International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com
Developments in Cellulose based Insulating Kraft Paper for Liquid Immersed Power Transformers
Vishavdeep Jindal1, Jashandeep Singh2 1Research Scholar, 2Professor, Electrical Engg. Department, I.K.G. Punjab Technical University, Jalandhar, Punjab, India [email protected], [email protected]
Abstract: Cellulosic materials such as paper have been used winding, winding(s) to ground, lead to lead, lead to in the insulation system of power transformers for a long ground spaces, and (iii) the winding insulation: section time due to their ease of manufacture and economical as to section, turn-to-turn, lead to winding [1]. compared to other materials. But the major disadvantage of cellulosic material for electrical use is its hygroscopic nature B. Insulating Materials: which needs to be maintained dry. To conquer this limitation, a new area to look forward is mixing of cellulose Insulating materials permits only a negligible current in fibers with artificial fibers. The current paper discusses the order of pico ampere (pA) to flow in phase with the development of cellulose based electrical grade insulating applied voltage. There are many properties like; paper for use in transformer, improvements/up-gradations resistivity, breakdown voltage, permittivity and made and particularly the employment of synthetic fibers to dielectric loss, on the basis of which insulating extend cellulose based insulation life and successively materials can be characterized. An ideal insulating increase the lifetime of transformer. material should have: high dielectric strength, Keywords: Kraft Paper, Transformer Insulation, Synthetic sustainable at elevated temperatures, good thermal Fibers, Polypropylene Paper, Oil Impregnated Paper, Epoxy conductivity and high tensile and shear strength. In Impregnated Paper. addition to these, the insulating material should have good mechanical properties such as; vibration, abrasion I. NTRODUCTION I and bending. Also, it should be able to withstand Power transformer is essential equipment in an chemical attack, heat, moisture and other adverse electrical power system and its reliability is of extreme conditions of service. Insulating materials are non- importance as its failure ends up with huge expenses metallic, organic or inorganic; uniform or and interruption of power delivery. In turn, heterogeneous in composition; natural or synthetic. transformer’s performance depends greatly on its Many of them are of natural origin like; paper, cloth, insulation system; therefore the insulation is may be paraffin wax and natural resins. Wide use is made of foremost critical part of power transformer. Through many inorganic insulating materials such as; glass, almost a century, cellulose based products: primarily ceramics and mica. In the recent years, new materials paper and pressboard are being used in combination whose composition and properties place them in an with oil as insulating materials in high voltage intermediate position between inorganic and organic technology such as power transformers due to its substances are being used. These are the synthetic excellent dielectric strength properties. But it degrades organo-silicon compounds, generally termed as as the materials age. Therefore, the degradation of the silicones. The most important property of these cellulosic materials is an important factor in materials is their high thermal stability and excellent determining the life of a transformer. Assessing the dielectric strength and they retain this property for a state of cellulose insulation in a functioning transformer sustained period even at high temperature for short is difficult. For this reason many studies have been periods of time. directed towards monitoring the cellulose degradation Out of all defined insulating materials the Cellulose products in the transformer insulation oil. But this insulation has been preferred as solid insulation in method is indirect, and thus suffers from the major power transformers, but not because it is best. In fact it disadvantage that it is dependent on knowledge of the would have never been the preferred material if it was history of the transformer and its components. The not available in plenty from natural renewable source - current paper discusses the development of cellulose softwood. based electrical grade insulating paper for use in transformer C. Cellulose Paper Insulation: A. Insulation System: Cellulose is a linear condensation polymer consisting of anhydroglucose joined together by glycosidic bonds On the basis of design of core, the transformer (see Figure 1) [2]. The cellulose molecules will construction is basically of two types: core-type and organize in both crystalline and amorphous regions. The shell type. In the oil immersed power transformer core main mechanism of cellulose depolymerization is form, the insulation consists of: (i) the main insulation, linked to acid hydrolysis, at least at the level of the typically utilizing mineral oil which is also acting as the amorphous regions, in conjunction with a pyrolysis-like cooling medium, (ii) pressboard barriers in winding to mechanism in the crystalline regions of cellulose [3]. 178
International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com
monitoring of the transformer oil temperature (maximum allowable limit 140°C) along with a thermal model of the transformer can also give an estimate of the loss of life of the transformer due to overheating. The accelerated electric stress is an important factor considered in the oxidation of the oil. The weight and dimension restrictions of the transformer lead to a decrease in insulation clearances. Consequently, the oil ducts also become narrower and hence the strength of Fig. 1. Cellulose Molecule Chain the electrical field increases which exerts a great effect on the oxidation process and depolymerization occurs. Cellulose based electrical grade insulation paper is Due to which cellulose chain length and mechanical made by the delignification of wood pulp by the kraft strength of paper become lower and as a result the process. In [4] the manufacturing process of kraft paper insulation finally becomes brittle and carbonaceous was presented. The major constituent of paper is with no short circuit withstand capability. This stage is cellulose, which is a natural polymer of glucose. It has termed as an absolute end of life of transformer [12]. about 90% cellulose, 6-7% lignin and the balance is hemi-cellulose. The natural water content of paper is 4- End of life may be dictated by any one factor or by a 5% by weight and the insulation is dried after winding combination of factors. Much attention has been given to less than 0.5%. The dried paper is impregnated with to a paper ageing as a cause of transformer failure. insulating oil, which increases its dielectric strength and While it is undoubtedly a factor in reducing life, it does also serves to cool the windings. Power transformer not automatically lead to failure; some other influence conductor windings are insulated by paper impregnated is normally required, such as a mechanical shock. Thus with insulating oil, which is expected to last the life of the topic developments in insulating paper for power the transformer (25 years minimum at an operating transformers will be the subject of review which covers temperature of 65-95 °C). A typical 600 MVA the available literature on production and transformer contains 12 tonns (12,192 kg) of paper with characterization of electrical grade insulating kraft 30-120 μm thickness having density 0.7-0.8 g/cm3 and paper and their application in power transformers. 45 tonns (40,642 kg) of mineral oil [5]. II. INSULATION IN POWER TRANSFORMER D. Insulating Degradation/ Need of Upgradation: A. Historical Background of Transformers: Mainly heat, water and oxygen degrade (depolymerize) The first practical transformer was invented in 1884 and the cellulose, reducing the polymer molecular chain patented in 1885 [13-16] by team of Hungarian length which directly affects the mechanical strength of engineers: K. Zipernowsky, M. Déri and O. Bláthy, the material. Moisture is a product of ageing. Its from Ganz Companies in Budapest (now part of presence in the transformer insulation, increase Crompton Greaves, CG). At the same time, practically cellulose conductivity, creates gas bubble; reduce simultaneously to the Hungarian inventors’ work, thermal stability of the insulation system during similar development of transformer took place in USA overload conditions. Thermal ageing of insulating by W. Stanley (working with G. Westinghouse) and by materials in transformer is linked with the chemical S.de Ferranti in England. During more than 125 years of reactions occurring inside them. These chemical transformer development the operating voltage reactions are caused by pyrolysis, oxidation and increased from several hundred volts to more than 1000 hydrolysis, and are speed up by enhanced levels of kV and the power rating from a few kVA to more than temperature, oxygen and moisture contents. There are 1000 MVA. This marked progress was possible due to two main sources of moisture inside the transformer: (i) advances in transformer engineering, technology and ingress of atmospheric air into the tank. The amount of manufacturing, including development of insulating water into the transformer's tank strongly depends on systems. It is not easy to predict what new requirements the design of the transformer. The rate of water will be raised in the years to come. In any case, the contamination in transformers with membrane-sealed present state of the art is surely an important link in the conservator preservation systems is 0.03 to 0.06% of chain leading into the future. It seems that in near future, water present in cellulose per annum whereas it is up to the transformer will still remain an important element of 0.2% per year in an open-breathing conservator [6] [7]. power systems. (ii) Thermal aging processes of insulation [8] [9] [10]. The best technique for evaluating the ageing of such B. Insulation Development in Transformers: thermally resistant papers is the determination of the The insulation system of a power transformer is degree of polymerization (DP) of the constituting understood as the complete internal assembly of cellulose chains. However, the technique turns out to be dielectric insulating materials. Electrical insulation impractical due to the difficulty in retrieving paper materials are a vital part of transformers, as they must samples in the field [11]. Continuous on-line 179
International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com
withstand the operating temperatures that occur during 120% and 200% etc. with coarse, medium or fine the transformer’s operating life. The lifetime of a creeping. On impregnating the paper with transformer transformer depends on the reliability of the insulation oil after drying and degassing, the voids in paper are used. Transformer insulation had developed filled-up with oil expelling the air. Drying process of concurrently with the transformer development, but it paper takes place in a warm vacuum chamber. In order took a few decades before the paper-oil combination to avoid damage to cellulose, slow drying is performed became reliable and well accepted. This combination of within a temperature of 120° C on achieving vacuum of oil and fibrous cellulose materials has dominated the 0.01 Torr, oil is impregnated at the reduced temperature technology of power transformer insulation since of about 60° C. In the composite insulation, the electrification began about a hundred years ago. This dielectric properties are improved considerably. Electric combination was used to satisfy the increasing strength improves from 10 or 11 kV/mm to 60 or 70 insulation requirements as the voltage readings kV/mm and tan , decreases from 0.1 or 0.2 to 0.004 to escalated. In the 1940s, kraft paper in combination with 0.006 at approximately 90°C. Standard values of oil was the dielectric material of choice for high voltage relative permittivity, loss tangent and electric strength transformers. During the last 50 years, several designs of paper impregnated with oil at 20° C are 3.5 - 3.9, of dry-type insulation systems have been used for 0.0026 – 0.003 and 60 - 70 kV/mm. Above results are distribution transformers of limited rating, up to a few considerably affected by the presence of moisture and MVA, and corresponding moderate system voltage ‘Service electric stress’. To give trouble free life to levels. But, for large, high voltage transformers the equipment, it is advisable to process and impregnate the traditional oil filled design still prevails [17]. Many new paper with transformer oil carefully and keep the insulation technologies have been proposed and tried. maximum 'Service stress' underv2 kV/mm. Synthetic dielectric materials which slowly developed C. Epoxy Impregnated Paper: in the late 1950s, began to replace cellulosic insulation in power cables and capacitors. Mixtures of cellulosic As per Atkinson & Thomas [22] in April, 1967 Epoxy- and synthetic materials are now used in many impregnated paper offers a solid insulation system transformer insulation applications [18]. having good thermal stability, high dynamic and static strengths both through and between laminations, high III. TYPES OF INSULATING PAPERS dielectric strength, and little change in power factor A. Insulating Kraft Paper: with temperature and voltage. It has good chemical and moisture resistance, and high impermeability to gases The cellulose material used in modern transformers is and liquids. It is a resin-rich (approximately 70 percent) known as kraft paper. Insulating Kraft Paper, Class Y material offering one of the few solid insulation systems insulation, are one of the earliest forms of composite that is void-free, resulting in high corona-inception dielectrics used in high voltage engineering since the voltages. end of the 19th century. Paper as such has very poor dielectric properties, but when impregnated with oil, the D. Polypropylene Paper: properties of the composite dielectric improve Buntin & Wesselhoft [23] checked the properties of considerably [19]. Paper is a product of 'Cellulose’ polypropylene paper in December, 1972 for possible obtained from soft wood pulp of mainly pine or spruce use as an insulating material for power transmission found in Scandinavia [20]. Cellulose is a complex cables carrying electric stresses excess of 500 kV for carbohydrate, which forms the chief constituent of the replacement of Kraft Paper. It has been recognized for cell walls of plants. The cell walls contain lots of some time that oil-impregnated cellulose paper will not contaminants like lignin and resins which are removed be a useful insulating material for cables carrying in by the sulphate process treatment and careful water excess of 500 kV because of prohibitive dielectric loss washing. Normal thicknesses of paper available in the heating and excessive increase of insulation thickness. market are 15, 20, 25, 50, 65, 75, 100, 125, 160, 200 or In order to reduce dielectric losses, increase dielectric 250 micrometers. It forms an excellent insulation strength, and hence reduce insulation thickness in oil- between primary and secondary windings. Additional filled cables, the cellulose paper has to be replaced by strength is provided during taping process so as not to some properly chosen synthetic material that exhibits allow presence of air pockets, which are trapped better dielectric properties than cellulose paper does. between layers. This additional strength is provided by Such material should also have sufficient mechanical use of crepe paper. strength and it should be compatible with liquid B. Crepe Paper: impregnants. Paper-like structures obtained from such polymeric materials as polyphenylene oxides [24] and Crepe paper is made of Kraft insulating paper by a polyolefin’s have shown considerable promise as process, which imparts irregular close crimps to it, thus replacements for cellulose paper in cable insulation. increasing its thickness as well as extensibility in the While polyphenylene oxides exhibit many attractive machine direction [21]. Crepe paper normally available properties for the contemplated use in oil filled cables, has nominal values for elongation break at 50%, 80%, 180
International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com
they are not yet available as an industrial product with The addition of nano-SiO2 increases the number of fixed specifications. hydrogen bonds in the modified aramid fiber insulation paper and reduces its free volume, which can improve E. Thermally Upgraded Insulating Paper: the mechanical properties, thermal stability, and In order to reduce the rate of molecular chain reaction electrical properties of aramid insulation paper. of cellulose and thus slow down the loss of paper tensile Nikjoo et al. modified the insulating paper with using strength during aging inside transformers, thermally adsorbed nano-particles such as Silica (SiO ) with two upgraded Kraft (TUK) papers were developed and 2 different concentrations i.e (2 wt% and 6 wt %) and introduced in transformers as early as 1950 [25, 26]. In zinc oxide (ZnO) with concentration (1 wt%) and 1962, the advantages of this type of paper were explained the partial discharge behaviour of modified officially recognized by the National Electrical paper samples and concluded that the papers with 2 wt% Manufacturers Association [27]. Three main SiO nanoparticles (with purification) and 6 wt% SiO2 methodologies have been found in literature to modify 2 nanoparticles improved the Partial discharge behaviour traditional kraft paper to produce TUK paper: (i) based against the impulse compared to the reference/basic on the chemical modification of reactive OH groups of paper and the modified paper samples with 1 wt% cellulose and hemi-cellulose to replace them with more silanized ZnO nanoparticles [34]. The method of stable groups (cyanoethyl) via cyanoethylation or addition of silica (SiO2) with concentration from 2.3 wt% acetylation during the paper pulping stage [28]. This to 15.3 wt % was discussed later on by Hollertz et al. process reduces hydrogen bridges and the mechanical using layer-by-layer technique and showed that layer- strength of paper [27]. This method was later discarded by-layer technique provides an efficient method to tailor due to environmental concerns [26], [29]. (ii) Addition the dielectric and mechanical properties of wood-fibre of nitrogen stabilizing agents (amide salts) such as urea, based electrical insulation materials [35]. melamine, dicyandiamide and polyacrylamide [29], [30]. The amide products, which are weak bases, react with G. Cellulose Insulating Paper Modified with Alumina: the cellulose degradation products, thus reducing the To improve the thermal stability of the cellulose oxidation of cellulose by partially consuming water and insulating paper used in power transformer, Al O partially neutralizing the acidity in oil [27] [28]. As a 2 3 nanoparticles were also used to modify traditional result, these stabilizing agents slow down the cellulose insulating paper [36]. Lei Wen et al. have autocatalytic reactions of cellulose aging. This method studied the impact of the addition of Al O is generally applied during the final stage of paper 2 3 nanoparticles and its relative content on the mechanical making, interacting mostly with paper surfaces [27, 28] properties of propylene [37]. Their experimental results or during the crinkle process (creping) in micro or demonstrated that, when the addition of nano Al2O3 creped papers. (iii) Addition of high-temperature meta- occupies 1 % of the PP mass, the increase in the aramid polymer (Nomex®) to the cellulose pulp magnitudes of the composite tensile strength, flexural resulting in a paper having better operating performance strength and impact strength are greatest. Chen Yufei et against oxidation at even higher temperatures, which al. have shown that, with an increase of nano Al O extends its lifetime [31]. 2 3 content, the corona resistance of polyimide material F. Thermally Upgraded Insulating Paper: increases significantly and the thermal decomposition temperature is 20oC higher than that before doping [38]. Aramid fiber exists in two forms: one is composed of Koner et al. have studied the impact of nano Al O on short-cut aramid fiber filaments, referred to as “short- 2 3 various properties of composites, where the results cut fiber”; the other form is fiber pulp, which is called show that the elastic modulus and the maximum tensile “precipitation fiber” [32]. Aramid insulation paper strength of the composites are significantly improved consist of short fibers and fiber pulp in the ratio of [39]. Wang Ke et al. have studied the impact that the 1.5:1.Chao Tang et al. prepared a composite insulation addition of nano Al O particles has on the mechanical paper by combining aramid insulation paper with nano 2 3 properties of ethoxyline resin, showing that when the particles of Silica (SiO ) with 1 wt.% and tested the 2 nano Al O content is equal to 1%, the tensile strength mechanical properties and breakdown strength of 2 3 and elongation are the highest and the tensile strength modified composite insulation paper in comparison and the elongation are the highest and the tensile with unmodified aramid insulation paper [33]. The strength will increase by 19% [40]. experimental results showed that, the tensile strength, breaking elongation, and the AC breakdown strength of IV. CONCLUDING REMARKS the modified aramid fiber with a nano-SiO2 content of 1 wt.% increased by about 7.5%, 10% and 14%, Till date cellulose based papers have been used in respectively as compared with the unmodified fiber. (3) electrical applications as an insulating material due to There exists a strong interfacial effect between the its excellent dielectric properties. However, the use of paper has certain disadvantages with regard to its nano-SiO2 and aramid fiber, and this effect is mainly reflected in the induction force and hydrogen bonds. moisture absorption and low thermal stability. It is desirable to review the use of cellulose in transformers 181
International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com
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