US3236979A

US3236979A - Damping resistor for electric current interrupters

Info

Publication number: US3236979A
Application number: US238903A
Authority: US
Inventors: Latour Andre
Original assignee: Merlin Gerin SA
Current assignee: Merlin Gerin SA
Priority date: 1961-11-27
Filing date: 1962-11-20
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22
Also published as: CH381300A; ES282469A1; GB964462A; DE1189607B

Description

Feb. 22, 1966 LATOUR 3,236,979

DAMPING RESISTOR FOR ELECTRIC CURRENT INTERRUPTERS Filed Nov. 20, 1962 I: Sheets-Sheet 1 Fig Feb. 22, 1966 L 'rou 3,236,979

DAMPING RESISTOR FOR ELECTRIC CURRENT INTERRUPTERS Filed Nov. 20, 1962 3 Sheets-Sheet 2 Feb. 22, 1966 A. LATOUR 3,236,979

DAMPING RESISTOR FOR ELECTRIC CURRENT INTERRUPTERS Filed Nov. 20, 1962 s Sheets-Sheet s United States Patent 3,236,979 DAMPING RESISTOR FOR ELECTRIC CURRENT INTERRUPTERS Andr Latonr, Grenoble, Isere, France, asslgnor t0 Etablissements Merlin & Gerin, Isere, France Filed Nov. 20, 1962, Ser. No. 238,903 Claims priority, application Belgium, Nov. 27, 1961, 610,790 Claims. (Cl. 200-144) This invention relates to electric interrupters compris ing a resistance and more particularly to electric med1um or high-voltage circuit interrupters comprising a damping resistance.

By shunting the arc of an interrupter through a resistor, a considerable reduction of the frequency of the restriking voltage across the terminals of the interrupter can be obtained so that the interruption of the current can be greatly facilitated in the absence of a restriking of the arc after the current has passed through zero, such re striking generally being caused by a too rapid rise of the recovery voltage across the contacts. An auxiliary interrupter is connected in series with the resistor in order to interrupt the residual current and to disconnect the resistance from the network. This solution gives good results but it is very expensive because one is obliged to give the resistor large dimensions in order that it may be capable of dissipating the considerable quantity of heat developed by the current passing through the resistor during the time the latter is inserted.

In the conventional solutions, the resistor is connected for an excessive time. For alternating current at 50 c./s. this time is at least of a second and may even be as much as several hundredths of a second. But the role of the resistor is justified only during the phenomena of arc quenching and of recovery of the voltage, which phenomena last only a few micro seconds. The known solutions therefore lead to a real wastage of material and apparatus.

To be economical, the shunting resistance should have at the beginning of the recovery voltage a low ohmic value, which would gradually increase and tend towards infinity when the voltage has reached its final value. By acting on the duration of this variation, it would be possible to adjust the rate of recovery of the voltage in order to make it less than the rate of reestablishment of dielectric strength across the contacts which is an essential condition of the breaking of the current. Finally, in order to be usable, a resistor of this kind should moreover have cooling means such that at any given moment the sum of the calories removed from it by the said cooling means, otherwise it will be very rapidly destroyed.

It is an object of the invention to provide an interrupter comprising a resistor which conforms with these conditions.

The resistor according to the invention consists of a thin film having a high negative temperature coetlicient and arranged in the zone exposed to the heat developed by the are on a supporting part of insulating material of good calorific conductivity, the whole arrangement being such that the resistance of the film becomes low under the effect of the heating produced by the arc, the cooling following the extinction of the are causing the return of the resistor to the insulating state.

The said resistor is thus constituted by a thin film of a substance having suitable electrical conductivity when hot and a much lower, even zero conductivity when cold. Preferably and still according to the invention, the film which is exposed to the heat of the arc during the existence of the latter, is subjected immediately after the arc extinction to the action of a cooling 3,236,979 Patented F eh. 22, 1966 means such as a current of air or a blast gas.

The circuit interrupter according to the invention functions in the following manner:

Immediately on separation of the contacts, the arc heats the film and renders it conductive; the result is that -a certain part of the current of the arc is diverted into the resistor. This part increases as the total current tends towards zero since at this moment the resistance of the arc increases, whilst the resistance of the film, heated by the diverted current, decreases. Finally the resistor takes all the current so that there is an anticipatory extinction of the are. From this moment the resistor coo-ls very rapidly since, on the one hand, the current passing through it tends towards zero and the are no longer exists and, on the other hand, its calories are rapidly removed by the support and by the current of cold gases. Consequently it tends to recover its insulating state and to brake, or even block, the return current generated by the recovery voltage. The final result is that the time elapsing between the moment the arc is extinguished and the moment a certain value of the recovery voltage appears can be considerably prolonged with the resistor according to the invention. This leaves the de-ionising means used time to act and build up a dielectric barrier capable of resisting the recovery voltage.

It is obvious that such a method of operation presup poses an extremely low calorific inertia of the resistor. This condition is fulfilled by the thin film.

Many substances with the exclusion of metals may be suitable to constitute the proposed film. Generally speaking, all those exhibiting the phenomenon of igneous electrolysis at a more or less high temperature are suitable. These are in particular the salts of numerous metals (on condition that they are neither hygroscopic or volatile), silicates, borates, borosilicates and aluminates of alkaline metals, of alkaline-earth metals or of common metals, certain oxides such as magnesia, certain semiconductors such as silicon carbide.

The film may be constituted by the material of the support itself. Thus, an arc chamber constituted partially or wholly by a tube of porcelain, the internal surface of which in contact with the arc vitrifies and forms a film, conductive at high temperatures and insulating when cold, meets the problem posed pretty well.

The pellicular resistor-can be constituted by a coating, enamel or cover deposited according to the usual methods on a support of ceramic material. Amongst the numerous substances mentioned above, there will be selected that which seems the most suitable at the temperatures between which the pellicular resistor is to vary, One must take into account in particular that the alkaline silicates are amongst the substances having an appreciable conductivity at relatively low temperatures. The same applies to lead borates and certain salts such as silver chloride. At the other end of the scale, that is to say getting towards high temperatures there are the oxides of magnesium, lithium, lanthanum, yttrium, zirconium, cerium, glucinium, calcium which may be used alone or in suitable mixtures.

As for the support there will preferably be selected a ceramic offering a good resistance to thermal shock associated with good dielectric properties. That does not exclude the use of porous ceramics which have the advantage of allowing a good adherence of the thermoconductive layer and which, when they are immersed permanently in a compressed gas, regain at least partly the dielectric strength and calorific conductivity which porosity generally takes away from them.

or compressed The invention will be better understood from the ensuing specification taken in conjunction with the drawings in which:

FIGURE 1 is an elevational view of an example of application of the invention to an air break circuit interrupter of the type known more particularly under the name of Solenarc.

FIGURE 2 is a sectional view along the line X-X' of FIGURE 1.

FIGURES 3 to 7 are sectional views of other examples of application of the invention to gas blast interrupters.

In FIGURES l and 2, a plate 11 of refractory insulating material, comprising a thickened peripheral portion, defines with an identical adjacent plate, not shown on the figures, an elementary arc chamber. 12 designates the electrode which cooperates with the electrode of the adjaoent plate to serve as support for an arc loop which develops in the elementary chamber. 13 is a flame arrester of insulating material retained by spacing members 14 and which limits the development of the arc upwards. The final path of the latter is a loop starting from the upper end of one of the electrodes, passing along the rim of the plate constituted by the thickened portion and the lower part of the flame arrester 13 to arrive finallyat the upper end of the other electrode. The arc extinction chamber comprises a plurality of these sub-assemblies so that all elementary loops formed by the arcs and by the paths of the currents in the electrodes constitute a solenoid. According to the invention, a suitable film 15 conforming with the abovementioned conditions is deposited in a thin layer on the plate 11 and preferably over a path similar to that of the arc.

During its development along the electrodes 12, the

are or the hot gases issuing from the arc heat considerably the coating 15 which, in consequence, becomes conductive. The moment the arc is extinguished, the coating has not yet had time enough to cool down and consequently allows a current to pass through the coating or layer 15 which reduces the rate of voltage recovery and limits the transient over-voltage. Doubtless this current causes heating in the pellicular resistor but the quantity of heat developed by this ohmic current is less than the quantity of heat removed from the pellicular resistor by its intimate contact with the supporting plate 11, on the one hand and, on the other hand, by the current of cool gas which, by suction action, follows the current of hot gas.

In FIGURE 3, the stationary and movable contacts of gas blast interrupter ars shown at 16 and 17, respectively in the open position, 17' showing in dotted lines the closed position of the movable contact. By way of example only tubular contacts have been shown. The movable contact may during the opening operation be retracted inside an insulating cap 18. The latter has in its central portion the form of a tube 19 concentric with the movable contact. According to the invention, the internal wall of this tube is lined with a thin thermo-conductive layer 20. The whole unit is immersed in an enclosure, not shown, containing compressed air or another gas either permanently or temporarily.

Exhaust pipes

21 and 22 of the contact communicate either with the atmosphere or with a reservoir at a lower pressure than that of the enclosure.

During the opening movement, the movable contact 17 separates from the stationary contact 16 whereby an arc is produced inside the tube 1?. During its lengthening, the arc heats the inner wall of this tube, that is to say the pellicular layer 20 which in consequence becomes conductive. The moment the arc is extinguished, that is to say the moment the current is cancelled, the layer 26 does not have time to cool down completely, it thus remains conductive, which has the effect of concentrating the transient recovery voltage between the stationary contact 16 and the end 23 of the, still conductive, layer. If the recovery voltage rises too rapidly with respect to the rate of re-establishment of dielectric strength of the interval comprised between stationary contact 16 and cap 18, a discharge flashes over between the stationary contact 16 and the end of the layer 23, thus inserting the pellicular resistor in the circuit. The resistor has the effect of limiting the current released by this discharge. Moreover, as the said pellicular resistor is rapidly cooled owing to the thermo-conductivity of its support, on the one hand, and on the other hand to the powerful blast of air which flows through the exhaust pipe 22, the current released follows the law of increase of the resistance and extinguishes rapidly.

To facilitate the striking between the end 23 of the thermo-resistor and the contact 16 and avoid the root of the discharge being sucked into the tube 19, one may provide at 23 one or more bosses directed towards the stationary contact. This is tantamount to reducing the breakdown interval and to placing the lower root of the discharge in the suction zone of the exhaust pipe 21.

It is not absolutely necessary to limit the thermoresistor to the inner surface of the tube 19. The thermoresistant layer may in fact be extended Wholly or partly over the upper surface of the cap 18 subjected to the radiation of the arc. If it is desired to increase the heating effect caused by the radiation of the arc, one may give this surface a concave shape, more particularly the form of a truncated cone as shown in FIGURE 4.

The pellicular resistor need not necessarily be fixed to the inside of the tube; it may also constitute the external surface of a solid or hollow rod. FIGURE 5 is an example of this type of construction, in which the support rod 24 is arranged concentrically with the stationary and movable contacts.

Whatever the arrangement of the resistor, it is understood that its ends may be metallised and/ or put in contact with any suitable metal part, so as to facilitate the passage and distribution of the current in the said resistor.

Instead of being heated directly by the current of the arc, the resistor may also be heated indirectly for example from hot gases issuing from the arc.

FIGURE 6 is an example of this arrangement. In this figure, the pellicular resistor 25 lines the inner wall of the tubular support 26 which forms at the same time the exhaust pipe of the stationary contact 16. At one of its ends the resistor is connected to the stationary contact 16 whilst the other end is in contact with a conductive part 27 connected through lead 28 to the movable contact 17. If it is desired to protect the resistor from the effect of the voltage which stands on permanently in the open position of the contacts, one may insert in the connection 28 an insulating interval which may be stationary or movable and which may or may not be temporarily or permanently immersed in the compressed gas. A similar effect may be obtained by depositing the thermo-conductive layer forming the pellicular resistor only on a part of its support, so that a breakdown interval exists between the upper end of the resistor and the part 27.

To improve heat exchanges betweenv the pellicular resistor and the exhaust gases, both during the heating and during the cooling period, and to improve the conditions of dielectric strength, a certain pressure may be maintained within the exhaust pipe 29, either by making it open into an enclosure where a pressure higher than atmospheric pressure prevails, or by partly obturating it by means of a valve 30 loaded by a spring 31.

It may be desirable to increase the surface of heat exchange between exhaust gas and pellicular resistor. In this case the interior section of the tube 26 may be appreciably enlarged, so that it becomes possible without reducing the exhaust section, to dispose parallel with the said tube one or more rods or one or more concentric tubes, or even a bundle of tubes, all these bodies being lined with a thermo-conductive layer. One may thus form a pellicular resistor capable of absorbing and dissipating a very larger quantity of energy.

The connection 28 of FIGURE 6 may be replaced by the pellicular resistor itself. This is shown in FIGURE 7. To this end, the exhaust gases coming from the conduit 32 are wholly or partly directed onto the pellicular resistor 33 which ensures temporary electric connection between the stationary contact 16 and the conductive part 34 connected to the movable contact. The support tube 35 may, as shown in the figure, be provided outside the enclosure 36 containing the contacts and the compressed gas. If necessary, and particularly when a large-sized enclosure is provided, it may also be disposed inside this enclosure.

Although the

exhaust pipes

32 and 37 of the stationary and movable contacts may be absolutely independent as regards their exhausts, it may be advantageous, as shown in the figure, to join them in a common exhaust conduit. This makes it possible to use only one arresting member for both the contacts (pressuremaintaining valve, intermediate pressure reservoir, flame arrester, or even, as shown in the figure at 38, obturating valve closing some time after the opening of the contacts).

Generally spreaking, all the arrangements provided in connection with FIGURE 6 are applicable here. In particular, so as to avoid the possibility of a striking inside the tube 35, it would be advisable to ensure a laminar flow or at least a flow divided into several parallel hot gases conveying channels. This may be done, vas indicated above, by placing inside the tube 35 one or more cylindrical or tubular bodies, or even an actual bundle of tubes covered internally and externally with a thermo-conductive layer.

What is claimed is:

1. In an electric current interrupter, a damping resistor consisting of a thin film having a high negative temperature coefiicient disposed in the zone exposed to the heat developed by the are on a supporting part of insulating material of good calorific conductivity, the resistance of the film becoming low under the effect of the heating produced by the arc, the cooling following on the extinction of the are causing said resistance to become very high.

2. An interrupter as set forth in claim 1, wherein said thin film is disposed substantially along the path of the arc.

3. In a device for extinguishing an electric arc to be drawn between separable contact members, a plurality of spaced parallel plates of insulating material defining between each other elementary arc extinction chambers, said plates being disposed transversely of the direction of separation of said contact members, conductive means mounted on the lower part of said plates and designed to divide the initial are into a certain number of elementary arcs and to draw said elementary arcs into said elementary arc extinction chambers causing said elementary arcs to form peripherally expanding loops in said elementary chambers, thin films having a high negative temperature coefiicient being deposited on said plates peripherally inside said elementary chambers to provide conductive shunting paths for said elementary arcs whereby the resistance of said thin films becomes rapidly very high as soon as said elementary arcs are extinguished.

4. In a device for extinguishing an electric arc to be drawn between separable contact members, a plurality of spaced parallel plates of insulating material, said plates being disposed transversely of the direction of separation of said contact members, conductive means mounted on the lower part of said plates and designed to draw the arc between said plates to cause it to form peripherally expanding loops, thin film-s having a high negative temperature coeflicient being deposited on said plates to provide conductive shunting plates for said arc, whereby the cooling of said films following the disappearing of the heating produced by said arc causes said films to become insulant.

5. In a gas blast interrupter for extinguishing an electric arc to be drawn between separable contact members, a damping resistor arranged in the interval separating said contact members in the open position, said damping resistor consisting of a thin film having a high negative temperature coefiicient disposed on a supporting part of insulating material of good calorific conductivity, the resistance of the film becoming low under the effect of the heating produced by the arc, the cooling of said film following the disappearing of the heating produced by the arc causing the resistance to become very high.

6. In a gas blast interrupter as set forth in claim 5, at least one tubular contact member, said film extending inside the hollow contact member.

7. A gas blast interrupter as set forth in claim 5, wherein said thin film comprises a cylindrical part having a conical extension.

8. In a gas blast interrupter for extinguishing an electric arc to be drawn between separable contact members, an exhaust canal for the hot gases, a damping resistor constituted by a thin film disposed on the inner Wall of said exhaust canal and connected electrically in parallel across said contact members, the resistance of the film becoming low under the effect of the heating produced by the arc, the cooling of said film following the disappearing of the heating produced by the are causing the resistance to become very high.

9. A gas blast interrupter as set forth in claim 8, wherein said damping resistor comprises a plurality of support parts of insulating material of good calorific conductivity, the outer surface of which is constituted by thin films having a high negative temperature coefficient.

10. A gas blast interrupter as set forth in claim 8, including means for maintaining a residual pressure in said exhaust canal during the arc extinction.

References Cited by the Examiner UNITED STATES PATENTS 4/1942 Grosse 200146 2/1962 Cobine et al 200-144 ROBERT K. SCI-IAEFER, BERNARD A. GILHEANY,

Examiners.