TECHNICAL FIELD

Surge arresters are used for protecting s itchgear and its apparatus against high overvoltages caused by lightning strokes or connections and disconnections in the switchgear. The present invention relates to the varistor blocks which are included in and constitute the active part of a surge arres- ter. The surge arrester normally comprises a stack of a plura¬ lity of cylindrical varistor blocks, preferably made of metal oxide, which 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.

BACKGROUND ART

Surge arresters of the above-mentioned kind are previously known from a plurality of patent documents, inter alia EP-A- 0 230 103. One drawback with this and several of these known designs is that, if, for example in the event of a fault on a varistor block, an arc is formed inside the surge arrester with an ensuing increase in pressure, parts of the surge arrester may spread explosion-like in a way which is harmful to the environment. Various attempts have been made to solve this problem.

In EP-A-0 335 480, a cross-wound cage with openings for pressure relief has been arranged around the stack of varistor blocks. Such a solution makes the manufacture both complicated and expensive.

In EP-A-0 683 496, the varistor blocks are axially surrounded by compression loops of insulating material to achieve the necessary contact pressure between the various varistor blocks in the stack. The varistor stack and the loops are then radially surrounded, on the centremost part of each varistor

block, by bursting-preventing annular bandages with a substan¬ tially square cross section made of insulating material with a thick layer of insulating polymer material between bandages and varistor blocks. This means that there are openings in the axial direction between the bandages for pressure relief in the event of a short circuit in the surge arrester.

SUMMARY OF THE INVENTION

The present invention aims to provide a varistor block and a surge arrester of the above-mentioned kind with improved short-circuit performance in relation to prior art designs by being able to withstand an electrical/thermal breakdown of the stack of varistors without mechanically falling apart. The varistor blocks and a surge arrester comprising these varistor blocks shall have a simple construction and be capable of being manufactured in a cost-effective manner. This is achieved according to the invention by a design with the characteristic features described in the characterizing part of claim 1.

To achieve the above-mentioned object, the invention comprises two principal designs.

The first principal design of the invention is that the varis¬ tor block around the circular-cylindrical envelope surface is surrounded by an electrically insulating ring of composite material with a ceramic matrix, for example of silicon carbide or boron carbide. The ring is fixed to the varistor block in connection with the manufacture thereof and is intended to constitute an integral part of and have the same axial length as the varistor block.

In an alternative embodiment, in order to further improve the short-circuit properties of the varistor block provided with the insulating ring, the ring may be wound with an organic fibrous material. The fibrous material may be so-called ara- mide fibres, for example kevlar or twaron.

The second principal design of the invention is that the varistor block around the circular-cylindrical envelope sur¬ face is surrounded by a bursting-preventing bandage of an organic fibre. The fibrous material may be so-called aramide fibre, for example kevlar or twaron. The bursting-preventing bandage is formed as a straight circular tube. The axial length of the bandage is somewhat shorter than the axial length of the varistor block. The bandage is axially placed such that the openings between the bandages for a plurality of axial varistor blocks, arranged one after the other, will be positioned right in front of the joints between adjoining varistor blocks. This results in a faster pressure relief at those points where the risk of arcing is greatest, and hence reduced stress on the rings. The scope of the invention allows for the bandage to be applied around the varistor block in a plurality of different ways.

In a preferred embodiment, the tubular bandage is wound direct onto the varistor block.

In an alternative embodiment, the bandage is wound, during the manufacture, around a cylindrical frame such that it becomes tubular. The tube consists of tangentially wound fibre. The inner diameter of the bandage is preferably made with a diame- ter which is a few millimetres larger than the outer diameter of the varistor block.

In still another embodiment, the bandage is wound onto a plastic ring shaped as the reel of a sewing-thread, that is, with collars provided with bobbins. Also in this embodiment, the inner diameter of the plastic ring is made with a diameter which is a few millimetres larger than the outer diameter of the varistor block.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows in a side view, half as a section, a preferred embodiment of a varistor block designed according to the in-

vention, according to the first principal design of the inven¬ tion.

Figure 2 shows in a side view, half as a section, an alterna- tive embodiment of a varistor block designed according to the invention, according to the first principal design of the in¬ vention.

Figure 3 shows in a side view, half as a section, a preferred embodiment of a varistor block designed according to the in¬ vention, according to the second principal design of the in¬ vention.

Figure 4 shows in a side view, half as a section, an alterna- tive embodiment of a varistor block designed according to the invention, according to the second principal design of the in¬ vention.

Figure 5 shows in a side view, half as a section, an addition- al alternative embodiment of a varistor block designed accor¬ ding to the invention, according to the second principal design of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figures 1 and 2 show embodiments according to the first prin¬ cipal design of the invention.

Figure 1 shows a circular-cylindrical varistor block 1 with an integrated electrically insulating ring 2 of composite mate¬ rial with a ceramic matrix according to the above and with the same axial length as the varistor block.

Figure 2 shows the same varistor block 1 with the integrated ring 2 as in Figure 1. In this embodiment of the invention, the ring is provided with an external annular layer 3 of organic fibre according to the above, wound around the ring.

As mentioned in the introductory part of the description, the varistor stack is surrounded by an elongated electrically insulating casing of rubber or other polymeric material. Preferably, the casing comprises an inner and an outer layer of different polymeric materials. The inner layer, which surrounds the varistor blocks and the insulating ring, com¬ prises a castable polymer material, for example a so-called RTV or LΞR polymeric material. The outer layer, which inter alia forms the shields of the surge arrester, consists of a die-cast polymer material, for example of a so-called HTV or EPDM polymer material.

Figures 3, 4 and 5 show embodiments of the second principal design 3 of the invention.

Figure 3 shows a circular-cylindrical varistor block 1 with a tubular bursting-preventing bandage 4 of organic material, wound directly onto the envelope surface.

Figure 4 shows the same varistor block. In this embodiment of the invention, the straight tubular bursting-preventing ban¬ dage 5 has been wound separately with an inner diameter a few millimetres larger than the outer diameter of the varistor block. This leads to the creation of an annular concentric space 6 between the varistor block and the bandage. The bandage is fixed concentrically to the varistor block in different ways according to the description below.

Figure 5 also shows the same varistor block 1. In this embo- diment, the tubular bursting-preventing bandage 6 has been wound onto a plastic ring in the form of a bobbin 6 provided with collars. The inner diameter of the plastic ring has a diameter a few millimetres larger than the outer diameter of the varistor block. In this way, an annular concentric space 9 is formed between the varistor block and the bobbin. The bobbin with the bandage is fixed concentrically to the varis¬ tor blocks in different ways according to the description below.

As mentioned under the Technical Field, a surge arrester comprises a plurality of varistor blocks stacked on top of each other and clamped between an upper and a lower end electrode. The axial compression of the varistor stack may suitably be accomplished with the aid of insulating com¬ pression loops which are clamped to the lower end electrode. The compression loops are designed such that, during the com¬ pression, for one thing the varistor blocks will have the same axial line, and for another the bandages according to Figures 4 and 5 will be fixed concentrically to the varistor blocks with a radial distance determined by the difference in outer diameter for the varistor block and the bandage and the inner diameter of the bobbin, respectively. As mentioned above, the bandage shall be axially placed around the varistor block such that the openings between the bandages for a plurality of axial varistor blocks, placed one after the other, will be positioned right in front of the joints between adjoining varistor blocks. To ensure this with an embodiment of the bandages according to Figures 4 and 5, a number of spacers 10, with a cross section as that of an HE-A beam or the like, are placed between the bandages and between confronting collars, respectively.

As mentioned in the introductory part of the description, the varistor stack is surrounded by an elongated electrically insulating casing of rubber or other polymeric material. Preferably, the casing comprises an inner and an outer layer of different polymeric materials. The inner layer, which surrounds the varistor blocks and the bandages, consists of a castable polymer material, for example of so-called RTV or LSR polymer material. The outer layer, which inter alia forms the shields of the surge arrester, consists of a die-cast polymer material, for example of a so-called HTV or EPDM polymer material.

The intention with the concentric space 6 and 7, respectively, according to Figures 4 and 5, which is formed between the varistor block and the bandage and between the varistor block

and the bobbin, respectively, is to allow it to be filled with the castable polymer material of the inner layer.