In conventional field control of dc voltage, only resistive field control is used. The electric potential between the live part and ground is distributed with the aid of a material with a suitable resistance. The disad- vantage of this known field control is that, at rapid voltage variations, the resistive material does not have time to control the field, which leads to high stresses in the material.

The object of the present invention is to improve the control of the electric field, at dc voltage, in order to reduce the risk of harmful charges building up and of high stresses arising at rapid changes of the voltage.

This object is achieved according to the invention by a combination of resistive field control, comprising at least one resistive layer which is arranged along the cable and which, at one end, is electrically connected to an internal live conductor of the cable and, at its other end, is electrically connected to ground potential, and geometrical field control, comprising a stress cone which is arranged at the above-mentioned other end of the resistive layer.

By combining the resistive field control with a geome- trical field control, a double control of the electric field is obtained, both at constant dc voltage and at

varying voltage. Voltage variations may arise during connection and disconnection or at similar pulse stresses.

According to the prior art, the insulation screen of the cable is removed before mounting of a termination, a connection, a joint or the like cable device. The stress of the electric field is highest in regions where the cable has less resistance to electric breakdown than in other regions. One such region is, consequently, where the insulation screen of the cable is mechanically removed.

There are reasons to assume that the problems which jeopardize the mode of operation of the installation, in connection with dc voltage, are caused, inter alia, by the build-up of space charges in those boundary layers which exist between the insulation and the accessories. To solve these problems, prior art cable devices were complicated and of unreliable design, since they had to handle the electrical stresses caused by dc voltage, but also the high-voltage transient overvoltages which may occur. The prior art cable devices were expensive and also involved a long time of delivery, since they had to be adapted in situ to the geometry and electrical stresses of the cable.

By means of the present invention, the cable devices will be simple and reliable, since they are prefabricated with necessary components for control of the dc voltage field and control of the high-voltage transient overvoltages.

The invention will now be described in greater detail with reference to the accompanying drawings, wherein: Figure 1 shows a schematic cross-section view of a cable connection,

Figure 2 shows a schematic cross-section view of a cable termination, and Figure 3 shows a schematic cross-section view of a cable joint.

In the drawings, Figure 1 shows a cable connection 1 and a cable 2, with an inner conductor 3, an outer conductive layer 4, and an insulation 5. The connection 1 accommo- dates a resistive layer 7, an insulating layer 8, an outer conductive layer 9, and a deflector 10. Also the boundary surface towards a bushing (not shown) may be provided with a field-controlling resistive layer 6.

The outer conductive layer 9 is in the form of a stress cone 11, for geometrical field control. The resistive layer 7 extends through the connection 1 along the cable 2. The inner deflector 10 is electrically connected to the inner live conductor 3 of the cable 2 and to one end 12 of the resistive layer 7. At its other end 13, the resistive layer 7 is electrically connected to ground potential, for example via the outer conductive layer 4 of the cable 2.

Likewise, the stress cone 11 is electrically connected to ground potential and extends with increasing diameter from a region in the vicinity of the above-mentioned other end 13 at least partially towards the above-mentioned one end 12 of the resistive layer 7. The inner limiting surface 14 of the stress cone forms an angle with the resistive layer 7 of about 5-30°.

Figure 2 shows a cable termination 15 and a cable 16, with an inner conductor 17, an outer conductive layer 18 and an insulation 19. The termination 15 exhibits a resistive layer 21 and an outer conductive layer 22. The outer con- ductive layer 22 is formed as a stress cone 23 for geome- trical field control. One end 24 of the resistive layer 21

is electrically connected to the inner live part 17 of the cable 16. At its other end 25, the resistive layer 21 is electrically connected to ground potential and is in con- tact with the outer conductive layer 18 of the cable 16.

Likewise, the stress cone 23 is connected to ground potential and extends with increasing diameter from the above-mentioned other end 25 at least partially towards the above-mentioned one end 24. The inner limiting surface 26 of the stress cone forms an angle with the resistive layer 21 of about 5-30°.

Figure 3 shows a cable joint 27, comprising a connecting portion 28 for the respective cable (not shown). Each connecting portion 28 exhibits a resistive field-con- trolling layer 29, an insulating layer 30 and an outer conductive layer 31. The outer conductive layer 31 is formed as a stress cone 32 for geometrical field control.

At one end of the connecting portion 28, that is, usually in the centre of the joint 27, an inner deflector 33 is electrically connected to the resistive layer 29 and an inner live conductor of the cable (not shown), and at the other end of the connecting portion 28, the resistive layer 29 is electrically connected to ground potential.

The deflector 33 exhibits a thickening of its end por- tions, which cooperate with the stress cone 32 at voltage variations. The stress cone 32 extends with increasing diameter from the other end of the resistive layer, which is nearest the ground potential, at least partially towards the above-mentioned one end of the resistive layer, which is nearest the live conductor. The inner limiting surface 36 of the stress cone 32 forms an angle with the resistive layer 29 of about 5-30°.

The joint 27 is suitably cylindrical, although other geometrical shapes are feasible, and is mainly made from

EPDM rubber, but also other plastics or elastomers are possible.