Cable termination

Abstract: The invention relates to high voltage surge arrestors which are installed in parallel with an electric apparatus to be protected from excessive high voltages, such as a high voltage power cable or a transformer. The arrestor consists of one or more annular bodies (9,19,32), through the hollow of which a high voltage conductor (3,31) is terminated. The arrestor may also be used with penetrators and transformer bushings.

Description: The present invention relates to high voltage surge arrestors which are installed in parallel with an electric apparatus to be protected from excessive high voltages, such as a high voltage power cable or a transformer. The simplest form of protecting device is a spark gap arranged in parallel between earth and the high voltage terminal. With newer technique the open spark gap device has been substituted by voltage arresters made of nonlinear material such as zinc oxide (ZnO). Examples of such arrestors are described in ASEA data sheet LB 25.1 of 1. Dec. 1984, 'ZnO - Ventilavledare typ XBE' and also in EP-A-0 281 945. These known surge arrestors consist of a number of cylindrical ZnO blocks which are stacked one upon the other within an insulating cover. The ends of the stack are interconnected between earth and the high voltage terminal, with one end as close as possible to said terminal. A weather tight surge-responsive electrical cable terminator (US 3,581,154) is known, having annular elements through which the cable is inserted. The annular body or ring elements are made of magnetic material. The object of the present invention is to simplify and improve surge arrestors for electric apparatus. The main features of the invention are defined in the accompanying claims. With a surge arrestor according to the invention the installation of high voltage electric apparatus such as power cables, will be greatly simplified. The arrestor is designed to accommodate a prepared cable end so as to provide a combined end terminal and surge arrestor. The design leads to cheaper cable installations as well as enhanced protection due to the compact construction. Above mentioned and other features and objects of the present invention will clearly appear from the following detailed description of three embodiments of the invention taken in conjunction with the drawings, which show surge arrestors according to various exemplary embodiments of the invention. In Figure 1 is schematically illustrated a power cable 1 having a number of armour or shielding elements (not shown) connected to an earthed base plate 2. The cable has a conductor 3 which is compressed or welded to a connector 4. The cable conductor 3 is provided with a layer of insulation material 5 and an outer semiconductor 6. A voltage stress relief device 7 is installed on the prepared cable end. The stress relief device 7 may take many forms. In Figure 1 it has the form of a layer of stress relieving material which is linear or nonlinear, whereas in Figure 2 it is based on the principles of the well known stress relief cone 8. Annular elements 9 of nonlinear material constituting the main surge arresting components are placed around the stress relief device 7. The annular elements 9 are compressed between two metal discs 10 and 11 by means of a spring 12. The spring also ensures good electrical contact between the top annulus and a high voltage terminal 13, and between the base annulus and the earthed base plate 2. Contact between the connector 4 and the high voltage terminal 13 may be achieved by tightening a top nut 14 so as to press the conical contact surfaces of these two components together. The surge arrestor assembly 9 is enclosed within an insulator 15 which can be made of any organic or inorganic insulator material. The assembly may be designed to fill all highly stressed cavities, or said cavities may alternatively be filled with insulating fluid or gas. Figure 2 shows a surge arrestor where the stress relief device 17 is based on other principles than that of Figure 1. Other components (19,20,21,22), including the annular rings 19, which are similar to those of Figure 1, are fitted to the device 17 and the stress cone 8. In Figure 3 is illustrated an alternative voltage surge arrestor 30 in which a conductor 31 is terminated through a nonlinear device 32 and a tightly enclosing sleeve 33 of insulating material such as silicon rubber or EPR rubber. The device 32 may be made in one piece as shown or it may consist of a number of annular rings similar to the rings 9 and 19. As an alternative the nonlinear device 32 may be given a form corresponding to a combination of the device 32 and the sleeve 33. In that case the outer surface of the combined nonlinear device should preferably be covered with a layer of insulation material such as silicon rubber. The end portions 34, 35 of the nonlinear device 32 are in direct contact with the terminals 2 and 13. The other components of Figure 3 correspond to similar components in Figures 1 and 2. The nonlinear devices 9, 19, 32 can for instance be made of silicon carbide SiC or a metal oxide such as ZnO. The combined voltage surge arrestor and cable termination can be preassembled and mounted at a desired position, for instance at the top of an overhead line pole or at some other suitable place, before inserting the prepared cable end through its hollow. The top terminal 13 may be lifted to allow filling of a compound into the cavities of the assembly and making the desired connection between the cable connector 4 and the terminal 13, or the combined surge arrestor may be prefilled and supplied with a plug-in contact, in order to optimize installation. The installation procedures, including time and expenses, of this combined unit, are very much simpler than having to install two separate units, one being the cable termination and the other being the excessive voltage arrestor. 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. Nor should it be limited to cable termination technology only. For example, the same principles may be applied to and used in penetrators and transformer bushings. In those cases the nonlinear rings are arranged around the penetrating conductor as they were arranged around the cable conductor.

Claims: High voltage surge arrestor for electric apparatus such as high voltage power cables or transformers said surge arrestor comprising an annular body formed of one or more annular sections (9, 19, 32) of high voltage surge arrestor material having an nonlinear voltage material characteristic, the interior of said annular body defining a hollow within which a high voltage conductor (3, 31) is terminated, a voltage stress relief device (7, 17) such as a cone or sleeve (8) connected between a first and a second end of the annular body, and high voltage connection means for effecting a first electrical connection between the second end of the annular body and the exposed end of the high voltage conductor and grounding means for effecting a second electric connection between the first end of the annular body and ground. Surge arrestor according to claim 1, characterized in that it consists of one annular section (32) which is provided with an insulating outer sleeve or layer (33). Surge arrestor according to claim 1, characterized in that the arrestor annular sections (9,19) are arranged within an insulator house (15). Surge arrestor according to claim 3, arranged to be installed betwen a power cable (1) having a conductor (3), insulation (5) and a shield/armour ((6), and a high voltage structure such as an overhead line comprising a bare or insulated conductor, characterized in that the surge arrestor is preassembled within the insulator house (15) so that the cable end (1) can be installed through the hollow of the annular sections (9,19,32) to the outlet of the insulator (15). Surge arrestor according to claim 1, characterized in that the annular sections (9,19,32) are made of a metal oxide such as zinc oxide (ZnO). Surge arrestor according to claim 1, characterized in that it is suitable for use in penetrators and transformer bushings.

Inventors: VARRENG JAN, VARRENG, JAN
Applicants: ALCATEL STK AS, ALCATEL STK A/S
Application year: 1990
Main classification: H02G15
IPC codes: H01C7, H02G15