The present invention relates to an extrusion plant for making a power cable having a conductor with extruded polymer insulation and semiconductors. The invention relates in particular to making of high voltage cables in a vertical extrusion plant, but is not limited to such plants. When extruding insulation layers having a wall thickness greater than some 20 mm, horizontal plants and catenary plants do, however, have certain drawbacks related to the problem of making concentric insulation systems.

It has been common practice for many years, see e g US 3,404,432, to extrude an inner semiconductive layer, a thicker insulation layer and a thin outer semiconducti e layer in one operation. When larger insulation diameters are concerned, the conventional extrusion plants have certain drawbacks.

The main object of the present invention is to provide a cable extrusion plant which is capable of extruding concentric polymer layers for power cables rated above 300 kV. This is obtained with the present invention and the features of the invention are defined in the accompanying claims.

Above mentioned and other features and objects of the present invention will clearly appear from the following detailed description of embodiments of the invention taken in conjunction with the drawings, where Figures 1A and IB illustrate high voltage power cable cores made with the present invention,

Figure 2 illustrates a vertical cable extrusion plant,

Figure 3 illustrates a closed material handling system,

Figure 4 and 5 schematically show partial cuts through an extrusion head,

Figure 6 shows details of the material flow within an extrusion head designed for a vertical plant, and

Figures 7 and 8 schematically illustrate a catenary line and a horizontal line respectively. In Figures 1A and IB are illustrated crossections of cable cores 1A and IB manufactured in the plant of the present invention. The core consists of an electrical conductor 2 of

copper, aluminium or other suitable material , and successive concentrical layers of polymer material , i.e. a first, inner layer of semiconductive material 3, at least two layers of insulation material 4,5 and 7,8,9 respectively, and a second, outer layer of semiconductive material 6. The first, inner layer of insulation 4,7 constitutes 10-75 % of the total insulation thickness.

Outer cable protective sheaths and armour are not shown as such items do not form part of the present invention. The reason for dividing the insulation system into at least two individual layers of insulation material is to simulate a lapped insulation system. Thereby it will also be possible to use differently graded polymers in the various layers. The innermost insulation layers 4 and 7 are subjected to the highest voltage gradient and should be carefully chosen.

As will be explained later, all the layers indicated in Figures 1A and IB are extruded in one extruder head. The layers are extruded directly on top of each other to establish a full integration between the layers. Individual extruder screws leading polymer material to the extruder head are normally arranged at angles to each other. Neighboring layers should preferably have different molecular orientation. By analyzing cable samples, an analyst will be able to distinguish a cable made in accordance the present invention from cables made in other plants.

In Figure 2 is schematically illustrated the main elements of a vertical plant or tower 10 for extrusion of concentrical layers of polymer material around an electrical conductor 11 in order to make a high voltage power cable core 12. Such an extrusion tower may be higher than 100 m.

The electrical conductor 11 is paid off a conductor reel 13 usually placed at ground level, or at a lower level, whereafter the conductor is lifted to a higher level 14 within the tower 10 and guided vertically down into an extrusion head 15. One of at least four extruder screws 16 for applying

layers of polymer material onto the conductor 11 is indicated. When all layers have been applied concentrically to the conductor 11 in the extrusion head 15, the cable is passed vertically downwards through a heating and cooling device 17 in order to crosslink the polymer material. The cable core 12 corresponding to the cores 1A or IB of Figure 1A and IB, is wound onto a reel 18 in the lower region of the tower.

Metal sheaths, corrosion protective covers and armouring are provided on the cable core in later manufacturing processes, not described here.

In connection with the extruder screw 16 feeding polymer material into the head 15 in Figure 2, this figure also indicates a drive unit 19 for the extruder as well as a closed clean material handling system 20 leading from a polymer container 21 to the extruder.

The polymer material storage containers 21 and conduits 20 are arranged at elevated levels relatively to the respec¬ tive extruder screws 16, so that the material is transported by gravity in individual closed material handling systems from individual containers to the screws.

One problem that may arise when making high voltage power cables is that the polymers may contain undesirable contaminants introduced into the materials in the supplier processes, or in own material handling systems. In order to make a complete insulation system for the cables, contaminants should not be allowed. Such contaminations can be removed by arranging fine mesh screen packs 22 at the inlet to the extrusion head. The fine mesh screen packs for the insulation polymer may apply 400 to 1400 mesh, whereas the screen packs for the semiconductor may apply some 200 to 400 mesh.

In Figure 3 is schematically illustrated in more detail one of the closed clean material handling systems 30 leading from a container 31 to an extruder screw 32. A bulk bag 33 or the like, delivered from a polymer supplier, which is transported to one of the top levels of the tower may be placed in the container 31. The bag outlet is connected with a

clean bin 34 by means of a clean room manipulating arrangement 35. The container 31 may however be left out, if a proper interconnection from the bag to the bin 34 can be obtained otherwi se. Figure 3 also indicates means 36 for circulating dry warm air or preferably nitrogen (N2) or other electrically inert gas, upstream through the bin 34. The air/gas is filtered in a filter 37 to remove particles larger than 0.3 /urn before entering the material handling system. A HEPA filter providing 99.999% clean air/gas is considered suitable. The whole material handling system and bulk bag are thus kept at an overpressure compared with the surroundings to avoid any possibility of contaminating the polymer. By using an inert gas, oxygen is excluded from being introduced into the polymer melt.

In Figures 4 and 5 are illustrated partial cuts through an extrusion head 40 corresponding to the head 15 in Figure 2, and showing the orientation of the four/five extruder screws. Figure 4 is a schematic cut through Figure 5, taken along lines IV-IV, whereas Figure 5 is a schematic cut through

Figure 4, taken along line V-V. A first and inner semiconduc¬ tive layer screw 41 is shown some 90 degrees angularly displaced from the screw 42 for the first and inner insulation layer 4,7. Screw 42 is some 90 degrees angularly displaced from screw 43 for the second insulation layer 5,8. These three screws 41, 42 and 43 may be arranged in the same vertical plane around the extrusion head 40. A screw 44 for the second and outer semiconductive layer 6 is arranged at a different vertical level than the screws 41-43 in order to simplify installation and dismantling of the extruder screws and head. The extruder screw 44 can be some 45 degrees displaced from the screw 43 or from the screw 41 as illustrated. A screw 46 for a third insulation layer 9 is also indicated. The screws should be arranged on the head so as to simplify installation and dismantling processes. In Figure 5, the conductor 11 and the cable core 12 are shown entering and leaving, respec-

tively, the extrusion head 40.

In Figure 6 is illustrated in some more detail the material flow within a 4-layer extrusion head 50 which is useful in connection with the present invention and a vertical plant. The drawing indicates the inlets to the head from four extruder screws, for respectively providing an inner 51 and an outer 52 semiconductive layer, and respectively for providing an inner 53 and an outer 54 insulation material layer. The extruder screw 54 may as shown be somewhat displaced from the plane of the extruder screws 51 and 53. The extruder screw 52 for the outer semiconductor is substantially displaced from the other three screws.

The drawing shows crossections of two different sets of dies. The dies 61-64 shown in the left hand area 55 is used for a large diameter cable, whereas the dies 65-68 shown in the right hand area 56 is used for a small diameter cable. The dies 61 or 65 are fed from the extruder screw 51, - the dies 62 or 66 are fed from the extruder screw 53, - the dies 63 or 67 are fed from the extruder screw 54 indicated at right angles to the extruder screws 51 and 53, whereas the dies 64 or 68 are fed from the extruder screw 52.

An electrical conductor (not shown) entering the top of the head 50 is provided with an inner semiconductive layer, two layers of insulation material extruded 90 degrees on each other, and finally an outer layer of semiconduct ve material. In this embodiment of the invention, - the inner semicon¬ ductive layer screw 51 is displaced some 180 degrees from the inner insulation layer screw 53.

The extrusion head illustrated is suitable for making cables with ratings within a wide range: 24 kV to 420 kV. When using the set of dies illustrated in the area 55, the cable has a rating of 420 kV, whereas the set of dies illustrated in the area 56 has a rating of 24 kV. The corresponding diameters of the cable components are:

Area 55 Area 56 Conductor: 35.6 mm 12.9 mm

Inner semiconductive layer: 39.6 mm 14.9 mm Inner insulation layer: 71.1 mm 20.4 mm Outer insulation layer: 102.6 mm 25.9 mm Outer semiconducti e layer: 104.6 mm 27.9 mm

The main advantages of using the present invention is obtained with large crossections and with cables rated for more than 300 kV..

As will be seen from this example, the two insulation layers are given the same thickness. As mentioned earlier, the inner insulation layer should constitute at least 10% of the total insulation thickness and the polymer of this layer could be graded to cope with a high voltage gradient.

Whereas the present invention mainly has been described in connection with vertical extrusion plants, the principles can also be applied to catenary lines as indicated in Figure 7 and to horizontal lines as indicated in Figure 8. The internal structure of the extruder head to be used in a particular extruder plant, must be carefully chosen or modified to obtain a fully concentric cable insulation system.

Figure 7 illustrates a catenary line 70. A cable conductor 71 which is paid off from a reel 72, is passed through an extruder head 73 applying two semiconducting layers 3,6 as well as at least two insulation layers 4,5; 7,8,9 to the conductor. The resulting cable core 74 corresponds to Figures 1A and IB. The extruder screws, one of which is indicated at 75, is catered with polymer material from a storage container 76 through a closed clean conduit system 77 like that described in connecti D-iwith Figure 3.

In Figure 8 we have indicated a horizontal extrusion plant 80 employing the principles of the invention described above. The above detailed description of embodiments of this invention must be taken as examples only and should not be considered as limitations on the scope of protection.