FAEAE2DDd01 - IEEE Transactions on Power Delivery Vol 9,No...

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IEEE Transactions on Power Delivery, Vol. 9, No. 1, January 1994 Multiple Stress Aging of Solid-Dielectric Extruded Dry-Cured Insulation Systems for Power Transmission Cables R.J. Densley R. Bartniltas B. Bernstein Fellow IEEE Fellow IEEE Fellow IEEE Institut de recherche d’Hydro-Qu&ec Toronto, Ontario Varennes, Quhbec Washington, D.C. Ontario Hydro Research Division Electric Power Research Institute 559 Abstract - A detailed examination of the aging processes, that may take place in solid-dielectric extruded-type high voltage transmis- sion cables under dry conditions, is presented. Particular emphasis is placed on the aging process as affected by the separate and simultane- ous action of four main aging factors, namely the electrical, mechanical and thermal stresses, and the physical and chemical environment. A number of pertinent aging models are considered and their validity and applicability to accelerated aging tests on solid-type transmission cables are discussed. INTRODUCTION Much work has been carried out in the past to study the aging characteristics of both low density high molecular weight polyethy- lene (HMWPE) and low density cross-linked polyethylene (XLPE) extruded insulation as applied to power distribution cables. These studies centered on the reasons for water-tree related failures, because many of the distribution cables that had failed in service had been ex- posed to water ingress and evidence had linked the incidence of water frees in the insulation to fallure occurrence [I, 21. Also more recently the influence of ions in water-tree propagation has been established. As interest developed in the use of polyethylene for higher voltage transmission cables, it became increasingly apparent that these cables should operate in a completely water-free environment to obviate the water induced failures observed with distribution cables. In North America, the upper operational limit thus far for extruded transmis- sion cables has been 230 kV, but in Japan the level has been extended to 275 kV and recently to 500 kV; in France an earlier level set at 220 kV was eventually raised to 400 kV [3,4). Stresses in these cables range usually from 7 to 14 kV/mm rms. Presently there is little under- standing on the nature of the a.ging mechanisms that may be operative at these stresses under dry conditions. The intent of this paper is to describe in detail the work carried out on an EPRI-sponsored project, which examined the problem of multiple stress aging of extruded in- sulation for transmission cables under dry conditions [5]. 3 Aging in polyethylene results from chemical and physical degra- dation processes, that cause irreversible changes in the material struc- ture of the dielectric; these changes are responsible for a failure rate that evinces an increase with time. Aging is caused by certain stresses and environmental variables, which act on the insulating system. The prime aging stresses that may induce aging in polyethylene cables op- erated under dry coiiditious are electrical, thermal and mechanical in nature. Each of these stress variables has a subset of variables; thus
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FAEAE2DDd01 - IEEE Transactions on Power Delivery Vol 9,No...

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