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