Surge arrester with movable supports to maintain its varistors.

The present invention relates to an improvement in the structure of the type "single column" arresters, whether post or distribution.

The arresters are electrical protection devices well known to be connected in parallel to an electrical device to be protected, in order to reduce the power surges that may occur across it. Specifically, surge arresters are electric systems normally having the form of an open circuit which is "transformed" into a parallel closed-loop device to be protected from an overvoltage significant appears across it. They thus reduce the level of insulation of electrical devices they protect and, consequently, their production cost.

Arresters presently available on the market are widely used in the transmission and distribution networks (station arresters and distribution) of electricity. These protectors are usually composed of a porcelain housing having the general shape of a cylindrical tube closed at one end sometimes, which envelope defines a cavity in which is disposed one or more columns of varistor being in the form of pellets stacked one another. It is well known that the varistors are electrically active elements consisting of metal oxide such as zinc oxide, or silicon carbide, whose impedance nonlinearly appears under the effect of an overvoltage to provide adequate protection.

Like any electrical device, surge arresters are sometimes subject to default. When such a fault occurs, one or more varistors are continuously shorted and an electric arc is formed inside the casing, which generates explosive pressures and temperatures exceeding the melting point of all known metals. To reduce the risk of explosion of the arrester housing following an internal short circuit, pressure limiters have been suggested, which are designed to transfer the arc to the outside using of a diaphragm and a nozzle orientation hot pressure gas generators. Obviously, these parts must be mounted on the porcelain shell, which makes the relatively expensive construction. This is why we find these especially in high-voltage transmission station arresters.

In Canadian Patent Application No. 526,139 filed December 23, 1986 to the Applicant, a new type of cylindrical housing has been proposed to replace the porcelain casings used so far for the construction of surge arresters. This new housing is advantageously composed of a synthetic insulating material capable of withstanding a high mechanical tension, such as concrete or concrete-epoxy-polymer. This envelope is molded to a thin tube preferably made of glass, disposed on an electrode.

The solution proposed in this Canadian patent application has the great advantage of eliminating all the drawbacks associated with the use of porcelain for making envelopes, and from there allow arresters for building distribution networks less subject to risk of explosion or shell breakage, and at a cost comparable to existing distribution arresters made of porcelain.

However, it was observed that if, as indicated above, the solution proposed in this Canadian patent application dramatically reduces the risk of explosion, it does not eliminate this risk in the particular case where the arrester is subject to a severe thermal shock.

To better understand this risk and the proposed solution under the present invention, it should further describe the basic structure of the single column-type arresters, and how they behave in case of default internal.

As previously indicated, any mono-column arrester comprises an envelope consisting of an insulating material such as porcelain or, as in Canadian patent application mentioned above, a concrete-epoxy or polymer, which envelope defines a generally cylindrical cavity provided with a wall and two ends, at least one is not closed. In the case of lightning arresters of the type "single column", a single column of varistors disposed inside the cavity. These varistors have the form of cylindrical pellets of diameter less than that of the cavity of the casing, which are stacked one above the other to form the single column. The means, generally consist of a spring coming squeeze on one of the bases of the column used to hold the pellets and stacked in the envelope. Electrical contacts or "electrodes" are disposed at both ends of the cavity of the casing to allow the circuit arrangement of the column of varistors in parallel to the terminals of an electric apparatus to be protected against overvoltage.

So as to allow transfer of the arc from inside to outside in case of failure of the arrester, a diaphragm is preferably disposed across each non-closed end of the cavity of the envelope. This diaphragm is made of an aluminum foil or any other easily rupturable material under the effect of excess pressure generated by an electric arc can be formed within the cavity in the event of internal failure of the surge arrester, thus enabling the evacuation "natural" hot gas generated by the arc to each non-closed end of the shell and, hence, reduce the risk of explosion of the latter. Usually, a nozzle is also used downstream of the diaphragm for guiding outward the hot gases escaping from the diaphragm is perforated.

In normal operating condition, the current from the external circuit to which the surge arrester is connected through the latter via a contact to each other via the varistor column.

In case of a failure of one or several varistors column disposed within the casing, an electric arc occurs inside the cavity and creates an internal pressure that leads to perforation of the diaphragm. When this perforation occurs, the hot gases inside the casing can escape and be directed by the nozzle towards another electrode, to thereby cause transfer of the arc from the inside to the outside the casing and hence, release the inside thereof excess pressures and temperatures which could cause an explosion.

For this safety mechanism operates properly, it is of course necessary that the electric arc which is created e naturally during the fault can "spreading out" within the cavity of the casing and that this arc can reach without obstruction diaphragm for perforating the latter and thus ensure the release of the hot gases required to transfer outside of the arc. It is therefore essential to have no mechanical blockage along the axis of the internal cavity of the casing, in the direction of the non-closed end thereof.

To this end, the usual practice has been to use varistors, pellet diameter smaller than the inner diameter of the cavity to allow sufficient air ring around the pellets to allow the arc to form and achieve diaphragm, provided that any obstruction to the passage of the arc reduces latter transfer efficiency to the outside and from there increasing very substantially the risk of explosion. However, the possibility of such an obstruction exists permanently in case of fragmentation of the pellets of varistor, for thermal or purely mechanical reasons. Thus, it was found that in case of failure of a varistor, a substantial leakage current is generated, which leads to a significant rise in voltage and a severe thermal shock that could lead to almost diametrical fragmentation of some pellets whose fragments are then obstruct the annular passage provided around the column.

The present invention provides an extremely simple structure device can solve the problem mentioned above and, hence, minimize the risk of explosion of the shell of an arrester in case of internal fault because lack of sufficient space in the cavity to allow the arc generated when the failure to develop and reach the diaphragm.

More specifically, the invention proposes to maintain the varistor wafers stacked in a single column in the cavity of the casing with at least three, and preferably four stakes made of a material which is both insulating and non elastic, such as porcelain, ceramic or concrete or concrete-epoxy-polymer. To ensure their function of holding the pellets, guardians each extend over the entire height of the column and are symmetrically arranged around the latter.

With this arrangement and this particular provision, tutors constantly form and maintain free passages identical in number to their own number, and all around the varistor column. Of course, each of these passages is in practice constituted by the air gap defined between two of the actual stakes, the wall of the column and the wall of the cavity.

As previously indicated, the main utility of these guardians is to keep the center of the cavity fragments varistor in case of fragmentation of one or more of these following a severe thermal shock. Tutors and guarantee non-obstruction of the passages they define around the column and from there the required expansion of the arc that can be established anywhere around the column and develop without any obstruction . Tutors also possible to channel the shock wave and hot gases to the diaphragm without any risk of obstruction and therefore explosion.

Preferably, each guardian has a isosceles trapezium shaped section and is arranged so that the major base of its trapezoidal section comes to bear against the periphery of the disks. This trapezoidal shape is interesting in that it maximizes the space inside the cavity of the housing and from there expand to the maximum the passages in which the fault arc can flourish.

So as to ensure good retention of the fragments of the varistor wafers in case of fracture of the latter, the large base of each trapezoidal section guardian is concave and same curvature as the periphery of the disks. In addition, this larger base has a length such that the sum of all wide bases of all guardians is slightly less than the perimeter of the pellets. In other words, the bases tutors are selected to surround almost continuous circumference pellets to prevent even small pieces can come closing passages defined between tutors for the development of the arc.

In a preferred embodiment of the invention, the tutors are mounted using readily sheared by simply fixing rods overheating or excess pressure around the periphery of a rigid core diameter greater than that of the pellets, the base being intended to be placed below the column of varistors. This particular arrangement has the advantage of giving enough flexibility guardians to allow expansion of the arc generated in case of default of the arrester, which arc is never straight. As can indeed understand, the arc formed in case of default of the arrester will end up funneled into an axial segment defined by two tutors. Tutors are held by one end to a same base, they can easily be extended to the other end which is preferably placed where the diaphragm so to ensure greater transition arc is. Furthermore, being maintained with easily sheared rods out of their base, guardians can be completely released as soon as the arc is formed and, as a result of the pressure of the bow, be pushed thus ensuring a maximum enlargement of the axial portion wherein the arc is formed.

As previously indicated, the invention is specifically designed to be used with single column type arresters, since the use of such guardians game has an advantage that when the SPD contains only one column of varistors. For cons, the present invention is not limited to a particular type of SPD and can be used interchangeably with post arresters or distribution arresters.

It is also worth mentioning that guardians whose use is proposed as part of the present invention should not be confused with calo-body mechanical carriers sometimes used to create a thermal bridge between the wall of the internal cavity of the envelope and the outer wall of the varistors in order to facilitate the cooling thereof. Indeed, heat transfer and mechanical supports used are usually made of rubber wherein the heat transfer additives are added. These supports are thus unlike stakes according to the invention, made of an elastic material so as to permit installation by clamping inside the cavity and thus to ensure a permanent and close contact between the wall of the varistors and the wall the cavity. In contrast, the stakes used in the invention must be made of non-elastic or rigid material so as to be movable without flex or deform under the effect of the pressure of an arc forming in one of the passages .

In the case of heat transfer mechanical supports, it is also necessary to have maximum contact with the wall of the cavity, since the main purpose of these supports is to ensure a transfer of heat varistors outward. In contrast, the stakes according to the invention preferably have the shape of a trapezoid whose small base is directed towards the wall of the cavity so as to ensure the maximum free space adjacent to said wall. In addition, according to the invention, contact between the tutors and the wall should be minimized to allow tutors to easily move from an arch.

It is obvious that we can not confuse both the utility point of view of the structural standpoint, the currently existing mechanical supports heat transfer structure with tutors and very specific proposed use according to the present invention.

Finally, it is clear that the use of the present invention should by no means be restricted to the arrester housing subject to Canadian Patent copending application mentioned above. In fact, as can be easily understood, the improvement of the present invention can be used with any type of existing arrester fitted with a porcelain shell or other similar material.

The invention will be better understood upon reading the nonlimiting description which follows of a preferred embodiment thereof, made with reference to the accompanying drawings in which:<ul><li>- Fig. 1 is a vertical sectional view of a surge arrester as described in Canadian Patent Application No 526 139, provided with the improvement according to the present invention;</li><li>- Fig. 2 is a perspective view of a column of varistors surrounded by a guardian of game according to the invention, mounted on a base;</li><li>- Fig. 3 is a view of the non-closed end of the arrester shown in Fig. 1, and</li><li>- Fig. 4 is a view identical to that of FIG. 3, showing the position of the stakes after a severe thermal shock which led to a diametrical fragmentation of the pellets and formation of an electric arc inside the cavity.</li></ul>

The single column arrester type shown in Fig. 1 of the drawings is described in detail in Canadian Patent Application No. 526,139 filed December 23, 1986 by the Applicant, HYDRO QUEBEC. Consequently, only the essential elements of this arrester understanding of the present invention will be described hereinafter.

The surge arrester 1 comprises an outer shell 3 made of an insulating material electrically. This envelope defines a cavity 5 of generally cylindrical shape, provided with a wall 7 and both ends of which at least one is closed by an electrode 9 acts as a first electrical contact.

The outer surface of the casing 3 is preferably provided with annular fins 11 serving on the one hand, to ensure a dielectric for the envelope in conditions of rain and pollution and, secondly, to increase the mechanical strength of the latter. The casing 3 is also provided with a plurality of anchors 13 and 13 '. The anchors 13 'are disposed on the side of the envelope where the cavity is closed by the electrode 9 serve for fastening the arrester to a same support 15 using bolts 17. anchorages 13 arranged in the other side of the casing, where there is the non-closed end of the cavity, mainly used for fixing a cover 19 using bolts 21.

The surge arrester 1 comprises a varistor 18 column each having the form of a cylindrical pellet of smaller diameter than that of the cavity 5. The varistors 18 are stacked one on another to form a single column disposed in the center of the cavity 5. Means constituted by a spring 23 mounted between the electrode 9 and the base of the varistor column 18 to keep it pressed against the cover 19.

This cover 19 is annular and is made of an electrically conductive material to thereby form the other contact electrode of the surge arrester or, at the other end of the column of varistors 20. This cover 19 is, for reasons of assembly composed of several parts which, in particular, a part 25 of centering and holding of the varistor column. This part 25 is itself provided with a plurality of peripheral holes 27 whose purpose is primarily to allow escape of hot gases when there is an overpressure in the cavity 5 within the envelope 3.

A diaphragm 29 is mounted on the cover 19 to close it. The diaphragm 29 is usually made of a thin sheet of metal capable of tearing when subject to a pressure generated by the shock wave and of an electric arc gases forming within the cavity 5 when internal fault of the arrester.

Finally, the surge arrester 1 comprises a nozzle 31 for discharging the hot gases to rise even the cover 19 to guide the hot gases escaping through the passage 27 due to the perforation of the diaphragm 29 in the direction of an explosive bolt 33 connected to the electrode 9, for, first, ensure in case of default transfer outside the arc created inside and, following this transfer, an explosion of the bolt 33 to isolate the surge arrester earth when the fault current is interrupted by the circuit breaker in effect in the power network.

The previously described surge arrester 1 is connected in parallel to the terminals of a device to be protected, by connecting the electrode 9 (usually by the explosive bolt 33) at one of the terminals of this device and the cover 19 by a bolt 35 to the other terminals of this unit. The other structural details and operation of the arrester described above are known and are described in detail in Canadian Patent No. 526,130 copending application.

According to the present invention, the arrester 1 above described is improved in that the means used to maintain the pellets varistor 20 stacked in columns in the cavity 5 of the casing 3 includes, besides the spring 23, and at least three preferably four stakes 41 made of a material which is both insulating and non-elastic. Guardians 41 which are preferably constituted by rods porcelain, ceramic or any synthetic insulator such as polymer concrete or concrete-epoxy, extend over the entire height of the column of varistors 18. These same stakes 41 are also arranged symmetrically around the varistor column 18, to almost completely surround it.

As was explained in detail in the preamble of the present specification, these guardians are primarily intended to form and maintain around the same number column in their passages 43, each passage being defined between the outer periphery of varistors 18, the side walls of two adjacent stakes and the wall 7 of the cavity 5.

The means for maintaining the pellets stacked in columns in the envelope cavity also include, in the illustrated embodiment, a rigid base 45 preferably consists of a metal disc whose diameter is greater than that of the pellets. The base 45 which serves to mount guardians 41 is advantageously arranged under the end of the column in contact with the spring 23. It is this base 45 which is in contact with the spring 23 and which ensure the electric transmission of current between the electrode 9 and the first varistor 18.

The stakes 41 are attached by their lower ends even in this base by means of small fastening pins formed by bolts 47 anchored to them, each bolt 47 passing through a hole 49 provided for this purpose in the base 45, before being held thereto with a nut 51.

Advantageously, the bolts 47 are selected of a material capable of melting or shear very easily in case of overheating, thereby enabling to immediately release the lower ends guardians 41. The bolts 47 can thus be made in NYLON or in any other similar material.

As clearly illustrated in Figs 2 and 3, each stake 41 has a general isosceles trapezium shaped section arranged so that its larger base 53 comes to bear against the periphery of the pellets of varistor 18. This large base 43 is concave and same curvature as the periphery varistors 18. This large base further has a length such that the sum of the length of all the major bases guardians surrounding the column is slightly less than the perimeter lozenges to surround so almost continues the periphery thereof and thus retain any fragments that may form in the event of breakage or breaking one or more of varistors 18.

This trapezoidal isosceles section is very advantageous in that it provides a perfect encirclement pellets of varistor 18 while maximizing the free space defined by the passages 43.

As noted above, the main use of tutors 41 is to hold the center of the cavity 5 fragments of varistor that may form in case of breakage of the latter following a severe thermal shock. Should such a defect, one or more varistors are diametrically will fragment as shown in Fig. 4 and will lead to the formation of an arc within the cavity 5. This bow course be formed and épandra within one of the passages 43 where it will be channeled through the adjacent stakes 41 to the cover 19, to tear the diaphragm and ensure the expulsion of hot gases into the nozzle 31.

With tutors whose large bases around almost continuity in the circumference of the column, no fragment may enter the space 43 in which will form the arc. As a result, the passage obstruction risks are virtually eliminated, which, in turn, minimizes the risk of explosion of the envelope 3.

Upon the formation of the arc, the heat generated and the shock wave will cause the bolts to shear 47 and from there guardians to completely break free from the base 45 to become mobile. Both tutors bordering the passage 43 in which the arc will be formed will be able to then slide in opposite directions under the influence of the arc. This spacing will open a maximum passage 43 and, from there, will ensure expansion of the arc without any constraints.

It is worth mentioning that the tests were carried out as part of a new standard of the company HYDRO QUEBEC, namely the standard B-31.19-03 of 1988. These tests were performed on arresters of the type described in the Canadian patent application No. 526,139 with co-pending mobile guardians made of concrete polymer were extremely positive.