05_infulence_tr_insu_g - IEEE Transactions on Dielectrics...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 12, No. 3; June 2005 423 Influence of Rise Time on Dielectric Parameters Extracted from Time Domain Spectroscopy in the Context of Generator Stator Insulation Eric David ´ Ecole de technologie superieure ´ 1100 Notre-Dame Street West Montreal, QC, H3C 1K3, Canada and Laurent Lamarre Ž. Hydro-Quebec Research Institute IREQ ´ 1800 Lionel Boulet Blvd. Varennes, QC, J3Y 8K6, Canada ABSTRACT The low-frequency dielectric response of generator stator winding insulation mate - rials was calculated from high voltage time domain spectroscopy measurements. These results were used to calculate the theoretical response after the application of a voltage function consisting of a ramped voltage followed by a constant level voltage. The rate of rise of the ramped portion of the voltage function was shown to () influence the well known and much used polarization index PI and insulation re - sistance IR parameters. Index Terms — Low-frequency dielectric response, generator winding insula - tion systems, polarization index, aging. 1 INTRODUCTION IME Domain Dielectric Spectroscopy TDS T is an efficient tool for the measurement of the low- frequency dielectric response of an insulation system. It is used more and more frequently as a diagnostic tool to assess the condition of insulating materials used in high voltage applications such as rotating machine insulation wx systems 1 ± 4 , the extruded insulation of underground ca- w x bles 5 ± 8 and oil r cellulose insulating systems 9, 10 . Fur- thermore, high voltage time domain spectroscopy allows the calculation of the low-frequency dielectric response under various electrical field levels, from low to high, in order to estimate the insulation system linearity, i.e., the variation of the dielectric response as a function of the electrical field. This non-linearity factor has also been re- ported as a valuable diagnostic tool 6 ± 8, 11 . The mainte- nance testing of generator stator insulation using high voltage dc dielectric tests is widely used to detect cracks, surface contamination or moisture absorption. These tests Manuscript recei ® ed on 6 January 2004, in final form 18 January 2005. fall under the category of time domain methods since they usually involve the recording of the current versus the time while a given voltage function is applied on the ground- wall insulation. The well known parameters, the polarisation index PI and the insulation resistance IR , are calculated from the charge current flowing through the insulation material af- ter the voltage is raised from zero to U and then left o constant for a given amount of time 12 . The polarization index is the ratio between the measured charge currents, 60 s and 600 s, respectively, after the constant voltage level is reached. The insulation resistance value is the ratio be- tween the constant voltage level and the measured cur- rent 60 s after this voltage level is reached. For the same dielectric response function, PI and IR will be affected by
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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05_infulence_tr_insu_g - IEEE Transactions on Dielectrics...

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