Materials commonly used in insulation systems are: (i) insulating fluid: mineral oil .... principles and assembly procedures for building reliable 1100 kV AC power.
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 19, No. 6; December 2012
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EDITORIAL
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 [6] 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].
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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
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Allan, D. J., “Power transformers – the second century”, IEE Power Engineering Journal, January 1991 Halacsy, A.A., “Von Fuchs GH “Transformer invented 75 years ago”, AIEE Transactions, Part III, Power Apparatus & Systems, June 1961 Jeszenszky, S., “History of transformers,” IEEE Power Engineering Review, Dec. 1996, pp. 9–12. T. Prevost, T.V. Oomen, “Cellulose Insulation in Oil-Filled Power Transformers: Part I—History and Development”, IEEE Electrical Insulation Magazine, Vol.22, No1, Jan-Feb 2006 Blume, L. F. et al., “Transformer Engineering”, J. Wiley and Sons, New York, 1959 Wilson, W. R. “A Fundamental Factor Controlling the Unit Dielectric Strength of Oil”, AIEE Transactions, Feb.1953, pp.68-74 Kappeler, H. “Recent Forms of Execution of 380kV Transformer Bushings”, CIGRE, Paper No.126, 1958 Hasterman, Z., Mosinski, F., Maliszewski, A., “Electrical strength of power transformers” (in Polish), WNT, Warsaw, 1983 Moser, et al., “Transformerboard”, Scientia Electrica, 1979 Krause, Ch., “Dielectric Strength of Transformer Oil”, in Proc. of WICOR Insulation Conference, Rapperswil, Switzerland, Sept.1996 Kuffel, E., Zaengel, W. S., Kuffel J., “High Voltage Engineering”, Butterworth-Heinemann, Newnes, Oxford-Boston-Melbourne, 2000 Nelson, J. K., “An Assessment of the Physical Basis for the Application of Design Criteria for Dielectric Structures”, IEEE Transactions on Electrical Insulation, Vol.24, No.5, Oct.1989 Lokhanin, A. K., Morozova, T. I., et al., “Problems of Coordination of Dielectric Strength of Extra High-Voltage Power Transformer Major Insulation”, CIGRE, 12-06, 1970 Tschudi, D. J., “AC Insulation Design”, in Proc. of WICOR Insulation Conference, Rapperswil, Switzerland, Sept.1996 Ziomek, W., Vijayan, K., Boyd, D., Kuby, K., Franchek, M., “High Voltage Power Transformer Insulation Design”, IEEE Electrical Insulation Conference Record, Annapolis, MD, USA, 2011 Ch. Krause, “Suitable pressboard insulation relevant dielectric design principles and assembly procedures for building reliable 1100 kV AC power transformers”, IEC/CIGRE UHV Symposium Beijing, 2007. Kachler, A. J., Hohlein, I., “Aging of cellulose at transformer service temperatures. Part 1: Influence of type of oil and air on the degree of polymerization of pressboard, dissolved gases, and furanic compounds in oil”, IEEE Electrical Insulation Magazine, Volume: 21 , 2005 Oomen, T. V., Prevost, T., “Cellulose Insulation in Oil-Filled Power Transformers: Part II —Maintaining Insulation Integrity and Life”, IEEE Electrical Insulation Magazine, Vol.22, No2, Mar-Apr 2006 Mehta, S., et al., “Power Transformers; technology review and assessments”, Electra No 236, 2008 Boss, P., “Presentation of Cigre SC A2 ‘Transformers’; Technical developments and inputs from present activities”, Electra No 242, 2009 Schaut, A., Eeckhoudt, S., “Identification of early-stage paper degradation by methanol”, CIGRE, Paper A2-107, 2012 Popov, M., et al., “Analysis of Very Fast Transients in Layer-Type Transformer Windings”, IEEE Transactions on Power Delivery, Vol.22, No.1, 2007 Glinkowski, M., et al., “Electrical environment of Transformers – Impact of fast transients”, Electra 218, 2005 Mitra, P., et al., “Response of EHV Grid Transformers to System-Originated Oscillatory Switching Transients”, IEEE Transactions on Power Delivery, Vol.27, No.1, 2012 Yang, Y. and Wang, Z., “Broadband Frequency Response Analysis of Transformer Windings”, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 19, No. 5; October 2012 Huang, D., Shu, Y, Ruan, J, Hu, Y., “Ultra High Voltage Transmission in China: Developments, Current Status and Future Prospects “, Proceedings of the IEEE, 2009 Nayak, R. N., et al., “Technical Feasibility and Research & Development Needs for ± 1000 kV and above HVDC System”, Cigre Session 2010, Paper B4105, 2010 Zehong, L., et al., “R&D progress of ±1100kV UHVDC technology”, Cigre Session 2012, B4-201, 2012
