AU603020B2

AU603020B2 - Surge arrester

Info

Publication number: AU603020B2
Application number: AU26354/88A
Authority: AU
Inventors: Bengt Johnnerfelt; Bengt Thors
Original assignee: Asea Brown Boveri AB
Current assignee: ABB AB
Priority date: 1988-12-06
Filing date: 1988-11-30
Publication date: 1990-11-01
Anticipated expiration: 2008-11-30
Also published as: AU2635488A; DE3889729T2; ES2056881T3; EP0372106A1; EP0372106B1; DE3889729D1

Description

COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 Application Number: Lodged: Complete specification: Lodged: Accepted: Published: FORM Class: Iut. Class 603020 4 0 0 a 4 q 4 4

I

"p 'S 9' S t I 4 4 4, b

S*

Priority: Related Art: Name of Applicant: ASEA BROWN BOVERI AB Address of Applicant: S-721 83 Vasteras, Sweden.

Actual Inventor/s: BENGT JOHNNERFELT; and BENGT THORS.

Address for Service: E.F. WELLINGTON CO., Patent and Trade Mark Attorneys, 457 St. Kilda Road, Melbourne, 3004, Victoria.

Complete Specification for the invention entitled: "SURGE ARRESTER" The following statement is a full description of this invention including the bost mothod of performing it known to us.

The present invention relates to a surge arrester comprising a plurality of cylindrical arrester elements of metal oxide varistor material, which are arranged one after the other in the axial direction of the arrester elements between two end electrodes in an elongated protective housing of a plastic material, which housing is resistant to deformation under the operating conditions for the surge arrester and makes contact -with the envelope surfaces of the arrester elements.

In the case of a passage of current, caused by overvoltages, through a surge arrester of^ the above-mentioned kind, the arrester elements are heated. When the passage of current is of considerable magnitude, the temperature may amount to 150- -200 OC. In surge arresters which are su~bjected to such considerable heating, this has reduced the choice of protective housing t~o a housing which, nearest the arrester elements, con- 0000 sists of a thermosetting resin, for example an epoxy resin in 00 the form of a casting around the arrester elements, or in the o 00 at form of an epoxy resin impregnated wrapping of a fibre material, 00 of a 9 to such as a woven glass fibre, around the arrester elements.

a 20 A pol ymer in the form of a shrinkable hose or a shrinkable tube with projections for extending creeep distance may be applied on the thermosetting resin.

According to the present invention, a protective housing which resistant to deformation under operating conditions f'or the 25 surge arrester and which remains intact when subjected to the O influence of arrester elements which have been heated to tern- @0 0 pera tures of the magnitude stated above, can be provided in a *considerably simpler way than -what has previously been possible.

According to the present invention, the favourable results are achieved by making the protectivo housing from cross..linked 11D polyethylene (High Density polyethylene), Which is shrunk onto the arrester elements stacked on top of each ohr, and 1 2 by arranging, between at least the main part of the arrester elements, heat-absorbing bodies of metallic material to make contact between confronting end surfaces of adjacent arrester elements. The wall thickness of the protective housing is made sufficient, preferably at least 2 mm, for the parts of the protective housing located furthest away from the arrester elements to attain a temperature, during maximum heating of the arrester elements, which is safely below the softening temperature of the cross-linked HD polyethylene; i.e. even if the inside of the protective housing is in contact with an arrester element which is briefly heated to a temperature in the vicinity of 200 0

C,

the outside is not to be heated, in the presence of the heatabsorbing bodies, to a higher temperature than one which is safely below 130 0 C. The outside i~i preferably heated to at most around 100 0 C. The heat-absorbing bodies have a total length in the longitudinal direction of the protective housing which is at least 10% and preferably 15-35% of the total length of the arrester olements in the same direction.

The varistor material in the arrester element may be of a known kind and preferably 70-97 mole per cent thereof consists of ZnO with additives of one or more oxides and/or carbonates of Bil Sb, Cr, Mn, Co, Ni, Si, B, Ba, Pb, Al, each in an amount of 0.01-10 mole per cent. The arrester elements are manufactured from a powder of the varistor material which unde r known conditions are moulded, pressed and sintered into bodies~ of the desired shape.

The electrodes on the end surfaces ot the arrester element may, inter alia, consist of layers of copper or aluminium which have boen applied by arc spraying or other spraying of metal, or of 44 30 varnish layers which are electrically conductive, for example 6664 0or epoxy resin containing powder of silver. Xt may also consist or surface layers in the variatOr Material itself wM~ch have Vo a beon made low-ohmic by Inner treatment.

Upon heating,, cross-linked RD~ polyethylene is tranaCormed from cryntalline to substantially amorphous state. Xn %'he amorphous 3 state, the shape of an object of the polymer material may be changed and, upon cooling, be brought to maintain the changed shape. If the object is heated again, the object resumes its original shape. This property of cross-linked H-D polyethylene is utilized when it is applied around the stack of arrester elements disposed on top of each other with the heat-absorbing bodies disposed therebetween.

The cross-linked HD polyethylene is suitably of the kind which is manufactured by silane grafting of linear HD polyethylene and a subsequent cross-linking of the grafted polymer after extrusion or other moulding by the moulded product being sub- Jected to moisture or water so that the hydrolyzable groups in the silane radical are hydrolyzed and provide siloxane bonds between the grafted polyethylene molecules. This cross-linked HD polyethylene has a softening temperature of around 130 0

C.

The end electrodes, which like the heat-absorbing bodies may 0:50 advantageously be of aluminium or copper, are preferably pro- 060 0 *4 vided with annular recesses or projections, into which and 0 a 0between which, respectively, parts of the protective housing 00 0 20 project.

0 be 0Particularly if the surge arrester is intended for outdoor use, 0 0' the protective housing is provided on the outside with means extending the creep distance. This can be done by covering the outside of the protective housing with a separate creep distance extending body, supported by the protective housing, preferably a body which in a known manner is formed with a plurality of projections arranged one after the other in the longitudinal *9 0 direction of the protective housing. It can also be done by 6* 660 forming the outside of the protective housing itself with a plurality or creep distance extending projections arranged one after the other in the longitudinal direation of the protective housing. The projections are then of the. same material as 'the protective housing and form a coherent unit with the rest of the protective housing.

r 26 The. invention will be explained in greater detail, by way of examples, with reference to the accompanying drawing, wherein Figure 1 shows an axial section through a surge arrester according to .the present invention without any creep distance extending means, Figure 2 shows an axial section through such a surge arrester with a creep distance extending means, and Figure 3 shows a modification of the surge arrester according to Figure A tube of cross-linked HD polyethylene (silane-grafted), which in one case, selected as example, has an inner diameter of 28 mum and a wall thickness of 3 mm, Is placed in a tube of steel or aluminium with an inner diameter of 38 mm. The tubes are heate.

4 to 150 0 C, whereafter the polyethylene tube is excpanded by compressed air supplied to the interior of the polymer tube, so that i.ts outer envelope surface makes contact with the inside of the metal tube. Thereafter, the tubes are cooled down in this expanded state of the polymer tube, whereby its inner diameter becomes 32 mm. The expansion can also be performed, inter alia, with a mandrel. When the metal tube has been removed, there are placed in the tube of H-D polyethylene (in shrunk state designated 10 in Fig. one after the other into a stack (shown in F~ig. a plurality of arrester elements 11 in the form of circular-cylindrical ZnO blocks provided with electrodes alternately with a plurality of heat-absorbing bodies 12 in the form C t 0 of circular-cylindrical blocks of aluminium and, in each end of' the stack$ an electrode 13 in the form of a subttantially circular.-cylindrical block of alumi-nium, In the exemp~lified case, the ZnO blocki' 11 have a diameter of 30 mm and a height of 47 mm, and the aluminium blocks 12 the some diameter and a height or 15 mm. That part 13a of' the aluminium blocks 13 which faces the interior of the surge arres~ter has a diameter of mm, and that part 13b or the aluminium olocka, 13 which faes away from the interior has a diameter of 28 n.mm The blocks 13 have a height of 40 mm. They are provided with annular slots or recesses 14 with a depth of 2 mm and, at the end facing the interior of the surge arrester, with a spring 15 which exerts a pressure on a washer 16, in the exemplified case of aluminium.

When the stack of the blocks 11, 12 and 13 has been placed in the expanded cross-linked tube of HD polyethylene, the blocks and the tubes are heated to a temperature of 150°C. This causes the tube 10 to shrink so as to make contact with the envelope surfaces of the blocks 11, 12 and 13, as is clear from Figure 1, and so as to penetrate into the slots 14 and form a protective housing for the arrester elements. The springs 15 ensure that the blocks 12 and 13, the latter via the washers 16, make contact, with an effective contact pressure, with the electrodes 11a secured to the end surfaces of the ZnO blocks and consisting of sprayed-on layers of aluminium. The wall thickness of the tube, after shrinking, is still 3 mm.

That part of the protective housing which makes contact with the part 13b on each end electrode 13 is surrounded with a binding strap or a hose clamp 17 to strengthen the fixing of the protective housing to the end electrodes. Over the outer portion of each end electrode there is arranged a cap 18, for example of aluminium, together with a seal, for example in the form of an O-ring or a sealing compound 19, such as silicone rubber. Finally, the surge arrester is provided with terminals 20. The surge arrester according to Figure 1 is designed for indoor use.

Surge arresters according to the present invention, which are designed for outdoor use, are provided, as is clear from Figure 2, with a creep distance extending body 21, which is formed with a plurality of projections 21a arranged one after the other in the longitudinal direction or the protective housing. The body 21 May be of an elastomer, for example ethylene propylene terpolymer (EPM rubber) and is passed over the protective houting It may also consist of a shrinking plastic, for example or a croso-linked ethylone-propyleno polymer or crots-linked RD polye thyleno applied on the protective housing 10 by shrin-

I

_"M

kage. The body 21 is applied on the protective housing before the cap 18 is fitted.

According to one embodiment of' the invention, a creep distance extending body 21 is formed with the same shape as that shown in Figu re 2 as part of the protective housing 10, s0 that the parts 10 and 21 form a coherent unit of cross-linked HD polyethylene manufactured in one piece. In the forming operation, the protective housing is then provided with a plurality of creep distance extending projections, arranged one after the other in the longitudinal direction of the protective housing, of a kind analogous to those designated 21a in Figure 2. The protective housing is then preferably manufactured by injection moulding, since a manufacture by extrusion of a thick-walled tube with subsequent milling away of material for forming projections would involve a considerable material waste.

Figure 3 illustrates an alternative embodiment of the surge arrester according to Figure 1. The surge arrester shown in 0009, Figure 3, with a creep distance extending body 21 in a manner 00a 0 00 "q similar to that shown in Figure 2, may be used for outdoor use.

0 20 In accordance with Figure 3, spring elements in the form of disc springs 22 are arranged between the arrester elements 11 and the heat-absorbing bodies 12 and between the arrester elemients 11 and the end electrodes 13 to ensure that an effective contact pressure is maintained between the parts 11, 12 and 13 in the entire stack under varying conditions. Instead of cup ~springs there may be used springs 15 of the kind shown in Figures 1 and 2 which ape built into the end electrodes in the manner shown in Figures 1 and 2 and in analogous mfanner~ into the heat-absorbing bodies.

The matter contained in each of the following claims is to be read as part of the general description of the present invention.

Claims

1. A surge arrester comprising a plurality of cylindrical arrester elements of metal oxide varistor material, which are arranged one after the other in the axial direction of the arrester elements between two end electrodes in an elongated protective housing of a plastic material, said protective housing being resistant to deformation under the operating conditions for the surge arrester and making contact with the envelope surface of the arrester elements, the end surfaces of the 10 arrester elements being perpendicular to the axial direction of the arrester elements and provided with electrodes secured to said arrester elements, characterized in that the protective housing is of HD polyethylene and is shrunk onto the arrester elements arranged one after the other and that between at least the main part of the arrester elements, heat-absorbing bodies of metallic material are adapted to make contact between confronting end surfaces of adjacent arrestor eolemeonts.

2. A curge arrestor according to claim 1, characterized in that the protective housing has a wall I thickneDs of at least 2 mm. 3, A Ourgo arrestor according to claim 1 or 2, charactorized in that the heat-oboorbing bo4die have a total longth in the lengitudinal diietion Of the protective housing which is t loast IC% of the tOtal S longth of the arroGter loeomrtis in the a.o d-tln. I. i S8 S4. A surge arrester according to any one of claims 1 to 3, characterized in that the end electrodes are at least substantially cylindrical and that the protective housing is shrunk onto the end electrodes. A surge arrester according to any one of claims 1 to 4, characterized in that the end electrodes are provided with annular recesses or projections, into which or between which parts of the protective housing S ,5 project.

6. A surge arrester according to any one of claims 1 to 5, characterized in that the protective housing is covered with a separate creep distance extending body, supported by the protective housing, preferably a body which is formed with a plurality of creep distance extending projections arranged one after the other in the longitudinal direction of the protective housing. i i

7. A surge arrester according to any one of claims 1 to 5, characterized in that the protective housing at the envelope surface is provided with a plurality of croop distance extending projections, S arranged one after the other in the longitudinal direction of the protective housing, said projections being of the came material ao the protective housing and forming a coherent unit with the ~ost of the protetivo housing.

8. A nurgo arrootor aeorde ng to any ene ef elaois 1 to 7, charaetriseod in that opriy oeloontt are arranged Lbtwoon the arroetor olon-onts and the hoat- atBorbing -iaon, I I I t 9

9. A surge arrester according to claim X, substantially as described herein with refeorence to any one of the embodiments illustrated in the accompanying draw in~s. DATED this 30th day of November, A. D. 1988 ASEA BROWN BOVERI AB, By its patent Attorneys, E. F. WELLJINGTON Co., 41 BFWCE S. WELLZINGTON