of High Voltage (HV) systems and smart grid revolution demanded a reliable and efficient insulating material with volatile and dynamic operating conditions [1]. The transformer used in the electrical system transforms voltage, current, and transfer energy; therefore, a potential failure of the transformer is destructive [2]. Most of the transformers in the power system (currently in use) are closer to or beyond the classical structure. The available statistics of transformer failure reveals that the average transformer’s operation failed due to insulation breakdown in ‘17.8’years [3]. The ability of a transformer to conduct heat describes its thermal conductivity. Mineral oil used in the transformer performs the primary function of insulation and heat transfer. Thermal conductivity is an important parameter considered in improving the heat transfer characteristics of the transformer. There is a pressing need to work with the fluid with higher thermal conductivity for transformer oil. Degradation of insulating mineral oil results because of heat and electrical discharge. Insulation and cooling features of mineral oil are reduced due to accelerated aging. Moreover, the addition of impurities and water particles in mineral oil form oxides and acidic products due to continuous usage, thus, reducing the breakdown strength of the transformer oil.

Recently, Nano-Particles (NPs) are added in transformer oil to improve the breakdown strength, insulation level, and age of transformer oil. Nano-sized particles are immersed in mineral oil; the smaller size of NPs subsequently leads to the increased interfacial area and ensures a larger zone of interaction between mineral oil and the NPs. Transformer oil-immersed with NPs is referred to as Nano-Fluid (NF). NPs not only improve the dielectric strength of oil but also the heat transfer characteristics of the mineral oil. Suspension stability is also improved in the case of NPs due to increased surface area, compared to conventionally used micro-sized particles [3].

The authors in [2] examined the AC breakdown strength of different NPs and concluded that the NPs enhanced the properties of base oil and performed better even at elevated temperatures. In [3], the authors found that the addition of Aluminum Nitride NPs in transformer oil increased partial discharge inception voltage up to 20%, compared to virgin transformer oil. Results further depicted a 3-7% increase in the thermal conductivity of the transformer oil. Transformer oil was modified by adding semiconducting NPs. Lightning and AC breakdown tests were carried out before and after modifying the transformer oil. Results revealed that the AC breakdown strength of NF was almost 1.26 times the base oil breakdown strength, while lightning breakdown voltage increased by almost 24% to that of base oil [4]. The authors in [5] reviewed the breakdown properties of transformer oil with NPs and concluded that the addition of nanoparticles in transformer oil improved the breakdown strength of the oil to large extent, compared to the virgin transformer oil. In [6], the authors analyzed AC breakdown strength of TiO₂ NFs with different concentrations (0.003g/L to 0.05g/L) of NPs and concluded that optimum results were achieved when the concentration of 0.01 g/L was used. AC breakdown strength increased with an increase in concentration up to a specified optimum concentration after that a notable decrease was observed mainly due to the agglomeration of NPs inside the fluid.

The authors in [7] investigated the AC breakdown strength of Fe₂O₃ with different concentrations (5% to 80% volume concentrations) of NPs. AC breakdown strength increased with an increase in concentrations but optimum results were obtained at 40% concentration. In [8]–​[10], Titanium Oxide (TiO2) NPs were added in transformer oil, compared with some suitable surfactants. AC breakdown strength of the oil showed an increase of about 27% in comparison to the base oil. AC breakdown strength and viscosity tests were performed on transformer oil containing SiO₂ NPs. Results showed a significant improvement of about 79% and revealed that the mass concentrations were unable to impose any effect on viscosity. Breakdown strength depends largely on the moisture content and decreases significantly with an increase in water content. Hydrophilic surface of SiO₂ NPs binds water on the surface, leading to minimum water, hence increasing the breakdown strength [11].

Semi-conductive TiO₂ NPs were added to mineral oil to improve its insulation characteristics. The volume concentration of NPs used was 0.075% of the base oil and the average particle size was less than 20nm. AC, partial discharge and lightning impulse breakdown voltage characteristics were measured for the mineral oil before and after modification with NPs. Results showed that the AC and lightning breakdown strength improved by almost 1.2 times due to the addition of NPs. The partial discharge also showed a significant increase with the addition of NPs and the improvement in results was contributed by the electron trapping property of the NPs [12]. In [13], the authors explained the effect of NPs on aging and breakdown of vegetable oil that is considered an alternative insulation medium to transformer oil. The authors in [14] investigated the AC dielectric strength of different colloids with varying particle mass fraction. TiO₂ NPs modified by different organic reagents were suspended in the base oil via the ultrasonic route. The suspensions were made by mixing TiO₂, silicon oil-treated TiO₂, and octadecanoic acid-treated TiO₂ NPs with transformer oil. The concentration of NPs was kept in the range of 0.003 to 0.005 g/ml and resulted in improved BV at optimum composition. Three types of NPs, namely conductive (Fe₂O₃), semi-conductive (TiO₂), and non-conductive (Al₂O₃) NPs were used to improve the BV of mineral oil. Different concentrations of NPs were used to check the impact of volume on the mineral oil breakdown strength. Results also revealed that the BV increased by increasing the concentration of NPs up to a specific concentration, after that the breakdown voltage decreased further. This may be contributed to the agglomeration of particles due to increased volume [15].

Despite extensive use of NPs in transformer oil to improve thermal and dielectric properties, the researchers failed to present a complete model set of tests to be carried out on NFs and to use NFs as an alternative to existing mineral oil. AC dielectric strength of NFs was measured in the past but other thermal and dielectric properties were not considered. The loss factor is one of the key parameters to account for the health of mineral oil was also not considered in the available literature. Other thermal properties like flash point and pour point that describes the behavior of oil at very low and high temperatures were not checked along with the breakdown voltage. Increased water content is the reason for the low dielectric strength of the oil that lacks in the previous work. Aging of the NFs being an important test to be performed was missing in the previous work. Aging gives the life expectancy of the oil. Detailed thermal properties of the NFs were unavailable in existing literature. Thermal property is considered important in defining the thermal conductivity of oil for cooling inside the transformer. Thermal aging was performed on a few NFs, while multi-aging (electrical and thermal) was not incorporated on NP additives.

In the light of above-mentioned issues, the primary objectives of this research are listed as:

• The selection of suitable NPs is accomplished by comparing the thermal and dielectric properties of the NPs including permittivity and measurement of electrical conductivity of the oil.

• The loss factor, as well as water content, is measured for the oil samples. Thermal properties namely viscosity, flash point and pour point are also analyzed that provide information about the oil performance under very low and high temperatures.

• Multi-aging is performed by applying thermal and electrical stresses at the same time for a period of 1000 hours approximating the age of NFs.

The rest of our paper is organized as follows: Section II describes the preparation of NFs and the aging apparatus. The detailed aging process of NFs is provided in Section III, Section IV presents the performance validation of the tests carried out on the oil samples. Critical analysis with recent State-of-the-Art work is presented in Section V. Section VI presents conclusions and future work.

SECTION II. NFs Preparation

Transformer oil was modified by adding NPs. Generally, there are two methods to prepare the NFs. The efficient and most broadly employed method is the two-step method. In this method, NPs prepared (either by chemical or physical methods) are suspended in the oil. Ultrasonic route and magnetic stirring mixing methods are used to fully prepare the NFs. Magnetic stirrer uses a rotating magnetic field to make a homogenous mixture. Application of external magnetic field to magnetic stirrers assists in mixing the solution that facilitates the rotation of small magnetic bars placed in a mixture of interest. Therefore, the rapidly rotating magnetic field results in the rotation of the bar magnet that ends up stirring the liquid. During this method, NPs are directly mixed with the carrier fluid and then the process of ultra-sonication is employed to finally prepare the NFs. Ultra-sonication is a mechanical technology used for sludge disintegration. Sonication agitates particles in a solution using sound waves. Sonication converts an electrical signal into a physical vibration to break substances apart. These disruptions can (a) accelerate the dissolution of a solid into a liquid, (b) mix solutions, and (c) remove dissolved gases from liquids. Periodical compression and rarefaction incur as a result of ultrasound waves propagation through the medium. Microbubbles are formed due to ultrasound waves propagation in the medium. The microbubbles violently collapse within a few microseconds after reaching a critical size and induce the occurrence of cavitation. The violent collapse of microbubbles leads to extreme conditions.

Thermal characterization of the NPs is carried out by conducting a test at room temperature (273K), 343K, 353K, and 363K separately and for 5 times at each temperature with a time interval gap of 4 minutes. The selection of suitable NPs to enhance dielectric and thermal properties is a difficult process, as the basic properties like electrical permittivity and conductivity affect the dielectric performance of oil significantly. Considering electrical stress, the NP exhibits an electron scavenging mechanism converting fast electrons into slow negatively charged particles in the insulating fluid. Based on conductivity, relaxation time constant, and electron scavenging, the NPs are generally classified into three types, namely semi-conductive, conductive, and magnetic NPs. The mechanisms exerted by the NPs are listed in Table 1.

TABLE 1 Mechanism of NPs

Three samples each of Nickel Oxide, Titanium Oxide, Aluminum Oxide, and Zinc Oxide NPs were prepared by adding 0.05g/L in virgin transformer oil. The average size of the NPs used was about 20nm. These NFs along mineral oil were aged both thermally and electrically. The temperature used for thermal aging was 100°C and a voltage of 1kV was applied to age the samples electrically. Samples were aged for a time period of 2 months.

SECTION III. Aging Process

The NF samples along virgin mineral oil are kept inside the aging chamber for accelerated aging. The aging period of the samples is 2 months. Multi-aging is performed on all the oil samples. The multi-aging term means that both electrical and thermal stresses are applied to the oil samples. Thermal aging and electrical aging are performed on the samples to check the level of deterioration of oil properties. The aging chamber has the dimensions of $5\times 3\times 2$ cubic feet. Bus bars are placed inside the chamber; the upper bar is connected to high voltage live wire while the lower bar is connected with neutral to apply electrical stresses. Heating rods are hanged inside the chamber for providing thermal stresses as shown in Figure 1.

FIGURE 1.

Aging chamber.

Temperature sensors continuously monitor the temperature of the chamber and are shown on the display unit. Heating rods are turned ON as the temperature falls below the setpoint and are turned OFF automatically as the temperature reaches the set point. The voltage applied to the samples is 1kV while the temperature used is 100°C for a period of 7 weeks. As in reality, mineral oil in the transformer is subjected to high voltages and currents, so the electrical stresses are applied to provide these conditions. During the operation, the transformer gets heated due to the heating effect of current and other power losses, so to provide the condition of higher temperature, thermal stresses are applied [16].

Three samples of each NF and virgin transformer oil are aged inside the aging chamber. Accelerated aging on oil sample is carried out to predict the life of the oil. In the accelerated aging process, intense conditions of electrical and thermal stresses are applied, compared to the stresses that oil undergoes during service life. These stresses are applied continuously for a longer period of time on transformer oil to check whether the oil can withstand those stresses in actual service life or not.

SECTION IV. Performance Validation

AC breakdown strength is the foremost property that needs to be checked for a liquid used as mineral oil. Water content and loss factor (tan delta) tests are important in determining the dielectric properties of the NFs. Other tests that are necessary to perform for examining the thermal properties of the oil are flash point and pour point. To check the physical condition of the oil, a viscosity test is performed to check its suitability of usage as a cooling liquid inside the transformer. All the mentioned tests are carried out on the oil samples before the thermal and electrical aging of the samples. BV test is performed at the HV Lab of COMSATS University Abbottabad. Other mentioned tests are carried out at Rawat HV Laboratories, Islamabad.

Equipment used to carry out tests for checking thermal and dielectric properties is shown in Figure 2 to Figure 7. Figure 2 represents the flash point measurement equipment. Tan delta is measured via equipment shown in Figure 3. Figure 4 shows the viscosity measurement equipment. Pour point is measured by the equipment shown in Figure 5. Figure 6 shows the measurement equipment of water content. Dissipation Factor is measured by equipment shown in Figure 7. The aforementioned tests were carried out before and after the accelerated aging of oil samples analyzing the variations in thermal and dielectric properties. Results were compared to virgin mineral oil and the recommended level (IEC 60296) of values for transformer oil. The oil sample passes the required test if acquired values lie within the standard limits.

FIGURE 2.

Flash point measurement equipment.

FIGURE 3.

Tan delta measurement equipment.

FIGURE 4.

Viscosity measurement equipment.

FIGURE 5.

Pour point measurement equipment.

FIGURE 6.

Water content measurement Equipment.

FIGURE 7.

Dissipation factor measurement equipment.

A. Test Cases

The aforementioned tests were performed to ensure that the sample NF can be considered as the possible replacement of mineral oil. A complete set of tests were performed on a sample for it to be suitable for consideration if the values of the result lie within the limits prescribed by the standard. Improved results show that the sample NF can be used as transformer oil. Aging is also an important factor to be considered to predict the life of the NFs. All these tests are necessary to conduct on oil samples before and after the aging of samples.

1) Breakdown Voltage (BV)

AC breakdown refers to the value of AC voltage that initiates disruption discharge in the liquid. The BV test is carried out by exposing the insulating mineral oil to the electric field applied between two electrodes dipped in the oil. The breakdown voltage of a liquid is actually the measure of its ability to withstand voltage stresses [17]–​[19]. An oil testing cup weighing 0.5kg and having dimensions of $140\times 100\times 110$ mm is used for BV measurement. Two movable spherical brass electrodes are fitted inside the oil testing cup. The separation between electrodes is kept 2.5mm and the volume of oil used is 300ml to completely dip the electrodes inside the oil. An applied voltage across electrodes is increased at the rate of 2kV/sec. This test helps in insulation properties characterization of the NPs also known as the Partial Discharge Inception Voltage (PDIV) test. Table 2 provides BV test results performed before and after the aging of the NFs. Among all the NPs, breakdown voltage for ZnO is the highest with less standard deviation. ZnO does not allow the streamer to accumulate and spreads the electric field developed. There is a significant 34.6% increase in the breakdown, compared to the virgin transformer oil. After the aging of oil samples, the BV decreased due to the addition of contamination and formation of oxides inside the oil that resulted in the early breakdown of the oil sample. It is visible from Table 2 that after aging, the BV for ZnO is the highest among all the oil samples and still very much above the recommended level. Higher the breakdown voltage, better is the NF.

TABLE 2 Comparatively Analysis of BV

NPs when added in the oil improve breakdown strength considerably as depicted in Figure 8. BV for ZnO NPs is greater among all other NFs. This is attributed to the fact that ZnO does not allow the electric field to accumulate. ZnO NPs disperse the developed electric field resulting in higher breakdown voltage. Figure 8 shows that the breakdown voltage of aged oil samples decreased but is still within the recommended level. The decrease in the breakdown strength after aging is due to the addition of contaminants and moisture in the oil. ZnO based NFs resulted in the highest BV among all the oil samples. ZnO NPs being conductive have very short relaxation time as compared to the streamer development time scale in mineral oil. So, these NPs get briskly polarized after the application of the electric field, converting high mobility electrons into slow negative charge carriers. Growth of net space charge at the streamer tip is delayed due to the slow-moving charges restraining streamer propagation in the oil resulting in higher breakdown performance.

FIGURE 8.

Comparison of BV.

2) Water Content (WC)

WC in an insulator is the measure of water particles/droplets present inside a liquid. WC is an important property that affects the breakdown of a liquid. Breakdown strength is reduced due to the presence of water particles. As mineral oil is an insulating material, the presence of WC reduces breakdown strength [4]. Megger KF875 is used for the measurement of WC. Table 3 describes the trend of WC in different NFs and mineral oil. Moreover, values of WC before and after accelerated aging were compared. WC usually increases after acceleration due to the production of oxides, so an increasing trend was observed in WC values after the aging of oil samples. WC content for all the NFs was within the prescribed limit of the recommended level. Further, comparing the water contents of mineral oil and other NFs revealed that ZnO based NF resulted in the lowest water content. ZnO NF also showed a minimal increase in WC even after aging. ZnO being inert towards oxidation reaction produces a smaller number of water particles as depicted from the values of ZnO NF in Table 3.

TABLE 3 Comparative Study of Water Content

Table 3 concludes that the water content of the NF samples reduces due to the reason that NPs absorb moisture due to the large surface area. Moisture content in the case of ZnO was low, which is in compliment with breakdown values in Table 2. Figure 9 provides the graphical representation of the results showing that water content is less than the required value even after the aging of the samples. The water content increased after aging as contaminants and water particles were added due to accelerated aging.

FIGURE 9.

Comparison of water content.

3) Tan Delta

Tan delta also known as the dissipation factor is the measure of insulation level. Tan delta (TD) provides details about the loss of insulation after the accelerated aging of the oil samples. Megger Foster OTS 100F/2 providing 100kV testing voltage is employed to measure TD. Table 4 depicts that the values of loss factor are within the standard limits for every oil sample. Less the value of TD, better is the dielectric strength of oil, this is true for ZnO based NFs that provided minimum value for TD even after accelerated aging of the oil samples. This is mainly due to the fact that fewer oxides are formed and low WC after aging in ZnO accounts for the minimum loss in insulation property of the oil. Among other proposed NFs, ZnO provides the lowest loss factor.

TABLE 4 Tan Delta Comparison

The values of TD for all the oils are less than the recommended level as shown in Figure 10. TD also measures the loss of insulation level after oil aging. After aging, the values of TD increased due to the addition of contaminants and moisture by continuous usage and aging of the oil. The minimum value of TD for ZnO indicates the high insulation level and less deterioration of ZnO.

FIGURE 10.

Comparison of tan delta.

4) Viscosity

Viscosity is the property of the oil that gives information about the resistance to the flow of liquid and considers the laminar flow of the liquid. Mineral oil in the transformer must have minimum viscosity to flow easily. Mineral oil performs two basic functions in the transformer, namely insulation and cooling of the transformer. As cooling inside a transformer is achieved through the conventional flow of mineral oil, therefore oil should have very low viscosity to flow easily without any hindrance. If the mineral oil has low viscosity only then the function of cooling could be performed efficiently [23]. KV6 viscometer bath is used to measure viscosity. In Table 5, the viscosity measurement of various oil samples shows that ZnO provided the lowest viscosity among the oil samples with a minimum increase in viscosity after aging. This is mainly due to the large surface area of the NPs.

TABLE 5 Viscosity Comparison

The addition of NPs accounts for the increase in viscosity as shown in Figure 11. The viscosity of ZnO NPs was lower and within the recommended level before aging. The viscosity of the oils increased after aging due to the addition of contaminants in the oil. The viscosity of other oil samples was more than the recommended level while in the case of ZnO, viscosity was slightly higher than the recommended level.

FIGURE 11.

Comparison of viscosity.

5) Flash Point

Flash point is a temperature at which vapors of a liquid ignite. Flash occurs due to an increase in temperature inside the transformer tank. When the temperature rises above the prescribed limit, liquid inside the transformer tank can have flashover. Flash point is used to categorize a liquid as flammable or inflammable. Tanaka APM-7 Automated Pensky-Martens Closed Cup flash point tester measures the flash point. Inside the equipment, oil is heated using a gas or electric heater. Flash point temperature is measured after a few minutes and is shown on the VFD (Variable Frequency Drive) module. Table 6 shows that the value of the flash point was very much above the standard value for all the oil samples because these NPs have high thermal conductivity. Among all the NFs, ZnO possessed the highest flash point after the aging of the oil samples.

TABLE 6 Flash Point Comparison

Flash point, if sustained for a longer time, the oil catches fire and results in hazardous circumstances. Figure 12 depicts that the addition of NPs enhances the thermal properties of the oil. NPs have a small size and large surface area that helps in the conduction of heat. The flash point of the oil was higher than the recommended value. The value of flash points decreased after the aging of the oil but was still higher than the recommended value. ZnO has the highest value of flash point due to the fact that ZnO NPs have the highest thermal conductivity, contrarily, Al₂O₃ gave the lowest value of flash point among all the proposed NPs but still the value of flash point was higher than the recommended level even after accelerated aging of oil.

FIGURE 12.

Comparison of flash point.

6) Pour Point Temperature

The liquid loses its flow properties and becomes semi-solid at the lowest temperature termed as pour point. It is due to the presence of high paraffin content present in the liquid. Paraffin content is in higher quantity in crude oil or the oils extracted from it, similar to mineral oil used in the transformer. If the pour point of the liquid is low, it is not a good property for oil to have, as with a small decrease in temperature it turns into solid-state. Viscosity increases at low temperatures offering more resistance to flow reducing the cooling efficiency of the oil [10]. Table 7 concluded that the pour point of all oil samples was close to the standard. Although PP increases from the standard value after aging that is not a big concern as these temperatures are not attainable in most parts of the world. Except for the TiO₂ NF sample, every sample had a pour point nearly equal to the standard value.

TABLE 7 Pour Point Comparative Analysis

Pour point is the property to be considered for transformers operating in freezing temperature regions. Pour point needs to have a minimum value as possible. Figure 13 shows that oils with Nickel and Zinc oxide NPs provided minimum pour point due to the fact that they absorb heat as these NFs possess very good thermal properties.

FIGURE 13.

Comparison of pour point.

SECTION V. Critical Analysis With Recent State-of-the-Art Work

In Table 8, our work is compared with recent state-of-art works related to the addition of NPs in mineral oil and the change in properties observed due to NP addition. After the preparation of NFs, dielectric and thermal properties were checked and compared with the recent work. ZnO based NFs were examined by checking the BV, viscosity, and pour point of the NF, and performed aging by applying thermal stresses in [6]. Moreover, the need for other tests to be performed on NF samples arises. In [7], TiO₂ NPs were tested only for BV and WC after applying electrical stresses during the aging of the NFs, leaving room for more tests to perform to make these NFs viable for the replacement of mineral oil. In [10], the investigation was carried out on Fe₂O₃ and TiO₂ NPs. BV and the acidity tests were performed. This showed that a complete set of tests were not performed on the NFs, for which further tests are still needed to be performed. In the case of [16], SiO₂ NPs were added to mineral oil to observe any change in the dielectric and thermal properties of the NF. The primary test performed was the BV test to check the dielectric strength of the oil. To check the physical property of the oil, viscosity was measured along the pour point test to check the thermal property of the NF.

TABLE 8 Comparison of Our Work With Recent Research Papers

The authors in [20] used other types of NPs, NiO₂, to put forward as the possible replacement to mineral oil after forming NFs. The research still lacked with tests like WC and acidity and electrical stresses were missing during the aging of NF samples. In the case of [21], insulative NPs, Al₂O₃ was used to prepare NFs and was tested for a better number of tests, but still lacked the important tests like the loss factor, pour point, and WC. In [22], Al₂O₃ NPs were used along TiO₂ to prepare the NFs that were tested for the BV and tan delta to check dielectric properties. WC test was used to determine the insulation property whereas to check chemical properties, acidity test was performed. Moreover, electrical stresses were applied to perform the aging of the NF samples.

Our samples included three different NPs that were used in specific proportions to make NFs. Every test required for the NF to pass for capable of replacing mineral oil was performed except for the acidity test as provided in Table 8. The ‘✔’ shows the presence of the test concluded in the selected study, while ‘✘’ shows the absence of the test in the referred study.

SECTION VI. Conclusion and Future Work

In this paper, dielectric and thermal characteristics of conductive (ZnO), insulative (Al₂O₃), and semi conductive (TiO₂) were investigated after carrying out a set of experiments. The addition of NPs enhanced the dielectric and thermal properties of the oil. Among the three NFs, ZnO and TiO₂ exhibit superior characteristics, but conductive NPs have the drawback of agglomeration. In the case of semi-conductive NPs, there was a decrease in the distortion of modified mineral oil due to the introduction of shallow traps that trapped high mobility electrons. Hence, increasing the dielectric strength of the NF as a smaller number of the electron was left to contribute towards streamer development. ZnO NFs showed better dielectric and thermal properties, compared to other NFs and mineral oil. ZnO NPs showed higher breakdown strength than other NPs and virgin mineral oil due to its ability to disperse the streamer and avoid an early breakdown to occur. The inert behavior of ZnO NPs accounts for the production of fewer oxides, preventing ZnO from having higher WC and viscosity and keeping the dielectric characteristics intact. The minimum value of tan delta or loss factor in the case of ZnO based NFs after accelerated aging concluded that ZnO NFs can be used as an alternative to mineral oil due to their superior thermal and dielectric properties.

In the future, NP composites will be tested for dielectric and thermal characteristics to find an even better composition for the replacement in transformer oil.

ACKNOWLEDGMENT

The authors would like to thank COMSATS University Islamabad, Abbottabad Campus, for providing the HV Lab Facility during all experimentation work. They would like to thank Prof. A. Rashid for his comments, suggestions, and valuable remarks. Furthermore, they want to express our gratitude to the Rawat HV laboratory crew for their support in whole lab work.