transformer development the operating voltage increased from several hundred ... typically utilizing mineral oil which is also acting as the cooling medium, and ...
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 19, No. 6; December 2012
TRANSFORMER ELECTRICAL INSULATION This Special Issue of Transactions on Dielectrics and Electrical Insulation is dedicated to transformer insulation. The transformer is a very essential apparatus in an electric power system and its reliability is of utmost importance as a transformer failure results in a very costly and difficult to predict interruption of energy delivery. In turn, transformer’s performance depends heavily on its insulation system; therefore the insulation is perhaps the most critical transformer part. The first practical transformer was invented in 1884 and patented in 1885 [1-4] by team of Hungarian engineers: K. Zipernowsky, M. Déri and O. Bláthy, from Ganz Companies in Budapest (now part of Crompton Greaves, CG). At the same time, practically simultaneously to the Hungarian inventors’ work, similar development of transformer took place in USA by W. Stanley (working with G. Westinghouse) and by S.de Ferranti in England. During more than 125 years of transformer development the operating voltage increased from several hundred volts to more than 1000 kV and the power rating from a few kVA to more than 1000 MVA. This marked progress was possible due to advances in transformer engineering, technology and manufacturing, including development of insulating systems. The modern electric power industry employs two basic transformer constructions: core-form, which is predominant, and shell-form. Insulation for the core-form, oil-immersed power transformer consists of: (i) the main insulation, typically utilizing mineral oil which is also acting as the cooling medium, and pressboard barriers in winding to winding, winding(s) to ground, lead to lead, lead to ground spaces, and (ii) the winding insulation: section to section, turn-to-turn, lead to winding. Materials commonly used in insulation systems are: (i) insulating fluid: mineral oil, synthetic, or vegetable oil (natural esters), (ii) conductor insulation: paper (kraft, Nomex®, enamel), (iii) ‘solid’ insulation, i.e. barriers, blocks, spacers, made of pressboard, transformer wood (densified wood), or wood (typically it is maple or beech wood). The development of a breakdown in liquid is complex and still under ongoing investigations. At present, there is no uniform theory explaining all processes leading to breakdown in oil. Theoretical studies established the following main mechanisms of breakdown in liquids: (i) electrical, with development of electron avalanche, leaders, and streamers, (ii) ionic, utilizing the ion conduction in contaminated liquid, (iii) suspended particles, which polarize in the field and concentrate, resulting in breakdown, (iv) gaseous, where presence of gas bubbles decreases the local dielectric strength, (v) electro-convection, involving dynamics of space charge in liquid and charge deposition on the cellulose insulation, see e.g. [5-14]. For practical purposes of estimating the withstand of insulating mineral oil, somewhat simplified approach is suggested [6, 7], where the dielectric strength was expressed as a function of inter-electrode gap, surface of electrodes, and the volume of oil contained between electrodes, where the latter is a result of two former phenomena. The volume effect  allows for comparison of the breakdown strength in different electrode systems. In general, the dielectric strength of an oil cellulose insulation system depends on the duration of voltage application, polarity of voltage, field enhancement factor, area and shape of electrodes, kind and degree of contamination of the oil, its temperature and pressure. The transformer insulation design should be prepared with careful consideration for all these aspects. Ongoing development of insulating structures utilizing the molded or formed pressboard parts allows for operation at higher electric stresses and results in reduction of size, weight and cost of transformer. The transformer designers optimize the pressboard barrier structures using the two- and three dimensional electric field calculations [e.g. 9, 14-16]. The transformer manufacturing processes are under continuous development. Physical phenomena influencing the status of insulation during assembly, drying, during and after oil impregnation are carefully considered to ensure the designed dimensions of windings and active part and required clamping forces. The transformer in operation is subjected to numerous phenomena affecting its insulation. Aging processes pyrolysis, hydrolysis, and oxidation, as well as partial discharges gradually weaken the solid insulation. Precise assessment of the status of the insulation is still a challenge to power utilities, as available methods (DGA, furan analysis, methanol analysis, partial discharge detection and location, etc.) are far from perfect [17-21]. There is a renewed interest in studying fast transient and very fast transients resulting from interaction between a transformer and a switching device (typically insulated with SF 6 or vacuum). These fast transient voltages, typically with smaller magnitude than the lightning strokes, pass under the protection level of surge arrestors and enter the transformer, causing significant excitation when they closely match the winding’s natural resonant frequencies. These localized internal resonance phenomena may result in significant amplification of voltage within the windings causing the breakdown of insulation [22-25].
New challenges are related to recent activities in UHV transformer applications, with ac voltages at 1100 kV and dc voltages at 800 kV, increasing to 1000 kV [26-28]. These efforts will require improvement and development in the area of design methods, insulating structures, manufacturing methods, testing techniques, field measurements, etc. It seems that the progress in transformer technology will never end… I would like to thank the Editor-in-Chief, Dr. Reuben Hackam, for his firm support and commitment to the publication of this Special Issue on Transformer Insulation. Waldemar Ziomek Guest Editor
                           
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