Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Patent
1997-06-02
1999-06-15
Walberg, Teresa
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
361126, H01C 710
Patent
active
059126116
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
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, electrically insulating outer casing of rubber or other polymeric material. The required contact pressure between the various instruments in the stack is provided achieved one or more clamping members extending between the two end electrodes and secured thereto.
DESCRIPTION OF RELATED ART
Surge arresters of the above-mentioned kind are previously known from the patent documents U.S. Pat. No. 4,656,555, U.S. Pat. No. 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 transverse to the surge arrester.
To carry large currents, a sufficient contact pressure must be achieved between the blocks in the stack. In the known devices, 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, applied to the upper end electrode, is 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. Such pressure reliefs give rise to insufficient contact pressure and cannot, therefore, be accepted. A known solution to this problem is to increase the prestressing 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 maintaining 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 constitute 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. When transverse load bears on such a surge arrester composed of several modules, a bending moment arises over the entire composite surge arrester. The varistor blocks in the lowermost arrester module are thereby subjected to very great compressive 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 provides 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 strap-shaped prestress elements secured to the end electrodes and by 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 th
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patent: 3710212 (1973-01-01), Marek et al.
patent: 4656555 (1987-04-01), Raudabaugh
patent: 4962440 (1990-10-01), Johnnerfelt et al.
patent: 5291366 (1994-03-01), Giese
patent: 5517382 (1996-05-01), Leupp et al.
Berggren Soren
Engstrom Jonas
Hellstrom Gunnar
Lindberg Bjorn
Lundquist Jan
Asea Brown Boveri AB
Pwu Jeffrey
Walberg Teresa
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