07_dielectric_dcramp - 1548 E. David et al.: Modeling of...

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E. David et al.: Modeling of the Dielectric Response of a Stator Winding Insulation from a DC Ramp Test 1070-9878/07/$25.00 © 2007 IEEE 1548 1548 Modeling of the Dielectric Response of a Stator Winding Insulation from a DC Ramp Test Eric David École de technologie supérieure 1100 Notre-Dame Street West Montreal, Quebec, H3C 1K3 Canada Thierry Godin, Jacques Bellemare Hydro-Québec Division appareillage de production Montréal, Canada and Laurent Lamarre Hydro-Québec Institut de recherche d’Hydro-Québec Varennes, Canada ABSTRACT The aging due to mechanical, thermal, electrical and environmental stresses of a stator insulation system inherently involves alterations of the material properties which are detrimental to its service operation. When these properties are deteriorated to the point where the material can no longer operate safely under normal stress conditions, it implies that it has reached the end of its useful life. To prevent such forced outages and even to improve the useful life, condition-based maintenance and diagnostic tests are periodically conducted on stator insulation system. Among the tests performed to assess the condition of winding insulation, various methods are commonly used like partial discharge measurements, hi-pot, step voltage and ramped voltage tests to name just a few. The I-V curve measured during a ramped direct voltage test, as the one obtained in a polarization/depolarization test, includes the contribution of various current components, the capacitive, the absorption, the surface leakage and the bulk conduction currents. These contributions, especially the absorption and the conduction (surface or volume) currents, are not as easy to separate in a ramped voltage test as in a polarization/depolarization test (step voltage test). A modeling of the I-V curve obtained with the ramped voltage test, allowing an easy separation of the leakage current from the absorption and the capacitive currents, is presented in this paper. This modeling is compared with a field measurement on complete stator winding and with a lab measurement on spare coil. The calculated values of the absorption and leakage currents obtained during a ramped direct voltage test are compared with the same components deduced from a polarization/depolarization test. Index Terms Machine insulation, dielectric response, time-domain spectroscopy, ramped voltage test 1 INTRODUCTION FOR machine stator winding insulation system, the search for a practical and reliable diagnostic method to assess the condition of large rotating machine stator insulation system has been ongoing for decades. In order to gain the insurance that the winding insulation can safely withstand the service voltage, ac and dc over-potential tests have been used for many decades [1, 2]. Diagnosis of electrical apparatus by dc testing has been a much appreciated method to get to know the state of the high voltage to ground insulation. There is a good reason for this regarding the pragmatic advantages of using dc testing equipment. However, the development of the
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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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07_dielectric_dcramp - 1548 E. David et al.: Modeling of...

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