EP0413618B1 - Surge arrester with movable supports to maintain its varistors - Google Patents

Abstract

Disclosed is a station or distribution surge arrester of the type comprising a single column of varistors stacked one upon the other and centrally arranged in a cylindrical chamber provided in a casing made of an insulating material. A plurality of movable props, made of a material which is both insulating and non-elastic, are used for holding the varistors in column form centrally in the chamber, even if the varistors break diametrically as is often the case during a severe thermal shock. By holding the varistors permanently at the center of the chamber, the props ensure the non-obstruction of several vertical passages around the column, which passages allow an arc created within the surge arrester to move freely toward a pressure limiting diaphragm during failure of the latter and, thereby, reduce explosion risks to a minimum. The props are advantageously secured over a base having a diameter greater than that of the varistors, by means of bolts or pins that are capable to shear at the least overheat or overpressure generated by the arc. The props are likewise preferably trapezoidal in cross-section and are arranged in such a way that the wide base of their trapezoidal cross-sectional come to rest against the periphery of the varistors, each wide base having a length such that the sum of all of the wide bases of all of the props be slightly shorter than the perimeter of the varistors.

Description

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

Surge arresters are well-known electrical protection devices intended to be connected in parallel to an electrical device to be protected, with the aim of reducing the overvoltages that may occur across the latter. More precisely, surge arresters are electrical systems normally in the form of an open circuit which "transforms" into a closed circuit parallel to the device to be protected as soon as a significant overvoltage appears at the terminals of the latter. They therefore make it possible to reduce the level of insulation of the electrical devices that they protect and, consequently, their production cost.

The surge arresters currently available on the market are widely used in transmission and distribution networks (post and distribution surge arresters) of electrical energy. These surge arresters usually consist of a porcelain envelope having the general shape of a cylindrical tube sometimes closed at one end, which envelope defines a cavity in which is placed one or more varistor columns in the form of pellets stacked the on top of each other. It is well known that the varistors are electrically active elements made up of metal oxide such as zinc oxide, or alternatively of silicon carbide, the impedance of which appears non-linearly under the effect of an overvoltage. way to provide adequate protection.

Like any electrical device, surge arresters are sometimes subject to default. When such a fault occurs, one or more of the varistors are permanently short-circuited and an electric arc is formed inside the envelope, which generates explosive overpressures as well as temperatures exceeding the melting point of all known metals. In order to reduce this risk of explosion of the arrester enclosure due to an internal short circuit, pressure relief devices have been suggested, which are intended to transfer the electric arc outside using a diaphragm and '' a nozzle for orienting the overpressure generating hot gases. Obviously, these elements must be mounted on the porcelain shell, which makes the construction relatively expensive. This is also why these elements are mainly found in the surge arresters of a high-voltage transmission station.

In Canadian patent application No. 526,139 filed December 23, 1986 in the name of the Applicant, a new type of cylindrical housing was proposed to replace the porcelain envelopes used until now for the construction of surge arresters. This new housing advantageously consists of a synthetic insulating material capable of withstanding a high mechanical tension, such as concrete-epoxy or concrete-polymer. This envelope is molded on a thin tube preferably made of glass, placed 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 the manufacture of envelopes, and, hence, allowing the construction of surge arresters for distribution networks much less subject to explosion or envelope breakage risks, at a cost comparable to that of existing distribution surge arresters made of porcelain.

It has however been observed that if, as previously indicated, the solution proposed in this Canadian patent application makes it possible to considerably reduce the risks of explosion, it does not entirely eliminate this risk in the particular case where the arrester is subject to severe thermal runaway.

To better understand this risk and the solution proposed in the context of the present invention, it is necessary to describe in more detail the basic structure of surge arresters of the monocolumn type, and how they behave in the event of a fault. internal.

As previously indicated, any single-column surge arrester comprises an envelope made of an insulating material, such as porcelain or, as in the Canadian patent application mentioned above, of an epoxy or polymer concrete, which envelope defines a generally cylindrical cavity provided with a wall and two ends, at least one of which is not closed. In the case of SPDs of the "single-column" type, a single column of varistors is arranged inside the cavity. These varistors are in the form of cylindrical pellets with a diameter smaller than that of the envelope cavity, which are stacked one above the other to form the single column. The means, generally constituted by a spring pressing on one of the bases of the column, serve to maintain the pellets thus stacked in the envelope. Electrical contacts or "electrodes" are arranged at the two ends of the envelope cavity to allow the electrical mounting of this column of varistors in parallel across the terminals of an electrical device to be protected against overvoltage.

In order to allow a transfer of the electric arc from the inside to the outside in the event of a fault with the arrester, a diaphragm is advantageously placed across each unclosed end of the envelope cavity. This diaphragm is made of aluminum foil or any other easily tearable material under the effect of any overpressure generated by an electric arc which can form within the cavity in the event of an internal defect in the arrester, thereby allowing the 'natural' evacuation hot gases generated by the arc at each unclosed end of the enclosure and, from there, reducing the risk of explosion of the latter. Usually, a nozzle is also used downstream of the diaphragm, to direct outward the hot gases escaping as soon as the diaphragm is perforated.

Under normal operating conditions, the current coming from the external circuit to which the arrester is connected crosses the latter, passing from one contact to the other via the varistors column.

In the event of a failure of one or more of the varistors arranged in a column inside the envelope, an electric arc occurs inside the cavity and creates an internal overpressure which leads to a perforation of the diaphragm. As soon as this perforation occurs, the hot gases inside the envelope can escape and be directed by the nozzle to another electrode, thereby causing a transfer of the arc from the inside to the outside of the envelope and, from there, releasing the interior of the latter from overpressures and temperatures which could cause it to explode.

For this safety mechanism to function properly, it is of course necessary that the electric arc which is naturally created during the fault can "spread" inside the cavity of the envelope and that this arc can reach without no obstruction of the diaphragm in order to perforate it and thus ensure the release of the hot gases necessary for the external transfer of the arc. It is therefore essential to have no mechanical blocking along the axis of the internal cavity of the envelope, in the direction of the unclosed end thereof.

To this end, the usual practice has always been to use varistors in the form of pellets of smaller diameter. the internal diameter of the cavity to leave a sufficient crown of air around the pellets to allow the arc to form and reach the diaphragm, it being understood that any obstruction to the passage of the arc reduces the transfer efficiency from the latter to the outside and, from there, very substantially increases the risk of explosion. However, the possibility of such an obstruction exists permanently in the event of fragmentation of the varistor pellets, for thermal or purely mechanical reasons. Thus, it has been observed that, in the event of a fault in one of the varistors, a substantial leakage current is generated, which leads to a significant rise in voltage and to a severe thermal runaway which can lead to an almost diametral fragmentation of certain pellets, the fragments of which then obstruct the annular passage provided around the column.

The present invention relates to an extremely simple structure device, making it possible to solve the problem mentioned above and, from there, to minimize the risks of explosion of the envelope of a surge arrester in the event of an internal fault due to a lack of sufficient space in the cavity to allow the arc generated during the fault to develop and reach the diaphragm.

More specifically, the invention proposes to maintain the varistor pads stacked in a single column in the cavity of the envelope using at least three, and preferably four stakes made of a material that is both insulating and not elastic, such as porcelain, ceramic or concrete-epoxy or concrete-polymer. To ensure their function of holding the pellets, the stakes each extend over the entire height of the column and are symmetrically arranged around the latter.

Thanks to this arrangement and this particular arrangement, the tutors permanently form and maintain free passages in numbers identical to their own number, and this all around the varistors column. Of course, each of these passages is, in practice constituted by the air space defined between two of the stakes proper, the wall of the column and the wall of the cavity.

As previously indicated, the main utility of these tutors is to maintain in the center of the cavity the fragments of varistor in the event of fragmentation of one or more of them following a severe thermal runaway. The stakes thus guarantee the non-obstruction of the passages which they define around the column and, from there, the required expansion of the arch which can be established anywhere around the column and develop without any obstruction . The stakes also make it possible to channel the shock wave and the hot gases up to the diaphragm, without any risk of obstruction and consequently of explosion.

Preferably, each stake has a section in the form of an isosceles trapezoid and is arranged so that the large base of its trapezoidal section comes to bear on the periphery of the pellets. This trapezoidal shape is interesting insofar as it makes it possible to maximize the free space inside the cavity of the envelope and, from there, to maximize the passages in which the defect arc can flourish.

In order to ensure good retention of the fragments of the varistor pellets in the event of fracture thereof, the large base of each stent of trapezoidal section is concave and of the same curvature as the periphery of the pellets. In addition, this large base has a length such that the sum of all the large bases of all the stakes is slightly less than the perimeter of the pellets. In other words, the bases of the stakes are chosen so as to almost continuously surround the circumference of the pellets to prevent fragments, even small ones, from blocking the passages defined between the stakes for the development of the arc.

According to a preferred embodiment of the invention, the stakes are mounted with the aid of easily shearable fixing rods by simple overheating or overpressure, all around the periphery of a rigid base with a diameter greater than that of the pellets, this base being intended to be placed below the varistors column. This particular arrangement has the great advantage of giving sufficient flexibility to the stakes to allow an expansion of the arc generated in the event of a defect in the arrester, which arc is never straight. As can be understood, the arc formed in the event of a lightning arrester defect will find itself channeled in an axial segment defined by two of the stakes. The stakes being held by only one end on the same base, they can easily widen at the other end which is preferably placed where the diaphragm is located, thereby ensuring a greater passage to the arch. In addition, by being held using easily shearable rods at their base, the stakes can be completely released as soon as the arc is formed and, under the effect of the pressure of the arc, be pushed to thereby ensure maximum widening of the axial segment in which the arc is formed.

As previously indicated, the invention is specially designed for use with surge arresters of the monocolumn type, since the use of such a set of stakes only has an advantage when the arrester contains only one column of varistors. On the other hand, the present invention is not limited to a given type of surge arrester and can be used indifferently with post surge arresters or distribution surge arresters.

It should also be mentioned that the stakes whose use is proposed in the context of the present invention should not be confused with the heat-carrying mechanical supports sometimes used to create a thermal bridge between the wall of the internal cavity of the envelope and the external wall of the varistors in order to facilitate the cooling of the latter. Indeed, the heat transfer mechanical supports thus used are usually made of rubber in which heat transfer additives are added. These supports are therefore unlike the stakes according to the invention, made of an elastic material so as to allow their installation by clamping inside the cavity and thus ensuring permanent and very close contact between the wall of the varistors and the wall of the cavity. In contrast, the stakes used according to the invention must be made of non-elastic or rigid material so as to be able to move without bending or deforming 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 the maximum contact with the wall of the cavity, since the essential purpose of these supports is to ensure a transfer of heat from the varistors to the outside. In contrast, the stakes according to the invention preferably have the shape of a trapezium whose small base is oriented on the side of the wall of the cavity so as to ensure the maximum free space near said wall. In addition, according to the invention, the contacts between the stakes and the wall must be reduced to a minimum, to allow the stakes to move easily as soon as an arc is formed.

It is therefore obvious that one cannot confuse, both from the utility point of view and from the structural point of view, the heat transfer mechanical supports currently existing with the very specific structure and use stakes proposed according to the present invention.

Finally, it is obvious that the use of the present invention should in no case be restricted to the housing of the surge arrester which is the subject of the Canadian patent application jointly mentioned above. In fact, as one can easily to understand it, the improvement according to the present invention can be used with any type of existing arrester, provided with a porcelain or any other similar material covering.

• la fig. 1 est une vue en coupe verticale d'un parafoudre telle que décrite dans la demande de brevet canadien no 526.139, pourvue du perfectionnement selon la présente invention;
• la fig. 2 est une vue en perspective d'une colonne de varistances entourée d'un jeu de tuteurs selon l'invention, montés sur une base;
• la fig. 3 est une vue de l'extrémité non fermée du parafoudre illustré sur la fig. 1, et
• la fig. 4 est une vue identique à celle de la fig. 3, montrant la position des tuteurs suite à un emballement thermique sévère ayant amené à une fragmentation diamétrale des pastilles et à la formation d'un arc électrique à l'intérieur de la cavité.

The invention will be better understood on reading the nonlimiting description which follows of a preferred embodiment thereof, made with reference to the appended drawings in which:

• fig. 1 is a vertical sectional view of a lightning arrester as described in Canadian patent application No. 526.139, provided with the improvement according to the present invention;
• fig. 2 is a perspective view of a column of varistors surrounded by a set of stakes according to the invention, mounted on a base;
• fig. 3 is a view of the unclosed end of the arrester illustrated in FIG. 1, and
• fig. 4 is a view identical to that of FIG. 3, showing the position of the stakes following a severe thermal runaway having led to a diametrical fragmentation of the pellets and to the formation of an electric arc inside the cavity.

The single-column surge arrester illustrated in fig. 1 of the accompanying drawings is described in detail in Canadian patent application no. 526,139 filed on December 23, 1986 by the Applicant, HYDRO QUEBEC. Consequently, only the elements of this arrester essential for understanding the present invention will be described below.

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

The outer surface of the envelope 3 is preferably provided with annular fins 11 serving, on the one hand, to ensure dielectric maintenance of the envelope in rain and pollution conditions and, on the other hand, to increase the mechanical strength of the latter. The envelope 3 is also provided with a plurality of anchors 13 and 13 ′. The anchors 13 ′ which are arranged on the side of the envelope where the cavity is closed by the electrode 9 are used for fixing the arrester to the same support 15 using bolts 17. Anchors 13 arranged from the other side of the envelope, where the unclosed end of the cavity is located, is used essentially for fixing a cover 19 using bolts 21.

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

This cover 19 is of annular shape and is made of an electrically conductive material so as to form the other contact or electrode of the arrester, at the other end of the varistor column 20. This cover 19 is, for reasons of 'assembly, composed of several parts including, in particular, a part 25 for centering and holding the varistor column. This part 25 is itself provided with a plurality of peripheral passages 27 whose purpose is essentially to allow the escape of hot gases when an overpressure occurs in the cavity 5 inside the envelope 3.

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

Finally, the arrester 1 comprises a nozzle 31 for evacuating the hot gases, mounted on the cover 19, to direct the hot gases escaping through the passage 27 following the perforation of the diaphragm 29 in the direction of an explosive bolt 33 connected to the electrode 9, on the one hand, in the event of a fault, ensuring a transfer outside the arc created inside and, following this transfer, an explosion of the bolt 33 to isolate the arrester earth when the fault current is interrupted by the circuit breaker provided for this purpose in the electrical supply network.

The arrester 1 described above is mounted in parallel with the terminals of an apparatus to be protected, by connecting the electrode 9 (usually by the explosive bolt 33) to one of the terminals of this apparatus and the cover 19 by a bolt 35 to the other of the terminals of this device. The other details of structure as well as the operation of the arrester previously described are known per se and described in detail in the Canadian patent application in co-instance no 526.130.

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

As explained in detail in the preamble to this specification, these tutors essentially aim to form and maintain all around the column a number identical to their number of passages 43, each passage being defined between the outer periphery of the varistors 18, the side walls of two adjacent stakes and the wall 7 of the cavity 5.

The means for holding the pellets stacked in columns in the cavity of the envelope also include, in the illustrated embodiment, a rigid base 45 preferably constituted by a metal disc whose diameter is greater than that of the pellets. The base 45 which is used for mounting the stakes 41 is advantageously disposed under the end of the column in contact with the spring 23. It is therefore this base 45 which is in contact with the spring 23 and which ensures the electrical transmission of the current between the electrode 9 and the first of the varistors 18.

The stakes 41 are fixed by their lower ends to the same base by means of small fixing rods constituted 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 in place by a nut 51.

Advantageously, the bolts 47 are chosen from a material capable of melting or shearing very easily in the event of overheating, thereby allowing the lower ends of the stakes 41 to be released immediately. The bolts 47 can thus be made of NYLON or any other similar material.

As clearly illustrated in FIGS. 2 to 3, each stake 41 has a section in the general shape of an isosceles trapezoid arranged so that its large base 53 comes to bear on the periphery of the varistor pads 18. This large base 43 is concave and with the same curvature as the periphery of the varistors 18. This large base also has a length such that the sum of the length of all the large bases of the stakes surrounding the column is slightly less than the perimeter of the pellets to come and almost surround continues the periphery thereof and thus retain any fragment which may form in the event of breakage or breakage of one or more of the varistors 18.

This isosceles trapezium-shaped section is very advantageous insofar as it makes it possible to obtain perfect encirclement of the varistor pads 18 while maximizing the free space defined by the passages 43.

As indicated above, the main use of the stakes 41 is to retain in the center of the cavity 5 the fragments of varistor which could form in the event of their breaking due to severe thermal runaway. In the event of such a defect, one or more of the varistors will fragment diametrically as illustrated in FIG. 4 and will lead to the formation of an arc inside the cavity 5. This arc of course will form and spread inside one of the passages 43 where it will be channeled thanks to the adjacent stakes 41 towards the cover 19, with a view to tearing the diaphragm and ensuring the expulsion of the hot gases in the nozzle 31.

Thanks to the stakes whose large bases surround the circumference of the column almost continuously, no fragment risks entering the space 43 in which the arch will form. Consequently, the risks of obstruction of the passage are almost eliminated, which, at the same time, minimizes the risks of explosion of the envelope 3.

As soon as the arc forms, the heat generated and the shock wave will cause the bolts 47 to shear and, from there, the stakes will completely free themselves from the base 45 to become mobile. The two stakes bordering the passage 43 in which the arc will form can therefore slide in opposite directions under the effect of the arc. This spacing will open the passage 43 to the maximum and, from there, will ensure an expansion of the arc without any constraint.

It should be mentioned that tests were carried out within the framework of a new standard from the company HYDRO QUEBEC, namely standard B-31.19-03 of 1988. These tests which were carried out on surge arresters of the type described in the Canadian joint patent application no. 526,139 with mobile stakes made of polymer concrete has been extremely positive.

Claims (8)

1. Surge arrester of the "single column" type, comprising:

   - a casing (3) made of insulating material, said casing having an inner wall defining a generally cylindrical chamber (5) having two ends at least one of which is not closed;

   - a single column of varistors (18) disposed inside said chamber, said varistors having the shape of cylindrical disks of a diameter smaller than that of the casing chamber, the disks being stacked one upon the other thereby to form the single column;

   - means for holding the disks stacked in column form in the casing; and

   - electric contacts (9,19) disposed at the two ends of the chamber of the casing to permit an electrical connection of said column of varistors in parallel on terminals of an electric apparatus to be protected against overvoltages, characterized in that said means for holding said stacked disks in column form in the casing include at least three props (41) made of a material which is both insulating and non-elastic, each of said props extending over the full height of the column and being symmetrically disposed therearound to form and hold, all around the same number of rectilinear passages (43) equal to the number of props in the case of disk breaking resulting from a severe thermal shock due to an internal failure of the surge arrester, said passages defined between the inner wall of the casing, the props and the column allowing any inner electric arc during the failure to spread within the chamber and hot gases generated by the arc to exhaust at each non-closed end of the casing, thereby reducing the risks of explosion of the casing.
2. Surge arrester according to claim 1, characterized in that said means for holding the disks stacked in column in the chamber of the casing further include a rigid base (45) disposed at one end of the column and over which the props (41) are mounted, this mounting ensuring by itself the holding of said props in position around the column.
3. Surge arrester according to claim 2, characterized in that the base (45) has a diameter greater than that of the disks and in that the props (41) are fixed by one end to this base by means of securing rods (47) capable of shearing under the smallest overpressure or overheat.
4. Surge arrester according to claim 3, characterized in that the securing rods capable of easily shearing are bolts made of nylon.
5. Surge arrester according to claim 1, 2 or 3, characterized in that the material from which the props (41) are made of is a rigid material selected from porcelains, ceramics and rigid synthetic insulating materials of the epoxy-concrete and polymer-concrete type.
6. Surge arrester according to claim 1, 2 or 3, characterized in that each prop (41) has a cross-section in the shape of an isosceles trapezium and is disposed so that the wide base (53) of said trapezium comes to bear against the periphery of the disks, said wide base being concave and having the same curvature as that of said periphery.
7. Surge arrester according to claim 1, 2 or 3, characterized in that each prop has a cross-section in the shape of an isosceles trapezium and is disposed so that the wide base (53) of said trapezium comes to bear against the periphery of the disks, said wide base being concave and having the same curvature as that of said periphery and having a length such that the sum of all wide bases of all of the props is slightly shorter than the perimeter of the disks.
8. Surge arrester according to claim 1, 2 or 3, characterized in that:

   - the props (41) are four in number and each one having a cross-section in the shape of an isosceles trapezium;

   - said props are disposed in such a way that the wide bases (53) of their trapezoidal cross-section come to bear against the periphery of the disks, said wide bases being concave and of the same curvature as said periphery and having a length such that the sum of all wide bases of all props is slightly shorter than the perimeter of the disks;

   - said means for holding the disks stacked in column form in the casing further include a spring (23) mounted between one (19) of electric contacts and the column of varistors (18);

   - said surge arrester further comprises a protection diaphragm (29) disposed across each non-closed end of the casing chamber, said diaphragm being made of a material that can tear easily by overpressure within the chamber whereby to allow escape of hot gases generated by an electric arc in case of a failure caused by lightning;

   - said surge arrester also comprises a nozzle (31) disposed above each non-closed end of the casing for guiding escaping hot gases in the case of internal failure and of tearing of the diaphragm.

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