Electrical surge arresters

Details Electrical surge arresters Electrical surge arresters

1999-11-03

2002-05-28

Jackson, Stephen W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

High voltage dissipation

C361S118000, C361S127000

Reexamination Certificate

active

06396676

ABSTRACT:

FIELD OF THE INVENTION
This invention concerns improvements relating to electrical surge arresters, also known as surge diverters, which, as is well known, are used in high voltage electric installations for providing a path to ground for surge overvoltages occasioned for example by lightning strikes and as switching transients.
BACKGROUND OF THE INVENTION
In GB-A-2188199 there is described a surge arrester which is primarily intended for use in distribution class applications, but can be coupled with other electrically matched arresters in a series parallel arrangement for higher voltage station class applications as described in GB-A-2230661. This surge arrester comprises a rigid core of great physical strength upon which there is provided a shedded outer housing of polymeric material, the core consisting of a plurality of ceramic varistor blocks stacked end to end and encased within a shield of reinforced rigid plastics material. The reinforced rigid plastics material shield is preferably bonded directly to the external surface of the varistor block stack, though proposals have been made to provide a thin spacing layer of Mylar™ between the varistor blocks and the shield. There may be a number of metal (eg aluminium) spacer blocks incorporated into the varistor block stack for the purpose of providing the stack with an overall length sufficient to avoid flashover externally of the arrester and terminal blocks are provided at opposite ends of the stack for enabling the coupling of the arrester to associated equipment.
As described in GB-A-2188199 the rigid core and polymeric housing of the surge arrester provides significant advantages as compared to porcelain housed surge arresters which have long suffered a fragility problem. Conventional porcelain housed surge arresters commonly have an inert gas filling which can literally blow the arrester housing apart if the arrester is subjected to an excessive overvoltage, littering the surrounding area with hot fragments and causing fires. Pressure relief rupturable diaphragms can be provided in the end cap arrangements of gas filled porcelain housed surge arresters, but these provisions complicate the arrester construction and the fragility problem of the porcelain housing still remains.
The rigid cored, polymer housed, gapless surge arrester of GB-A-2188199 is manufactured to avoid gaseous inclusions and retains its structural integrity even under failure. Additionally, it enables support insulators to be dispensed with, since the rigid core of the arrester is sufficiently strong to support the loading which conventionally is supported by the provision of support insulators. The inherent strength of the arrester of GB-A-2188199, which results from its rigid core and gapless construction, enables it to be coupled into a series parallel array capable of handling station class voltages as described in GB-A-2230661. As a result of these and other advantages, the surge arrester of GB-A-2188199 has enjoyed considerable commercial success.
Notwithstanding the success of the surge arresters of GB-A-2188199 and GB-A-2230661 we have, as a part of our continuous improvement strategy, considered how component costs, assembly times and manufacturing process times might be reduced. The manufacture of surge arresters in accordance with GB-A2188199 requires curing processes to cure the reinforced epoxy material that is applied to the varistor block stack in an uncured state and, if a silver loaded adhesive is provided between the contiguous faces of the stacked blocks to improve electrical contact therebetween, additionally requires a curing process for the adhesive. After curing, the core assembly is then sleeved with its polymeric housing in a separate operation and finally end caps are fitted. The manufacturing time required for these processes is considerable and demands the availability of curing ovens, and a manufacturing technique which could eliminate the curing process would show considerable benefit in manufacturing space and time. Any reduction in component costs would be an added bonus.
OBJECTS AND SUMMARY OF THE INVENTION
It is, accordingly, the object of the present invention to provide an electrical surge arrester which can be more readily manufactured than the surge arrester of GB-A-2188199 without prejudice to the advantages demonstrated by that surge arrester.
The present invention stems primarily from the realization that the external passivation coating that is invariably provided by manufacturers of metal oxide varistor blocks on all surfaces of the blocks other than their metallized contact surfaces can in fact be dispensed with, thereby leading to a reduction in component costs. Varistor blocks conventionally are solid circularly cylindrical in shape with metallized contact coatings on their circular axially end faces and passivation coatings on their cylindrical external surfaces, the passivation coating comprising an epoxy resin or glass or a separately fitted electrically insulating collar. The passivation coating enables a varistor to be used to its full electrical capability by avoiding external flashover problems between the contact surfaces, and its provision is also useful to the manufacturer in that it enables classification and quality control operations to be conducted. However, the provision of the passivation coating significantly contributes to the manufacturing cost of varistor blocks.
If the need for a passivation coating can be dispensed with, then the limitations that the need for a passivation coating have imposed upon the shapes of varistor blocks can also be dispensed with. The process by which passivation coatings are most economically provided on varistor blocks is to spray an epoxy resin or glass material on to the surface of the varistor block and then to bake the varistor in an oven until the resin is cured on to the varistor surface or until the glass material has sintered on to the varistor body. This process has hindered the production hitherto of varistor blocks with through holes, since the manufacture of a varistor block with a through hole and with an effective passivation coating on the surface of the through hole would impose additional difficulties for the varistor block manufacturer.
By dispensing with the passivation coating, varistor blocks with through holes, for example an axial through hole, can be manufactured at reasonable cost. A stack of such varistor blocks can then be retained in face-to-face contact between end terminations by means of an electrically-insulating rod appropriately secured to the end terminations and the present invention proposes to take advantage of such an arrangement. A surge arrester comprising varistor blocks with axial through holes which are retained in a stacked configuration and supported by an electrically insulating rod has previously been proposed in GB-A-2073965 but has not been manufactured. Other surge arresters wherein varistor blocks have a supportive rod extending through holes in the blocks are proposed in U.S. Pat. No. 4,262,318, EP-A-0141239, U.S. Pat. No. 4,825,188 and WO-A-95/10846. The surge arresters of U.S. Pat. No. 4,262,318 and EP-A-0 141 239 are of the previously mentioned porcelain housed type wherein the physical strength of the arrester, such as it is, is provided by the porcelain housing. The surge arrester of U.S. Pat. No. 4,825,188 has a similar construction to a surge arrester embodying to the present invention as hereinafter described, but U.S. Pat. No. 4,825,188 does not disclose or suggest the present invention. Likewise WO-A-95/10846 discloses a similar surge arrester but does not disclose or suggest the present invention.
In accordance with the present invention, a stack of non-passivated varistor elements is retained in face-to-face contact between end terminations by means of an electrically insulating rod passed through through-holes in the varistor elements and secured to the end terminations, the through-holes and the rod are sized such that there is a clearance between the surface of the rod and the surrounding

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