The present invention relates to a high voltage cable comprising a conductor, an insulating layer surrounding said conductor, a screen surrounding said insulating layer and said conductor, and a protective cover or jacket surrounding said screen and being made of e . g. plastics or rubber.

A conventional high voltage cable includes one or more conductors transmitting the electrical power, insulating layers insulating the conductor or conductors relative to one another and relative to the environment, and an outer cover which is intended to protect the insulating la er(s) and the conductors ) against being damaged and which may comprise a metal screen and a protective jacket.

Normally, the metal screen is maintained at the earth potential and constitutes an earth conductor and furthermore, the protective jacket is intended to protect the screen against corrosion and mechanical damage. Normally, the conductor or conductors is/are made from copper or aluminium . Nowadays , cross-linked polyethylene is most often used as an insulation between the conductor and the screen while oil impregnated paper was previously used for the same purpose. The screen may be made of aluminium, copper, steel, or lead, the the protective jacket is nowadays most often made from plastic- or rubber-tape-windings or from an extruded plastic jacket preferably of poly inylchloride , while the protective jacket previously consisted of impregnated jute braidings and asphalt.

However, the application of high voltage cables having protective jackets made of plastics has proven to cause problems in connection with providing an adequate earth connection . Therefore, additional earth conductors and associated earth rods have to be employed in connection with plastic jacket high voltage cables in order to guarantee the provision of the required earth connection .

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Therefore, it is an object of the present invention to provide a high voltage cable of the aforementioned kind utilizing the advantageous characteristics of the plastic jacket in relation to corrosive and mechanical protection, while at the same time eliminating the requirements of employing additional earth conductors and associated earth rods for the provision of the required earth connection.

This object is obtained in accordance with the present invention which is characterized in that the plastic or rubber material of the protective jacket is electrically conductive in itself or rendered electrically conductive by the incorporation of evenly distributed particles of a conductive material in order to provide electrical conductive connection between the screen and the outer surface of the protective jacket. In this way the whole screen of the high voltage cable is used for the provision of the required earth connection so that the employment of additional earth conductors and associated earth rods is eliminated.

For example, the required conductivity may bfe o.btained by making the protective jacket of carbon- containing polyvinylchloride or crosslinked polyethylene. Thus , the specific resistance of the carbon- containing polyvinylchloride or crosslinked polyethylene is preferably approximately 2000 Ω cm.

In accordance with another embodiment of the invention the required conductivity may be obtained by embodying the particles incorporated in the protective jacket as metallic chips or swarfs . In this embodiment the protective jacket may be made of any appropriate material, e.g. polyvinylchloride or crosslinked polyethylene .

Conventionally, high voltage cables are employed for the transmission of three-phased A. C . power and consequently , in another embodiment, the high voltage cable according to the invention comprises a total of three conductors , each provided with an insulating layer and a screen, and being arranged, in cross

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sectional view, at the vertices of an equilateral triangle . This embodiment of the invention may be modified by surrounding the three conductors with a common screen . For the transmission of three- or multi-phased A . C . power several single conductor high voltage cables according to the invention may also be employed.

In an especially advantageous embodiment of the invention , a filling substance is arranged between the jacket and the screeen or the screens of the high voltage cable and mutually between these screens , the filling substance being made of a material which is electrically conductive in itself or rendered electrically conductive by the incorporation of evenly distributed particles of an electrically conductive material. In this way an advantage both in relation to saving material and thus weight is obtained, and a reduction of costs , since the filling substance may be of a less dense- and thus lighter type than the protective jacket so that the price of the cable may be reduced. Furthermore, the arrangement of the filling substance results in a further advantage since the . flexibility of the cable is increased as a result of the arrangement ₀ f relatively soft material between the relatively hard and also stiff material of the protective jacket and the screen(s) .

In order to further increase the flexibility of the cable , it is preferred that the screen or screens is or are made of helically wound tapes of aluminium or copper. The screen or the screens may instead be made of one or more helically wound cores .

In order to increase the overall sectional area of the earth conductor, not to be confounded with the above mentioned problem of providing the earth connection in connection with known plastic jacket high voltage cables , it is preferred that the cable includes one or more earth conductors arranged in electrically conductive connection with the screeen or the screens .

in all embodiments of the invention the conductor or each of the conductors may be composed of several individual, helically wound cores and may have a substantially circular cross section .

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Alternatively, the cross section of the conductor or each of the conductors may be sector-formed, or the conductor or conductors may be constructed as a solid conductor having a circular or sector-formed cross section.

The invention will now be further described in detail with reference to the accompanying drawings , in which Fig. 1 illustrates an embodiment of a three-conductor high voltage cable according to the invention, Fig. 2 a cross section of the embodiment shown in Fig. 1 , and

Fig. 3 a cross section of a slightly modified embodiment of a three-conductor high voltage cable according to the invention.

In Figs . 1 and 2 a three-conductor high voltage cable is shown which is intended to be used for transmission of high voltage three-phased A. C. within the interval of 6 - 36 kV. Thus, the high voltage cable comprises three conductors designated 1, 2, and 3, respectively. Each of the conductors 1, 2, and 3 are constructed of eighteen helically wound metal cores, preferably of aluminium or copper. As will be apparent from the figure, the individual cores of the conductors 1, 2, and 3 are arranged in such a manner that the cross section of the conductor is substantially circular. The interspaces between the individual cores are filled with a semi-conductive material. Furthermore, layers 4, 5, and 6 of said semi-conductive material are arranged surrounding each of the conductors 1, 2, and 3, respectively. As shown in Fig. 1, insulating layers 7, 8, and 9 are arranged surrounding the semi-conductive layers 4, 5, and 6, respectively, and externally relative to said insulating layers graphite layers 10, 11, and 12, respectively, and semi-conductive tape layers 13, 14, and 15, respectively, are arranged. A phase-indication 16, 17, and 18 is arranged on each of the semi-conductive tape layers 16, 17, and 18, respectively, for identification of the individual conductors or phases of the high voltage cable. Finally, screens are arranged on the outside of the semi- conductive tape and the phase identifications which, corresponding to the conductors 1 , 2, and 3, respectively, are designed by the reference numerals 19, 20 , and

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21 , respectively . The screens 19 , 20, and 21 are made of helically wound tapes of aluminium or copper. In a manner known per se these screens act as earth conductors . However, as will appear from the figure, the overall cross sectional area of the screens is significantly smaller than the cross sectional area of each of the phase conductors and, consequently, the high voltage cable further includes three earth conductors 22, 23, and 24. The screens 19, 20, and 21 and the earth conductors 22, 23 , and 24 are in electrically conductive connection with one another, as will be explained below.

On the outside of the screens 19, 20, and 21 and the additional earth conductors 22, 23, and 24, a jacket 25 is arranged serving several different purposes. Firstly, the jacket physically supports the other components of the high voltage cable , secondly, the jacket functions as a protective cover for the other components of the high voltage cable since it possesses an appropriate hardness , and thirdly , as has already been mentioned above, the jacket provides electrically conductive connection between the screens 19 , 20, and 21 and the additional earth conductors 22, 23, and 24.

While the insulating layers 7, 8 , and 9 preferably are made of crosslinked polyethylene, which is a plastic type showing high insulating properties , the jacket 25 is made of carbon-containing polyvinylchloride or a different kind of plastic, e. g. crosslinked polyethylene or of plastic, e . g. polyvinylchloride or crosslinked polyethylene, incorporating particles of a conductive material, e. g. metal chips or swarfs .

In Fig. 3 a cross section is shown of a modified embodiment of a three conductor high voltage cable according to the invention . In Fig. 3 components , which correspond to components shown in Figs . 1 and 2, are designated by reference numerals identical to the reference numerals of Figs . 1 and 2.

The embodiment of the invention shown in Fig . 3 comprises a jacket 27 being formed as a cylindrical outer layer having uniform thickness in contradistinction to the jacket 25 shown in Figs . 1

and 2. The internal spaces between the inner surface of the jacket 27 and the screens 19, 20 , and 21 and mutually between the screens are filled up with a filling substance 26. The filling substance 26 is preferably made of carbon-containing, relatively soft polyvinylchloride or of relatively soft polyvinylchloride incorporating particles of an electrically conductive material, e. g. metallic chips or swarfs , and is adapted to provide electrically conductive connection between the screens 19, 20 and 21. However, the jacket 27 is made of carbon-contain fn g , relatively hard polyvinylchloride or crosslinked polyethylene, or of relatively hard polyvinylchloride or crosslinked polyethylene incorporating particles of an electrically conductive material, e. g. metallic chips or swarfs , and is adapted to support the other components of the high voltage cable, to protect the other components of the high voltage cable, and to provide electrically conductive connection between, the outer surface of the cable and the earth conductor just as the protective jacket 25 shown in Figs . 1 and 2.

As will be appreciated, the embodiment of the invention shown in Fig. .3 may further comprise additional earth conductors corresponding to the earth conductor 22, 23, and 24 shown in Figs . 1 and 2.

Although the invention has been described and explained with reference to two specific embodiments of a three-conductor high voltage cable, it will be understood that several amendments and modifications may be proposed by a person skilled in the art within the scope of the invention. Thus , the high voltage cable according to the invention may comprise further or less conductors having corresponding insulating layers , screens , etc . , and an arbitrary number of additional earth conductors .

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