SURGE ARRESTER

Abstract: A surge arrester comprising a stack of a plurality of cylindrical varistor blocks (10), preferably made of metal oxide, which are arranged one after the other in the axial direction of the varistor blocks between an upper end electrode (11) and a lower end electrode (12) and surrounded by an elongated, electrically insulating external casing (19) of rubber or other polymeric material. The end electrodes (11, 12) are interconnected by means of at least three clamping members (15) of insulating material to achieve the required contact pressure between the different elements (10-14) in the stack. Between the lower end electrode (12) and the first block (10) in the stack of varistors, there is arranged a pivot means comprising a centrally placed pivot member (17), projecting from the lower end electrode (12), said pivot member (17) making contact with a pressure plate (14), resting against the lowermost block (10) in the stack of varistors, to achieve articulation in all transverse directions.

Description: Surσe arrester TECHNICAL FIELD The present invention relates to a surge arrester comprising a stack of a plurality of cylindrical varistor blocks which are preferably made of metal oxide and are arranged one after the other in the axial direction of the varistor blocks between two end electrodes and surrounded by an elongated, electri- cally insulating outer casing of rubber or other polymeric material. To achieve the required contact pressure between the various instruments in the stack, the surge arrester is provi¬ ded with one or more clamping members extending between the two end electrodes and being secured thereto. BACKGROUND ART Surge arresters of the above-mentioned kind are previously known from the patent documents US-A-4 656 555, US-A-5 291 366 and EP-A-0 230 103. A drawback with these known designs is that they exhibit little resistance to mechanical influence in a direction transversally to the surge arrester. To carry large currents, a sufficient contact pressure must be achieved between the blocks. In the known solutions, this contact pressure is achieved by prestressing the varistor stack with an external mechanical, electrically insulating joint. The varistor stack is very stiff in relation to the prestress elements, and a transverse external load, which applies to the upper end electrode, is therefore absorbed as a bending stress in the varistor stack. This bending stress entails a force distribution over the surface of the varistor blocks which provides a compressive stress, increasing towards the edge, in the direction of deflection and a corresponding pressure relief in the opposite direction. Pressure reliefs give rise to insufficient contact pressure and cannot, there¬ fore, be accepted. A known solution is to increase the pre¬ stressing force such that sufficient contact pressure is obtained over the whole surface of the varistor blocks. However, the varistor blocks are brittle, so they can easily crack as a result of great compressive stresses at the edges. The known solutions therefore strike a balance between main- taining a sufficient contact pressure and not exceeding the strength of the varistor blocks with respect to bearing pressure. A surge arrester of the kind described above may alone consti- tute the active part in a surge arrester for medium-voltage systems. A plurality of surge arrester may also, like modules, be connected together into a composite (series-connected) surge arrester intended for higher system voltages. At a transverse load on such a surge arrester composed of several modules, a bending moment arises over the whole of the compo¬ site surge arrester. The varistor blocks in the lowermost arrester module are thereby subjected to very great compress¬ ive stresses and tensile stresses, respectively. A further drawback with these known designs is, therefore, that the resistance to external transverse forces is greatly reduced when joining surge arrester modules together. SUMMARY OF THE INVENTION The invention aims to provide a surge arrester of the above- mentioned kind which has better resistance to external, transverse loads than prior art arresters. This is achieved according to the invention by introducing strap-shaped pre- stress elements secured to the end electrodes and by intro- ducing a pivot means between the varistor stack and the lower end electrode. The prestress elements are arranged such that the end electrodes are connected to each other at at least three points, such that in all directions a bending moment, caused by deflection, is absorbed as tensile and compressive forces, respectively, in the prestress elements. Also an external bending moment, attacking the upper end electrode, will be absorbed as tensile and compressive forces in the prestress elements. A surge arrester composed of a plurality of modules is therefore capable of withstanding considerably greater external transverse forces than a corresponding surge arrester composed of the known design. A surge arrester module with a pivot means which is loaded with an external transverse force gives rise to a bending moment in the upper end electrode. This leads to the creation of a mechanical stress distribution over the surface of the uppermost varistor block, which is of the same magnitude as in the case without a pivot but directed in the opposite direc¬ tion. In the lower end of the varistor stack, no moment can be transmitted in case of an ideal pivot, and therefore no stress distribution arises over the lowermost varistor block. By constructing the pivot means such that it is partially capable of transmitting a bending moment, an additional advantage is obtained. In this case, a bending moment can be transmitted from the lower end electrode to the lower part of the varistor stack, which bending moment is directed in the opposite direc¬ tion of the moment in the upper part of the varistor stack. The bending moments thus arising may be dimensioned to balance each other such that a considerably lower bearing pressure over the surface of the varistor blocks arises. Greater exter¬ nal transverse loads may thus be withstood. The solution described above may be achieved by allowing the pivot means to consist of an elastic plate. According to the invention, it may also be achieved by a pressure plate resting against the varistor stack and a pivot making contact with the pressure plate, the pivot being formed with a plane surface towards the pressure plate. This results in an additional advantage in that the pivot point, when the surge arrester is deflected, is displaced in the deflected direction, whereby the torque arms to the prestress elements are changed. The torque arm belonging to the prestress element which is under tensile load becomes longer during the deflection, which results in a lower tensile load in the prestress element. In this way, the surge arrester is also given an initial stiff- ness, which means that a certain bending moment must be over¬ come before a greater deflection occurs. The prestress elements may advantageously consist of straps, continuously wound of glass-fibre strand and embedded into polymer. The straps are clamped onto shoulders projecting from the end electrodes, for example as shown in the non pre- published German patent application P 43 06 691 7. Through the pivot means, the surge arrester will have a larger deflection amplitude at transverse forces than in prior art designs. This means that, upon deflection, the straps resting against the shoulders projecting from the lower end electrode are subjec¬ ted to an unfavourable force distribution in the direction of deflection. Upon such a deflection, the end electrodes are not only displaced in parallel but they are also positioned at an angle to each other. The displacement and the angular adjust¬ ment mean that a cross section of a strap in an axial plane parallel to the direction of deflection will become subjected to different forces at the inner and outer edges of the cross section. The edge load thus arising becomes dimensioning for the total load-absorbing ability of the strap. A problem then arises in that the load-absorbing ability of the strap is thus reduced if, at the same time, deflection is to be allowed. According to the invention, the above-mentioned problems are solved by pivoting a lower load-absorbing part of the shoulder from an upper part of the shoulder, integrated with the end electrode, by means of a joint in the tangential direction. That part of the shoulder which makes contact with the strap then has a force transmission which is evenly distributed in relation to the cross section of the strap. The force can then be transmitted in a torque-free manner to the fixed part of the shoulder through the joint, which may consist of a rounding of the lower part of the shoulder. ERIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail by descrip¬ tion of embodiments with reference to the accompanying drawings, wherein Figure 1 is a three-dimensional picture of a surge arrester according to the invention with part of the casing of the arrester being cut away, Figure 2 shows such a surge arrester in an axial section, and Figure 3 shows an alternative embodiment of the lower part of the above-mentioned surge arrester. DESCRIPTION OF THE PREFERRED EMBODIMENTS The surge arrester modules shown in Figures 1 and 2 comprise a stack of a number of varistor elements 10 in the form of circularly cylindrical ZnO blocks. The varistor stack is clamped between an upper end electrode 11 and a lower end electrode 12 with intermediate pressure plates 13, 14. The end electrodes and the pressure plates may suitably be made of a electrically conducting material, for example aluminium. The axial compression of the varistor stack is brought about with the aid of four electrically insulating straps 15, which are wound of continuous glass-fibre strand with a plurality of turns and embedded into thermosetting resin. The straps are secured to the end electrodes, which for this purpose are provided with four radially projecting shoulders 16 with circularly cylindrical contact surfaces. The straps may be prefabricated and then be clamped on the stack composed of varistor blocks, washers and electrodes by tightening a screw 17 which is screwed into the lower end electrode and which at the same time functions as a joint screw or terminal. The upper end electrode of the arrester module is provided with a threaded hole 18 for a screw to be screwed (series connection) to a similar module or for external connection. At each end the surge arrester is provided with an end yoke 20 comprising four lugs 21 arranged on a washer, each lug over¬ lapping a shoulder 16 and making contact with a projecting support 22 at each shoulder. The task of the lugs 21 is to reduce the deflection of the surge arrester and to counteract lateral contraction forces in the straps 15. The end yokes 20 are also intended to be able to transmit a torque when screwing together the surge arrester modules or the end connection. The surge arrester module is provided with a casing 19 applied by casting, preferably an elastomer, for example silicone rubber or ethylene propylene terpolymer (EPDM rubber) . Figure 3 shows an alternative embodiment of the lower end electrode 12. The shoulders 16 projecting from the end elec¬ trode each comprise an upper fixed part 16a, integrated with the end electrode, and a lower pivoted part 16b which comprises the semicircular contact surface facing the strap 15. In a normal plane to the surge arrester, on a level with the support 22, the fixed part 16a is formed with a plane contact surface 30. The pivoted lower part 16b of the shoulder 16 is, in the same plane, formed with a cylindrical contact surface 31 resting against the contact surface 30, which contact surface 31 has a direction tangential to an axial plane through the centre of the shoulder. In this way, the contact surfaces 30 and 31 form a joint, through which forces from the strap 15 may be transmitted in a torque-free manner to the end electrode 12. For this reason, no uneven load of the cross section of the strap when deflecting the surge arrester occurs. The plate 14 abutting the screw 17 differs from the preceding example in that its edges are made concave. According to an advantageous development of the invention, the pivot means is made so stiff that it is able partially to transmit a bending moment. The bending moment arising at the lower end of the varistor stack can thus be dimensioned to partially counteract the bending moment at the upper end of the varistor stack. Through this design, the surge arrester can take up considerably greater transverse forces than in the known case without exceeding the allowed bearing pressure in the varistor blocks. This property may be achieved by repla- cing the pivot means with an elastic plate, inserted between the pressure plate 14 and the end electrode 12, with a modulus of elasticity corresponding to a few hundred MPa. When an insulated foot is desired, the elastic plate may be made of an electrically insulating, elastic material. In this embodiment, the electrical connection may be connected to the pressure plate 14. The property of being able partially to transmit a bending moment may also be achieved by forming the screw 15, which is arranged through the end electrode 12, with a plane contact surface. The plane contact surface of the screw must then be given a sufficient diameter, so that a small torque arm is formed from the centre to the edge of the screw, by which torque arm it is possible to transmit part of the external bending moment to the varistor stack. In this way, the pivot point is laterally adjusted in the direction of the deflec¬ tion, whereby the torque arms to the straps 15 are favourably influenced such that smaller tensile forces arise in the straps 15.

Claims: CLAIMS 1. A surge arrester comprising a stack of a plurality of cylindrical varistor blocks (10), preferably made of metal oxide, which are arranged one after the other in the axial direction of the varistor blocks between an upper end elec¬ trode (11) and a lower end electrode (12) and surrounded by an elongated, electrically insulating external casing (19) of rubber or other polymeric material, wherein the end electrodes (11, 12) are interconnected by means of clamping members (15) of insulating material to achieve the required contact pressure between the different elements (10-14) in the stack, characterized in that the clamping members (15) comprise at least three straps of continuously wound strand and that, between the lower end electrode (12) and the lowermost block (10) in the stack of varistors, there is arranged a pivot means comprising a centrally placed pivot member (17), projec¬ ting from the lower end electrode (12), said pivot member (17) making contact with a pressure plate (14), resting against the lowermost block (10) in the varistor stack, to achieve pivoting in all transverse directions. 2. A surge arrester according to claim 1, characterized in that the pivot means comprises a screw (17) for prestressing the stack of varistors. 3. A surge arrester according to claim 1, characterized in that the pivot member (17) comprises at least one lining (shims) . 4. A surge arrester according to any of the preceding claims, characterized in that the pressure plate (14) is formed with oblique edges. 5. A surge arrester according to claim 4, characterized in that the pressure plate (14) is formed with concave edges. 6. A surge arrester according to any of the preceding claims, characterized in that the pivot member (17) is formed with a plane surface making contact with the pressure plate (14) . 7. A surge arrester according to any of claims 1-5, characterized in that the pivot member (17) or the pressure plate (14) is formed with a convex contact surface. 8. A surge arrester according to any of claims 1-5, characterized in that the pivot means comprises an elastic plate arranged between the pressure plate (14) and the end electrode (12) . 9. A surge arrester according to claim 8, characterized in that the elastic plate is electrically insulating and the pressure plate (14) is provided with an electric connection member. 10. A surge arrester according to any of the preceding claims, characterized in that the end electrodes are provi¬ ded with radially projecting shoulders (16), with which the clamping members (15) make contact, wherein each shoulder (16) radially projecting from the lower end electrode (12) compri¬ ses a fixed part (16a) , integrated with the end electrode (12), and a pivoted part (16b) in relation to the fixed part, whereby the joints (30, 31) are able to transmit, in a torque- free manner, compressive stresses from the clamping members (15) to the end electrode (12) . 11. A surge arrester according to any of the preceding claims, characterized in that at each end electrode (11, 12) an end yoke (20) is arranged comprising lugs (21) , by means of which the deflection of the surge arrester can be reduced, the lateral contraction effect of the clamping members (15) can be counteracted and, when joining surge arrester modules, torque can be transmitted.

Inventors: BERGGREN SOEREN, ENGSTROEM JONAS, HELLSTROEM GUNNAR, LINDBERG BJOERN, LUNDQUIST JAN, MORITZ BERTIL, WIECK HAAKAN, BERGGREN, SOEREN, ENGSTROEM, JONAS, HELLSTROEM, GUNNAR, LINDBERG, BJOERN, LUNDQUIST, JAN, MORITZ, BERTIL, WIECK, HAAKAN
Applicants: ASEA BROWN BOVERI, BERGGREN SOEREN, ENGSTROEM JONAS, HELLSTROEM GUNNAR, LINDBERG BJOERN, LUNDQUIST JAN, MORITZ BERTIL, WIECK HAAKAN, ASEA BROWN BOVERI AB, BERGGREN, SOEREN, ENGSTROEM, JONAS, HELLSTROEM, GUNNAR, LINDBERG, BJOERN, LUNDQUIST, JAN, MORITZ, BERTIL, WIECK, HAAKAN
Application year: 1995
Main classification: H01C7
IPC codes: H01C1, H01C7