Use of vegetable oils as transformer oils – a review

Renewable and Sustainable Energy Reviews 52 (2015) 308–324

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Use of vegetable oils as transformer oils – a review M. Rafiq a,c,n, Y.Z. Lv a,b, Y. Zhou a,c, K.B. Ma a,c, W. Wang b, C.R. Li a,c, Q. Wang a,c a b c

Beijing Key Laboratory of High Voltage and EMC, School of Electric and Electronic Engineering, North China Electric Power University, Beijing 102206, China School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China State key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China

art ic l e i nf o

a b s t r a c t

Article history: Received 29 January 2014 Received in revised form 17 May 2015 Accepted 8 July 2015

The mineral or synthetic oil is mostly being used in conjunction with paper as the dielectric medium in most of the high voltage equipment. However, impact on environment, lack of petroleum oil reserves and disposal problems with used oils, have prompted researchers to direct their focus onto biodegradable and renewable insulating materials. The new insulating liquid materials development is guided by multiple factors such as environmental requirements and other safety and economic considerations. Therefore transformers manufacturer have to face new specifications related to these new requirements. The Vegetable-oil based transformer fluids increasingly replacing mineral oil-based products in the market place. They are successful because they perform better than mineral oil products and they provide definite environmental and safety gains. This paper reviews the current status of vegetable oils use as transformer oil, including their production, processing, and characterization. The vegetable oils most used as transformer oils are presented and their main advantages described in comparison with mineral oil. The various experimental work carried out in different countries is described, giving an overview of the current research carried out on the vegetable oils. In addition scope and challenges being faced in this area of research are clearly described. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Vegetable oil Biodegradable oil Renewable materials Dielectric strength

Contents 1. 2.

3.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Vegetable oil as transformer oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 2.1. Viability of vegetable oil as transformer oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 2.1.1. What is vegetable oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 2.1.2. The problems with mineral oil and application of vegetable oils as transformer oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 2.1.3. Properties of transformer-grade vegetable oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 2.1.4. Historic evolution of vegetable oil as transformer oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 2.2. Production and usage of vegetable oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 2.2.1. Base fluid selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 2.2.2. Oil processing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 2.2.3. Stabilization of the oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 2.2.4. Improving pour point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 2.2.5. Chemical composition of vegetable oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 Performance comparison of vegetable oil vs. mineral oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 3.1. Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.1.1. Flash, fire point and viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.1.2. Gases produced after aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.1.3. Oxidation stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.1.4. Breakdown strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.1.5. Streamer initiation and propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

Corresponding author. Tel.: þ 8613641117142.

http://dx.doi.org/10.1016/j.rser.2015.07.032 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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309

3.1.6. Partial discharge and thermal aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 3.1.7. Recent research work carried on vegetable oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 4. Advantages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 4.1. Environment friendly and adapted to sensitive areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 4.2. High sustainable efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 4.3. Less flammable product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 4.4. Customer benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 4.5. Strengthen agricultural economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 4.6. Renewable fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 5. Technical difficulties and challenges and research gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 5.1. Technical difficulties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 5.2. Special challenges and research gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

1. Introduction The development of future low carbon network and smart grid has raised high demands on the reliability and performance of insulation materials used in electrical power system plants to cope with more dynamic and volatile operating conditions. A transformer, which transforms voltage and transfer energy, is one of the most vital components of a power network. Mineral oils have been used as coolant and insulator for over a century [1]. However, due to limitation of sources of mineral oils, sustainable production of transformer oil is being hotly debated worldwide since it is increasingly acknowledged that first generation mineral oil, primarily produced from petroleum products, are limited in reserves and have several other drawbacks such as, non-biodegradability (the level of biodegradability for mineral oil is not more than 30%), low flash point, non-renewable and could cause a serious problem if there is a spillage [2–4]. Also the enhanced industrialization and motorization of the society has led to a huge rise in demand of petroleum products. The above mentioned concerns have increased the attention to look for alternate, which can be produced from materials available abundant in nature and which potentially can offer greatest opportunities in the longer term. Liquid filled transformers use billion of liters of insulating fluid. They come in various sizes: large, medium and small. Power as well as distribution transformers use oil for insulation and cooling purposes. The distribution smaller units are numerous than larger units because distribution is more widespread by definition and hence smaller units hold much more fluid in total as compared to larger units. Mineral oil is most commonly used transformer fluid and has been used for more than a century. Small units used in confined areas like shopping centers may use fire resistance fluids such as silicone, high temperature mineral oil and synthetic ester fluids. In the recent years, environmental concerns have been raised on the use of poorly biodegradable fluids in electrical apparatus in areas where spills from leaks and equipment failure could contaminate the surroundings. Contamination of the water supply is more serious as compared to the contamination of soil [5]. The vegetable oils are thought to be a suitable alternate of mineral oil in transformers. The vegetable oils are naturally obtained from seeds as well as form flowers. Many researchers and industries are performing investigations on vegetable oils for providing them as insulating oils in transformers and pollution free environment [6]. Vegetable oils have the properties like High biodegradable (495%), low toxicity, high flash points (4300 1C), fire points (4300 1C), provide lower flammability and it is considered more environmental friendly fluids [7–9]. In addition, these vegetable oils absorb more moisture compared to mineral oils [10,11]. However, high concentration of unsaturated fatty

acid makes them unstable and prone to oxidation [12]. These fatty acid hydrocarbons chains and their degree of un-saturation affect the dielectric and physiochemical characteristics of vegetable oils. Vegetable oils have higher acidity than mineral oils [13] due to hydrolysis reaction which forms above mentioned acids (reaction that does not occur in mineral oils) and to the different chemical structure of the two oils. Also, the nature of the contained acids in both oils is different. Vegetable oils mainly contain highly molecular weight acids (HMA) like stearic and oleic acids whereas the mineral oil contains low molecular weight acids (LMA) like acetic, formic and levulinic acids [14–16]. Research efforts started in mid 1990s to develop a fully biodegradable liquid due to the utility interest. The R&D labs stated efforts in this direction and initiated oil development work. Vegetable oil was considered the most likely candidate for a fully biodegradable insulation liquid. Vegetable oil is available in plenty as a natural resource. It was considered a biodegradable and a good insulator [17]. Vegetable oils have emerged as an increasingly common mineral oil alternative. They offset all the main risks associated with common mineral oil, such as high flammability and environmental impact. They are made from renewable biological sources such as vegetables. It is biodegradable, non-toxic and possesses low emission profiles. Also, the use of vegetable oil liquids is environmentally beneficial. Only recently transformer-grade vegetable oils become available. The first commercial product was BIOTEMPs, patented in September 1999 by ABB in US [18]. The base fluid was high oleic oil with 80% oleic content. These oils were produced from seeds which have been developed by selective breeding; recently gene manipulation techniques have also been used. Unstable triunsaturates were minimized by additional step of partial hydrogenation. The BIOTEMPs fluid is now in use in distribution transformers in some sensitive areas. Later in September 1999, another U.S patent was issued, for transformer oil obtained from regular soybean oil prepared by Waverly Light & Power in Iowa [19]. It is not high oleic oil. In March 2000, the Cooper industries, Inc in Milwaukee, WI under the trademark Envirotemp FRs 3 [20]. This fluid is being used in some commercial distribution transformers and is from standard grade oleic base oils. In August 2001, a second patent was issued to ABB inventors on BIOTEMPs [21]. Except BIOTEMPs, the fluid development details are not available, on which a dozen of technical papers have been published. For BIOTEMPs, the starting oil is high oleic oil, such as sunflower oil, containing 80% or more oleic content. Canola oil upgraded to this level of oleic content also been tested for use [22]. This paper gives a comprehensive review of the methods used for producing vegetable oil, experimental investigation on different oils, characterization, merits, demerits and challenges faced by vegetable oil are described.

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Fig. 1. Breakdown voltage of impregnated cellulose [26].

Table 1 Water solubility in transformer liquids [26]. Class

Fire point

Class

Net calorific value

O K L

r300 1C 4300 1C No measureable fire point

1 2 3

Z 42 MJ/kg r 42 MJ/kg and Z32 MJ/kg o 32 MJ/kg

Table 2 Fire and flash points of different insulating liquids [26].

Fig. 2. Comparison of unit life of transformer for oil and natural ester [28].

Fluid type

Flash point

Fire point (ºC)

Class

Mineral oil Silicone fluid Low viscosity silicone fluid Natural ester Synthetic ester

160–170 4 300 268 4 300 4 250

170–180 4350 312 4350 4300

O K3 K3 K2 K3

2. Vegetable oil as transformer oil 2.1. Viability of vegetable oil as transformer oil 2.1.1. What is vegetable oil Vegetable oils are triglyceride normally obtained from a plant. The vegetable oils have been used by human since centuries. The term “vegetable oil” can be defined as plant oil that is liquid at room temperature. Vegetable oils consist of triglycerides. Although many plant parts may yield oil, commercially, oil is extracted primarily from seeds. 2.1.2. The problems with mineral oil and application of vegetable oils as transformer oils The mineral oil as transformer oil can generate poisonous substances due to oxidative instability. The disposal and clearance after equipment failure and spillage is also a very difficult exercise. The leakage of mineral oil transformer can post a serious threat to environment if leakage or spillage. Due to these above mentioned negative points attached with mineral oil, its use is highly questionable in many countries. The use of silicon oil has some better properties like, high flash point (low flammability) but they are very expensive and also non-biodegradable [23]. Vegetable oil on the other hand is environmental friendly, biodegradable, renewable, cheap, highly available and safer

alternative insulating and cooling medium for transformers. The characteristics of commercially available vegetable oils vary with the product and typical representative values of the most vital characteristics for the use of vegetable oils as transformer oils are presented in the following sections. 2.1.2.1. Electrical properties. All transformer oils are required to meet the AC withstand voltage, lightening impulse and switching impulse standards. A lot of tests have been carried out by researchers to breakdown voltages and discharge characteristics of vegetable oils and mineral oils from various suppliers [24]. The most important and common requirement that an insulating liquid must fulfill is the AC breakdown voltage, which is defined as the value of an applied AC voltage at which disruptive discharge begins. A number of standard test methods are normally used where a small volume of oil is subjected to an almost homogeneous electric field between two electrodes immersed in the insulation field. The voltages are raised until the breakdown happens. The standard international test is that described in IEC 60156, which uses a electrode gap of 2.5 mm and a voltage rise rate of 2 kV/s [25]. The AC breakdown is extremely sensitive to the impurities existing in a transformer liquid, such as particulates, excessive moisture and air or gas bubbles. Consequently the measured AC breakdown voltage of an insulating fluid mostly

M. Rafiq et al. / Renewable and Sustainable Energy Reviews 52 (2015) 308–324

represents the oil quality rather than oil properties itself. Moisture can be found mainly in two forms in insulating fluids: free water or dissolved water. Polar fluids tend to create hydrogen bonds with water molecules so that water can dissolve easily. Therefore polar liquids have extremely high water tolerance. On the other hand, non-polar mineral oil and slightly polar silicone oil is highly sensitive to absolute moisture content. The Fig. 1 shows the breakdown voltages of synthetic ester, natural ester, silicone and mineral oil at ambient temperature. It can be clearly seen that even a small amount of moisture in mineral oil caused a rapid deterioration in breakdown voltages whereas both types of ester oils indicated high breakdown strength at high dissolved moisture level. The comparison of unit life transformer with mineral oil and natural ester is shown in Fig. 2.

 Pour point



2.1.2.2. Physical properties



 Viscosity Viscosity of insulating liquid affects the ability to transfer heat by conduction. Conduction cooling is major heat removal mechanisms in transformers and higher viscosity would be expected to result in higher hot spot temperatures within the transformer. Experimental tests have indicated that application of vegetable oils in transformers resulted in increased temperatures of between 1–3 1C [27].

311

The pour point is the temperature at which transformer oil just flow under the prescribed conditions. Pout point is a useful measure to see how transformer oil will perform at low temperatures, particularly when it is required to cold start a transformer at very low temperature conditions. Vegetable oils have higher pour point than mineral oils typically in the range  15 to  25 1C [29], but test have indicated the successful cold start down to  30 1C. Operating temperature The operating temperature of the transformer also influences the lifetime of the paper insulation, which degrades at rates depending on both the insulating liquid and temperature. The experimental test results have indicated that it is possible to operate a transformer at higher temperature using vegetable oils than with the mineral oil. The temperature is measured not as the average but as a “hot spot” temperature in the transformer windings. Higher operating temperature means increased loading of the transformer, an important consideration when looking at an existing plant. Water absorption The water absorption of different transformer liquids at room temperature is shown in Table 1 i.e. the total amount of moisture content which a liquid can hold without free water being deposited. The solubility of free water increases with temperature in all liquids. The natural esters oils can increase the thermal stability of paper, as they remove more moisture

Table 3 Properties of transformer fluids (Typical values/Limits) [31]. Vegetable oil

High temp. mineral oil

Silicone 561 fluid

Physical Appearance Specific Gravity at 25 1C Kinematic viscosity. (cSt) 0 1C 25 1C 40 1C 100 1C Pour point, ºC Interfacial tension (IFT), dynes/cm Flash point ºC Fire point ºC Moisture content, ppm dry oil (water solubility at 25 1C)

Light yellowa 0.91–0.92

Light yellow 0.89

Colorless 0.96

170–250 55–75 33–45 8–10  15 to  25 25 310–325 354–360 50–100 1200

2200 300 125 13  20 max. 40–45 275 min. 160–180 10–25b 60

95 50 38 16  50 max. 25 300 min. 340 50 200

Thermal constants Heat capacity, cal/g ºC Thermal conductivity, W/mK Coefficient of expansion/ºC

0.50–0.57 0.17a 0.0007

0.488 0.13 0.00073

0.363 0.15 0.00104

Ester 0.06a Passa

Hydrocarbon 0.01 Pass

Organo-silicon 0.01 Pass

Electrical Dielectric constant at 25 1C Volume resistivity at 25 1C, Ohm cm

3.1 1014

2.2 1014–1015

2.71 1014

Breakdown voltage, kV ASTM D 1816,2 mm gap electrodes Impulse breakdown voltage, kV(needle negative)

74a 116a

60 145

– 136

Dissipation factor (%) 25 1C 100 1C Grassing tendency-ASTM D2300

0.25a 1.00a  50a

0.05 max. 0.3 max.  10 to 20

 0.01 – N/A

Biodegradabilityc CEC-L-33 (21 days)

97–99

30

Very low

Chemical Chemical type

Note: a b c

For BIOTEMP fluid. Varies with transformer rating. See below.

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Fig. 3. Development of natural esters as transformer insulating liquids [26].





from solid insulation more effectively as compared to mineral oil thus allowing either higher spot temperature or increased equipment life. Stability and degradability Vegetable oils are degradable and this property is both an advantage and disadvantage, as if oil spill benefit from degradability, oil must not be degrade inside the transformer in the presence of oxygen. The oxidation stability of vegetable oils for transformer is a major concern of end users. Natural esters are more susceptible to oxidation as compared to mineral oils. To ensure and maintain optimum performance of vegetable oils, exposure to oxygen and moisture must be minimized. Thus hermetic sealing against ambient air is the best way to benefit from the properties mentioned before [30]. Flammability-flame point and flash point The flammability of transformer oil is major safety concern in recent years. There have been many cases of transformer explosions resulting extensive fires which are difficult to extinguish and which spread to surrounding areas as oil leaks out. The fire-point or flame point is defined as the temperature at which the liquid surface emits enough vapor to sustain a fire for five seconds in the presence of flame, and the flash point as the temperature at which the fluid surface emits enough vapor to ignite in the presence of a flame. The comparative values for different insulating fluids are shown in Table 2. It can be seen that the natural esters have values more than 300 1C compared to the 160–170 1C range for mineral oils.

2.1.2.3. Environmental properties. Environmental safety is determined by two basic factors: biodegradability and low toxicity. Generally, the liquids which have high biodegradation rate and demonstrate low toxicity are classified as “environmental friendly”. These factors are very important when considering the application of liquids in environmentally sensitive areas, such as water courses, to avoid contamination. The term “biodegradability” reflects the extent which the fluid is metabolized by natural occurring microbes in soil or water ways, in the event of leak or spillage. Clearly it is good if the spilt liquids disappear quickly naturally without the use expensive clean up processes. Vegetable oils are classified as being “readily biodegradable”. 2.1.3. Properties of transformer-grade vegetable oil Physical, chemical and electrical properties of vegetable oil specifically developed for transformer use are shown in Table 3 Comparison data is also given for high temperature mineral oil and silicone fluid used in transformers [31]. 2.1.4. Historic evolution of vegetable oil as transformer oil Power transformers have evolved significantly since their innovation a century ago, especially in terms of their functionality

and size. But one very important thing remained unchanged during all this time, the use of mineral oil as insulating and cooling medium. The global oil crisis of the 1970s forced the initial search for alternatives and growing desire for environment friendly, safe, reliable and clean power solutions. In 1892, ester oil extracted from seeds was tested to be used as transformer oil, but it showed no operational improvement on mineral oil and was not commercially successful. The major problem with this oil was high pour point and high oxidation relative to mineral oil [32]. No serious efforts were made regarding vegetable oils until the severe oil crises of 1970, which forced the decision makers and researchers to think for renewable transformer oil. The last century mid eighties saw the emergence of appliances and equipment using vegetable oils as insulating liquids. In 1984, transformers using synthetic esters were manufactured. Due to their compact design and dimensions, they had remote heat exchanger and forced circulation. These synthetic ester oils have good lubricity, low pour point and high fire point but they had very less market acceptance to their high cost as compared to other dielectric fluids [31]. As a result of environmental regulations and liability risks associated with non-edible oils, an extensive research work began after 1990, which led to revisit the use of vegetable oils as transformer oils. They have excellent dielectric and fire safety properties and they are classified as edible oils. In addition, they are biodegradable because they have organic composition and most importantly, they are more economical and available than synthetic esters [33,34]. A summary of development of vegetable oils as transformer liquids is presented in Fig. 3. All large number of experiments was carried out with vegetable oil as a replacement of transformer mineral oil liquid by researchers from various parts of the world. Most of these experiments were reported from US, UK, China, Japan, India, Malaysia and Europe. A summary of these experimental results is given below. Oommen et al. [31,35] tested the new, fully biodegradable fluid from oleic vegetable oils sources for use in distribution transformers. This new build transformer fluid fulfilled the demand for an environmental friendly fluid for transformers. A number of qualifying tests were performed including usual acceptance tests for ordinary transformer oils. Oxidation stability tests were performed as per ASTM. Other advanced test included was life testing, biodegradability and decomposition studies. Electrical and thermal decomposition were also studied. Thermal study was performed with air and without air environment. The fluid was found 97–98% biodegradable. The decomposition products under electrical and thermal stress were similar to that ordinary transformer oil, but CO and CO2 produced were high in amount. This was an exploratory investigation to determine the effect of fluid on transformer performance. The testing proved that the

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biodegradable fluid could be used as a feasible alternative liquid. Bandent et al. from University of Karlsruhe, Germany [36] developed high oxidation stability rape seed oil with co-operation of German oil mill. The developed oil was classified as “in general not hazardous to water” according to German Federal Water Act (WHG). Degradation rate was more than 80% within 28 days test period. Hence developed rape-seed oil had superior health and environmental characteristics. The investigations showed that AC, Impulse strength were as high as Shell Diala D mineral oil. The entire transformer test was passed successfully. Y. Bertrand et al. [37] presented insulating and cooling liquid which was prepared from conventional crop plants products to be used in distribution transformers as alternative to mineral oil liquid. This dielectric fluid provided a good balance between high functional performances inside the unit versus low impact on the environment in the event of release. The results were compared with unused mineral oils and organic esters specified values. The electrical properties of vegetable oils were close to the conventional insulating liquids. Specifically AC breakdown strength was as high as other mineral oils. The thermal properties (specific heat and expansion coefficient) were also good. Al-Ammar et al. [38] compared the breakdown characteristics of three vegetable oils (corn, canola and palm olein oils) with conventional mineral insulating oil. The results indicated that corn and canola oils exhibit higher AC breakdown voltage than mineral oil. Divakaran et al. [39] investigated impulse voltage, other electrical and thermal characteristics of virgin and aged vegetable oils and showed that vegetable oils have better insulation performance. Coconut and palm oil have better dielectric strength, viscosity, safe fire and flash points and impulse voltage properties superior to mineral transformer oil. Guo et al. [40] analyzed and concluded that relative permittivity of mineral oil and vegetable oil at power frequency (50 Hz) decreases with the temperature rise. However, permittivity their relevant papers at 50 Hz was increased firstly and then decreased with increase of temperature. The permittivity (ɛ) of vegetable oil and vegetable oil-impregnated papers was high than that of mineral oil and mineral oilimpregnated papers respectively, which is quite helping in reducing the size of transformer. Dissipation factor (tanδ) at 50 Hz increased for both kinds of oil and their corresponding oil impregnated papers. The rising rate of dissipation factor of the vegetable oil was high as compared to mineral oil whereas tanδ of mineral oil impregnated papers was a little greater than that of vegetable oil impregnated papers, which proved that vegetable oil can slow down the aging of transformer insulation. Han et al. [41] presented a new type of high fire resistance transformer filled with camellia oil. The physical, electrical and chemical properties were measured and compared with that of mineral oil. The results showed that camellia insulating oil has good electrical properties and is fire resistant and thus can be a good candidate to be used as substitute for ordinary mineral oil. No-load and load loss of camellia oil filled transformer were 30% and 3% lower than the request value respectively. The results indicated that temperature rise of transformer filled with camellia oil was slightly more than that of mineral oil filled transformer suggesting that the oil gallery structure must be improved. Kanoh et al. [42] from Japan developed a new vegetable oil based insulating transformer fluid called PFAE (Palm Fatty Acid Ester). This specific oil has 1.3 times higher dielectric constant and 0.6 times less viscosity as compared to mineral oil. It means this vegetable based oil has better insulating and cooling properties. PFAE indicated high biodegradability than the conventional mineral oil. The flash point of PFAE is also higher as compared to the mineral oil. The oxidative stability was also found better than mineral oil. It was concluded that PFAE would be much better and safer for environment than mineral oil. Kano et al. [43] investigated oxidative stability of PFAE with

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another vegetable oil containing unsaturated fatty acids. PFAE showed outstanding biodegradability and oxidation stability. Kojima et al. [44] studied PFAE and showed advantages of high permittivity, lower kinetic viscosity, and higher flash point as compared to conventional mineral oil. It was revealed that the essential charge mechanism behavior was similar by comparing charge behavior and electric field but the relaxation time of PFAE was much lower than that of mineral oil. Therefore it was concluded that PFAE is a better insulating liquid for compact and environment friendly transformers. Suwarno et al. [45] investigated the effects of temperature on dielectric properties of Rhicinnus oils (obtained from rhicinnuscomunnis seeds) and methyl ester obtained from rhicinnus oil. The results showed that the refined rhicinnus oil with treatment and methyl ester obtained from rhicinnus oil. Have outstanding dielectric properties and compliant almost IEC standards. Hosier et al. [46] from UK studied the thermal ageing of vegetable based oils by various analytical techniques. The color becomes more yellow as compared with data of mineral oils [47] and dodecylbenzene [48] on aging, more so with copper but the vegetable oils did not generate any solid particulates unlike mineral oils. Sunflower and Envirotemp oils behaved similarly optically whereas olive oil seemed to be affected due to chlorophyll presence by aging with copper. The viscosity of the oils was increased with aging showing most effect on sunflower oil whereas least on the olive oil. With aging, the dielectric loss was increased as expected [49,50,5] and Envirotemp oil indicated the least effect of aging on dielectric loss. Sunflower and olive oils were affected by almost same degree by ageing. Marulanda et al. [51] performed some programmed test. Through these test, properties and characteristics of different types of oil were analyzed. The obtained results indicated that the useful life of transformer with vegetable oil was 42.86% greater than the useful life of transformer filled with mineral oil. The test results showed that the use of vegetable oil improved the performance of transformers, prolonged useful life, lowered rates of failure.Jung-II Jeong et al. [52] thermally aged transformer filled with vegetable oil and mineral oil and concluded results that vegetable oil have better insulation characteristics as compared to the mineral oil. Alexandra Ciuriuc et al. [53] presented comparative study related to the ageing of vegetable and mineral oils used in power transformers. The samples of vegetable and mineral oil were accelerated thermally aged and their dielectric properties (real part of complex permittivity, dissipation factor and resistivity) were found at different ageing times. The results showed values of relative permittivity and loss factor for vegetable oil were high as compared to the mineral oil while the resistivity of vegetable was lower than that of mineral oil. The color of mineral oil samples was changed with the ageing time whereas in the case of vegetable oil samples, the color change was not significant. The variation rate of vegetable oil's real part of permittivity, dissipation factor and resistivity decreased with time while that of mineral oil had increasing tendency. Dumitran et al. [54] showed the effect of moisture content on the electrical properties of vegetable oil and mineral oil. For mineral oil, a correlation was found between the variation of moisture content and dielectric properties. But in case of vegetable oil, it was observed that there is no evident relationship between water content with the dielectric properties. It meant that moisture content is not suitable diagnostic factor to estimate the aging condition of vegetable oil. A possible description could be that moisture content did not contribute to a variation in dielectric properties of the oil but the variation is due to the degradation products resulting from chemical reactions that take place between moisture and oil. Hemmer et al. [55] from Germany investigated and showed that stabilized rape-seed oil RAPSOLT have good compatibility with conventional used transformer board. AC breakdown voltage

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Fig. 4. Oil seeds used to extract oil.

Table 4 Typical Fatty acid composition of some vegetable oils [5]. Vegetable oil

Saturated fatty acids (%) Unsaturated Fatty Acids (%)

Canola oila Corn oil Cottonseed oil Peanut oil Oilve oil Safflower oil Safflower oil, high oleic Soyabean oil Sunflower oil Sunflower oil, high oleic

7.9 12.7 25.8 13.6 13.2 8.5 6.1 14.2 10.5 9.2

Mono-

Di-

Tri-

55.9 24.2 17.8 17.8 73.3 12.1 75.3 22.5 19.6 80.8

22.1 58 51.8 51.8 7.9 74.1 14.2 51 65.7 8.4

11.1 0.7 0.2 0.2 0.6 0.4 – 6.8 – 0.2

Fig. 6. Flash point andfirepoint of different insulating liquids used transformers [64].

a low erucic acid variety of rape-seed oil; more recently canola oil containg over 75% monounsaturate content has been developed.

Fig. 7. Fire point of different samples at different ageing conditions [39].

Fig. 5. Degree of biodegradability [63].

of pressboard impregnated with RAPSOLT was quite high. Moreover, accelerated ageing of rape seed oil impregnated paper insulation did not influence dielectric losses as much as for mineral oil impregnation. From this point of view, rape seed oil can be suitable impregnate for use in HV transformer. Liao et al. [56] found that BIOTEMP oil could decrease rate of degradation of paper and extend the life-time of paper insulation of transformers. McShane et al. [57] found that thermally upgraded Kraft paper ages considerably slower in the ester dielectric liquid than in ordinary mineral oil under similar thermal stress. Paper aged 5–8 times longer in the natural ester to reach the same end-of-life points as paper aged in mineral oil at 170 1C in sealed vessels. Yang et al. [58] also studied the effects of vegetable oil on the aging of Kraft paper using the accelerated thermal aging test. It was demonstrated that the useful life of Kraft paper can be increased by using vegetable oil for insulation. Gasser et al. [59] subjected high density pressboard to accelerated ageing tests with different vegetable oils, mineral oils and synthetic ester and found that the initial rate of aging was the same for mineral oils and ester oils. For ester oils, reaction with water was observed after certain aging time, depending on the aging condition. From then on, the aging of pressboard in ester oils slowed remarkably. At high temperatures and high moisture content, the ester oils consumed some of water

Fig. 8. Flash point of different samples at different ageing conditions [39].

hydrolysis, thus preventing the rise of moisture content in the pressboard and slowing down the deterioration of cellulose. 2.2. Production and usage of vegetable oil The researchers soon recognized that vegetable oils required further improvements to be used as transformer liquid. The liquid in sealed transformer remains in the unit for many years (30–40 years if not changed in between). Vegetable oil inherently has some components that degrade in relatively short time. The degree of unsaturation in an indication of thermal instability becomes more unstable as the degree of saturation progresses from mono to tri unsaturation. The relative instability to oxidation is roughly 1:10:100:200 for saturated, mono, di, and tri unsaturated C-18 triglycerides [60].The presence of copper in transformer enhances oxidation tendency so powerful oxidation inhibitors are needed for the oils used in transformers. Another factor is purity of the oil. The oil has to be free of conducting particles to acceptable levels, and commercial-grade oils are not available with this purity level.

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Fig. 12. Gases produced by vegetable oil and mineral oil [5].

Fig. 9. Viscosity of different samples at different ageing conditions [12].

Fig. 10. Percentage of gas generated by BIOTEMPs and Envirotemp FR3s [38].

Fig. 13. Comparison of oxidation stability of vegetable oil and mineral oil [42].

needed. Since the standard clay treatment is not enough so high absorbent neutral clay is used until the conductivity becomes to an acceptable value (usually below 1pS/m).

Fig. 11. Gases produced by vegetable oil (BIOTEMPs) [5].

The development of vegetable oil is a multistage process. First step is to select suitable oil and second is to purify it to electrical grade. Finally it is stabilized for the harsh environment in the transformer. These stages are explained briefly as follows. 2.2.1. Base fluid selection Vegetable oil seeds have two main components, the oil part and the solid part having protein called meal part. The oil is extracted from crude base by a process designated as RDB which stands for ‘Refined’, ‘Bleached’ and ‘Deodorized’. The commercially available RDB grade is the starting material. To extract the oily part, hydrocarbon solvents are used and then the solvents are removed. It is followed by refining and bleaching which involves treatment by absorbent clay and filtration. In deodorization process superheated steam is used to remove odor causing volatiles. In addition a winterization process is used to remove easily freezing saturate fats [5]. Fig. 4 shows typical oil seeds used from which oils are extracted and processed for transformer use. 2.2.2. Oil processing The RDB has conducting impurities above an acceptable limit to be used as transformer oil. Therefore further purifications are

2.2.3. Stabilization of the oil The vegetable oils become degraded when air oxidation will occur due to the inherent unsaturation of all vegetable oils. The oxidation rate is low for monounsaturated oils and high for triunsaturate oils (e.g. linseed oil with high triunsaturate content, a typical drying oil). The antioxidants can be used to protect vegetable oil from oxidation. The vegetable oil used as transformer oil should be subjected to the oxidation test. Though transformers are usually sealed, leaks and periodic maintenance operations could expose the oil to atmosphere. Over a period of years the oil could degrade. 2.2.4. Improving pour point Vegetable oils freeze at higher temperatures as compared to the mineral oil which has pour pint less than 40 1C. The freezing temperatures would vary from oil to oil due to the different composition of every oil. The high saturate content in oil would enhance the freezing point. Unsaturated oils have pour point in the range of  10–20 1C. The depressant derived from polymethylacrylate (PMA) can be used to further lower the pour point of the oil. The pour point can be decreased by 101 without affecting conductivity by using depressant value below one percent in concentration. It may be noted that the addition of depressants and antioxidants into the oil make it unsuitable for food consumption. 2.2.5. Chemical composition of vegetable oil The crude oil obtained from oil seeds is dark in color and also contain solid constitutes such as fibers, proteins and liquid (oil and fats). Both oil and fats are triglyceride esters of fatty acids, but fats contain relatively high percentage of saturated triglycerides and would freeze to solid below room temperature. The oily part normally remains liquid above 0 1C; oils with high unsaturation may remain as liquid at  15–30 1C. The triglyceride ester

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Fig. 14. AC breakdown strength of different types of oils with respect to moisture [65].

Fig. 15. Impulse breakdown strength of different oils [65].

Fig. 16. Breakdown voltage values of virgin oil samples at different gap distance [12].

Fig. 17. Breakdown voltage values of 30 days thermally aged samples at different gap distance [12].

molecule may be represented as

3. Performance comparison of vegetable oil vs. mineral oil

CH2  OOCR1

Mineral oil, used as cooling insulating liquids in power transformers, are obtained by petroleum distillation and followed by treatment with sulfuric acid refinery. The final characteristics of conventional mineral oil depend on the chemical composition [61]. On the other hand, vegetable oils are related to a group of organic compounds which are produced by reaction of an acid with the alcohol. Vegetable oils are natural ester molecules with triglyceride structure, produced from the chemical linkage of three fatty acids to one glycerol molecule [62]. The use of vegetable oils is increasing due to its advantages over mineral oils. Vegetable oils have high biodegradability (Fig. 5), therefore they are more environment-friendly [63].

j CH  OOCR2 j CH2  OOCR3 Where R1, R2 and R3 are fatty acid chains of same or different types. Saturated fatty acids with 8 to 22 carbon atoms are found in oils. Fatty acids with one unsaturated bond have 10 to 22 carbon atoms. Fatty acids with di-and tri-unsaturation mostly have 18 carbon atoms [5]. In Table 4, the fatty acid composition of some vegetable oils is shown.

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Fig. 18. (a) mineral oil impregnated pressboard and (b) Ester impregnated pressboard [40].

Fig. 21. Relative permittivity of aged and unaged samples [55]. Fig. 19. Breakdown voltage vs. kind of impregnation fluid comparison of unaged and aged test samples [55].

Fig. 20. Dissipation factors of aged and unaged samples [55].

3.1. Biodegradability 3.1.1. Flash, fire point and viscosity The flash point andfirepoint of different insulating liquids used transformers is shown in Fig. 6 [64]. Divakaran et al. [39] also studied and investiagted thermal charactiritics of mineral oil (virgin and aged) and vegetable oils (virgin and aged) and the result showed vegetable oils have better thermal performance. Coconut and palm oil have safe fire, flash pints, fire point and viscosity as shown in Figs. 7–9. 3.1.2. Gases produced after aging The liquids used in transformers for cooling and insulation experience thermal and electrical stress, so it is very important to determine the influence of these stresses.Gas generation is the most easy meausered property, and it is meaningful to study generation of gas after aging in the presence of Cu for certain period of time. Fig. 10 shows the percentage of gas generated in a test carried out by Double Engineering Lab on some specific vegetable oil fluids for 22 days and 250 1C [38].

The major difference in the decomposition products, as compared to hydrocarbons liquids, is the production of CO and CO2 in large amount. This is because ester liquids contain carbonyl group-COO, which breaks down to give CO and CO2. Hydrogen is produced due to additives in the oil as in case of FR3 fluid. The hydrogen, methane, CO and CO2 are main products under partial discharge conditions. The methane and hydrogen generation is same as their production from mineral oils and result from extraction of hydrogen atoms from molecular framework under electric field. Fig. 11 shows Gases produced by BIOTEMPs fluid and compares with transformer oil degradation [39]. The gases produced for mineral oil-based transformer oil are: hydrogen and acetylene under arcing conditions whereas CO and CO2 are produced in large quantities in addition for vegetable oils. The comparison of gas generation for vegetable oils (BIOTEMPs) and mineral oil is shown in Fig. 12 [39]. It was concluded that total gas produced by vegetable oil was only one-fourth of the gas produced from conventional transformer oil. This indicates the arc-quenching capability of vegetable oils is high. 3.1.3. Oxidation stability Takaaki Kanoh et al. [42] performed oxidation stability experiments and showed that there was a little change in both total acid value and breakdown voltage of vegetable oil (PFAE). Therefore, oxidation stability of PFAE was determined to be superior to mineral oil as shown in Fig. 13. 3.1.4. Breakdown strength The breakdown strength of oil is affected by water content present in the oil. The AC breakdown strength of different types of oils with respect to moisture (ppm) and moisture (%RH) is shown in Fig. 14. The impulse breakdown strength of different types of oils with respect to moisture (ppm) and moisture (%RH) is shown in Fig. 15. It is concluded from above figures that dielectric strength of ester oils less affected by moisture than mineral oil. Dijin Divakaran et al. [40] also studied and investiagted dielectric charactiritics of mineral oil (virgin and aged) and

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Table 5 Comparison transformer dielectric fluids-typical values [22].

Dielectric breakdown, kV Relative permittivity at 25 1C Viscosity at 0 1C, mm2 s  1 “ at 40 1C,” “100 1C” Pour point, ºC Flash point, ºC Fire point, ºC Density at 20 1C,kg dm3 Specific heat,J g  1 K  1 Thermal conductivity, W m  1 K  1 Expansion coefficient, 10–4 K  1

Mineral oils

Silicone oils

Synthetic esters

Vegetable oils

Test method

30–85 2.1–2.5 o76 3–16 2–2.5  30 to  60 100–170 110–185 0.83–0.89 1.6–2.0 0.11–0.16 7–9

35–60 2.6–2.9 81–92 35–40 15–17  50 to  60 300–310 340–350 0.96–1.10 1.5 0.15 10

45–70 3.0–3.5 26–50 14–29 4–6  40 to  50 250–270 300–310 0.90–1.00 1.8–2.3 0.15 6.5–10

82–97 3.1–3.3 143–77 16–37 4–8  19 to  33 315–328 350–360 0.87–0.92 1.5–2.1 0.16–0.17 5.5–5.9

IEC 60156 IEC 60247 ISO 3104

ISO 3016 ISO 2592(1) ISO 3675 ASTM E1269 (DCS) ASTM D1903

(1) Cleveland open cup procedure.

Fig. 22. Examples of streamer in JMEO (a) and mineral oil (b) under negative impulse voltage, d ¼30.

vegetable oils (virgin and aged) and the result showed vegetable oils have better dielectric insulation performance. Coconut and palm oil have dielectric strength and impulse voltage characteritics as shown in Figs. 16 and 17. The results of AC stress test for mineral oil pressboard and ester-pressboard are shown in Fig. 18; these results are in favor of the ester impregnated pressboard [40]. Hemmer et al. [55] investigated different samples of mineral oil and rape-seed oil and showed that rape-seed oil RAPSOLT have good compatibility with ordinary transformer mineral oil. Especially the AC breakdown strength of pressboard impregnated with RAPSOLT was quite high. Moreover, accelerated aging of RAPSOLT impregnated paper insulation did not influence dielectric losses as much as for mineral oil

impregnation. From this point it can be concluded that rape-seed oil can be a good alternate for insulation in high voltage transformers. The results of breakdown, dielectric loss and relative permittivity comparison of mineral oil and vegetable oil samples is shown in Figs. 19–21. Bertrand et al. [22] demonstrate the characteristics of different oils to be used in transformers. A summary of the results is shown in Table 5 along with the properties of different existing dielectric liquids. 3.1.5. Streamer initiation and propagation Sitorus et al. [77] studied experimentally streamer phenomenon in jatropha curcas methyl ester oil (JMEO) and mineral oil under positive and negative lightening impulse voltages. The streamer pattern is shown in Figs. 22 and 23.The negative streamers in mineral oil are

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Fig. 23. Examples of streamer in JMEO (a) and mineral oil (b) under positive impulse voltage, d ¼30.

highly filamentary and luminous as compared to those in JMEO. There was not obvious difference between positive streamers shapes in both of the liquids. Base on the shape and stopping length, the author concluded that JMEO could constitute a potential substitute for mineral and other synthetic oils for electric insulation particularly in high voltage power transformers.

3.1.6. Partial discharge and thermal aging Chandrasekar et al. presented a comparative assessment of the PD characteristics of thermally aged samples (natural esters and mineral oil). Time and frequency domain analysis of PD pulse are used. This work collected typical PD patterns from natural ester liquid in order to see its suitability high voltage power transformer applications [7]. The figures PD properties of natural esters are shown in Figs. 24 and 25. The influence of thermal aging on breakdown strength seems much less on natural ester liquids as compared to mineral oil. Lower PD activity was seen in palm and corn oils as compared to mineral oil.

3.1.7. Recent research work carried on vegetable oils A lot of research work has been carried out recently on vegetable oils as transformer oils by different researchers. A summary of current work on vegetable oils as transformer oils is shown in Table 6.

4.1. Environment friendly and adapted to sensitive areas The vegetable oil immersed transformers have become as one of the cleanest types of product available in the market today. The vegetable oil is made of food-grade seeds and is not considered as dangerous by international authorities such as EPA (Environmental Protection Agency) and OSHA (Occupational Safety and health Administration). It is biodegradable and non-toxic with recycling rate of more than 99%. It can be used near water points, fields and forests because of its water hazard classification of zero. It contains low aromatics and sulphur content and hence is environment friendly. 4.2. High sustainable efficiency As compared to the conventional mineral oil, the vegetable oil has a better health and environmental profile. It is biodegradable, so its spill away management solutions are easy. It also has ability to absorb moisture contained in the aging paper, so increase the insulation life of paper. It also helps chemically to prevent long cellulose paper molecules from aging due to heat exposure. These properties can increase the transformer insulation life and overloading capability. It can be used within the existing electric power infrastructure (with minor or no modification of transformer). 4.3. Less flammable product

4. Advantages From the literature review available in the field of vegetable oil usage, many advantages are noticeable. The following are some of the advantages of using vegetable oil as transformer oil [5,36,22,39,46,66–85]

Vegetable oil dielectric liquid has a high fire point as is classified as less-flammable dielectric coolant. Thus vegetable oil liquid does not contribute to an external fire and the products of combustion are nontoxic. Therefore vegetable oil is well adapted to installation in hazardous industrial area with high risk levels such as steel plants, Oil and Gas off-shore installations, Wind power farms, etc.

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4.4. Customer benefits

4.6. Renewable fuel

The vegetable oil filled transformers are normally installed indoors and tightly spaces outdoor, typically without additional fire safety measures. They also provide great risk mitigation of collateral damage from fire or explosion. They provide enhanced fire behavior, low risks to human health and increased overload withstand.

The vegetable oil is a renewable fuel that can be obtained from agricultural crops and other feed stocks that are considered as waste.

5. Technical difficulties and challenges and research gap 4.5. Strengthen agricultural economy

5.1. Technical difficulties

The development of vegetable oil industry would strengthen the domestic and specially the rural agricultural economy of agricultural based countries and it will help to reduce the costly mineral oil imports.

The major technical areas (with respect to the use of vegetable oils as insulation liquid in transformers), which need further attention is listed as following [5,42–45, 66–85]:

Fig. 24. Partial discharge pattern obtained for rod-plane electrode, configuration of unaged (left) and 30 days thermally aged (right) oils, (a) mineral oil (b) palm oil and (c) corn oil. Test voltage 25 kV.

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Fig. 25. Partial discharge pattern obtained for rod-plane electrode, configuration after 45 days of thermal stress, without copper (left) and with, copper (right) (a) mineral oil (b) palm oil and (c) corn oil. Test voltage 25 kV.

(i) More studies and research is needed to the reduce the production cost, develop low cost feed stocks and identify potential markets in order to balance cost and availability. (ii) Continued transformer performance and durability in a variety of transformer types and sizes need to be developed to increase manufacture and consumer confidence. (iii) Environmental advantages offered by vegetable oil over mineral oil must be advertised. (iv) Develop of additives for improving of cold flow properties, prevention of oxidation while storage and material compatibility etc. (v) Study of the effects of oxidized liquid on transformer performance and durability.

5.2. Special challenges and research gap The major challenges that face the use of vegetable oil as transformer oil are listed as follows [1,42,66–78]: (i) The price of vegetable oil dependent on the feed stock price. (ii) Storage and handling of vegetable oil is difficult. (iii) Homogeneity of the product depends on the supplier, feed stocks and production methods. (iv) Acceptance by transformers manufacturers in another issue. (v) Continuous availability of the vegetable oil needs to be assured before embarking on major use of it in transformers.

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Table 6 Summary of the recent research work carried out on vegetable oils as transformer oils. Properties

Kiyoshi Takamoto et al.

Villarroel et al. [83] TM

Rapsead oil

Mineral oil

Bio Electra

Dielectric strength (kV) Relative permittivity Tan δ Volume Resistivity Physical Viscosity mm2/s cSt Flash point 1C Fire point 1C Pour point 1C Density g/cm3 Kg/m2 Environmental Total Acid value (mgKOH g  1) Toxity Biodegradability source Appearance

81/2.5 mm 2.95@80 1C 3.1  10–3@80 1C 7.1  1012 5.06@40 1C

74/2.5 mm 2.86@80 1C 8.3  10–2@80 1C 4.4  1012 36@40 1C

70–75/2.5 mm 2.2@80 1C 1  10–3@80 1C 7.6  1015 8.13@40 1C

65 – – –

176 –  32.5 0.86@20 1C

334 –  27.5 0.92@20 1C

152 –  45 0.88@20 1C

– Non-toxic high – –

– Non-toxic high – –

– Slightly toxic low – –

Properties

Achmad Susilo et al. [81]

Electrical

Electrical

Physical

Environmental

Dielectric strength (kV) Relative permittivity Tan δ Volume Resistivity Viscosity mm2/s cSt Flash point 1C Fire point 1C Pour point 1C Density g/cm3 Kg/m2 Total Acid value (mgKOH g  1) Toxity Biodegradability source Appearance

Bio Temp

Shuhan Yaol et al. [84]

Zhengjiang Wang et al. [85]

Geminix

FR3

Cammellia oil

Mineral oil

56 3.0@90 1C 4  1010 @90 1C 4  1010 @90 1C r34.1@40 1C – 316 –  21 – 0.91@20 1C 0.04 – – – Light green

– – – – 39.9@40 1C

– – – – r 13@40 1C

322 –  28 – 0.90@20 1C 0.04 – – – Light yellow

4135 – o  22 – r 13@20 1C r 0.03 – – – Transparent

Z 35 2.2@90 1C r 0.1@90 1C 1  1012 @90 1C r 13@40 1C 39.2@40 1C 45@40 1C – 330 330 Z 135 –  26  15 o  22 – – – o 0.895@20 1C – – r 0.03 – – – – – – Sunflower seeds Sunflower seeds – – – Transparent 65 – – –

Gómez et al. [82]

Yanuar Z. Arief et al. [78]

Mineral oil

PFAE

Bivolt A

Bivolt HW

FR3

Mineral oil

Palm oil

Coconet oil

Sunflower oil

70–75/2.5 mm 2.2@80 1C 0.001@40 1C 7.6  1015 8.13@401C

85 2.95@80 1C 0.31@80 1C 7.1  1012 5@401C

50

50

48

57

1.6@100 1C

0.77@100 1C

1.9@100 1C

0.08@100 1C

75 3.1@40 0.03@25 1C

60 2.79@20 1C 0.08@20 1C

38–45 3.1@25 1C 0.0093@25 1C

300@251C

29.8 ̴ 31.6@40 1C

43@401C

36.6@40 1C 308 342

40.1@40 1C 308 338

36.7@40 1C 314 338

10@40 1C 138 148

4 220

225

o 330

0.9@15 1C 0.07

0.917@20 1C 0.02

0.919@20 1C 0.02

152 –  45 0.88@40 1C

176 –  32.5 0.86@40 1C

0.04 o0.01 – –

r0.03 0.05 – –

Corn flower

Sunflower

Soya oil

Petroleum

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PFAE OIl

TM

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(vi) The vegetable oil usage in transformers that are exposed to cold weather has been as issue. The pour point of vegetable oils does not go below  30 1C even after adding depressants. Without additives, the fluid could freeze at subzero temperatures. (vii) The transformer units having vegetable oil should be sealed well to prevent air and moisture from entering the unit. Antioxidants in sufficient amount should be present in the sealed unit because of possible entry of air and moisture during the life of the unit. 6. Conclusion Researchers in many countries carried out a lot of experimental works using vegetable oils as transformer oil substitutes. The use of vegetable oil as transformer oil can play a vital role in helping the world to reduce the environmental impact of mineral oils. The development of vegetable oil fluid fulfills current requirements for an environmentally friendly transformer liquid. Though environmental benefits are of great current interest, in the future, when petroleum products are eventually going to run out, and there could be serious shortages even by the mid-twenty first century. Fortunately the groundwork has been laid already by the development of suitable transformer liquid. Other possible applications for this liquid would be circuit breakers, tap changers, cables and capacitors. Further exploitation of these liquids for these applications need further study and tests.

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