Arcing contact assembly for medium and/or high voltage circuit breakers

Abstract

 An arcing contact assembly for medium and/or high-voltage circuit breaker, comprising a fixed contact (1) and a moving contact. The fixed contact (1) is hollow in order to allow for the escape of gaseous by-products from the arc chamber, and shaped in such a way as to minimise the flow resistance.

Description

[0001] The present invention relates to an arcing contact assembly for medium and/or high voltage circuit breakers which contains, as electric arc quenching means, a fluorinated dielectric fluid, in particular a perfluoropolyether (PFPE).

[0002] It is known that medium- and high-voltage circuit breakers comprise an insulating casing in which there is an interruption chamber with interruption mechanisms constituted by at least one fixed contact and at least one moving contact. Opening/closure manoeuvres of the circuit breaker are performed by engaging/disengaging the fixed contacts with respect to the moving contacts by using known actuation devices.

[0003] Generally, during opening the mutual separation of the moving contact and of the fixed contact is accompanied by the generation of an electric arc between the two contacts; these arcs are quenched by using known kinds of dielectric fluid whose use, in the current state of the art, entails various drawbacks and disadvantages.

[0004] For example, one of the most common fluids used to quench the electric arc in medium and/or high-voltage circuit breakers is constituted by a mineral oil: mineral oils, however, in addition to being dangerously flammable, can sometimes lead to unacceptable drawbacks, such as the generation of hydrogen, methane and carbon residuals.

[0005] Many applications commonly use gaseous substances such as nitrogen, noble gases, compressed air, sulphur hexafluoride (SF₆) and mixtures thereof. With these substances it is indispensable to use devices for monitoring the pressure of the gas used and for replenishing it in order to maintain the dielectric performance of the system.

[0006] Another drawback is due to the fact that both when using gaseous fluids and when using liquid fluids it is necessary to adopt particular refinements and to use safety systems in order to avoid and/or indicate any losses and leaks of said fluid. In this case, the losses and consequent leaks of the fluid might in fact cause malfunctions of the circuit breaker and environmental contamination problems. This obviously affects the constructive complexity of the circuit breaker and its overall reliability.

[0007] It is also known in the technical literature to use non-flammable liquids, such as for example mixtures of chlorofluorocarbons (CFC) or of perfluorocarbons (PFC); these liquids are highly limited in their use because in the presence of electric arcs they undergo a decomposition, which produces reactive and toxic by-products. Since these reactive substances are unacceptable from the point of view of environmental impact, they require the adoption of disposal procedures typical of toxic waste.

[0008] It is also known to use, as arc quenching media, particular fluorinated fluids of the perfluoropolyether (PFPE) family. The decomposition of said fluids, following an electric arc, brings about the formation of reactive gaseous substances, a proportion of which remain dissolved in the liquid contained in the arc chamber. Consequently, the said reactive species attack the arc contacts, as well as the construction materials of the chamber itself, eventually reducing the lifespan of the circuit breaker. Stable gaseous substances are also produced as an effect of the electric arc. These stable substances, being non-reactive, cannot be easily removed from the fluid.

[0009] The aim of the present invention is to provide a medium and/or high voltage (i.e. voltages greater than 1000 V) circuit breaker, which is intrinsically safe, i.e. which does not use as arc quenching medium flammable fluids and in which the production of potentially hazardous substances is reduced.

[0010] Within the scope of this aim, an object of the present invention is to provide a medium and high voltage circuit breaker, in which ageing phenomena are reduced.

[0011] A further object of the present invention is to provide a medium and high voltage circuit breaker which does not require frequent maintenance interventions.

[0012] This aim, these objects and others which will become apparent hereinafter are achieved by the suitable design of the arcing contact assembly according to the invention.

[0013] According to the present invention, the arcing contact assembly, for medium and/or high voltage circuit breakers having an arc chamber containing a fluorinated dielectric fluid for quenching electrical arcs, comprises a fixed contact and a moving contact. The fixed contact has an internal canal, which allows the flow of fluids from said arc chamber. The dielectric fluid is a perfluoropolyether having oxygen dissolved therein.

[0014] The arcing contact assembly thus conceived solves many of the drawbacks of the prior art. The presence of oxygen dissolved in the perfluoropolyether allows altering the recombination reaction of the gases produced by the decomposition of the fluorinated fluid so as to avoid producing the stable compounds. Also, the presence of the canal inside the fixed contact maximises the amount of gaseous by-products that leave the arc chamber.

[0015] Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the arcing contact assembly according to the invention, illustrated only by way of non-limitative example in the accompanying figure 1, in which a schematic view of a fixed contact is given.

[0016] With reference to figure 1, the fixed contact of the arching contact assembly according to the invention is designated by the reference numeral 1. Inside the fixed contact, there is a canal 10, which allows the flow of fluids from the arc chamber (not shown in figure).

[0017] The movable contact can be shaped according to the state of the art, and therefore is not presented in figure 1.

[0018] In order to maximise the flow from the arc chamber, the internal canal is shaped as divergent Venturi tube. The principle of Venturi tubes are well known in the technical art and will not described here.

[0019] Advantageously, the canal has a conical shape, with a cone angle of about 15 degrees. In this way, it is possible to avoid additional losses due to flow separation.

[0020] The presence of oxygen dissolved in the perfluoropolyether would tend to produce, after many switching operations, oxidation by-products deriving from the material of the contact (e.g. CuO2, Cu3WO6). In order to minimise this effect, the fixed arcing contact is conveniently featured as a two-part body, each part being made of a different conductive material.

[0021] In particular, according to the embodiment of figure 1, the fixed contact present a first part 20 which is directly subjected to the effect of the electric arc and which is normally designated as contact finger. The second part 30 of the fixed contact has a conical shape and constitutes the main body of the fixed contact itself.

[0022] Conveniently, the contact finger 20 is made of a conductive alloy having at least one component which does react with oxygen. In a preferred embodiment, said alloy is a silver/tungsten alloy in a ratio from 20/80 to 30/70. The main body 30 of the fixed contact can be conveniently made of a standard Ni/Cr alloy. In this way, possible phenomena of oxidation of the arching contacts are minimised.

[0023] The fixed contact according to the embodiment of figure 1 further comprises a flat portion 32, which is mechanically and electrically coupled to the current-carrying flanges of the circuit breaker. The coupling can be made by nuts and bolts or by screw.

[0024] The flat portion 32 is connected to the main body 30 through a curved portion 31. The curvature radius of the portion 31 chosen to minimise the losses at the outlet of the Venturi tube.

[0025] Examples of perfluoropolyether are those belonging to the family having the following formula:

        endcap-O-(C₃F₆O)n-(CF₂O)m-endcap;     (a)

or

        endcap-O-(C₂F₄O)n-(CF₂O)m-endcap.     (b)

[0026] The endcaps can be constituted by HCF₂ (one or both) or CF₃ (preferably no more than one). In particular, the presence of the HCF₂ endcap (hydrogen-endcapped perfluoropolyether) is highly important for environmental impact, since it makes the molecule reactive in the upper atmosphere and therefore does not contribute to the greenhouse effect. In general, perfluoropolyethers can be chosen from a wide range of molecular weights; for example, for molecular weight 2,000 it is possible to have m and n equal to approximately 11, or m and n can be chosen in a ratio of 1:3.

[0027] The arcing contact assembly thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. All the details may furthermore be replaced with other technically equivalent elements.

[0028] In practice, the materials used, so long as they are compatible with the specific use, as well as the dimensions, may be any according to the requirements and the state of the art.

Claims

1. An arcing contact assembly for high and/or medium voltage circuit breakers having an arc chamber containing a fluorinated dielectric fluid for quenching electric arcs, said dielectric fluid being a perfluoropolyether having oxygen dissolved therein, the said assembly comprising a fixed contact and a moving contact, wherein and said fixed contact has an internal canal allowing the flow of fluids from said arc chamber.

2. An arcing contact assembly according to claim 1 wherein said internal canal has the shape of a divergent Venturi tube.

3. An arcing contact assembly according to claim 2 wherein the cone angle of said divergent Venturi tube is about 15 degrees.

4. An arcing contact assembly according to any of the previous claims wherein the fixed contact consists of a two-part body made of different conductive materials.

5. An arcing contact assembly according to claim 4 wherein the two part body consists of a contact finger and a main body of conical shape.

6. An arcing contact assembly according to claim 5 wherein said contact finger is made of a conductive alloy having at least one component which does not react with oxygen.

7. An arcing contact assembly according to claim 6 wherein said alloy is a silver-tungsten alloy in a ratio from 20/80 to 30/70.

8. A high and/or medium voltage circuit breaker comprising an arcing contact assembly according to any of the preceding claims.

Drawing