Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01C—RESISTORS
  • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
  • H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
  • H01C7/12—Overvoltage protection resistors

Definitions

• 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.
• the surge arrester is provi ⁇ ded with one or more clamping members extending between the two end electrodes and being secured thereto.
• 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.
• 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.
• 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.
• 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.
• no moment can be transmitted in case of an ideal pivot, and therefore no stress distribution arises over the lowermost varistor block.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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
• Figure 3 shows an alternative embodiment of the lower part of the above-mentioned surge arrester.
• 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.
• 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) .
• FIG. 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.
• 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.
• 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.
• 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.
• 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.
• the elastic plate may be made of an electrically insulating, elastic material.
• 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.
• 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.

Landscapes

• Engineering & Computer Science (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Thermistors And Varistors (AREA)
• Emergency Protection Circuit Devices (AREA)
• Lift Valve (AREA)

Priority Applications (7)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20394941

Family Applications (1)

Country Status (10)

Cited By (5)

Families Citing this family (25)

Citations (1)

Family Cites Families (5)

• 1994
  • 1994-08-29 SE SE9402745A patent/SE504075C2/sv not_active IP Right Cessation
• 1995
  • 1995-08-25 JP JP50866296A patent/JP3612571B2/ja not_active Expired - Lifetime
  • 1995-08-25 EP EP95930762A patent/EP0777904B1/en not_active Expired - Lifetime
  • 1995-08-25 CN CN95195756A patent/CN1083138C/zh not_active Expired - Lifetime
  • 1995-08-25 RU RU97104882A patent/RU2145743C1/ru active
  • 1995-08-25 DE DE69502620T patent/DE69502620T2/de not_active Expired - Lifetime
  • 1995-08-25 US US08/793,516 patent/US5912611A/en not_active Expired - Lifetime
  • 1995-08-25 AU AU34023/95A patent/AU683770B2/en not_active Ceased
  • 1995-08-25 WO PCT/SE1995/000963 patent/WO1996007186A1/en active IP Right Grant
  • 1995-08-25 BR BR9508648A patent/BR9508648A/pt not_active IP Right Cessation

Patent Citations (1)

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