Experimental Investigation of Insulating Properties of Vegetable Oil under High Voltage

Due to concerns over the world’s energy necessity and environmental impact of mineral oil, these conditions induce many researchers to search for substitute sources for insulating oil. Alternatives insulating oil with biodegradable characteristics, environment friendly and presented in different countries including Bangladesh such as vegetable oils have been proposed for high voltage applications. In this paper, a relative measurement of breakdown voltage through experimental investigation of coconut, mustard, soybean, and palm oil and their blend (which is available in Bangladesh and cost effective) is presented. Break down voltage was measure with different electrode configuration by changing gap distance. The results show that the blend of (50% coconut oil + 50% palm oil) got high breakdown voltage in mushroom-mushroom electrode, and other side in plane-plane type pure soybean oil got high breakdown voltage, compared with transformer oil. The presented result illustrate that the proposed mixed oil provides better performance than the rise husk oil.


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Abstract-Due to concerns over the world's energy necessity and environmental impact of mineral oil, these conditions induce many researchers to search for substitute sources for insulating oil.Alternatives insulating oil with biodegradable characteristics, environment friendly and presented in different countries including Bangladesh such as vegetable oils have been proposed for high voltage applications.In this paper, a relative measurement of breakdown voltage through experimental investigation of coconut, mustard, soybean, and palm oil and their blend (which is available in Bangladesh and cost effective) is presented.Break down voltage was measure with different electrode configuration by changing gap distance.The results show that the blend of (50% coconut oil + 50% palm oil) got high breakdown voltage in mushroommushroom electrode, and other side in plane-plane type pure soybean oil got high breakdown voltage, compared with transformer oil.The presented result illustrate that the proposed mixed oil provides better performance than the rise husk oil.

Index Terms-Breakdown Voltage; Electric Field Strength;
Electrode Configuration; Vegetable Oil; Viscosity; Synthetic Oil; Transformer Oil.

I. INTRODUCTION
The transformer plays an important role in providing a reliable and efficient electricity supply and is one of the most critical apparatus's in electric power transmission and distribution systems [1].The maximum of high voltage transformers is filled with mineral oils or transformer oil.Oil works as an electrical insulation as well as a heat transfer medium.Transformer mineral oil has been substituted by alternative oils such as silicon oil or esters (natural ester or synthetic ester) ester oil have high flash point, high relative permittivity than silicon oil.Additionally, ester oil got abundant resource.Ester oil is highly biodegradable, renewable and environment friendly nature over mineral oil.
The ester oil is excellent in the surface break down characteristics, and has high flash point over mineral oil [2].In contrast, silicon oil was excellent in the point disaster prevention.
The statistical analysis of the AC breakdown strength of synthesizes ester in comparison with crude palm kernel oil sample is discussed in [3].The breakdown test was carried out in accordance with ASTM 1816 test method using bespoke cell.Result displays that, mean AC breakdown Published on January 9, 2019 S. Islam, M. S. Hossain, M. F. Reza and M. M. Rashid are with the Electrical & Electronic Engineering Department, Rajshahi University of Engineering & Technology, Bangladesh (e-mail: islam.sanjida@yahoo.com,engg.selim@gmail.com,farhamdur@gmail.com,mrashid.eee@gmail.com).field of ester oil was higher than mineral oil and observe that eater oil got low viscosity and high oxidative stability, they summarized that; the synthesized ester may serve as an alternative to mineral oil as a transformer fluid.
The pre-breakdown and breakdown properties at high voltage and large gaps (10cm) in several natural and synthetic esters is presented in [4], and a comparison is made with the well-known mineral transformer oil.They concluded that, the properties of ester were less favorable for high voltage insulation than in mineral oil.The electrical and chemical properties of coconut oil as an alternative transformer oil due to the environment consideration is analyzed in [5] and have shown that pure coconut oil provide well concert in chemical properties.
Dielectric properties of palm oil under temperature variation is discussed in [6] and compared the performance with mineral oil and silicone oil.Breakdown voltage measurement was performed in accordance with IEC 156 standard, whereas, the dissipation factor and dielectric constant measurement were conducted based on IEC 60247 standard test methods.They summarized Breakdown voltages and dissipation factors of all tested oils were increased, while their dielectric constants were slightly decreased with the increase of temperature.Breakdown voltage of the insulating oil examined with the effect of the voltage types, different electrode formations, and the impurities on oil [7].Experimental result shows that using the t-test, the significant difference of the measurements between the pure oil and oil with impurities can be observed.The non-uniform electric field was developed in the point-plate electrode hence; the dielectric strength of the oil was low at another electrode formation.The curvature of the rod electrode mitigated the electric field and hence, the breakdown voltage enhanced rather than in plate-plate and point-plate electrodes.
The breakdown voltages of transformer oil and oilimpregnated pressboard under alternating current (AC) and direct current (DC) voltages, using three types of electrodes and the breakdown voltages were measured in both no bubbles and bubbles [8].The results showed the bubbles have the minimum effect on the non-uniform electric field.For AC, the cone-plan electrodes have the largest breakdown voltages in the oil without bubbles.In DC, the influence of bubble decrease of the breakdown voltages of the cone-plan and sphere-plan electrode.The breakdown voltage of rice-husk oil using different electrode and varying gap distance and concluded that pure rice-husk oil can use in low voltage application as a dielectric [9].However, it is not effective and it cannot be used for high voltage applications. To

II. EXPERIMENTAL SETUP
In order to measure breakdown voltage, high voltage oil tester set, model OTS-E series (semi-automatic) are used which is available in RUET HV Laboratory as shown in Fig. 1, the oil tester was equipped with adjustable 36mm diameter mushroom electrodes.An Ac Voltage was applied using a 60KV step up transformer for the breakdown test.The maximum output voltage capacity of 60KV was possible for this instrument.Breakdown voltage was determined by applying 50Hz AC voltage to the electrodes.The dimension of oil test cup was approximately 15 cm length, 10 cm in width and 12cm in height, the cell was designed to take small sample volumes with a gap distance of 2.5mm, 5mm, 7.5mm and 10mm as shown in Fig. 2.During each experiment the applied voltage was increased manually from zero at a rate of approximately .4KV/sec.until the breakdown occurred.A transformer control until (TSU) monitored the cell current and interrupted the supply voltage to the step-up transformer when breakdown occurred in the sample test cell.The different types of electrodes are used to find out the suitable configuration in the real time implementation.

A. Mushroom Electrode
The mushroom electrode which is used in this work, this electrode is fitted in oil tester; it is specially manufactured for measuring breakdown voltage in high voltage operations.It is made by company.Fig. 3 and 4 show the mushroom electrode.

B. Plane electrode
The plane electrode which was used in this project, this electrode is made by ordinary manual process.These electrodes are manufactured by normal steeliness steel shit, and then electroplated the electrode.There are three types of plane electrode are used in this project.The length, diameter and width are the same of three electrodes just shape is different.Three types of electrode are shown below.For each of the vegetable oil samples four breakdown measurements were carried out by varying gap distance (2.5mm, 5mm, 7.5mm and 10mm) at room temperature of 32º C. the samples were purchased from the local market and were used without processing and filtration allowing a pause of at least 2 min after each breakdown before reapplication of voltage or until there is no gas bubbles present within the electrode gap.For each sample, breakdown voltages were measured.At the same time video recording was performed to examine breakdown phenomena, after finishing each experiment and the clash was disassembled cleaned and dried at room temperature.

A. Breakdown Voltage Measurement
The results of breakdown voltage (BDV) measurements for transformer oil and several oils are listed in Table 5-1.Breakdown voltage measurement was performed in such a way that seven samples were used and breakdown tests were conducted on each sample with four varying gap distance and unfixed four types of electrode pairs.Thus, there were 16 breakdown voltages obtained for each sample.The results of each test listed in Table I.
A graphic presentation of the breakdown voltage comparison between mineral oil and vegetable oil along the varying gap distance and different electrode is described in Fig. 11 and Fig. 12. From Fig. 11, it is observed that the breakdown voltage is increasing with the increasing gap distance between the mushroom-mushroom electrodes, all oil samples provide maximum breakdown voltage with mushroom-mushroom electrodes.It is seen that, blend 50%coconut oil with 50% palm oil breakdown voltage(58KV) is high at 7.5 mm gap distance, however at 10mm gap distance breakdown is not observed cause the maximum output voltage 60KV Fig. 12 that soybean oil got high breakdown voltage at 10 mm gap distance between plane-plane type1.14 also shows that blend 50%coconut oil with 50% palm oil breakdown voltage is higher than other oil, even compared with transformer oil.Mushroommushroom electrodes show highest value than plane-plane electrodes reason of mushroom electrodes is manufactured by company for high voltage application; on the other hand, side plane-plane electrodes are prepared manually.From the above discussions, its low voltage application (less than 11KV) to use as dielectric fluid.Blend 50% Coconut oil with 50% palm oil could be considered as a probable dielectric.itseen that among all pure oil sample, coconut oil has a good dielectric property.

B. Electric Field Strength Estimation
Electric field strength is defined as the electro statistic force 'F' per unit positive test charge 'q' placed at a point 'P' in a dielectric.It is denoted by 'E' and expressed unit "Newton's per coulomb" that is the force per unit charge.The electric field strength is often more specifically mentioned as "electric stress" when experienced by a dielectric or an insulating material.
The maximum magnitude of electric field strength is given by the maximum value of the rate of charge of potential with distance [10].
The qualitative definition of electric field strength of a dielectric is the maximum electric stress a dielectric can withstand.
The results of electric field strength for transformer oil and vegetable oils are listed in Table II.From Fig. 15, it is seen that, electric field strength is more in all sample oil for smaller gap distance(2.5mm) and fall in 5mm gap distance, however after 5mm gap distance the all oil electric field strength is rise smoothly except mustard oil, its electric field strength is decreased.From Fig. 16, all oil electric field strength is falling after 2.5mm gap distance between electrodes, other than soybean oil field strength is rising.And it shows high field strength than other oil.

C. Viscosity
Viscosity is a measure of the fluid's resistance, generally viscosity decreases as the temperature is increased.Viscosity is a measure of particle bond strength.
Table III illustrates the viscosity comparison of different vegetable oil.One can observe that the Palm oil has high viscosity.This work is focused to find out alternative dielectric fluid of availably and low cost in Bangladesh.For that reason, four available oil and their blend was investigated.Other hand another obtainable oil is rice-husk oil, which is addressed in [9].Rice-husk oil has high viscosity (80 at 30֯ ) than blend 50% Coconut oil+50% palm oil (51 at 30֯ ), and breakdown voltage of rice-husk oil is 11kVA at 2.5mm gap distance where is 50%Coconut oil+50% palm oils breakdown voltage was 16 at 2.5 mm gap distance, which is shown in Fig. 19.In this work, vegetable oils use as substitutes of the transformer oil, the submission of mineral oil in transformer also brings a problem when there is an outflow during operation and the oil become a pollutant to the environment due to its less biodegradable characteristic.On the other hand, Vegetable oil has been recognized as good quality substitute material for transformer oil due to its high-quality biodegradability characteristic, low pour point, high flash point, and high solubility.The experimental results show that the proposed mined oil has better breakdown strength that the existing rice bask oil and other vegetable oil.

Fig. 11 .
Fig. 11.Comparison of breakdown voltage for all samples across mushroom-mushroom electrodes.

Fig. 15 .
Fig. 15.Comparison of Electric field strength between mushroommushroom electrodes for all samples

Fig. 17 .
Fig. 17.Comparison of Electric field strength between plane type2-plane type2 electrodes for all samples.

Fig. 18 .
Fig. 18.Comparison of Electric field strength between plane type3-plane type3 electrodes for all samples overcome the aforementioned difficulties, this work is deal with breakdown voltage of vegetable oils under high Experimental Investigation of Insulating Properties of Vegetable Oil under High Voltage Sanjida Islam, Md.Selim Hossain, Md.Farhamdur Reza, and Md.Mamunur Rashid voltage.The selection of oils was depending on availability and lower cost in market, four types of vegetable oil (Coconut oil, mastered oil, palm oil, soybeans oil and their blend), commonly available in local market are use in this work to investigate the insulating properties.There were two types of electrode use to measure the insulating properties in this thesis.The Spherical-Spherical (mushroom) and planeplane (type1, type2 and, type3) electrodes with variable gap distance are used for measuring breakdown voltage.

TABLE - I
: BREAKDOWN VOLTAGE MEASUREMENT OF OIL