Surge arrester arrangement
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surge arrester arrangement comprising a surge arrester and a a cut-out device arranged in series with the arrester for automatic disconnection of the arrester in the event of an arrester failure. 
     2. The Prior Art 
     Surge arresters with cut-out devices are previously known, for example from U.S. Pat. No. 2,305,436 and EP-B-0 013 401, and are used, inter alia, in transmission lines to protect against back flashover caused by thunder or against switching surges. The arresters are placed in the towers of the transmission line and are connected between the respective phase line and the grounded tower construction. For a surge arrester connected between phase and ground, a failure in all probability results in a permanent ground fault. However, if the arrester is provided with a cut-out device which automatically disconnects the arrester in the event of a failure, the operation of the line can continue and the faulted arrester can be located and replaced at some suitable time. It is, however, important that the disconnected connection between the line and the arrester does not get near the line in case of hard wind, thus causing a ground fault on the line. Hitherto, there has been no good solution to this problem, and it is therefore common to arrange in line surge arresters, instead of cut-out devices, an open spark gap in series with the arrester. However, this normally provides an inferior protective effect, since it is difficult, from the point of view of voltage, to coordinate an arrester and a series-connected, separate spark gap. Another drawback with this solution is that there is no indication if the arrester has failed. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide a surge arrester arrangement the arrester is connected to the power line via a connecting link with the following properties: 
     good flexibility such that the insulator chain is able to swing in case of wind load without stressing the arrester 
     heavy dimensions with respect to corona discharges, that is, a large cross section diameter 
     a low weight 
     spring back property such that the disconnection becomes efficient upon a failure 
     remains suspended straight under the arrester even in case of hard wind after a failure. 
     These properties are achieved according to the invention by designing the link as a flexible electric conductor enclosed in a tube divided into several parts and including resilient members adjacent the tube for keeping the link at insulation distance from the power line when the arrester has been disconnected by means of the cut-out device. 
     According to an especially suitable embodiment of the invention, the connecting link comprises a tube divided into several parts, preferably of aluminium, and provided with an inner helical spring extending between the ends of the tube, and with a continuous cable inside the helical spring to attend to the electrical contact between the line and the arrester. By imparting to the helical spring a considerable prestress, the link will remain straight at the bending stresses which may arise because of wired stresses after a failure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in greater detail by describing embodiments with reference to the accompanying drawing, wherein 
     FIG. 1 shows in side view a first embodiment of a surge arrester arrangement designed according to the invention, during normal operation, 
     FIG. 2 shows the same surge arrester arrangement after an arrester failure, 
     FIG. 3 shows in axial section a part of a connecting link which is part of a surge arrester according to FIG. 1, and 
     FIG. 4 shows in side view a part of an alternative embodiment of such a connecting link. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a power line 1 which, by means of a suspension insulator chain 2, is suspended from a power line tower, of which only an end portion of the horizontal cross beam 3 of the tower is shown in the figure. At the outer end of the cross beam, at insulation distance from the line 1, a surge arrester 4 is arranged. The lower end of the surge arrester is connected to the line i via a connecting link 5 and a cut-out device 6. 
     The surge arrester 4 may be of the frequently used design having an elongated insulating casing, in which a number of preferably cylindrical ZnO blocks are arranged between two end electrodes. The cut-out device 6 may, for example, be of the design described in Swedish patent application 9200525-5. 
     The connecting link 5 is made in the form of an aluminium tube 7 (FIG. 3) consisting of a plurality of tube parts 7a, 7b, arranged one after the other, and provided with an inner helically wound tension spring 8 which extends between the inner ends of the tube. The tube parts 7a, 7b are provided with end inserts 9, preferably of metal. A continuous cable 10 inside the helical spring 8 attends to the electrical contact between the line 1 and the arrester 4. 
     In the event of overload and failure of the arrester 4, the arrester is disconnected with the aid of the cut-out device 6, the connecting link 5 thus assuming the vertical position shown in FIG. 2. In that way, the operation of the power line can continue undisturbed in spite of the arrester failure. At the same time, a clear indication of failure of the arrester is obtained. Faulty arresters can therefore be easily located by inspection from the ground or from a helicopter and be replaced at some suitable time. 
     By imparting to the helical spring 8 a considerable pre-stress and by making the end surfaces of the inserts 9 plane, the connecting link 5 will remain straight at the bending stresses which may arise as a result of wind stresses after a failure. When the link 5 is connected to the line 1, it will exert a sufficient tensile force on the cut-out device 6 to ensure a fast disconnection upon an arrester failure. 
     FIG. 4 shows an alternative embodiment of the resilient member required for straightening of the connecting link 5. Instead of the long, through-going spring 8 shown in FIG. 3, there is used in the embodiment according to FIG. 4 an outer, relatively short close-wound helical spring 11 at each joint between two adjacent tube parts 7a and 7b. The spring is fixed in the axial direction by inserting the ends of the spring wire into fixing holes in the respective tube part.