10_freq_dielctric - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 3; June 2010 1070-9878/10/$25.00 © 2010 IEEE 799 On the Frequency Domain Dielectric Response of Oil-paper Insulation at Low Temperatures I. Fofana 1,2 , H. Hemmatjou 1,2 , F. Meghnefi 1,2 , M. Farzaneh 2 , A. Setayeshmehr 3 , H. Borsi 3 and E. Gockenbach 3 1 Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), University of Quebec at Chicoutimi, Qc, Canada 2 International Research Centre on Atmospheric Icing and Power Network Engineering (CenGivre), University of Quebec at Chicoutimi, Qc, Canada 3 Institute of Electric Power Systems, Division of High Voltage Engineering, Schering- Institute; University of Hanover, Germany ABSTRACT Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. Because field measurements, last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care are required to interpret the results when performing tests at relatively low surrounding temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behavior of the insulation system in response to the ambient conditions. In this contribution, the dielectric behavior of a composite oil paper insulation system has been explained from the properties of Debye basic model. A series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. The equivalent circuit parameters of a laboratory made oil paper condenser bushing model were obtained using a non-linear optimization procedure. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. It was shown that the poles can be regarded as parameters able to be used for insulation condition assessment. Index Terms - Dielectric spectroscopy, Frequency domain, conductivity, moisture content, low temperatures, oil-paper insulation, poles. 1 INTRODUCTION POWER transformers are considered capital investments in the infrastructure of every country in the world. They are the “heart” of any electric power distribution and transmission systems and it is essential that they function properly. This importance is either raised due to the increasing demand of electric energy. Nowadays, a large number of power transformers around the world are approaching towards the end of their design life. Replacing them with new ones - only because of their age - is clearly uneconomic, since some of these transformers are still in good condition and could be used for many more years. For these reasons, transformer life management gained an ever increasing interest over the past decade, due to both economic and technical reasons [1-8].
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10_freq_dielctric - IEEE Transactions on Dielectrics and...

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