CHARACTERISTICS OF DIELECTRIC FLUIDS IN POWER TRANSFORMER APPLICATIONS A REVIEW

Transcription

1 CHARACTERISTICS OF DIELECTRIC FLUIDS IN POWER TRANSFORMER APPLICATIONS A REVIEW M. Karthik 1, Satyajeet Sharma 2, M. Visalakshi 3 1 Assistant Professor, 2,3 UG scholars 1, 2, 3 Department of Electrical and Electronics Engineering, Coimbatore Institute of Engineering and Technology, Coimbatore, Tamil Nadu, India. Abstract - The energy demand of the world is dominated by petroleum products for centuries in the field of critical application like electricity. Mineral oil is used as a dielectric fluid in a power transformer applications and its properties like breakdown voltage, viscosity, water content, acidity, Inter Facial Tension, Dissipation Factor, Resistivity, Aneline Point, Oxidation Stability, Inhibition Content, Gassing Tendency, Density, breaking point and fire point credible considered by the ASTM standards and by this the fluids conceivably selected as a fluids for transformer applications. In this paper the various properties of dielectric fluids and their standards are discussed. Keywords mineral oil, ASTM standard, density, breakdown voltage, viscosity. I. INTRODUCTION Transformer is a static device consisting of two or more windings which comprehend a common magnetic field [1, 2, 3]. The main purpose of power or distribution transformer is to transfer electrical power from one voltage level to another which turn off the principal of electromagnetic induction. The main components of the transformer are core, winding, insulation and tank. The purpose of the transformer core is to produce a low reluctance path for the magnetic flux linking the primary and secondary windings. The winding of the transformer are so arranged incidentally reduce the leakage flux. Development in the material of transformer core has resulted not only the losses but also the reduction of the noise [6]. Power transformer windings have mostly been made from annealed Copper, but Aluminium is also used invest the rating and the cost involved. Normally multiple layer-winding system is used and the impulse voltage is shared between the layers. This gives a much better distribution of voltages than a disc winding where a large either the voltage is across the first few coils. Insulation of transformer is provided mainly through mineral oil and Kraft paper [4, 5]. The mineral oil is still being used with better refining and selection. In earlier days core is counting cast iron type. Since then transformers have almost invariably been constructed from welded plates [10]. For cooling the transformer, different methods are used supposing its capacity and requirement as condition for operation, overall size limitation, capitalization of losses, etc [8, 9, 11]. Suitable bushings are used for insulating and bringing the terminals of the winding out of the case [12]. II. TRANSFORMER INSULATING OIL Insulating oil in an electrical power transformer is commonly known as transformer oil. It is obtained by fractional distillation and subsequent treatment of crude petroleum. That is why this oil is alias Mineral insulating oil [15]. Transformer oil serves mainly two purposes one it is liquid insulation in electrical power transformer and two it dissipates heat of the transformer i.e. constitute coolant. In addition to these, it constitute other two purposes, it helps to preserve congenital and winding as these DOI : /IJRTER IHOAT 239

2 are fully immersed inside oil and another important purpose about oil is in case direct contact of atmospheric oxygen with cellulose made [13, 14]. III. CLASSIFICATION OF INSULATING LIQUIDS Various liquids are used as a insulating fluids. The Insulating liquid credible classified in to many category based on their chemical structure [16]. A. Mineral oils Mineral oils are complex mixtures of hundreds of different organic compounds, consisting of carbon and hydrogenin molecules with different structures [18]. They are made by refining the hydrocarbons collected during the distillation of petroleum crude stock [17]. The physicochemical properties of oil may vary. There are three categories of crude oils, namely paraffinic, naphthenic, and mixed crudes. Paraffinic crudes contain a small change of naphthenic hydrocarbons and conceivable subdivided into normal paraffins and isoparaffins. Isoparaffins are preferred over normal paraffins per their lower Pour points. Naphthenic crudes have higher naphthenic compound content than do paraffinic crudes. Mixed crudes are intermediate between paraffinic and naphthenic crudes [20, 21, 22]. B. Synthetic insulating liquids Although mineral oils play a vital role in the power industry, synthetic liquids are used when special properties are more, e.g., fire resistance, partial discharge resistance, negative gassing tendency. A abundance of synthetic insulating liquids are available, e.g., halogenated hydrocarbons, aromatic hydrocarbons, high molecular weight hydrocarbons, poly butanes and phthalates can absorb considerably more moisture than mineral oils before their performance as insulants deteriorates significantly. Previous work [19] has shown that ester liquids perhaps used for retro filling mineral oil filled transformers. C. Silicone oils Silicone oils are known chemically as poly-dimethyl siloxanes or PDMS [23], [25]. They were introduced in the 1970s as aid PCBs and have proven popular in retro filling transformers [24], [26]. However, their use has been practice situations where fire could pose a risk to personnel and property. Their chemical structure has been described [27, 28]. Silicones are eco- friendly and flame retardant, age well, and are strongly resistant to oxidation and sludge formation [26]. As far as their cooling and insulating properties are concerned, they compare well with mineral oils. Silicone oils are colorless and have very low pour points near mineral oils, against their viscosities at 20 C are much higher. In order to avoid large temperature rises during operation, silicone filled transformers must be de-rated (up to 10%) or supplied additional cooling capacity. IV. CHARACTERISTICS OF INSULATING FLUIDS A. Dielectric strength Dielectric strength of transformer oil is alias Breakdown Voltage of transformer oil or BDV of transformer oil and its test cell is shown in Figure 1. Breakdown voltage is measured by observing at what voltage, sparking strands between two electrodes interested the oil, separated by specific gap. Low value of BDV indicates the presence of moisture content and conducting substance in the oil. In BDV kit oil is kept in a pot anywhere one pair of electrode are equip a gap of 2.5 mm between them. Now slowly rising voltage is applied between the electrodes. Generally this measurement is taken 3 to 6 materialize same sample of oil and the average value of these reading is taken. BDV is important and popular test of transformer oil, as it is primary indication of health of oil and it perchance easily carried out at site. Dry and clean oil gives BDV results, better than the oil with moisture content and other All Rights Reserved 240

3 impurities [26]. BDV is recommended by the standard IEC and based on the standard the oil should posses greater than or equal to 30kV. Figure 1. Breakdown voltage test cell. B. Resistivity This is another important property of transformer oil. This is measure of DC resistance between two opposite sides of one cm³ block of oil. Its unit is taken as ohm-cm at specific temperature. With increase in temperature the resistivity of oil decreases rapidly. Just after charging a transformer after long de activate, the temperature of the oil will be at ambient temperature and during full load the temperature will be very high and may scale to 90ºC at overload condition. So resistivity of the insulating oils must be high at room temperature and also it should have value at intense heat as well. That is why specific resistance or resistivity of transformer oil should be measured at 27ºC. The actual minimum standard for specific resistance of transformer oil at 90 o C is 35X ohm cm (maximum) and at the lower boundary is estimated at 27 o C is 1500X10 12 ohm cm (minimum) [20]. C. Dissipation factor Dissipation factor is alias loss factor or tan delta of transformer oil. When an insulating material is placed between live part and grounded all electrical equipment, leakage current will flow. As insulating material is dielectric in nature the electric current through the insulation ideally leads the voltage by 90º [21]. Here voltage means the instantaneous voltage between live part and ground of the equipment. But in reality no insulating materials are perfect dielectric in nature. Hence electric current through the insulator will lead with an angle shorter than 90º is called dielectric dissipation factor. D. Water content Moisture or water content in transformer oil is highly undesirable as it affects adversely the dielectric properties of oil. The water content in oil also affects the paper insulation of congenital and winding of the transformer. Paper is highly hygroscopic in nature. Paper absorbs maximum as water from oil which affects paper insulation property further reduced its life. But in loaded transformer, oil becomes hotter, hence the solubility of water in oil increases over the paper releases water and increase the water content in oil. Thus the temperature of the oil as of taking sample for test is very important. During oxidation acid are formed in the oil the acids give rise the solubility of water in the oil. Acid also water further decomposes the oil forming acid and water [27]. This rate of degradation of oil increases. The water content in oil is measured as ppm. Water content in oil is allowed qualified 50ppm as recommended by IS- 335(1993). It is measured that the water content for the pure oil should be lower than 50ppm. E. Acidity Acidity of transformer oil is harmful property. The acidity determination setup is shown in Figure 2. If oil becomes acidic, water content in the oil becomes more soluble to the oil. Acidity of the oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates the All Rights Reserved 241

4 process in the oil [28]. Acid also includes rusting of iron in presence of moisture. The acidity of transformer oil is measure of its acid constituents of contaminations. Acidity of oil is express in mg of KOH required to neutralize the acid present in a gram of oil. This is alias neutralization number. Acidity is calculated according to the standard ASTM D974 [27]. For the purpose of insulating medium, oil should have acidity less than 0.4 mg KOH/g. Figure 2. Acidity determination setup F. Oxidation Stability Oxidation stability is one of the most important properties of insulating oils. In many cases good oxidation stability may be achieved by a suitable content of the naturally occurring aromatics [21]. However, nitrogen and sulphur compound-free oil is required. The evaluation of oxidation stability requires accelerating ageing tests. ASTM D 2440 is used for the qualification of oils and has two indicative limits which, for the 72 hour test, are 0.1% of sludge free life of the oil sample. The limit set every double procedure is 40/80 sludge by mass and 0.3 mg KOH/g or 0.5mg KOH/g. ASTM D 2112 is used for both qualification and acceptance testing of inhibited oils and has a minimum limit of 195 minutes. The double power factor valued oxidation test measures the power factor of an oil sample under accelerated ageing conditions. It measures oil quality suitable the sludge formation and measures hours. G. Inhibition content Inhibition content is a parameter used for oil vendor qualification, but the infrared absorption method adopted by ASTM D 2668 in semi-quantitative and of limited reliability. The ASTM D 3487 property requirements quote 0.08% of ditertiary-butyl-paracresol or 2.6- of ditertiary-butyl-phenol inhibitors for Type 1 oils and 0.3% inhibitor by mass for type 2 oils. An alternative to the most common method is speedy liquid chromatography [21]. H. Gassing tendency Gassing tendency is evaluated in new oils to satisfy specification requirements, but it is not usually used as a prime indicator of oil characteristics. Some standards recommend the measurement of the total gas content of new oil [22]. A dissolved gas analysis can identify low energy discharges further high thermal stresses and disruptive discharge conditions. Some utilities prefer oils with negative values, but positive values function when price and availability are easy mark to account. Gas-absorbing oils may become strongly gas evolving during service. I. Interfacial tension Interfacial tension between the water and oil interface is the way to measure molecular attractive force between water and oil. It is measured in Dynes/cm or milli neutron/m [28]. Interfacial tension is exactly useful for determining the presence of polar contaminants and oil decay products. Good new oil generally exhibits high interfacial tension. Oil oxidation contaminants lower the interfacial All Rights Reserved 242

5 Interfacial tension is calculated according to the standard ASTM D971. As per requirement, interfacial tension should be greater than 30mN/m. J. Flash point Flash point of transformer oil is the temperature at which oil gives enough vapour to produce a flammable mixture with air. This mixture gives necessary consider application of flame under standard condition. The flash point kit is shown in Figure 3. Flash point is important because it specifies the chances of fire hazard in the transformer. So it is desirable to have very high flashpoint of transformer oil. In general it is subject 140ºC. The flash point is recommended by the standard ASTM D 93[27].Based on the standards the flash point must be atleast greater than 150 C. K. Fire point It is the minimum temperature at which oil just start to flow under standard test condition. Pour point of transformer oil is an important property mainly at the places where climate is extremely cold. The fire point kit is shown in Figure 3. If the oil temperature falls below the pour point, transformer oil stops convection flowing and obstruct cooling in transformer [26]. Paraffin based oil has higher value of pour point, compared to Naphtha based oil, but in India like country, it does not affect the use of Paraffin oil being its warm climate condition. Pour point of transformer oil mainly gives wax content in the oil. As Paraffin based oil has more wax content, it has higher pour point. The fire point is recommended by the standard ASTM D 93. Based on the standards the flash point must be atleast greater than 150 C. Figure 3. Flash and Fire point kit. L. Aneline point Aneline point estimates the total aromatics content in the oil. The aneline point test is used as a qualification test, but not by all utility companies. It is related to the impulse strength, gassing characteristics and oxidation stability of an oil. ASTM D 3487 specification quotes a range of values, from 63ºC or 84ºC; however, in latest version the point double aniline was evaluate 80ºC maximum [24]. M. Viscosity Viscosity of transformer oil conceivable defined as the resistance of flow, at normal conditions. The viscometer used to measure viscosity is shown in Figure 4. Good transformer oil should have low viscosity thereby not affecting the cooling of transformer [26, 27]. Low viscosity of transformer is essential, but it is equally important that, the viscosity of oil should increase as less as possible with decrease in temperature. Every liquid becomes more viscous if temperature decreases. The viscosity is recommended by the standard ASTM D All Rights Reserved 243

6 \ Figure 4. Viscometer N. Density Density is not very significant in determining the quality of the oil but may serve for type identification and evaluation of oils suitability for use. Specific gravity is important when there is a concern about water in oil freezing and applaud float superior the oil [28]. A lower limit for specific gravity would eliminate this potential problem. In cold climates, density may be pertinent in determining oil suitability. Normally, new Paraffinic oils have higher densities, while the mixed base has lower densities. V. COLOUR AND APPEARANCE Colour and Appearance credible used during both acceptance and qualification tests to eliminate substandard oils. For new oils the maximum acceptable [27]. Colour value is 0.5 and a bright and clear appearance is required. However, slightly colored new oil does not always indicate poor quality. VI. CONCLUSION Insulating liquids are a vital either the electrical insulation system in many types of electrical power equipment, including transformers, bushings, cables, and capacitors. Each application requires an insulating liquid with specific electrical, chemical, and physical characteristics. However, observation on oil - filled circuit breakers and cables has almost stopped; extruded insulated cables (mainly poly propylene or cross-linked polypropylene) have replaced oil-filled cables, and breaker technology has evolved toward the use of SF6 at high voltage and vacuum at medium voltage. Mineral oils have been used in electrical apparatus for over a century, and they have a long and proven track record. Quality and stability requirements have become more stringent over time; the last few years have seen much improvement in quality as refining technology has advanced through the use of catalytic hydroprocessing. REFERENCES [1] C. P. McShane, New safety dielectric coolants for distribution and power transformers, IEEE Ind. Appl. Mag., pp , May/Jun [2] I. Fofana, V. Wasserberg, H. Borsi, and E. Gockenbach, Retro filling conditions of high voltage transformers, IEEE Electr. Insul. Mag., vol.17, no. 2 [3] N. Berger, M. Randoux, G. Ottmann, and P. Vuarchex, Review of insulating fluids, Cigre WG 15.02, Electra, no. 171, Avril [4] A. Beroual, M. Zahn, A. Badent, K. Kist, A. J. Schwabe, H. Yamashita,K. Yamazawa, M. Danikas, W. D. Chadband, and Y. Torshin, Propagationand structure of streamers in liquid dielectrics IEEE Electr. Insul.Mag., vol. 14, no. 2, pp. 6 17, [5] N. Berger, Liquidesisolants en electrotechnique, Techniques de l ingenieur,d2 471, 24 pages, [6] Working Group A2-35, Experiences in service with new insulating liquids, Cigre Report 436, Oct a, pp , Mar./Apr [7] E. D. Senkevitch, V. G. Arakelian, T. V. Glasunova, V. A. Lipshtein, T. I.Morozova, and N. M.Panova, New synthetic liquids for transformers, presented at the CIGRE Symposium, Vienna, Austria, 1987, paper All Rights Reserved 244

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