Possibility of Blending Sesame Oil with Field Aged Mineral Oil for Transformer Applications D.U. Bandara, J.R.S.S. Kumara, M.A.R.M Fernando Department of Electrical and Electronic Engineering, University of Peradeniya Peradeniya, Sri Lanka
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[email protected] Abstract—Different vegetable oils have been proposed over the last few years as an alternative to the mineral oil used in transformer liquid insulation applications. Previous studies on coconut, sesame and castor oil revealed that physical, chemical as well as electrical properties of these oils needs improvements to be used as liquid insulation in transformers. This paper presents an investigation of electrical, chemical and physical properties of blends of treated sesame and field aged mineral oil to identify the required improvements to use these oils for refilling of mineral oil filled transformers. The tested properties were polarization & depolarization current, dc conductivity, breakdown voltage, viscosity, acidity and color. Result shows that the conductivity should be the most important property when mixing treated sesame oil with aged mineral oil. Conductivity was varied hugely from the acceptable limit after mixing. Hence treating should be more precise to control the conductivity to the limits. Keywords—sesame oil; mineral oil, field aged; blends; dc coductivity; polarization
I.
INTRODUCTION
Power transformer is a key equipment in the power system to deliver a reliable power supply to consumers. Different reasons cause the transformer failures and insulation failure has been reported as one of the significant failure type. The transformer oil (liquid) and paper/pressboard (solid) are the main insulation mediums used inside the transformers. Environmental conditions (temperature, moisture), aging and different stresses (electrical, chemical, mechanical etc.) are hugely affecting the insulation conditions. The mineral oil has been used more than 70 years as transformer oil due to good electrical, chemical and physical properties [3]. The exploitation of petroleum oil is running out of demand and in the near future, oil scarcity exists. It will be serious shortages even by the mid-twenty first century [8]. In addition, mineral based transformer oil is poorly biodegradable [7]. Hence due to recent demand of sustainable concepts, attention has been paid on alternative natural oils as replacement for mineral oil. Different vegetable oils such as soya bean, sunflower, olive etc, have been tested during last decade and their results seem to be promising [9, 1, 2]. With modification to raw
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C.S. Kalpage Department of Chemical and Process Engineering, University of Peradeniya Peradeniya, Sri Lanka
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vegetable oils and with suitable additives, commercially available alternative oils have been produced i.e. ENVIRONTEMP [1], BIOTEMP [2] etc. By considering availability in Sri Lanka, authors investigated the potential of using coconut, sesame and castor oils in results are presented in [10,4]. On the other hand, these oils cover range of chemical, physical and electrical properties due to inclusion of saturated and unsaturated FFAs. It was found that none of these oils have all required physical, chemical and electrical properties in the suitable levels for transformer applications. Thus demanding modifications to chemical structures or blending with other oils [4]. In this paper, possibility of blending sesame oil with field aged mineral oil was studied. Better understating about such blends will be practically important in case of refilling existing transformers with vegetable oils. In order to study the worst case scenario, 3 field aged mineral oil samples, which are in service more than 40 years was selected. First field aged mineral oil as well as virgin sesame oil was treated and tested. Then blends were prepared and tested. Several physical (color, viscosity), chemical (acidity) and electrical properties (polarization current, depolarization current, dc conductivity and breakdown voltage) of oil blends were measured and compared. II.
PROPERTIES OF SESAME
Sesame oil contains mainly oleic acid and linoleic acids i.e. about 40% each. Oleic acid (octadec-9-enoic acid) is a monounsaturated omega 9 fatty acid having C double bond at the 9th place. Linoleic acid (9,12 Octadecadienoc acid) is a polyunsaturated omega 6 fatty acid having two cis C bonds at the 9th and 12th places of the hydrocarbon chain. In addition to those acids, sesame oil contain two saturated acids: palmitic (7- 12%) and stearic (0.35-6%) acids [10,4]. Melting point largely depends on saturated fatty acids as they are having zigzag hydrocarbon chains with very close inter-molecular interaction. Pour point of sesame oil is about -9 ºC and comparatively low compared to other vegetable oils. The
viscosity of the oil depends on the length of the fatty acids. This is comparatively low in sesame oil (36 cst at 40ºC). Low viscosity is a good property related to the cooling performance in transformer applications. The breakage of triglyceride determines the acidity level of the oil. It is around 4.6 (mg KOH/ g oil) for sesame and this is comparatively high due to the various impurities and moisture content. According to the chemical properties, the sesame oil has an acceptable coverage to use as an insulating material [10]. III. TEST METHODS A. Physical and Chemical Tests The tested physical and chemical tests were color, acidity and viscosity. The color observation of samples was done by comparing the color of the oil samples with ASTM D-1500 color code. The acidity was estimated by titrating with 0.1N NaOH (eq). First 1 ml of oil was measured and then it was mixed with 10 ml (98%) Iso-Propyl alcohol. Phenolphthalein was used as the indicator when titrating with NaOH. Added NaOH (eq) was measured when the mixture got stabilized with pink color for 10 seconds. The Redwood viscometer was used to measure the viscosity. The outer container was filled with water and it was heated to 80 ºC and waited until water gets thermal stability with oil in the inner container. Then time to fill 50 ml of oil was measured. The viscosity values were estimated at 70 °C. IEC standards are defined at 40 °C. But obtaining the viscosity at 40 °C was not possible with the available lab setup. B. Electrical Tests The Polarization and depolarization current (PDC) measurements, dc conductivity, and breakdown (BD) voltage were taken as the electrical tests. The KEITHLEY electrometer (Model 6517A) together with a three electrode oil cell was used to take the PDC measurements. As these measurements are highly sensitive to humidity variation, a desiccator was used to avoid adding moisture to the treated oil samples while taking measurements. NaOH (s) was kept inside the desiccator as it can absorb the remaining moisture. PDC measurements were performed by applying 100 V DC and current readings were taken until current gets stabilized. In the depolarization process, test cell was short circuited through the ammeter to obtain the depolarization current. The schematic view and photograph of the laboratory setup are shown in Fig. 1 and Fig. 2 respectively. Here path 1 is the polarization path and at that time switch 1 is closed while switch 2 is open. Path 2 is the depolarization path and here the opposite of previous switching happens.
Fig. 1: Schematic diagram of PDC measurements
Fig. 2: Laboratory setup for PDC measurements
By considering the continuity of electric current, it can be proved that the total current through a dielectric is addition of resistive, capacitive and polarization current components as illustrated in equation (1). (1) Here I(t) is total current: v(t) is applied voltage; f(t) is the dielectric response function of the material: σ is conductivity and εα is permittivity at high frequencies. Polarization and depolarization current can be separated as in equation (2) and equation (3) respectively [5]. (2) (3) Using above two equations, dc conductivity can be calculated as, (4) Where v0 is the applied dc voltage, i.e 100 V in this study. C0 is the geometric capacitance and it is 70 pF for the oil test cell used in this experiments. After conducting all the tests, the breakdown voltage was obtained according to IEC 60156 with a 2.5 mm gap. The breakdown voltage values were obtained five times with a time lapse of two minutes between two consecutive voltage applications. IV.
TREATING AND MIXING METHODS
As mentioned in section II, sesame oil contains triglycerides of different free fatty acids. Hence these oil of fats should be transesterificated. Biodiesel and Glycerin can be separated through this process. This separated biodiesel can be tested to use as transformer oil. Here Methanol is using as the catalyst for this process [6]. First 200 ml of methanol was mixed with 150 ml (1 N) NaOH. This solution is known as Sodium methoxide. Next, sodium methoxide was added to 1 liter of sesame oil, which
was preheated about 65 ºC. Then the mixture was shaken for 5 minutes slowly and left for 48 hours inside a glass container. Two layers were formed after 48 hours and the upper layer is biodiesel while the lower layer is glycerin as explained in Fig. 3. The upper layer was separated using gravity separation method.
that adding sesame oil to an aged mineral oil only slightly increase the viscosity. Maximum allowable viscosity for transformer oil is 12 cts according to the IEC standards at 40ºC. But all the above oil samples have values above that limit. Color of the sample was the only physical parameter in testing procedure. Mixing has improved the colors of the mineral oil compared to their old color level as shown in the table IV. However it is worth to mention that the validity of color codes, which are originally defined for mineral oil, to interpret vegetable oil is questionable. TABLE 2. Comparison of acidity, viscosity and color
Figure 3: Transesterification process
Then collected biodiesel was taken into a beaker. Hot water (40 ºC) was poured into the biodiesel slowly. Then the mixture was shaken slowly about 10 minutes and was kept about 4 hours in stable position. A layer of soap has formed in the bottom of beaker. Then the biodiesel was collected using siphoning method. This method has been repeated 4 times to reduce the soap content in the biodiesel. Finally the separated biodiesel from sesame was heated at 105ºC for 6 hours to remove the moisture content. This treated sesame oil was stored inside a desiccator to avoid ingress of moisture [6]. The mixing of treated sesame with mineral oil was done inside the desiccator. 125 ml of mineral oil and 125 ml of treated sesame oil were mixed to get 1:1 ration mixture. These mixed samples were stored inside the desiccator until the tests were started. V.
PROPERTIES OF MINERAL OIL AND BLENDS
In order to use an oil as a liquid insulation of a transformer various parameters related to insulation as well as cooling should be within a certain appropriate range. IEC 60296 specifies limiting values for various physical, chemical and electrical properties. Limiting values for four critical parameters, which are considered in this study, are listed in Table 1.
Sample Mineral sample 01 (Age 41 years) Mineral sample 02 (Age 44 years) Mineral sample 03(Age 47 years) Treated Sesame Mineral 01 + Treated Sesame Mineral 02 + Treated Sesame Mineral 03 + Treated Sesame
Acidity/ (mg NaOH/ g Oil)
Viscosity/ (cts)
Color
4.198
15.407
4.5
4.224
15.407
6
4.051
15.169
4.5
4.720
28.451
2
4.800
18.965
1.5
5.335
18.254
3.5
5.405
17.305
3
B. Electrical Results Measured polarization and depolarization currents were plotted in log-log scale in fig. 4. Three plots are corosponds to three field aged mineral oil samples. According to the Fig. 4, polarization currents of mixed oils are comparatively higher compared to the polarization of mineral oil without mixing. This is because the polarization of the treated sesame has influenced more to the characteristics of the mixed oil. Depolarization currents are not showing any particular pattern. Hence, only the steady state value of the depolarization current can be obtained as an accurate parameter to calculate the conductivity.
A. Physical and Chemical Results According to the IEC standards, acidity should be below 0.01 [mg NaOH/ g Oil]. But as shown in Table 2 all the tested individual as well as oil blends has acidity much higher than that level. Also it can be noted that blends has acidity values higher than individual oils used for the blending.
DC conductivity was derived from PDC measurements using equation (4). According to the IEC standards, it is acceptable to have conductivity below 5 pS/m. But both all the tested oils and blends has much higher conductivity levels than specified level. However conductivity of sesame oil has significantly reduced due to blending with aged mineral oil.
Mixing of sesame oil with field aged mineral results viscosity values lower than individual sesame oil. However it is slightly higher than the individual mineral oil. Thus indicating
The other measured electrical parameter was BD voltage. Breakdown voltage of transformer oil indicates its ability to resist electrical stress in electrical equipment. It also depends highly with moisture content of the sample and the humidity of the testing area. The following results were obtained at dry temperature of 27ºC and wet temperature of 22ºC. Relative humidity at those conditions is 64.5%. The pressure was considered as 720 Hgmm. A correction factor was considered due to the humidity factor inside the laboratory.
TABLE 1. IEC standards for transformer oil Property BD voltage/ kV Viscosity at 40ºC/ cst Acidity /(mg NaOH/ g Oil) Conductivity/ (pS/m)
Minimum Value 30 -
Maximum Value 12 0.01 5
TABLE 3. Comparison of BD voltage
Breakdown voltages are not in the limits for all the oil samples. The reason could be the humidity inside the laboratory. Blends has slightly higher BD voltages than individual mineral oils.
Sample Mineral 1 (Age 41 years) Mineral 2 (Age 44 years) Mineral 3 (Age 47 years) Treated Sesame Mineral 01 + Treated Sesame Mineral 02 + Treated Sesame Mineral 03 + Treated Sesame
VI.
(a) PDC Measurements for mineral Oil Sample 1 and mixed mineral oil with treated sesame oil
BD Voltage/ (kV) 15 16.5 15.2 26.5 15.5 17 16
DC Conductivity/ (pS/m) 15.353 16.927 5.693 486.4 34.134 86.248 65.174
CONCLUSIONS
Mixing of sesame oil with field aged mineral oil results variations of acidity, viscosity, conductivity and breakdown voltages. Specifically by mixing sesame oil with mineral oil, its viscosity and conductivity can be significantly reduced, which is in favor of transformer applications. It was observed that blends has acidity levels higher than individual oils used for mixing. On the other hand breakdown voltage of the mixture is lower than individual sesame oil. But it is higher than mineral oil. Thus it can be concluded viscosity and breakdown voltage is mainly depend on quality of mineral oil, whereas conductivity depend on sesame oil. In overall it can be concluded that mineral oil should be well treated and sesame oil need to be improved before using in blends. Such improvements should mainly concern on dc conductivity. REFERENCES [1]
(b) PDC measurements for mineral oil sample 2 and mixed mineral oil with treated sesame oil
(c) PDC measurements for mineral oil sample 3 and mixed mineral oil with treated sesame oil Fig. 3: PDC measurements of different samples before mixing and after mixing
D.Martin, J. Wijaya, N. Lelekakis, and N. Heyward, “Thermal analysis of two transformers filled with different oils”, IEEE Electr. Insul. Magazine, Vol. 30, No. 1, pp. 39-45, 2014. [2] G.K. Frimpong, T.V. Oommen, and R. Asano,“ A survey of aging characteristics of cellulose insulation in natural ester and mineral oi”l, IEEE Electr. Insul. Magazine, Vol. 27, No. 5, pp. 36-48, 2011. [3] I. Fofana, ‘‘50 years in the development of insulating liquids’’, IEEE Electr. Insul. Magazine, Vol. 29, No. 5, pp. 13-15, 2013. [4] J.R.S.S Kumara, M.A.R.M Fernandoand C.S Kalpage, “Comparison of Coconut/Sesame/Castor Oils and Their Blends for Transformer Insulation”, submitted to 12th IEEE Int. Conf. Industrial and Information Systems (ICIIS2017), Kandy Sri Lanka, 2017. [5] K.M.K.S. Bandara, “Condition Monitoring of Biodegradable Oil Filled Transformer”, PhD Thesis, The University of Queensland, 2016. [6] Md. A. Hossain, S. M. Chowdhury, Y. Rekhu, K.S. Faraz, M.U. Islam, “Biodiesel from coconut oil: A renewable alternative fuel for diesel engine”, International Journal of Environmental, Chemical, Ecological, Geological and Geophysical Engineering, Vol. 6, No.8, 2012. [7] R. Rajesh Kanna, M. Ravindran, ‘‘Investigating the performance of transformer using insulating fluids’’, International Conference on Circuit, Power and Computing Technologies [ICCPCT], 2014. [8] T.O. Rouse, ‘‘Mineral insulating oil in transformers’’, IEEE Electr. Insul. Magazine, Vol. 14, No. 3, pp. 6-16, 1998. [9] T.V. Oommen, ‘‘Vegetable oils for liquid-filled transformers’’, IEEE Electr. Insul. Magazine, Vol. 18, No. 1, pp. 6-11, 2002. [10] W.M.L.B Naranpanawe, M.A.R.M Fernando, J.R.S.S Kumara, E.M.S.N Naramapanawa, and C.S Kalpage, “Performance Analysis of Natural Esters as Transformer Liquid Insulation – Coconut, Castor and Sesame oils”, 8th IEEE Int. Conf. Industrial and Information Systems (ICIIS2013), Kandy Sri Lanka, pp. 105-109, 2013.
