95_montanari - IEEE Bansactions on Dielectrics and...

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IEEE Bansactions on Dielectrics and Electrical Insulation Vol. 2 No. 6, December 1995 1095 Comparison of Electrical Aging Tests on EPR-insulated Minicables and Ribbons from Full-sized EPR Cable G. Mazzanti, G. C. Montanari Dipartimento di Ingegneria Elettrica, University of Bologna A. Motori Dipartimento di Chimica Applicata e Scienza dei Materiali, Facoltl di Ingegneria, University of Bologna, Italy and P. Anelli Pirelli Cavi S.p.A., Milano, Italy ABSTRACT The results of electrical endurance tests performed on ribbons cut from full-sized EPR cables are compared with those ob- tained by tests performed on cable models, having the same insulation as the full-sized cable. It is shown that the voltage endurance coefficient estimated on the basis of the short-term life test data has approximately the same value for cable models and ribbons, according to both inverse-power and exponential models. However, the surface roughness resulting from cutting the specimens can affect the result at high stresses. 1. INTRODUCTION LECTRICAL and multistress endurance characteri- E zation of HV cables is generally investigated by ac- celerated life tests, that is, short and long-term tests per- formed at stresses higher than those expected in service. Use of appropriate life models allows us to derive pa- rameters which are characteristic of material stress en- durance, such as halving interval HIC, and temperature index TI, for thermal life models, voltage endurance coef- ficient n, and stress-compatibility index SCI, for electri- cal and multistress life models [l]. However, realization of accelerated tests on full-sized HV cables requires an expensive experimental setup, so that they can seldom be performed. On the contrary, production of new ma- terials, particularly copolymers, or introduction of new additives in the already-used insulating materials for HV application is raising the need to carry out comparative tests, possibly short-time tests, at reasonable costs. For this purpose, cable models of reduced size are often used. For example, a typical cable model is realized by copper conductor 1.8 mm in diameter, conductor shield 0.5 mm thick, insulation 1.5 mm thick, and insulation shield [2]. The problem with this solution is that the technologi- cal curing processes are not the same as those occurring in full-sized cables, so that the results obtained for cible models must be very carefully extended to full-sized ca- bles (even if appropriate statistical treatment of data, as well as dimensional effect estimates, are made). Therefore, another solution has been alternatively cho- sen, which consists of peeling strips from full-sized cables by cutting tools, and use these ribbons as flat specimens for endurance and electric strength tests. Examples of re- 1070-9878/95/ $3.00 @ IEEE Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY KANPUR. Downloaded on August 11,2010 at 12:20:49 UTC from IEEE Xplore. Restrictions apply.
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1096 Mazzanti et al.: Electrical Aging Tests on EPR-insulated Minicables sults obtained from short-term tests carried out on spec-
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95_montanari - IEEE Bansactions on Dielectrics and...

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