AU692856B2 - Power cable, extrusion plant and method - Google Patents

Description

Power Cable, Extrusion Plant and Method 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 semiconductive layer in one operation. When larger insulation diameters are concerned, the conventional extrusion plants have certain drawbacks.

This US patent represents one of several publications describing a 3-layer extruder head forming the background for our development. The problem of material contaminations is not mentioned 1 5 EP-A 0 277 111 relates to extrusion of colored insulation sheaths and -cripes on small conductors. There are two main 'full layer' extruders and two stripe extruders.

FR-A 2 704 176 relates to a modified 3-layer extruder head for extrusion of a inner semiconductor, one layer of insulation material and an outer semiconductor.

DE-A 35 38 527 relates to a crosslinking (verneizung) process for making a 3layer cable, the three layers being an inner semiconductor, a crosslinked layer of insulation material and an outer semiconductor.

DE-A 19 14 478 describes the difficulties that arise with a 3-layer extruder when the thickness of the single insulation layer is increased in order to produce :'25 cables with higher power and voltage ratings. Difficulties with contaminations in the raw material are also mentioned (page 2 line This document also points out (page 2 section 3) that similar difficulties may arise even if the insulation layer is divided into two or three coaxial parts.

This document has solved the mentioned problems at least on paper -by A recommending an insulation body consisting of several (at least 3, preferably 5-100)

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1 0 4 oi IT separately extruded thin (at most 2 mm, preferably 0.2-2 mm) layers of insulation material.

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.

DE-A 19 14 478 specifies a high voltage power cable comprising successively a conductor, an inner layer of semiconductor material, not less than three (the corresponding US 3,580,987: more than two)(the corresponding GB 1 237 173: at least three) layers of insulation material and an outer layer of semiconductive material.

This document disclases that the layers may be applied by passing the cable core through a multiple-head extruder which applies the layers to the conductor during a single passage. There is however no indication of how the extruders and heads could be arranged. This document does not disclose the features of having only one head which ensures that the layers are fully integrated with each other. The document states that each layer 'is extruded at a predetermined distance from the point of extrusion of the preceding layer' in order that 'each layer before being covered by a subsequent one lie and settle in a certain position so that their i: individuality as layers is maintained' in the final product. There is not obtained a full 2Q integration of the layers.

This document does not disclose a head which is provided with a number of individual extruder inlet screws for the layers. It hardly discloses anything about material handling. One of the main ideas resulting in our invention is to have full control of the material flow all the way from the material storage containers to the 99 ,i cable surface.

The document specifies that the thickness of each layer should be not smaller e than 0.2 mm and not greater than 2.0 mm, whereas in the present invention the .9.9.9 thickness of the inner insulation layer should beat least 2.75 mm.

Above mentioned and other features and objects of the present invention will 7- clearly appear from the following detailed description of embodiments of the ~~lrr 3 invention taken in conjunction with the drawings, where Figures 1A and 1 B illustrate high voltage power cable cores made with the present invention, Figure 2 illustrates a verticai 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 1B are illustrated crossections of cable cores 1A and 1B 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 1B 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 0« polymer material to the extruder head are normally arranged at angles to each other.

Neighboring layers should preferably have different molecular orientation. By analysing cable samples, an analyst will be able to distinguish a cable made in accordance the present invention from cables made in other plants.

1O04 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 1 6 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 1 0 through a heating and cooling device 1 7 in order to crosslink the polymer material.

The cable core 12 corresponding to the cores 1A or 1 B of Figure 1A and 1 B, 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 1 6 feeding polymer material into the head 15 in Figure 2, this figure also indicates a drive unit 1 9 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 S^ elevated levels relatively to the respective 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 A4 some 200 to 400 mesh.

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a 10 4 "7X T 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 otherwise.

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 /um 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 a preferred embodiment of the invention the insulation layer is made up of two layers where the inner layer constitutes 46 to 60 of the total insulation thickness.

The extruder head outlets are arranged so closely together that all layers are fully integrated with each other on the cable conductor. The thickness of each insulation layer is at least 2.75 mm.

In Figures 4 and 5 are illustrated partial cuts through an extrusion head :1 corresponding to the head 15 in Figure 2, and showing the orientation of the -our/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 5 and inner semicondudctive layer screw 41 is shown some 90 degrees angularly i: i displaced from the screw 42 for the first and inner insulation layer 4,7. Screw 42 is sme 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 AZ layer 6 is arranged at a different vertical level than the screws 41-43 in order to

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'0104 I, II I~ 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, respectively, the extrusion head In Figure 6 is illustrated in some more detail the material flow within a 4-layer e-xtrusion 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 degrees on each other, and finally an outer layer of semiconductive material. In this embodiment of the invention, the inner semiconductive layer screw 51 is displaced :74 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 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: 1 104 X-fT O' 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 semiconductive layer: 104.6 mm 27.9 mm The main advantages of using the present invention is obtained with large crossections and with cables ruted 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 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 1 B. The extruder screws, one 0 of which is indicated at 75, is catered with polymer material from a storage container S 76 through a closed clean conduit system 77 like that described in connection with Figure 3.

In Figure 8 we have indicated a horizontal extrusion plant 80 employing the 4. principles of the invention described above.

The above detailed description of embodiments of this invention must be taken Sas examples only and should not be considered as limitations on the scope of protection.

(TsEC 104 17 a

Claims (14)

1. An extrusion plant for making a high voltage power cable including a conductor and an insulation system consisting of concentrical layers of extruded polymer insulation and semiconductors, including a material handling system and a single extrusion head for extruding the layers on top of each other, onto the conductor, wherein polymner material storage containers and material handling systems are arranged at elevated levels relatively to the respective extruder and its screws, so that the material is transported by gravity in individual closed systems from individual containers to at least four individual extruder inlet screws of said single extrusion head, at least two of said 1 0 inlet screws for the insulation layers and two of said inlet screws for the seiniconductive layers.

2. An extrusion plant as claimed in claim 1, including means for filtering and circulating dry warm air or inert gas upstream through the closed systems, so that the whole system is pressurized by filtered clean air/gas.

3. An extrusion plant as claimed in claim 1 or 2, wherein fine mesh screen packs are arranged between each individual extruder inlet screw and the extrusion head.

4. An extrusion plant as claimed in claim 7, wherein the at least four individual S extrusion inlet screws of said single extrusion head are arranged close together so that all layers are fully integrated with each other on the cable conductor. 20 5. An extrusion plant as claimed in any one of claims 1 to 4, wherein the at least four individual extrusion inlet screws are angularly displaced relatively to each other about said extrusion head's longitudinal axis.

6. An extrusion plant as claimed in any one of the preceding claims,. wherein of the said at least two inlet screws for the insulation layers, one inlet 25 screw is for extruding an inner insulation layer and the other inlet screw is for extruding an outer insulation layer, and wherein of the two of said inlet screws for the semiconductor layers, one is for extruding an inner semiconductor layer and the other is for extruding an outer semrninconductor C ayer. i~u "Le 0' 0'

7. An extrusion plant as claimed in claim 6, wherein the inner semiconductive layer inlet screw is angularly displaced relatively to the inner insulation layer inlet screw, about said extrusion head's longitudinal axis.

8. An extrusion plant as claimed in claim 6, wherein the outer semiconductive layer inlet screw is angularly displaced relatively to the outer insulation layer inlet screw by 45 degrees, about said extrusion head's longitudinal axis.

9. An extrusion plant as claimed in claim 8, wherein the outer semicondudctor layer inlet screw is axially displaced from the other inlet screws. An extrusion plant as claimed in any one of the claims 1-9, wherein the extrusion plant is arranged vertically within a tower.

11. A method for making a high voltage power cable comprising a conductor and an insulation system consisting of concentrical layers of extruded polymer insulation and semiconductors, where the insulation system is installed on the conductor in an extrusion plant including a single extrusion head for extruding successive layers on top of each other, onto the conductor, including -he steps of by providing the single extruder head with at least four individual extruder inlet screws at least two for the insulation layers and two for the semiconductive layers and supplying polymer material from storage containers and material handling systems which are arranged at elevated levels S relatively to the respective extruder and its screws, so that the material is transported by gravity in individual closed systems from individual containers to the extruder screws.

12. A method as claimed in claim 11, further including the step of arranging the extruder head outlets so closely together that all layers are fully 'integrated with each other on the cable conductor.

13. A high voltage power cable made by the method as claimed in claims 1 I 25 or 12, said cable comprising a conductor and an insulation system consisting of concentrical layers of extruded polymer insulation and semiconductors, including a first, inner layer of semicondudctive material, two or more layers of insulation material, and a second, outer layer of semiconductive material, on top of each other, on the conductor, wherein all layers are fully integrated with each other on the cable conductor and that c the first, inner layer of insulation constitutes 10-75 of the total insulation thickness.

14. A cable as claimed in claim 13, wherein the thickness of each insulation layer is at least 2.75 mm. An extrusion plant substantially as herein described with reference to the accompanying drawings.

16. A method substantially as herein described with reference to Figs. 7 to 8 of the accompanying drawings.

77. A cable substantially as herein described with reference to Figs. 1 to 8 of the accompanying drawings. .0 DATED THIS TWENTY-THIRD DAY OF APRIL 1998 ALCATEL KABEL NORGE AS o ABSTRACT This invention relates to an extrusion plant for making a high voltage power cable including a conductor (2,11) and an insulation system consisting of concentrical layers of extruded polymer insulation and semiconductors, including a material handling system and a single extrusion head (15,40,50) for extruding the layers on top of each other, onto the conductor. The polymer material storage containers (21;31/33) and material handling systems (20;30) are arranged at elevated levels relatively to the respective extruder and its screws (1 6;32,41 -44;51 so that the material is transported by gravity in individual closed systems from individual containers to at least four individual extruder inlet screws (41-44) of said single extrusion head, at least two of said inlet screws for the insulation layers and two of said inlet screws for the semiconductive layers. The invention also relates to a method for making a high voltage power cable with such a plant, and to a power cable made with the pluit. ea a A *EC o ef e i II