SURGE ARRESTER

Abstract: An open surge arrester comprises, on the one hand, and ac­ tive part (1,2,3) with a number of surge arrester elements (10) of a metal oxide varistor material, arranged between two end fittings (4,5), and, on the other hand, a retaining part with at least one rod (6) of an insulating material for mechanically holding the surge arrester together. Both, the active part and the retaining part are exposed to the envi­ ronment surrounding the surge arrester and are individually profiled to provide an extended creep distance between the end fittings (4,5).

Description: The invention relates to a surge arrester according to the precharacterising part of Claim 1. The surge arrester is primarily intended for voltages of the order of magnitude of 100 kV and thereabove, but the same embodiment, in principle, may be used also for lower voltages. A surge arrester of the above-mentioned kind is previously known from the US-A-4 262 318. In this surge arrester a plurality of surge arrester elements in the form of zinc oxide varistors are arranged in a stack and enclosed within a housing of porcelain with a gap between the housing and the varistor stack. In the event of a fault inside the surge arrester with a resultant short-circuit current and violent gas development, a high pressure may be built up in the afore-mentioned gap. In such a design, therefore, special and often expensive measures for pressure relief have to be taken to prevent the housing from bursting in an uncontrollable manner, causing secondary damage by fragments being thrown around. Other drawbacks of this design are the requirement for relatively large space and the limited number of parallel varistor stacks that can be accommodated within one and the same porcelain housing. From the GB-A-2 073 965 a surge arrester is previously known in which the arrester elements are provided with a tightly surrounding housing of a shrinkable plastic or rubber material. As mechanical reinforcement of this surge arrester it has been proposed to arrange an insulating rod between the end fittings of the surge arrester. This rod extends through an axial hole in the surge arrester elements. Such a design presupposes a gastight housing which shields the rod from the outside environment, since there is otherwise a risk of creeping currents which may cause a flash-over in the gap which is inevitably formed between the rod and the wall of the hole in the arrester elements. For higher voltages this surge arrester is less suitable. The invention aims at a surge arrester of the above-mentioned kind which does not suffer from the drawbacks associated with the above-mentioned prior art designs. To achieve this aim the invention suggests a surge arrester according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. By using insulating pull rods for mechanically holding the surge arrester together, and by profiling the pull rods and the active surge arrester elements to obtain a sufficiently long creep distance, the outer porcelain housing can be dispensed with. This results, inter alia, in the following advantages in comparison with a surge arrester with a porcelain housing: a) There is no enclosed gas volume in the surge arrester and consequently there is no risk of explosion caused by a pressure increase in the event of a fault inside the surge arrester. Accordingly, no fragments of a burst surge arrester can be ejected and cause secondary damage. b) The surge arrester can be constructed with, in principle, an unlimited number of parallel legs (stacks of surge arrester elements) at a relatively low cost, since there is no space-confining porcelain housing. c) The surge arrester will have a smaller overall height than a surge arrester with a porcelain housing, which, inter alia, is due to the fact that the latter requires larger end fittings. d) The weight of the surge arrester is reduced. e) Cooling of the active parts of the surge arrester is improved. The necessary creep distance of the active part of the surge arrester can be achieved in several different ways: For example, the envelope surfaces of the surge arrester elements can be tightly surrounded by mutually overlapping protective rings and guide rings, which are profiled and made of a material resistant to creeping current. Another possibility is to design the actual surge arrester elements as an elongated element with a plurality of creep distance extending annular projections, arranged one after the other in the axial direction of the surge arrester element, which projections are of the same material as the material in the rest of the surge arrester element and which form, together with the rest of the surge arrester element, a coherent unit (in principle, as shown in EP-A-0 196 370). The envelope surface is suitably provided with a protective coating of an electrically insulating material. According to a further embodiment, the active part, which may comprise a plurality of surge arrester elements stacked on top of each other, is provided with a tightly surrounding casing of a plastic or rubber material resistant to creeping current, which casing has been applied by shrinkage and is provided with grooves. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 a side view of an embodiment of a surge arrester designed according to the invention, Figure 2 a cross-section through the surge arrester taken along line II-II in Figure 1 , Figure 3 an axial section through three series-connected varistor blocks with envelope protective means associated with the surge arrester. The surge arrester shown in Figures 1 and 2 has an active part consisting of three parallel stacks 1,2,3 of cylindrical, coaxially arranged ZnO varistor blocks 10 (Figure 3). The stacks are arranged between a bottom plate 4 and a top plate 5 and are held together mechanically by three insulating pull rods 6 of, for example, glass fibre, which are evenly spaced from each other around the periphery of the surge arrester. Instead of using three rods, it is also possible to use, for example, one single rod 6' arranged between the three varistor stacks along the longitudinal axis of the surge arrester. The rods 6 are provided with end nuts 7. To achieve the necessary contact pressure between adjacent varistor blocks, a spring package 8 is arranged around each pull rod 6 between the top end plate 5 and the associated nut 7. Alternatively, these spring packages 8 may be arranged at one end of each varistor stack. The surge arrester shown may have a length of, for example, 2 m, and each stack may comprise, for example, about 70 varistor blocks. For mechanically reinforcing the surge arrester, a number of metallic support plates 9 are arranged between the stacks and are evenly spaced between the end plates 4,5. These support plates 9 are made with recesses which are adapted to the cross-section of the varistor blocks to form seats for the abutting blocks. Thus, the plates 9 extend across the stacks and form electric parallel connections therebetween. Figure 3 shows how the varistor stacks are built up between the support plates 9. The end surfaces of the circular-cylindrical varistor blocks 10 are provided with electrode coatings, for example in the form of layers of copper or aluminium, applied by plasma spraying, whereby the varistor blocks in the stacks are series-connected. Further, the blocks are provided with electrically insulating envelope protective means, in principle, the same design as that shown in US-A-4 352 140. The envelope protective means consists of protective rings 11, for example of silicon rubber or EPDM rubber, tightly surrounding the varistor blocks, and of separate guide rings 12 of, for example, polypropylene. These guide rings 12 enable the varistor blocks to be stacked on top of each other with guidance in the lateral direction. At the same time the guide rings seal the varistor stack, so that ionized gas which may be generated by glow discharge between two adjacent blocks is unable to spread outwards. Each guide ring 12 is provided with a radially projecting, surrounding fin 13 for extending the creep distance in the axial direction of the stack. The protective rings 11 have almost the same axial extension as the varistor blocks and are provided with an external, surrounding elevation 14 to secure the guide rings 12 in the axial direction. Furthermore, the creep distance-extending fin 13 of the guide rings 12 also result in increased mechanical stability of the guide rings 12, which entails the possibility of considerably reducing the number of metallic support plates 9, or even of omitting these plates completely. To achieve a sufficient creep distance along the rods 6, which are arranged for mechanically supporting the surge arrester, these rods 6 can be profiled, for example by providing them with grooves or threaded slots. The profile of such a rod should be such that the creep distance along the rod between the end plates of the surge arrester is at least 30% longer than for a corresponding rod with a smooth circular-cylindrical shape. The rods 6 may also be plain and the necessary creep distance be obtained by, for example, shrinking a grooved shrinking hose, available on the market, on each rod. The surge arrester shown in the drawing is primarily designed for indoor use as overvoltage protection for electric high voltage equipment set up in a protected environment, for example thyristor valves for high voltage direct current. However, the design principle suggested according to the invention may advantageously be used for outdoor surge arresters as well. The invention is not limited to the embodiment described but can be materialized in many ways within the scope of the appended claims.

Claims: Surge arrester comprising an active part (1,2,3) including at least one substantially cylindrical surge arrester element (10) of a metal oxide varistor material, arranged between two end plates (4,5), and a retaining part including at least one rod (6) of insulating material extending between the end plates (4,5) for mechanically holding the surge arrester together, characterized in that both the active part (1,2,3) and the retaining part are exposed to the environment surrounding the surge arrester and that both said parts are individually profiled to increase the creep distance between the end plates (4,5). Surge arrester according to claim 1, characterized in that said rod (6) of the retaining part is threaded or grooved in such a way that the creep distance along the rod between the end plates of the surge arrester is at least 30% longer than for a corresponding rod with a smooth circular-cylindrical shape. Surge arrester according to claim 1, characterized in that said rod (6) of the retaining part consists of a plain rod which is covered with a creep distance-extending, shrunk-on casing in the form of a grooved shrink hose. Surge arrester according to any of the preceding claims, characterized in that the active part (1,2,3) comprises a plurality of surge arrester elements (10) arranged coaxially in a stack, the end surfaces of said surge arrester elements being provided with electrode coatings. Surge arrester according to any of the preceding claims, in which the envelope surfaces of the surge arrester elements (10) are tightly surrounded by annular protective members (11,12) of insulating material, which are formed and arranged so as to overlap each other around adjacent surge arrester elements, characterized in that the protective members are formed with at least one creep distance-extending annular projection. Surge arrester according to claim 5, characterized in that each varistor block is tightly surrounded by a protective ring (11) of almost the same axial extension as the varistor block and that the protective rings (11) of adjacent varistor blocks are surrounded by guide rings (12) which are provided with a radially projecting surrounding creep-distance extending fin (13), the protective rings (11) being provided with an external surrounding elevation (14) to secure the guide rings (12) in the axial direction and the protective and guide rings being of an insulative material. Surge arrester according to any of claims 1 to 4, characterized in that the surge arrester element (10) is formed at its envelope surface with a plurality of creep distance-extending annular projections arranged one after the other in the axial direction of the surge arrester element, said projections being of the same material as the material in the rest of the surge arrester element and forming together with the rest of the surge arrester element a coherent unit. Surge arrester according to Claim 7, characterized in that the envelope surface of the surge arrester element (10) is provided with a protective coating of an electrically insulating material, said coating being formed and secured on the surge arrester element. Surge arrester according to Claim 8, characterized in that the coating consists of a layer of a polymer or elastomer, or of a layer of glass, or of a ceramic material. Surge arrester according to any of claims 1 to 4, characterized in that the active part (e.g. 1) is provided with a tightly surrounding casing, applied by means of shrinkage and provided with grooves, of a plastic or rubber material, shrinkable by heating and being resistant to creeping current. Surge arrester according to any of the preceding Claims, characterized in that its active part comprises a plurality of columns (1,2,3) arranged side-by-side in parallel between the end plates, each of said columns including one or more surge arrester elements (10).

Inventors: JOHNNERFELT, BENGT, STENGARD, PETER, THORS, BENGT
Applicants: ASEA BROWN BOVERI AB
Application year: 1988
Main classification: H01C7
IPC codes: H01C7