Cable installation method

Abstract: The invention relates to interruption of screens of power cables (1,2) with double stress cone systems (23,30). Such interruption systems have hitherto always been made at the same place as the cable core joints.

Description: The present invention relates to a method for installation of high-voltage power-cables and in particular to a method for sectionalizing the outer screens on single core cables. The currents or electromotive forces which are induced in the sheaths and screens of the three single core cables making up a three-phase cable installation can be intolerably high, but they can be eliminated by dividing the screens into isolated sections and crossbonding them. So far, the screen interruptions have always been made at the same site as the cable joints. The screens are crossbonded at the interruption sites and the screens are usually grounded for every third cable section. In Cigre-report No. 21-08 of 1986 there is described design, manufacturing and installation of XLPE-cables in Denmark. In connection with sectionalizing joints, there are described three different principles for interruption of the screens in such cables. Of the three different systems, 1 (a double stress cone system), 2 (a screen over-lap system), and 3 (a high resistivity system), the first one is in our opinion the best alternative. As mentioned in said article special care must be taken to ensure that the metallic connections from the stress cone to the metal sheath of the cable and to the bonding leads do not mechanically influence the position of the stress cone. It is also mentioned that allowance has to be made for the methane-gas from the vulcanized joint insulation to escape without forming air-pockets in the interface between the XLPE-insulation and the stress cone. The object of the present invention is to provide a sectionalizing and crossbonding system which makes use of a double stress cone system, but which does not involve the risk of forming gas pockets in the interface between the applied XLPE insulation and the stress cone. The main features of the present invention are defined in the attached claims. By displacing the screen interruption from the conductor joint and from the area where the XLPE-insulation material is re-applied, we have obtained a cable installation which satisfies the requirements. Our defined screen interruption may not only be used on XLPE-cables, but it may also be used on oil-filled cables and mass-impregnated cables. As the screen interruption takes place outside the jointing area of the cable core, crossbonding of the sheaths without jointing of the main cables is another option which is readily offered by the described method. When it will be possible to make longer cable sections, our method will allow sectionalizing and crossbonding of the screens independent of the cable section joints. 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 Fig. 1 shows a double stress cone screen interruption similar to that described in the mentioned Cigre-report, Fig. 2 shows our screen interruption system used in connection with a sectionalized cable core joint, and Fig. 3 shows our screen interruption system used on a non-jointed cable core. In Fig. 1 is illustrated a joint between two XLPE-cables 1 and 2, each having a conductor 3 and 4 and a layer or layers of XLPE-insulation 5 and 6 as well as outer conductive or semiconductive screens 7 and 8. The conductors 3 and 4 are jointed at 9 and the XLPE-insulation is re-applied at 10. The two screens 7′ and 8′ are re-applied from the surface of the cables 1 and 2 over the re-applied XLPE-insulation material 10, to meet in double stress cone 11. The double stress cone 11 has two electrodes 12 and 13 which are molded into a piece of insulation material 14. The re-applied XLPE-insulation material may be cross-linked or vulcanized prior to or after application of the re-applied screens 7′ and 8′. After the vulcanizing process methane-gas will escape from the insulation material. This may, as mentioned, quite often result in gas pockets at the interface between the re-applied XLPE insulation and the double stress cone. In Fig. 2 the two cables 1 and 2 each having insulated conductors with outer screens are jointed in nearly the same manner as described in connection with Fig. 1. However the semiconducting or conducting screen 7 of cable 1 is terminated outside the conductor joint, whereas the re-applied semiconductor 8′ from the cable 2 is completed over the conductor joint area 24 and terminated in the same area as the screen of the cable 1. The two screens 7 and 8′ are terminated in contact with electrodes 20 and 21 which together with a molded piece of insulation material 22 make a double stress cone 23. When the re-applied insulation material 10 of an XLPE-joint is to be vulcanized or cross-linked, the methane gas is free to escape from the joint along the whole surface of the joint. It would be clear that this double stress cone system may be used also for other types of cables such as oil-filled cables and mass-impregnated power cables. For XLPE cables the double stress cone may be made of an elastomeric material, whereas for example epoxy may be used for paper insulated oil filled cables. When the outer semiconductor screen is to be terminated for crossbonding near a joint of the cable core as illustrated in Fig. 2, the molded stress cone ring 23 is placed over cable 1 before jointing of the cable core. If the sectionalizing and crossbonding of the screen is to be made distant from a cable core joint, it will have to be flexible (including the electrodes), so that it can be expanded to a larger diameter than the cable. In the expanded condition to about 1.4 times of the cable diameter it may be placed on a sleeve which can be pulled over the cable,- or the cable may be pulled through the sleeve at the position where the crossbonding is to take place. In Figure 3 is illustrated a double stress cone 30 placed on a cable surface distant from cable core joints. The screen 7 of the cable 1 (or the screen 8 of the cable 2) is interrupted at a place required and the two screen ends are terminated in the double stress cone 30. The crossbonding itself is not shown. 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.

Claims: Method for interrupting the outer screen of a power cable made up of at least two sections (1, 2) to be jointed together, each section comprising : an inner conductor (3, 4), an insulating layer (5, 6) surrounding said inner conductor, an outer screen (7, 8) surrounding said insulating layer, said method including the following steps : taking off the insulating layers (5, 6) and outer screens (7, 8) of two adjacent sections (1, 2) from the place where the junction (9) is to be made, called the joint area (24), jointing the two conductors (3, 4) of said two adjacent sections (1, 2), covering the joint area (24) with new insulating material (10) extending up to said insulating layers (5, 6) at either side of said junction (9), locating screen interrupting means (23) at the ends of said screens (7, 8) and being characterized in that : the screen (7) of the first (1) of said two sections (1, 2) is terminated outside said joint area (24), the screen (8') of the second (2) of said two sections (1, 2) is pulled back, the junction (9) is carried out, the pulled back screen (8') is re-applied in order to cover the entire joint area (24) upon said new insulating material (10) and to terminate, through a first of said screen interrupting means (23), in the same area as said screen (7), said area being called first screen interrupting area and being longitudinally displaced from said new insulating material (10) and from said joint area (24). Method according to claim 1 further characterized by the steps of : interrupting once more said screen (7) of one of said two sections (1, 2) in order to form a second screen interrupting area separating two screen portions at a location longitudinally displaced from the joint area (24) and from said first screen interrupting area and locating a second of said screen interrupting means (30) around said second screen interrupting area. Method according to anyone of claims 1 or 2 characterized in that said screen interrupting means consist of a double stress cone system (23, 30). Method according to anyone of claims 1 to 3 characterized in that said outer screen (7, 8) is a semiconductive screen. Method according to anyone of claims 3 or 4 characterized in that said double stress cone (23, 30) is ring shaped and made of flexible material, and in that one of said two sections (1, 2) is inserted in said double stress cone (23, 30) while said double stress cone is expanded, at the place where said double stress cone is to be located. Power cable comprising at least two sections (1, 2) jointed together, characterized in that the outer screen (7, 8) of said cable is interrupted according to the method of anyone of claims 1 to 5.

Inventors: WARRENG JAN, WARRENG, JAN
Applicants: STANDARD TEL KABELFAB AS, STANDARD TELEFON OG KABELFABRIK A/S
Application year: 1988
Main classification: H02G15
IPC codes: H02G15, H02G1