Electricity: conductors and insulators – Conduits – cables or conductors – Combined
Reexamination Certificate
1997-10-14
2001-07-24
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Combined
C174S07400A, C174S07500C, C174S07500C
Reexamination Certificate
active
06265663
ABSTRACT:
The present invention relates to a terminal for an electric cable, in particular to a terminal for a medium and high-voltage electric cable for external use, that is adapted to be used outdoors, being exposed to the air and possible polluting agents.
To the ends of the present invention by the-term “medium and high voltage” it is generally meant that voltages in a range of 10 kV to 245 kV or more are taken into account.
Cable terminals for external use are known, for example, for connecting a cable to an overhead line, usually comprising an insulating envelope provided with means for connection to a support pylon inside which the end portion of a cable is housed, said end portion being deprived of its outer protection layer and provided with a field control element, the conductor of said cable extending as far as the end of the insulating envelope, to be connected to the overhead line.
According to an example of the known art, the insulating envelope is made up of a porcelain element which ensures an insulation between the end portion of the live cable conductor and the bearing structure of the terminal, said bearing structure being connected to earth and having a surface extension sufficient to restrain the current passage along the outer surface of the terminal.
It is known that an electric cable of the type provided with an extruded insulator, mainly comprises an inner conductor, consisting for example of a metal cord made of copper or the like, coated with a semi-conductive layer, an insulating layer, an electrically earthed screen, and a protection sheath.
At the terminal inlet the cable is deprived of the sheath and screen, and field control elements are used for restraining the electric field gradients at the interruption of the screen.
By field control elements, known per se, it is meant either a suitably radiated conductive body, usually referred to as an electrode or baffle, normally made of a semiconductive elastomeric material incorporated in a body of an insulating material, the assembly being conveniently shaped in order to keep the electric field gradient within acceptable limits, or a material having a variable dielectric constant and conductivity depending on the electric gradient, or yet a capacitor system.
In an embodiment known from “New Prefabricated Accessories for 64-154 KV Crosslinked Polyethylene Cables” (Underground Transmission and Distribution Conference, 1974, pages 224-232), a terminal for external use is in particular comprised of a base plate, to which the base of a ribbed porcelain body is linked, at the upper end of which the cable conductor is connected, through appropriate support and connection means, an earth electrode and a field control cone, of elastomeric material, is forced into the surface of the cable insulator within a cylinder of epoxy resin, at the point it enters the ribbed body, whereas the free space within the ribbed body is filled with insulating oil.
This insulating oil within the porcelain envelope aims at eliminating the air which is subjected to a possible ionization phenomenon where the electric field has a higher value, which involves impairing of the terminal integrity.
Such a terminal is mounted upright, being linked to a bearing structure at its base.
Also known are terminals for electric cables provided with a solid insulator, for example, from “IEE Power Cables and Accessories 10 kV -180 kV”, London, November 1986, pages 238-241, in which the semi-conductive screen of a cable is removed over a certain length thereof starting from the cable end. The screen end is covered with a conductive paint extending over a portion of the cable insulator to which heat-shrinkable field-control pipes having a controlled impedance are applied. Said pipes and the uncovered portion of the cable insulator are therefore covered with a heat-shrinkable pipe of a weather-resistant material, incorporating an insulating profile located at the broken end of the cable screen. Several heat-shrinkable annular ribs are therefore fitted onto the pipe.
Such a terminal, being devoid of rigid elements capable of supporting mechanical stresses in a direction transverse to the conductor axis, is suspended from a frame, via an insulator.
A terminal of the so-called “synthetic” type provided with upper connection means, that is adapted to enable a so-called overhead mounting, is also described in CIGRE' 1992, 21-201, being entitled “Assessment of Service Life”, said publication states that a terminal of the “synthetic” type is, among other things, devoid of burst and fire risks in case of inner discharges, as compared to traditional porcelain terminals containing the insulating fluid.
Also known is U.S. Pat. No. 4,757,159 disclosing a terminal comprising a cable portion having an uncovered insulator, on which a multi-layered structure of a high-dielectric-constant insulating material, a semiconductor material having a non-linear coefficient, is disposed.
By high-dielectric-constant insulating material it is meant a material having a dielectric constant relative to the air higher than about 10.
Over the whole length of the uncovered insulator portion and beyond the end of the conductor core of the cable, an armouring extends that comprises several rigid rods of epoxy resin reinforced with unidirectional fiber glass and an external support and protection pipe.
In this solution, in order to inhibit the formation of air pockets, a dry deformable filler material, a putty in particular, is provided to be introduced in order to completely fill all free spaces between the rods, the outer pipe and the high-dielectric-constant layers. The ends of the rods and the pipe are confined by two insulating plates provided with appropriate inlet and outlet openings for enabling passage of the cable in the terminal.
In the '159 patent description it is stated that the reinforcing rods in combination with the insulating plates give the terminal a great stiffness, as well as resistance to compression, drawing and bending moments.
Several cone-shaped discs are disposed around the above armouring and in contact with the outer surface of the pipe. Both the pipe and discs are made of an elastomeric material.
Cable terminals for external use according to the known art therefore either possess a rigid structure in terms of resistance to bending, as in the case of a porcelain element or the assembly disclosed in U.S. Pat. No. 4,757,159 (although in the last-mentioned case the obtainable flexional rigidity is actually limited, as it is ensured by rods substantially independent of one another and of reduced sizes, for bulkiness reasons) inside which the cable end is housed so that the cable conductor projects from the upper end thereof, or they possess a flexible structure elastically fitted on the end portion of the cable.
In the first case, the use of a filler between the cable and the terminal body is necessary. The filler consists of an oil or a particular putty to be applied in the field and is subjected to escape thereby impairing the terminal integrity. In the second case, the lack of flexional rigidity of the terminal requires mounting in a overhead configuration and therefore a more complicated bearing structure is required, as well as interposition of an insulator.
Terminals for indoor use are also known, that is terminals adapted for use in a closed environment, the outer surface of which is designed to operate in contact with a gas or dielectric oil, as shown for example in “Power Cables and Accessories 10 kV-500 kV”, Nov. 23-25, 1993, page 275. In such terminals no insulating fluid between the insulator and field control element is provided, and latter element advantageously consists of an elastomeric sleeve.
Such terminals are particularly advantageous for indoor application but are not adapted for external use, in that, in the presence of air and possible polluting agents deposited on their surface, they are unable to offer a sufficient insulation between the uncovered live end and the area connected to earth, due to the reduced length possessed by them.
In
Cicogna Francesco
Parmigiani Bruno
Quaggia Dario
Brooks L. P.
Norris & McLaughlin & Marcus
Paladini Albert W.
Pirelli Cavi S.p.A.
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