Electricity: conductors and insulators – Insulators – Sectional – multi-part – composite – or coated
Reexamination Certificate
1997-07-30
2002-01-29
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Insulators
Sectional, multi-part, composite, or coated
C174S178000, C174S196000
Reexamination Certificate
active
06342679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to insulation or protection coverings for electric components, such as terminations for electric cables, voltage dischargers or composite insulators. In particular, the present invention concerns coverings adapted to be fitted and elastically clamped to an electric component. It also pertains to the related manufacturing method. More specifically, this invention relates to elastic terminations for cables comprising said coverings and the method of mounting them.
2. Description of the Related Art
In an embodiment known from “New Prefabricated Accessories for 64-154 kV Crosslinked Polyethylene Cables” (Underground Transmission and Distribution Conference, 1974, pages 224-232), an outdoor termination comprises a base plate to which the base of an insulating covering formed of a finned body made of porcelain is linked. A cable conductor is connected to the upper end of the outdoor termination through appropriate support and connection means. A ground electrode and a field control cone of polymeric material is forcedly fitted onto the surface of the cable insulator within a cylinder at its entry into the finned body. The cylinder is made of an epoxy resin. The free space within the finned body is filled with an insulating oil.
However, filling with insulating oil may cause some problems. This insulating oil within the porcelain covering aims at eliminating air. The air may be subjected to an ionization phenomenon causing the electric field to be higher. Such ionization will impair the termination integrity.
CIGRE' 1992, 21-201, entitled “Synthetic Terminations for High Voltage Cables-Assessment of Service Life” teaches a method of replacing the porcelain material in the termination by tubes made of fibre-glass-reinforced plastics coated with anti-tracking insulating rubber. The tubes may shape like a cylinder and/or a cone. The tubes must both ensure protection for the underlying portion against humidity penetration and provide the necessary leak line (profile and fins) for surface currents. In this case, however, insulating oil is still used.
For the purposes of the present application, “tracking”, or surface track, means an irreversible degradation of the insulating material surface, involving formation of conductive paths even under dry conditions. This definition is consistent with the specification of IEC 1109 of 1992 entitled “Composite insulators for a.c. overhead lines with a nominal voltage greater than 100 OV. Definitions, test methods and acceptance criteria.”
Examples of coatings made of anti-tracking insulating rubber for electric components are known in the art. These coatings for extruded-insulator cables are called “dry terminations” because they are devoid of both the porcelain covering and the insulating oil. These coatings are described in an article entitled “Outdoor and incorporated terminations for extruded synthetic cables up to 400 kV” by F. Gahungu, J. M. Delcoustal, J. Brouet, presented to Jicable 1995. This article describes applications for internal use in cabins (for voltages up to 90 kV), as well as applications for temporary outdoor connections.
European Patent Application No. EP 95 106 910.3 describes a self-bearing dry termination for an electric cable intended for outdoor use for voltages of 10 to 245 kV and above. The electric cable is provided with a covering of elastomeric material fitted onto a tubular element. However, when high-voltage applications are concerned, involving voltages as high as 60 kV and above, this covering can have an early decay if its fitting takes place under strong interference conditions.
Among the materials used to make insulating coverings for applications to outdoor terminations, two fundamental types can be identified: elastic materials and heat-shrinkable materials. The elastic material consists of elastomers having the capability to make manufactured articles adapted to be interference-fitted onto the cable end or a similar support body and to keep a preestablished pressure against it. On the contrary, the heat shrinkable materials enable manufactured articles to be pre-expanded at the factory by using heat sources, generally in the form of a gas flame.
Examples of known devices adopting heat-shrinkable materials include those described in an article entitled “Heat-shrinkable terminations for 66 kV polymeric cables” by J. W. Weatherley R. A. John, M. H. Parry, presented to the IEE London Power & Accessories 10 kV-180 kV, November 1986.
Although heat-shrinkable products are still in use, they are not quite desirable. Employment of a flame as required by heat-shrinkable products involves problems not only for practical-use and safety reasons, but also because of the difficulty in ensuring a correct elastic recovery of the covering to the starting sizes, especially when heating is not carried out with particular care and skill.
“Elastic” or “cold-shrinkable” products preferably comprise EPR-based blends or silicone rubber-based blends.
For the purposes of the present description, EPR means blends based on ethylene-propylene polymers in particular comprising EPM (ethylene-propylene copolymer) or EPDM (ethylene-propylene-diene terpolymer) based blends.
European Patent Application No. 92203797.3 describes a tubular element which is formed by a wound strip having adjacent coils made by cutting the surface of the element itself.
European Patent Application No. 90119273.2 describes a storable element of a device to make cable joints. The cable joints consist of a tubular support over which a covering for conductor connections is fitted under elastic expansion conditions. The covering is suitable to make joints between cables selected in a wide range of diameters.
This covering comprises two coaxial and superposed sleeves. The radially inner one has a residual deformation under imposed elongation conditions. The residual deformation of the inner sleeve is lower than that of the radially outer sleeve. The inner sleeve also has a greater elastic modulus.
However, this covering is not adapted to construct terminations and the like which are exposed to phenomena such as surface discharges (tracking).
SUMMARY OF THE INVENTION
The invention comprises a tubular elastic covering for electric components. The elastic covering has predetermined values of mechanical and electric/environmental requirements. The mechanical requirements comprise expandibility and elastic recovery in a radial direction. The electric/environmental requirements comprise resistance to tracking and resistance to solar radiation. The covering comprises an inner insulating layer and an outer insulating layer. The inner and outer insulating layers are formed coaxially with each other, superposed and bonded together. The inner and outer insulating layers are made of blends of cross-linked polymeric materials. The cross-linked blend forming the inner layer has predetermined values of the mechanical requirements. The cross-linked blend forming the outer layer has predetermined values of the electric/environmental requirements.
This application has solved the problems by replacing porcelain with polymeric and composite materials in particular in medium- and high-voltage outdoor terminations of extruded cables. Several reasons for using the polymeric and composite materials include:
reducing to the minimum explosion risks in case of fire or internal electric discharge;
reducing weight;
reducing brittleness, so as to prevent damages caused by accidental impacts or, for example, vandalic acts;
increasing transportation and installation simplicity and facility;
increasing safety margins under high-pollution conditions.
This application has considered coverings of the elastic type to be applied to the surface of a cable insulator. However, the requisite qualities to be simultaneously met by the elastic coverings of the above described type are hardly compatible with each other. In particular, it has been noted that an insulating covering for cable terminations of the elastic type must
Portas Francesco
Vallauri Ubaldo
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi S.p.A.
Reichard Dean A.
Walkenhorst W. David
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