A Comparative Study of DGA and PD Test on Mineral and Biodegradable Based Transformer Insulation Oil

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1 A Comparative Study of DGA and PD Test on Mineral and Biodegradable Based Transformer Insulation Oil Mohd Fikri Hilmi Mohd Taib, Yanuar Z. Arief, Nor Asiah Muhamad, Zuraimy Adzis Institute of Voltage & Current (IVAT) Fakulty of Electrical Engineering Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia Abstract Power transformer is one of the most important equipment in electrical power system due to its cost and failure consequence. Mineral oil has been widely used as dielectric insulating fluid in transformers due to its excellent performance in service. Mineral oil is petroleum-based oil, which means it can potentially be hazardous to the environment as waste. Due to concerns on environmental issues and petroleum resources viability, the alternative insulating oil with biodegradable characteristics has been attracting lots of attention for the past couple of decades. This paper deals with the comparative study of the previous research on dissolved gas analysis (DGA) and partial discharge (PD) investigation done on biodegradable oil as transformer insulating oil. Keywords - Transformer insulation oil, biodegradable oil, mineral oil, dissolved gas analysis, partial discharge I. INTRODUCTION In high voltage power transmission, large amounts of electrical energy are transmitted from generating stations to the consumers [1]. The transformer is one of the most important equipment used in power transmission to step up or step down the voltage [2]. Normally, the petroleum based mineral oil is used in transformer as oil insulation and cooling system [1, 2]. Transformer oil is petroleum-based (mineral oil), widely known and used well in insulating liquid since 1900 s It has low viscosity, good ageing behavior, cheap and readily available [3, 4]. However, this type of oil is non-renewable since it was mined from the inside of the earth. In addition, this mineral oil gives bad impact to the environment and ecosystem due to the difficulty in decomposing it [2, 3, 5]. Due to concerns about being environmentally friendly, cost and its long term working life span, many researches are done in the previous decade to replace the use of petroleum based oil with a new type of insulation oil. The organic based oil (such as vegetable, soya bean, sunflower and etc.) are environmentally friendly, biodegradable, nontoxic and better fire safety features are showing good potentials for this replacement [5-17]. The insulating oil are considered good when it has low kinetic viscosity, high flash point, high breakdown voltage and low partial discharge [3]. The kinetic viscosity needs to be low to ensure rapid circulation inside the transformer [3]. flash point means that the oil will only give enough vapors to produce flammable mixture with air only at high temperatures [3]. Having high breakdown voltage means that the oil are able to withstand high electrical stress before breakdown [3]. Partial discharge (PD) needs to be low to minimise chemical changes and heat generation in oil due to partial discharge activities [3]. The comparative study on dissolved gas analysis (DGA) and partial discharge (PD) investigation of mineral and biodegradablebased oil is described in this work. II. ISSUES, CHALLENGES AND TRENDS A. Issues The usage of biodegradable oil are still limited on medium voltage transformer [5]. Silicone oil are studied in order to replace the mineral oil use due to the potential in being environmentally-friendly and flame retardant insulation [18]. The silicone oil is insulating oil made from silicon, which is the second most common element in the world and is chemically stable. The agricultural ester insulating oil is made from edible seed, widely produced and with high biodegradable [18]. However, it was found that the ester oil have more potential compared to silicon oil in term of charge density and breakdown voltage at high temperature[18]. Besides that, the operation of transformer may rise the insulation oil temperature. It s may reduce the insulation integrity [1, 19]. Due to present of defect and high electric stress, the partial discharge will present in the insulation oil [1, 20, 21]. This condition lead to degrade insulation oil and may lead to failure or breakdown [1]. DOI /IJSSST.a ISSN: x online, print

2 B. Challenges The ester oil are one of the biodegradable oil which is having problem in term of long stability and it has possibility to approach solid form when heated for long time [18]. The transformer oil are the most widely as electrical insulation liquids in the world [22]. It has been used more than 100 years. There have been many research are conduct in order to replace or as substitute oil for the transformer [11]. More consideration made for new insulation oil to maintain less dielectric loss, low values of degradation and high breakdown strength compare to previous oil (mineral oil base) [22]. It is found that silicone oil, ester oil are having some different characteristics from mineral oil, and this new substitution oil cannot directly replace mineral oil in service or repaired units [22]. Biodegradable oil like vegetable oil has high viscosity and low in long-term PD activity, which does not allow it to directly replace the mineral oil unless improvements are made [2]. C. Trends Nowadays, more development are achieved towards the use of artificial network as pattern recognition and decision making method. It is now able to recognize the PD pattern on transformer in order prevent the serious damage using artificial neural network as one method of artificial intelligent (AI) [23]. The artificial intelligent technique are been adopted into the electrical fields in many applications [24-30]. Online test is better compared to offline test for transformers. The acoustic emission are one of the method that can be used to detect the online PD detection compare to using shutdown period of equipment [23]. The pattern recognition of their faulty type can easily be determined by using statistical and neural network [23, 24]. As the PD pattern can be used to diagnosis the insulation condition in high voltage. By analyzing the PD pulse can show the PRPD (phase-resolved partial discharge) pattern and also got the (time vs. Frequency) map [5]. The cause of PD activity due to corona discharge, surface discharge and internal discharge can be found by analyze the PD distribution in time frequency map [5]. This work can be further extended by utilizing the pattern recognition system using this neural network method. The biodegradable oils are able to challenge the existing mineral oil performance using the polymer nano-composite as additive. There are various electrical properties of polymer nano-composites that gained interests among these researchers such as partial discharge (PD) resistance, tracking resistance, dielectric breakdown strength, permittivity, and loss tangent [31]. III. TYPE OF TESTS A. Dissolved Gas Analysis (DGA) During the faults, transformer insulation oil will release hydrocarbon gases that dissolved into the oil. Types and quantities of hydrocarbon gases produced give early indication of inception faults in transformer. Five types of hydrocarbon gases that commonly dissolved in transformer oil are H 2 (hydrogen), CH 4 (methane), C 2 H 6 (Ethane), C 2 H 4 (ethylene) and C 2H 2 (acetylene) [32]. Common faults can be detected using this method are partial discharge, overheating, and arcing or breakdown faults. Overheating fault can be detected by present of hydrogen together with methane, ethane and ethylene. The key gas for this type of fault is ethylene gas [1, [32] DGA is one of method able to determine the type of fault from the gas generated according to IEEE standard C : IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers. There are common methods to evaluate or analysis the DGA, which is:- 1. Duval Triangle method, 2. Key Gases method, 3. IEC method, 4. Rogers Ratio method, 5. Nomograph method, 6. Doernenburg Ratio method 7. CIGRE Method. For both thermal and electrical faults, coconut oil contained only H 2, CO and CO 2 gases while transformer oil contained additional gases such as CH 4, C 2 H 4 and C 2 H 6. In most cases, gases such as H 2, CO and CO 2 found in coconut oil were in the same level as those in transformer oil. Usually CO and CO2 gases are formed as a result of the ageing of pressboard whereas H 2 is formed by overheating of oil. Therefore, formation of CO and CO 2 gases in both coconut oil and transformer oil was due to the added pressboard. to make the samples wet. When comparing the gas levels at different temperatures, one could expect high gas level for the coconut oil samples aged at 200 C. The results were opposite showing that even at a lower temperature level with higher duration gave high dissolve gas levels [17]. There is some information found:- There are negligible amount of hydrocarbon produce from coconut oil samples during the electrical and thermal faults [17]. The vegetable oil releases large amount of CO and CO 2, but the dielectric strength decreased lower compared to the mineral oil on the real transformer. The degradation of insulation paper can be determined by indicating the CO and CO 2 amount. Then the amount of CO and CO 2 of mineral oil and vegetable oil are differing due to different of kinetic DOI /IJSSST.a ISSN: x online, print

3 viscosity of them. The heat transfer from the insulation paper to the insulation oil are low than mineral oil due to high kinetic viscosity of vegetable oil [16]. The prediction for overheating fault in oil filled transformer using this method will give wrong fault interpretations when using Roger s ratio when applied for transformer using biodegradable oil as insulation compare to mineral oil which due to the methane amount are low compare to the hydrogen [32]. Figure 1. Arcing test without pressboard for normal oils [33]. Interpretation method Duval s Triangle Method Roger s Ratio Method Key Gas Method TABLE 1. DGA FAULT GAS ANALYSIS [33]. Oil Type Mineral Oil Biodegradable Oil Mineral Oil Biodegradable Oil Mineral Oil Biodegradable Oil Point to plane test (Without pressboard) Normal Dried Wet Low Low Low (Sparking) (Sparking) (Sparking) Low (Sparking) - - Low (Sparking) No Prediction - - Low (Sparking) No Prediction - - The prediction using Roger s Ratio Method are unable to detect the arching fault from the DGA data. This due to the smallest amount of methane produce compare to hydrogen from the biodegradable oil. [33] Figure 2. Arcing test without pressboard for dried oils [33]. Figure 3. Arcing test without pressboard for wet oils [33]. Figure 1, 2 and 3 show the amount of hydrocarbon gas produce by the arcing test without the pressboard. The arching (Breakdown voltage) test are done on point to plane followed the ASTM D877. The data from figure 1, 2, and 3 are analysis using Duval s Triangle Method, Roger s Ratio method and Key Gas Method are been shown in Table 1. B. Partial (PD) Partial discharge is the electrical discharge which are not completely bridging the distance between two electrodes under high voltage stress [24]. This PD can be seen as small electrical spark that happen within the electrical insulation of electrical equipment. This PD magnitude are normally small but it have potential to cause the ultimate failure [24, 34, 35]. PD may cause the insulation ageing, destroy the insulation and lead to electrical equipment failure [24]. Luckily, the prevention method can be taken at the early stage of PD presenting. In addition, the PD occurrence in electrical equipment emitted the several signals like electrical signal, chemical reaction and acoustic pulse, i.e. heat, gas, light and sound [24]. The occurrence of PD signal can be detect using electrical, chemical and acoustic detection [24, 36]. The basic partial detection is normally according to the test circuit as shown in Figure 4 below. DOI /IJSSST.a ISSN: x online, print

4 Figure 4. Basic PD detection measurement [24]. Where: U: high voltage supply, Z mi: input impedance of measuring system, CC: connector cable, C a : test object, C k: coupling capacitor, CD: coupling device, MI: measuring instrument, Z f: filter. PD can be classified into 4 types which are internal discharge, surface discharge, corona discharge and discharge in electrical trees [24, 34, 36]. One of the consequences of PDs is chemical change of material. (i.e. oil, solid and gas) [24, 37]. Acoustic detection of PD is based on the detection of the mechanical waves propagated from the discharge site to the surrounding medium. Acoustic detection has been widely used in diagnostics of transformers. The primary advantage of using acoustic detection is position information is readily available from acoustic systems using sensors at multiple locations [24, 38-40]. Electrical PD detection methods are based on the appearance of a PD pulse at the terminals of a test object. Electrical detection includes two methods: Pulse Current Method and Ultra Frequency Method (UHF) [24]. Pulse Current Method: This method gets the apparent charge by detecting the PD current in detecting impedance [24, 36, 41]. Pulse Current Method is easy for quantitative measurement and it has high sensitivity [24]. Ultra Frequency Method (UHF): UHF detection which is based on the detection of electrical resonance at ultrahigh frequencies can be applied to realize not only the phenomena but also the location of a PD source [24, 42-44]. PD detection can be done during off-line and on-line test on transformer. It is possible to determine the PD using Acoustic emission without shutdown the transformer [23]. Figure 5. PD charge trend during one hours of 10kVrms injection voltage[3]. Figure 6. PD charge trend during one hours of 30kVrms injection voltage [3]. Figure 5 and 6 show the PD charge trend of difference biodegradable oil (Soybean (SOY) Oil and Palm Fatty Acid Ester (PFAE) Oil) in 10 kv and 30 kv for during 1 hours injection time using electrode according to IEC It found that biodegradable oil have low PD charge trend compare to mineral oil in low voltage ad high voltage application [3]. IV. FUTURE OF BIODEGRADABLE OIL AS DIELECTRIC INSULATING FLUID IN TRANSFORMERS Many researches have been conducted on insulation oil with many tests including physical test, chemical test and electrical test. Future tests needs to be conducted on oil PD test on transformers online as it is necessary to investigate the PD activities instead just only using laboratory samples [3]. Biodegradable oils such as palm oil and corn oil lacks in viscosity and PD activity properties compared to mineral oil [2]. The improvement on viscosity and long-term PD are required for palm oil and corn-based oil to have potential as electrical insulation applications [2]. DOI /IJSSST.a ISSN: x online, print

5 The vegetable oil is comparable with mineral oil as insulating oil and need to be improved further for future investigation [5]. The electrical performance of real palm-based transformer has been investigated by the authors. The main objective of the research works is to compare the efficiency of the real power distribution of palm-based oil compared with mineral-based oil. Other characteristics such as DGA and polarization and depolarization current of the transformer also been investigated. Figure 7 shows the picture of two type of the power transformer which is observed in the research work. While Figure 8 shows the typical result for heat population of palm-based oil transformer under certain load condition. V. CONCLUSION The comparative study of the pass research on dissolved gas analysis (DGA) and partial discharge (PD) test done on biodegradable oil as transformer insulating oil has been performed in this work. Common faults can be detected using DGA method are partial discharge, overheating, and arcing or breakdown faults. PD detection can be done during off-line and on-line test on transformer. It is possible to determine the PD using acoustic emission without shutdown the transformer. There are needs a future test conduct on oil PD test on real transformers is necessary to investigate the PD activities instead just only using samples which mean it is better to conduct an on-line test beside just only off-line test. The vegetable oil is comparable with mineral oil as insulating oil and need to proceed for future investigation. ACKNOWLEDGMENT Authors gratefully acknowledge the Malaysia Ministry of Education (MOE) and Universiti Teknologi Malaysia (UTM) for the use of facilities and by awarding Research University Grants (RUG) under vote number Q.J H21. Figure 7. Research work on electrical performance of two type distribution power transformers, namely palm-based oil and mineral-based oil. Figure 8. Heat population at palm-based transformer in certain load condition REFERENCES [1] H. Sutikno, "Effects of temperature on the breakdown voltage and partial discharge patterns of biodegradable oil," in Electrical Engineering and Informatics (ICEEI), 2011 International Conference on, 2011, pp [2] S. Chandrasekar and G. C. Montanari, "Analysis of partial discharge characteristics of natural esters as dielectric fluid for electric power apparatus applications," Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 21, pp , [3] M. R. Ramli, Y. Z. Arief, S. A. Azli, N. A. Muhamad, K. Lau, M. Farhan, et al., "Partial discharge characteristics of palm fatty acid ester (PFAE) as high voltage insulating material," in Power Engineering and Renewable (ICPERE), 2014 International Conference on, 2014, pp [4] I. Fofana and J. Sabau, "Application of petroleum-based oil in power transformer," Natural gas research progress, p. 23, [5] S. Banumathi, S. Chandrasekar, and G. C. Montanari, "Investigations on PD characteristics of vegetable oils for high voltage applications," in 2013 IEEE 1st International Conference on Condition Assessment Techniques in Electrical Systems (CATCON), [6] K. Kiasatina, M. Kamarol, M. Zulhilmey, and Y. Arief, "Breakdown characteristics of RBDPO and soybean oil mixture for transformer application," in Electrical, Control and Computer Engineering (INECCE), 2011 International Conference on, 2011, pp [7] A. Rajab and S. Aminuddin, "Properties of RBDPO Oleum as a candidate of palm based-transformer insulating liquid," in Electrical Engineering and Informatics, ICEEI'09. International Conference on, 2009, pp [8] A. Suleiman, N. Muhamad, N. Bashir, N. Murad, Y. Arief, and B. Phung, "Effect of moisture on breakdown voltage and structure of palm based insulation oils," Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 21, pp , [9] F. S. Suwarno, I. Suhariadi, and L. Imsak, "Study on the characteristics of Palm Oil and its Derivatives as Liquid Insulating Materials," in 7th International Conference on Properties and Application of Dielectric Materials. Nagoya, 2003, pp [10] C. Perrier and A. Beroual, "Experimental investigations on insulating liquids for power transformers: mineral, ester, and DOI /IJSSST.a ISSN: x online, print

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