WO2017032667A1

WO2017032667A1 - Electrical switching device and process for cooling a switching medium in an electrical switching device

Info

Publication number: WO2017032667A1
Application number: PCT/EP2016/069538
Authority: WO
Inventors: Mahesh DHOTRE; Javier MANTILLA FLOREZ; Jean-Claude Mauroux; Oliver Cossalter; Sami Kotilainen; Xiangyang Ye; Stephan Grob
Original assignee: Abb Schweiz Ag
Priority date: 2015-08-21
Filing date: 2016-08-17
Publication date: 2017-03-02
Also published as: CN108140501B; EP3338289A1; CN108140501A

Abstract

The invention relates to an electrical switching device with a switching chamber (10), which comprises at least two arcing contacts (192, 192) movable in relation to each other and defining an arcing region (22) in which an arc (20) is formed during a current breaking operation, with the switching chamber (10) being filled with a switching medium (20) for arc quenching and for dielectrical insulation. The switching chamber (10) further comprises an exhaust volume (40, 62) fluidically connected to the arcing region (22) to allow the switching medium heated by the arc (20) to flow out of the arcing region (22) to the exhaust volume (40, 62), thereby transferring heat to a surface area of a metal component of the switching chamber (10). The device is characterized in that at least a portion of a surface contained in the switching chamber (10) is covered with a porous layer (72).

Description

Electrical switching device and process for cooling a switching medium in an electrical switching device

The present invention relates to an electrical switching device comprising at least one switching chamber, according to the preamble of claim 1, specifically to a circuit breaker or a generator circuit breaker. The present invention further relates to a process for cooling a switching medium in an electrical switching device, specifically a circuit breaker or a generator circuit breaker. In conventional circuit breakers, the arc formed during a current breaking operation is extinguished using a switching gas (also referred to as "arc-quenching gas") . For this purpose, the circuit-breaker comprises one or more series- connected switching chambers, which are filled with the switching gas and operate on one of the conventional principles for extinguishing the arc generated in the arcing region, e.g. by way of e.g. a self-blasting mechanism or conventional puffer-assisted mechanism. The hot gas created during arc extinction flows from the arcing region in direction to an exhaust volume whereby it needs to be cooled down sufficiently before entering the tank volume.

EP 0 836 209, e.g., discloses a circuit breaker comprising a switching chamber which is filled with e.g. SF₆ as the arc- quenching gas. According to EP 0 836 209, an arc is generated between the two main contacts during the breaking operation and is quenched by the arc-quenching gas. The hot and ionized gases which are produced in the arcing region are transported downstream, i.e. in direction to an exhaust volume, with a portion of the hot gases being stored in a self-blast volume and being used later in a known manner to assist the quenching process. The remaining hot gases are transported through the tubular main arcing contacts into an exhaust volume .

Aiming at an improvement in the interruption performance, EP 1 403 891 discloses a circuit-breaker having a switching chamber, which is filled with a switching gas, contains an arcing region and has at least two arcing contacts. At least one of the arcing contacts is in the form of a hollow tubular contact, which is provided for transporting hot gases out of the arcing region into an exhaust volume connected to the switching chamber volume.

As mentioned, SF₆ is typically used as arc-quenching gas. This is also the case for the circuit breakers according to EP 0 836 209 and EP 1 403 891.

Aiming at improved interruption capability and at the same time simple and economic construction and operation of the circuit breaker, a switching medium comprising an organo- fluorine compound selected from the group consisting of a fluoroether, a fluoroamine, a fluoroketone and mixtures thereof has been suggested in WO 2013/087687. These alternative "non-SF₆" switching media allow for a good dielectric strength and at the same time exhibit a very low Global Warming Potential (GWP) and an Ozone Depletion Potential of about 0. Despite of the environmental-friendli^¬ ness of the alternative switching media according to WO 2013/087687, issues might occur due the organofluorine compound decomposing when being subjected to the high temperatures present during burning of the arc. Contrary to SF₆, which after decomposition readily recombines again, the decomposition of the organofluorine compounds of interest, in particular the fluoroketones , is an irreversible process. In order to maintain a high interruption performance and a long lifetime of the circuit breaker, decomposition of organo- fluorine compound shall be minimized. Apart from this and irrespective of the switching medium used, there is an ongoing need to minimize the size of the exhaust volume to allow for a compact design and, ultimately, for a reduction in costs. The object of the present invention is thus to provide a circuit breaker and cooling method which allows for a high interruption performance also when choosing a very compact design. In particular, the circuit breaker shall allow for a high interruption performance and a prolonged lifetime also when using a non-SF₆ switching medium. The problem is solved by the electrical switching device and method or process of the independent claims. Embodiments of the invention are defined in the dependent claims and claim combinations.

According to claim 1, the electrical switching device comprises at least one switching chamber, which comprises at least two contacts movable in relation to each other and defining therebetween an arcing region in which an arc is formed during a current breaking operation. The arcing region thus relates to the region which lies between the axially distanced contacts when being disconnected.

At least a portion of the switching chamber is filled with a switching medium (or "arc-extinction medium") for quenching the arc and for providing dielectrical insulation. The switching medium is typically a switching gas (or "arc- extinction gas") ; the term "switching medium" can, however, also encompass media at least a part of which is liquid and particularly encompasses two-phase systems comprising both a gaseous and a liquid portion. As will be shown in further detail below, the switching medium can comprise or essential- ly consist of SF₆ or of an alternative non-SF₆ dielectric compound, in particular an organofluorine compound, or a mix^¬ ture of SF₆ and the alternative non-SF₆ dielectric compound. The switching chamber further comprises an exhaust volume, which is fluidically connected to the arcing region, to allow the switching medium heated by the arc to flow out of the arcing region in direction to the exhaust volume, thereby transferring heat to a metal component of the switching chamber. Thus, heat is absorbed from the switching medium via the surface area of the metal component, which conducts heat absorbed away and out of the system.

According to the invention, at least a portion of a surface contained in the switching chamber is covered with a porous layer. In particular, at least a portion of the metal component is covered with the porous layer. In other words, at least a portion of the area that is subject to heat radiation emitted by the heated switching medium during its flowing out is covered with the porous layer.

More particularly, this applies to the surface or a portion thereof, which without being covered would be in direct contact with the heated switching medium. In other words, the porous layer is, thus, preferably in contact with at least a portion of the heated switching medium.

Herein, the term "metal component" as used according to the present invention relates to any metal component or part thereof that is able to absorb heat and to conduct the heat absorbed out of the system. Further, the term "surface" as used in the context of the present invention encompasses also a surface which is not directly exposed, but which is covered, specifically with the porous layer of the present invention.

In still more specific terms and with regard to the basic design suggested in EP 1 403 891, at least a portion of the inner surface of the hollow tubular contact and any other component arranged downstream thereof in direction of the outflowing switching medium, e.g. an intermediate chamber or any baffle arranged therein and/or the exhaust volume or any baffle contained therein, can be covered with the porous layer. The inner surfaces mentioned (or the portion thereof) can thus be considered to be the metal component within the meaning of the present invention. This will be specified in more detail below.

According to a specific embodiment of the present invention, the porous layer is thus in direct contact with at least a portion of the heated switching medium.

With regard to the cooling achieved by the exhaust volume, different cooling mechanisms apply: Firstly, the hot switching gas entering the exhaust volume is cooled by being mixed with switching gas of lower temperature contained in the exhaust volume. Secondly, the hot switching gas is further cooled by transferring heat via convection and radiation to the exhaust volume wall or any baffle contained in the exhaust volume. Without wanting to be bound by the theory, it is assumed that due to the presence of the porous layer, absorption of heat, in particular of heat radiation, is increased, resulting in a substantially increased cooling of the hot switching gas. Heat energy transferred to the exhaust volume by the in- flowing switching medium is thus substantially reduced, thus allowing for a more compact design of the exhaust volume and also a more compact overall design of the whole device.

According to a further aspect, at least a portion of the tank wall delimiting the tank volume is covered by a porous layer. The embodiment according to this aspect is specifically designed for augmenting absorption of heat from the tank volume generated under normal conditions by ohmic heating under nominal current. Also with regard to this non-switching condition of the switching device, increased heat absorption and consequently increased cooling of the switching gas can be obtained by the present invention, thus allowing for a more compact design of the device than possible when the porous layer is absent.

Specifically, hot gas generated by ohmic heating impinges on the porous layer applied on the tank wall, thus allowing for particularly efficient heat absorption. Preferably, the porous layer is thereby applied on the tank wall where the electrical field stress is not critical, for example in an electrical field shadow. In the embodiment of the present invention, in which at least a portion of the tank wall is covered by a porous layer, the porous layer is therefore preferably applied in a region of the tank wall where the electrical field stress is sub-maximal, i.e. lower than in at least one other region of the tank wall.

The term "porous layer" as used in the context of the present invention is to be interpreted broadly and encompasses any layer having pores, be it an open-cell layer or closed-cell layer .

Preferably, the porous layer contains or essentially consists of a porous insulating or porous metal material, particularly a metal foam, and/or a ceramic porous material. The choice of the preferred material depends on the specific surface, onto which the porous layer is to be applied, as well as on the specific aim to be achieved, as will be discussed in further detail below.

For embodiments, in which a high temperature peak heat shall be absorbed, as is typically the case when the hot switching gas generated by the arc shall be cooled below a predetermined threshold value, the porous layer preferably contains or essentially consists of a porous metal or porous insulating material.

A metal foam is particularly preferred. A metal foam is a cellular structure consisting of a solid metal, frequently aluminum, as well as a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam) . The morphology structure is defined by the porosity (ε) and pore density (ω) wherein the pore density relates to the pore number in a unit length, namely pores per inch (ppi) .

For embodiments, in which heat of a bulk gas or a solid component is to be absorbed, as it is typically the case for the heat generated by ohmic heating at normal conditions under nominal current, the porous layer preferably contains or essentially consists of a ceramic porous material.

In embodiments, which are combinable with any other embodi^¬ ments disclosed herein, the porous layer has a characteristic property selected from the group consisting of: a porosity of at least 45%, a pore density ranging from 15 ppi to 70 ppi, a mean pore diameter in the range from 0.7 mm to 2.0 mm; a thickness of more than 1 mm; a thickness of less than 50 mm; and combinations thereof. Selecting porous layers with such characteristic property allows to achieve good cooling efficiency (requiring high porosity, i.e. many or dense or large pores) while avoiding negative impact on the flow behaviour .

In embodiments, the porous layer has a porosity of at least 45%, preferably at least 65%, more preferably at least 85%, and most preferably at least 95%. Throughout this disclosure, the term "porosity" refers to the fraction of the volume of voids or pores to the total volume of the material or foam. In particular in view of the electrical apparatus being a high-voltage circuit breaker, the pore density preferably ranges from 15 ppi (pores per inch) to 70 ppi.

According to embodiments, the porous layer has a mean pore diameter in the range from 0.7 mm to 2.0 mm, corresponding to a pore density range of 30 ppi (pores per inch) to 60 ppi. More preferably, the mean pore diameter is in the range from 1 mm to 1.5 mm, most preferably from 1.1 mm to 1.3 mm. Such porous layers have been found to be of particular relevance for the purposes according to the present invention.

It has been found that the influence of the pore density of the porous layer is inverse to the cooling efficiency. In aiming at a high cooling efficiency, a relatively low pore density is thus preferably chosen. For the specific case, where the porous layer has a pore density of about 30 ppi, a 37% lower pressure build-up was determined compared to the reference case where no porous layer is present. This implies a cooling efficiency of 37%.

However, for the cooling of a heated switching medium of relatively high temperature, a relatively high pore density can be preferred, since this allows a porous layer of relatively high thermal resistance to be provided. Thus, for the cooling of a heated switching medium of 2000 K or more, the pore density is preferably 50 ppi or higher. According to one specific embodiment of the present invention, the heated switching medium has a temperature of 2000 K at most, the porous layer having a pore density of 50 ppi at most. More specifically, the heated switching medium has according to this embodiment a temperature of 1500 K at most, the porous layer having a pore density of 30 ppi at most. According to an alternative embodiment, the heated switching medium has a temperature of higher than 2000 K, the porous layer having a pore density of higher than 50 ppi.

According to a very specific preferred embodiment, the porous layer has a porosity of about 95 % and a pore density of about 45 ppi.

Metal foams suitable for the purpose of the present invention can preferably be obtained by metallic sintering, electro- deposition, chemical vapor deposition (CVD) or metal deposition through evaporation.

As mentioned above, the exhaust volume is delimited by an exhaust volume wall, at least a portion of the inner surface of the exhaust volume wall being preferably covered with the porous layer. According to a further embodiment, an exhaust volume baffle is arranged in the exhaust volume, at least a portion of the surface of the exhaust volume baffle being covered with the porous layer. Due to the presence of the baffle, the retention time of the switching gas in the exhaust volume is increased, thereby allowing for a very high amount of heat to be absorbed by the respective surfaces. Specifically, the exhaust volume baffle can be in the form of a circumferential wall portion on which the radially outflowing switching gas impinges . In the preferred case where the baffle is arranged in a manner such that it is directly impinged on by the switching gas flowing in a direction at least approximately perpendicu^¬ lar to the baffle, a particularly intensive contact of switching gas and baffle surface and thus a very efficient heat absorption is obtained. According to embodiments, at least one baffle that is covered with the porous layer is arranged such that it functions as a filter for removing dust particles from the outflowing switching medium, specifically the switching gas. This effect is in particular achieved when the baffles are arranged at least approximately perpendicular with respect to the flow direction of the switching gas, as mentioned above.

According to embodiments, the electrical switching device, and in particular the circuit breaker, further comprises an intermediate chamber, which, when seen in direction of the outflow of the heated switching medium, is arranged between the arcing region and the exhaust volume, said intermediate chamber being delimited by an intermediate chamber wall. A respective surface breaker is e.g. described in EP 1 403 891. According to preferred embodiments of the present invention, at least a portion of the inner surface of the intermediate chamber wall is covered with the porous layer.

Given the relatively high temperature of the switching gas entering the intermediate chamber, the increased absorption of heat radiation achieved according to the present invention is of particular relevance for this specific embodiment. Surprisingly, it has been found that the covering of the intermediate chamber wall or a portion thereof does not negatively affect the flow behaviour of the switching gas, but rather has a positive influence on the flow behaviour.

In further embodiments, an intermediate chamber baffle can be arranged in the intermediate chamber. In this case, at least a portion of the surface of said intermediate chamber baffle is preferably covered with the porous layer. Preferably, the surface, which is covered with the porous layer, forms part of or corresponds to the surface of the switching chamber, in particular forms part of or corresponds to a surface selected from the group consisting of: the inner surface of the exhaust volume wall, the surface of the exhaust volume baffle, the inner surface of the intermediate chamber wall, the surface of the intermediate chamber baffle, any portion of such surface (s), and combinations thereof. In particular with regard to the covering of any of these surfaces, the porous layer preferably contains or essentially consists of a porous insulating or porous metal material. According to a particularly preferred embodiment, the porous layer preferably contains or essentially consists of a porous metal material containing or essentially consisting of a metal, in particular selected from the group consisting of: copper and aluminum, and/or a metal alloy, in particular an iron/carbon alloy, more particularly steel, or a copper/zinc alloy, more particularly brass, or a nickel alloy in porous form.

As also mentioned, the exhaust volume opens out into a tank volume delimited by a tank wall. According to a further preferred embodiment, at least a portion of the inner surface of the tank wall is covered with the porous layer.

If the switching device comprises a self-blast volume (or a "puffer volume" or "fixed heating volume") for building up pressure of the switching medium and thus for assisting the quenching process, it can also be that at least a portion of the inner wall of the self-blast volume is covered with the porous layer. According to a specific embodiment, the switching device, thus, further comprises a self-blast volume for building up pressure of the switching medium, at least a portion of the inner wall of said self-blast volume being covered with the porous layer. This allows for a particularly efficient heat extraction, ultimately allowing for an efficient lowering of the temperature of the switching medium used for blowing the arc. In particular with regard to the covering of the inner surface of the tank wall or a portion thereof, the porous layer contains or essentially consists of a ceramic porous material containing or consisting of alumina ceramic in porous form, in particular porous alumina ceramic having a porosity of at least 45%, preferably at least 65~6 , more preferably at least 85%, and most preferably at least 95%.

Irrespective of the material of which it is made of, the porous layer preferably has a thickness of more than 1 mm, preferably more than 2 mm, more preferably more than 3 mm, and most preferably more than 4 mm. It has been found that a porous layer of this thickness allows a very high cooling performance to be achieved. Given the thickness, any effect on the cooling performance caused by pores getting clogged by particles contained in the gas stream, is negligible.

Oweing to the fact that according to the present invention a porous material forms a layer on at least a portion of a surface contained in the switching chamber, there is no negative impact on the flow behaviour of the switching gas, as mentioned above. In this regard, the present invention differs from technologies according to which a porous material forms a body arranged in the flow path of a gas. In particular, the porous layer of the present invention differs from the trap according to EP 1 895 558 primarily aiming at filtering the gas and from the quenching gas cooling device according to DE 1 98 32 709 passed through by the quenching gas, since both the trap of EP 1 895 558 and the quenching gas cooling device of DE 1 98 32 709 do not relate to a porous layer applied on a surface contained in a switching chamber. In view of reliably ensuring that there is no negative impact on the flow behaviour of the switching gas, it is further preferred that the porous layer has a thickness less than 50 mm, preferably less than 40 mm, more preferably less than 20 mm, most preferably less than 10 mm, and specifically of about 5 mm. Thus, the thickness of the porous layer is preferably in a range from 1 mm to 50 mm, preferably from 2 mm to 40 mm, more preferably from 3 mm to 10 mm, and most preferably from 4 mm to 5 mm. According to embodiments, the porous layer is directly applied on the portion of the surface which it is covering. In other words, there is in these embodiments no intermediate layer between the portion of the surface, which is typically made of metal, and the porous layer, allowing for a specifi- cally efficient transfer of heat absorbed. Alternatively, an intermediary layer is formed between the surface and the porous layer when the porous layer is applied by brazing. This embodiment is also encompassed by the present invention.

As mentioned, the switching medium is specifically a switching gas. Preferably, it comprises or essentially consists of an organofluorine compound. In this regard, the switching medium preferably comprises or essentially consists of an organofluorine compound selected from the group consisting of: fluoroethers (including oxiranes) , in particular hydrofluoromonoethers , fluoroketones , in particu^¬ lar perfluoroketones , fluoroolefins , in particular hydro- fluoroolefins , fluoronitriles , in particular perfluoro- nitriles, and mixtures thereof.

It is thereby particularly preferred that the switching medium comprises a fluoroketone containing from four to twelve carbon atoms, preferably containing exactly five carbon atoms or exactly six carbon atoms or mixtures thereof. The advantages achieved by the present invention are particularly pronounced when the switching medium comprises a fluoroketone as defined above, since any problem, which might otherwise arise from the ketone group being subject to nucleophilic substitution, can be avoided. The term "fluoroketone" as used in this application shall be interpreted broadly and shall encompass both perfluoroketones and hydrofluoroketones , and shall further encompass both saturated compounds and unsaturated compounds, i.e. compounds including double and/or triple bonds between carbon atoms. The at least partially fluorinated alkyl chain of the fluoro- ketones can be linear or branched, or can form a ring, which optionally is substituted by one or more alkyl groups. In exemplary embodiments, the fluoroketone is a perfluoroketone . In further exemplary embodiment, the fluoroketone has a branched alkyl chain, in particular an at least partially fluorinated alkyl chain. In still further exemplary embodiments, the fluoroketone is a fully saturated compound.

In embodiments, the switching medium comprises a fluoroketone containing exactly five carbon atoms or exactly six carbon atoms or mixtures thereof. Compared to fluoroketones having a greater chain length with more than six carbon atoms, fluoroketones containing five or six carbon atoms have the advantage of a relatively low boiling point. Thus, problems which might go along with liquefaction can be avoided, even when the apparatus is used at low temperatures.

According to embodiments, the fluoroketone is at least one compound selected from the group consisting of the compounds defined by the following structural formulae in which at least one hydrogen atom is substituted with a fluorine atom:

(la) 

Fluoroketones containing five or more carbon atoms are further advantageous, because they are generally non-toxic with outstanding margins for human safety. This is in contrast to fluoroketones having less than four carbon atoms, such as hexafluoroacetone (or hexafluoropropanone) , which are toxic and very reactive. In particular, fluoroketones containing exactly five carbon atoms, herein briefly named C5K, and fluoroketones containing exactly six carbon atoms are thermally stable up to 500°C. In embodiments of this invention, the fluoroketones, in particular C5K, having a branched alkyl chain are preferred, because their boiling points are lower than the boiling points of the corresponding compounds (i.e. compounds with same molecular formula) having a straight alkyl chain. According to embodiments, the C5K is a perfluoroketone, in particular has the molecular formula C₅Fi₀O, i.e. is fully saturated without double or triple bonds between carbon atoms. The fluoroketone a) may more preferably be selected from the group consisting of 1, 1, 1, 3, 4, 4, 4-heptafluoro-3- (trifluoromethyl) butan-2-one (also named decafluoro-2- methylbutan-3-one) , 1,1,1,3,3,4,4,5,5, 5-decafluoropentan-2- one, 1 , 1 , 1 , 2 , 2 , 4 , 4 , 5 , 5 , 5-decafluoropentan-3-one and octafluorocylcopentanone, and most preferably is

1,1,1,3,4,4, 4-heptafluoro-3- (trifluoromethyl) butan-2 -one . 1, 1, 1, 3, 4, 4, 4-heptafluoro-3- (trifluoromethyl) butan-2-one can be represented by the following structural formula (I) :

1,1,1,3,4,4, 4-heptafluoro-3- (trifluoromethyl) butan-2 -one with molecular formula CF₃C (0) CF (CF₃) ₂ or C₅Fi₀O, has been found to be particularly preferred for high and medium voltage insulation applications, because it has the advantages of high dielectric insulation performance, in particular in mixtures with a dielectric carrier gas, has very low GWP and has a low boiling point. It has an ODP of 0 and is practically non-toxic.

According to embodiments, even higher insulation capabilities can be achieved by combining the mixture of different fluoroketone components. In embodiments, a fluoroketone containing exactly five carbon atoms, as described above and here briefly called C5K, and a fluoroketone containing exactly six carbon atoms or exactly seven carbon atoms, here briefly named fluoroketone c) , can favourably be part of the dielectric insulation at the same time. Thus, a switching medium can be achieved having more than one fluoroketone, each contributing by itself to the dielectric strength of the switching medium.

In embodiments, the further fluoroketone c) is at least one compound selected from the group consisting of the compounds defined by the following structural formulae in which at least one hydrogen atom is substituted with a fluorine atom:

o (lid),

O (Hf), and

as well as any fluoroketone having exactly 6 carbon atoms, in which the at least partially fluorinated alkyl chain of the fluoroketone forms a ring, which is substituted by one or more alkyl groups (Ilh); and/or is at least one compound selected from the group consisting of the compounds defined by the following structural formulae in which at least one hydrogen atom is substituted with a fluorine atom:

(Hie) ,

,

and

(ΙΙΙη), e.g. dodecafluoro-cycloheptanone, as well as any fluoroketone having exactly 7 carbon atoms, in which the at least partially fluorinated alkyl chain of the fluoroketone forms a ring, which is substituted by one or more alkyl groups (IIIo) .

The present invention encompasses each compound or each combination of compounds selected from the group consisting of the compounds according to structural formulae (la) to (Ii), (Ha) to (Ilh), (Ilia) to (IIIo), and mixtures thereof. Depending on the specific application of the apparatus of the present invention, a fluoroketone containing exactly six carbon atoms (falling under the designation "fluoroketone c) " mentioned above) may be preferred; such a fluoroketone is non-toxic, with outstanding margins for human safety. In embodiments, fluoroketone c) , alike C5K, is a perfluoro- ketone, and/or has a branched alkyl chain, in particular an at least partially fluorinated alkyl chain; and/or the fluoroketone c) contains fully saturated compounds. E.g., the fluoroketone c) is or contains decafluorocyclohexanone . In particular, the fluoroketone c) has the molecular formula C₆Fi₂0, i.e. is fully saturated without double or triple bonds between carbon atoms. More preferably, the fluoroketone c) can be selected from the group consisting of: 1, 1, 1, 2, 4, 4, 5, 5, 5-nonafluoro-2- (trifluoromethyl) pentan-3-one (also named dodecafluoro-2-methylpentan-3-one) ,

1, 1, 1, 3, 3, 4, 5, 5, 5-nonafluoro-4- (trifluoromethyl) pentan-2 -one (also named dodecafluoro-4-methylpentan-2-one) ,

1, 1, 1, 3, 4, 4, 5, 5, 5-nonafluoro-3- (trifluoromethyl) pentan-2 -one (also named dodecafluoro-3-methylpentan-2-one) , 1, 1, 1, 4, 4, 4-hexafluoro-3, 3-bis- (trifluoromethyl) butan-2 -one (also named dodecafluoro-3 , 3- (dimethyl) butan-2-one) , dodecafluorohexan-2-one, dodecafluorohexan-3-one, and particularly is the mentioned 1, 1, 1, 2, 4, 4, 5, 5, 5-Nonafluoro-2- (trifluoromethyl) pentan-3-one .

1,1,1,2,4,4,5,5, 5-Nonafluoro-2- (trifluoromethyl) pentan-3-one (also named dodecafluoro-2-methylpentan-3-one) can be represented by the following structural formula (II) :

(ID 1,1,1,2,4,4,5,5, 5-Nonafluoro-4- (trifluoromethyl) pentan-3-one

(here briefly called "C6-ketone", with molecular formula C₂F₅C (0) CF (CF₃) ₂) has been found to be particularly preferred for high voltage insulation applications because of its high insulating properties and its extremely low GWP . Specifically, its pressure-reduced breakdown field strength is around 240 kV/ (cm*bar) , which is much higher than the one of air having a much lower dielectric strength (E_cr = 25 kV/ (cm*bar) . It has an ozone depletion potential of 0 and is non-toxic (LC50 of about 100' 000 ppm) . Thus, the environmental impact is very low, and at the same time outstanding margins for human safety are achieved.

In additional or alternative embodiments, the switching medium, in particular the switching gas, comprises at least one compound being a hydrofluoroether selected from the group consisting of: hydrofluoro monoether containing at least three carbon atoms; hydrofluoro monoether containing exactly three or exactly four carbon atoms; hydrofluoro monoether having a ratio of number of fluorine atoms to total number of fluorine and hydrogen atoms of at least 5:8; hydrofluoro monoether having a ratio of number of fluorine atoms to number of carbon atoms ranging from 1.5:1 to 2:1; pentafluoro-ethyl-methyl ether; 2 , 2 , 2-trifluoroethyl- trifluoromethyl ether; and mixtures thereof.

As mentioned above, the organofluorine compound can also be a fluoroolefin, in particular a hydrofluoroolefin . More particularly, the fluoroolefin or hydrofluorolefin, respectively, contains at least three carbon atoms or contains exactly three carbon atoms .

According to particular embodiments, the hydrofluoroolefin is thus selected from the group consisting of: 1 , 1 , 1 , 2-tetra- fluoropropene (HFO-1234yf; also named 2, 3, 3, 3-tetrafluoro-1- propene) , 1, 2, 3, 3-tetrafluoro-2-propene (HFO-1234yc) ,

1, 1, 3, 3-tetrafluoro-2-propene (HFO-1234zc) , 1 , 1 , 1 , 3-tetra- fluoro-2-propene (HFO-1234ze) , 1 , 1 , 2 , 3-tetrafluoro-2-propene (HFO-1234ye) , 1 , 1 , 1 , 2 , 3-pentafluoropropene (HFO-1225ye) , 1 , 1 , 2 , 3 , 3-pentafluoropropene (HFO-1225yc) , 1 , 1 , 1 , 3 , 3-penta- fluoropropene (HFO-1225zc) , ( Z ) 1 , 1 , 1 , 3-tetrafluoropropene (HFO-1234zeZ; also named cis-1, 3, 3, 3-tetrafluoro-l-propene) , (Z) 1, 1, 2, 3-tetrafluoro-2-propene (HFO-1234yeZ) , (E)l, 1,1,3- tetrafluoropropene (HFO-1234zeE; also named trans-1, 3, 3, 3- tetrafluoro-l-propene) , (E) 1,1,2, 3-tetrafluoro-2-propene (HFO-1234yeE) , ( Z ) 1 , 1 , 1 , 2 , 3-pentafluoropropene (HFO-1225yeZ ; also named cis-1, 2, 3, 3, 3 pentafluoroprop-l-ene) ,

(E) 1 , 1 , 1 , 2 , 3-pentafluoropropene (HFO-1225yeE; also named trans-1, 2, 3, 3, 3 pentafluoroprop-l-ene) , and mixtures thereof.

As mentioned above, the organofluorine compound can also be a fluoronitrile, in particular a perfluoronitrile . In particu^¬ lar, the organofluorine compound can be a fluoronitrile, specifically a perfluoronitrile, containing two carbon atoms, three carbon atoms or four carbon atoms . More particularly, the fluoronitrile can be a perfluoro- alkylnitrile, specifically perfluoroacetonitrile, perfluoro- propionitrile (C₂F₅CN) and/or perfluorobutyronitrile (C₃F₇CN) .

Most particularly, the fluoronitrile can be perfluoro- isobutyronitrile (according to the formula (CF₃)₂CFCN) and/or perfluoro-2-methoxypropanenitrile (according to the formula CF₃CF (OCF₃) CN) . Of these, perfluoroisobutyronitrile is particularly preferred due to its low toxicity.

According to embodiments of the invention, the switching medium further comprises a carrier gas. More preferably, the switching medium comprises the organofluorine compound, particularly a fluoroketone having exactly five carbon atoms, at a partial pressure corresponding to the vapour pressure of the organofluorine compound at the minimum operating temperature of the apparatus, with the remainder of the switching medium being or comprising the carrier gas. Thus, the organofluorine compound, particularly a fluoroketone having exactly five carbon atoms, is present in fully gaseous phase in the insulation space. In embodiments, the carrier gas comprises air or an air component. In particular, the carrier gas shall be selected from the group consisting of carbon dioxide (C0₂) , oxygen (0₂) , nitrogen (N₂) , and mixtures thereof. In particular, the carrier gas can be a mixture of N₂ and 0₂, or the carrier gas can be a mixture of C0₂ and 0₂. Most preferably, the carrier gas is air. Alternatively or additionally, the carrier gas can also comprise a noble gas, and/or nitric oxide, and/or nitrogen dioxide.

According to embodiments, the carrier comprises 0₂, since this allows to efficiently avoid or reduce the formation of harmful decomposition products. When the carrier gas comprises 0₂, the partial pressure of 0₂ is preferably at least about twice that of the partial pressure of the organo- fluorine compound. Since the present invention relates to a switching device, the carrier gas preferably comprises C0₂.

According to further embodiments, the surface covered by the porous layer relates to an inner surface of a hollow body designed to be passed through by at least a portion of the heated switching medium. In this regard, the term hollow body encompasses any compartment of the switching device or portion thereof enclosing an inner space. This embodiment is therefore in accordance with the above mentioned preferred embodiments, in which at least a portion of the inner surface of the exhaust volume wall, the intermediate chamber wall and/or the tank wall is covered with the porous layer.

According to a still further embodiment, the surface covered by the porous layer is a surface other than the surface of a nozzle arranged in the switching device. This embodiment is therefore even further distinct from the voltage breaker according to WO 2015/039918, in which a nozzle is coated in order to protect from an influence of the arc. In addition to the electrical switching device, the present invention relates, according to a further aspect, also to a process for cooling a switching medium in an electrical switching device, specifically a circuit breaker or a generator circuit breaker, whereby the switching medium after being heated by an arc generated during a current breaking operation in an arcing region flows out of the arcing region in direction to an exhaust volume. The process is characterized in that during flowing out, the switching medium transfers heat to a porous layer applied on a metal component of the switching device. As described above, increased heat absorption efficiency achieved according to the process of the present invention allows to minimize the size of the exhaust volume to allow for a compact design and, ultimately, for a reduction in costs.

According to embodiments of the process, heat is transferred at least partially by heat radiation. Alternatively or additionally, also other heat transfer mechanisms can take effect, in particular heat transfer by heat conduction.

The invention is further illustrated by means of the attached figures of which:

Fig. 1 shows a longitudinal section of a circuit breaker according to a first embodiment of the present invention during a current breaking operation,

Fig. 2 shows a longitudinal section of a circuit breaker according to a second embodiment of the present invention during a current breaking operation, Fig. 3 shows a longitudinal section of a circuit breaker according to a third embodiment of the present invention during a current breaking operation,

Fig. 4a-d show the temperature development in the nozzle

(Fig. 4a), the intermediate chamber (Fig. 4b), the exhaust volume (Fig. 4c) and the tank volume (Fig.

4d) (in this order) of a test device following a switching operation, both for the case where the inner surface of the exhaust volume is covered by a porous layer according to the present invention in comparison to the case where there is no porous layer, said temperature development having been determined by numerical experiments using a simulation model,

Fig. 5 the pressure development measured in the intermediate chamber of the test device following a switching operation, both for the case where the inner surface of the exhaust volume is covered by a porous layer according to the present invention in comparison to the case where there is no porous layer.

As shown in Fig. 1 to 3, the circuit breaker of the present invention comprises a switching chamber 10, which in the embodiments shown is rotationally symmetrical and extends along a longitudinal axis L. The switching chamber comprises a tank wall 11 which delimits a tank volume 13.

The switching chamber 10 comprises two nominal contacts 12 movable in relation to each other in the axial direction, specifically a main contact as a first nominal contact 121 and a contact cylinder as second nominal contact 122. The second nominal contact 122 surrounds a concentrically disposed nozzle arrangement 14 comprising a nozzle 16 and further surrounds a conducting portion 18 forming the wall of a self-blast volume 17. The nozzle arrangement 14 further surrounds two concentrically disposed arcing contacts 19, one in the form of a hollow tubular contact 191 and the other in the form of a pin contact 192.

In the embodiment shown, the second nominal contact 122 is exemplarily designed as a movable contact, whereas the first nominal contact 121 is designed as a stationary contact. This may also be vice versa or both contacts 122, 121 may be moveable .

During a current breaking operation, the second nominal contact 122 is moved in axial direction L away from the first nominal contact 121 from a connected (or closed) state to a disconnected (or open) state. Thereby, also the hollow tubular contact 191 is moved in axial direction L away from the pin contact 192 and eventual^¬ ly disconnected, whereby an arc 20 is formed in the arcing region 22 located between the arcing contacts 191, 192. An actuating rod 24 is linked to the nozzle arrangement 14, said actuating rod 24 being connected to the pin contact 192 by means of an angular lever 26, adapted such to pull the pin contact 192 in a direction away from the hollow tubular contact 191 during current breaking, thereby increasing the speed of disconnecting the arcing contacts 191, 192.

The arc 20 formed is quenched by means of a self-blasting mechanism blowing the heated switching gas into the arcing region 22 and outwards through the nozzle 16. Thereupon, some of the heated and pressurized switching gas flows out of the arcing region 22 through the hollow tubular contact 191, whereas some switching gas flows out of the arcing region 22 in the opposite direction trough a nozzle channel 28 arranged concentrically to and extending along the pin contact 192. The flow direction of the hot switching medium away from the arcing region 22 is depicted with respective arrows.

On the side of the hollow tubular contact 191, a first intermediate chamber 30 is optionally present. It is disposed concentrically relative to the hollow tubular contact 191 and at a distance from the arcing region 22. The first inter- mediate chamber 30 is fluidically connected with the hollow tubular 191 contact by openings 32 provided in the wall 34 of the hollow tubular contact 191. Specifically, a row of e.g. four openings 32 having same cross section and being radially disposed over the circumference of the hollow tubular contact 191 are provided in the embodiment shown.

The first intermediate chamber 30 is delimited by a first intermediate chamber wall 36 comprising a proximal side wall 361 facing the arcing region 22, a distal side wall 362 arranged opposite to said proximal side wall 361 and a circumferential wall 363. The first intermediate chamber wall 36 is preferably made of metal, for example steel or copper, although it may also be composed of a comparatively highly thermally conductive plastic.

In the specific embodiment shown, two rows of circumferen- tially disposed radial openings 38 of same cross-section are arranged in the first intermediate chamber wall 36, one in direct proximity to the proximal side wall 361 and one in direct proximity to the distal side wall 362. The openings 38 open into a first exhaust volume 40 arranged concentrically with respect to the first intermediate chamber 30.

The openings 32 in the hollow tubular contact 191 are arranged offset with regard to the openings 38 in the first intermediate chamber wall 36 so that the swirled gases flowing in the radial direction cannot flow further directly through the openings 38 into the first exhaust volume 40. However, it can also be feasible for at least one of the openings 32 in the hollow tubular contact wall 34 to be provided such that it is entirely or partially coincident with a respective opening 38 in the intermediate chamber wall 36, in order to deliberately ensure a direct partial or complete flow from the hollow tubular contact 191 into the first exhaust volume 40. The shape, size, arrangement and number of the openings 32 and 38, respectively, are optimally configured, and are matched to the respectively operational requirements .

The first exhaust volume 40 is delimited by an exhaust volume wall 42. In the embodiment shown, the exhaust volume wall comprises a proximal sidewall 421, a distal sidewall 422, an outer circumferential wall 423 and an inner circumferential wall 424, the circumferential walls 423, 424 being displaced axially from each other. Specifically, the inner circumferential wall 424 extends from the distal side wall 422 leaving a gap 44 between its free end and the proximal side wall 421, whereas the outer circumferential wall 423 extends from the proximal side wall 421 in a manner such that it overlaps with the inner circumferential wall 424. Thereby, an annular channel 46 is formed between the circumferential walls 423, 424, said channel 46 opening into the tank volume 13 delimited by the tank wall 11 and being filled with switching gas of relatively low temperature. The tank wall 11 is designed in a gas-tight manner and is made of a metal.

Following the heating of the gas caused by the current breaking operation, a portion of the heated pressurized switching gas flows out of the arcing region through the hollow tubular contact 191, as mentioned above. The gas flow indicated by the arrow A10 is deflected by an approximately conical deflection device (not shown) , as indicated by radially deflecting arrows, into a predominantly radial direction. The gas flow passes through the openings 38 into the first intermediate chamber 30, in which the switching gas is swirled.

The swirled switching gas is then allowed to pass through the openings 38 in the first intermediate chamber wall 36 in the radial direction into the first exhaust volume 40, as also indicated by arrows.

The switching gas that has entered the first exhaust volume 40 then flows through the gap 44 or first gap volume 44 and the annular channel 46 formed by the circumferential walls 423, 424 into the tank volume 13. On the side of the pin arcing contact 192, a second intermediate chamber 52 is arranged, with the distal end 54 of the pin contact 192 and the angular lever 26 being arranged in the interior of the second intermediate chamber 52 being delimited by a second intermediate chamber wall 60. One row of circumferentially disposed radial openings 58 is arranged in the circumferential wall 603 of the second intermediate chamber 52 in direct proximity to its distal sidewall 602. These openings 58 open into the second exhaust volume 62.

Like the first exhaust volume 40, also the second exhaust volume 62 is delimited by an exhaust volume wall 64 comprising a proximal sidewall 641, a distal sidewall 642, an outer circumferential wall 643 and an inner circumferential wall 644, the circumferential walls 643, 644 being displaced axially from each other. Also with regard to the second exhaust volume 62, the inner circumferential wall 644 extends from the distal side wall 642 leaving a gap 66 or second gap volume 66 between its free end and the proximal side wall 641, whereas the outer circumferential wall 643 extends from the proximal side wall 641 in a manner such that it overlaps with the inner circumferential wall 644. Thereby, an annular channel 68 is formed between the circumferential walls, said annular channel 68 opening into the tank volume 13, as described above for the first exhaust volume 40.

During the current breaking operation, a second portion of the heated and pressurized switching gas flows through the nozzle channel 28 extending along the pin contact 192, as illustrated by arrows A20. This second portion of pressurized switching gas flows partly directly into the second exhaust volume 62 by passing openings 70 and partly into the second intermediate chamber 52 and from there into the second exhaust volume 62 by passing openings 58. Thereby, the portion flowing out of the second intermediary chamber 52 is deflected in the second exhaust volume 62 by means of the inner circumferential wall 644, before flowing out into the tank volume 13 containing switching gas of relatively low temperature, as described above for the first exhaust volume 40. Like the inner circumferential wall 424 of the first exhaust volume wall 40, also the inner circumferential wall 644 of the second exhaust volume wall 64 thus functions as an exhaust volume baffle.

In the embodiment shown in Fig. 1, the inner surface of the first intermediate chamber wall 36 and the second intermediate chamber wall 60, which function as a metal component for absorbing heat, is covered by a porous layer 72, particularly of a porous insulating or a porous metal material made of copper, aluminum, steel, brass or a nickel alloy. Specifically, this includes the inner surface of both sidewalls 361, 362 and the circumferential wall 363 of the first intermediate chamber 30 and the inner surface of the distal sidewall 602 and circumferential wall 603 of the second intermediate chamber 52.

Also, the inner surface of the first exhaust volume wall 42 and the second exhaust volume wall 64, which also function as a metal component for absorbing heat, is covered by the porous layer 72, preferably by the same porous insulating or porous metal material covering the inner surface of the intermediate chamber walls 36, 60.

Thus, the embodiment shown in Fig. 1 is specifically designed for augmenting heat transfer from the hot switching gas during its flowing out after a current breaking operation. During its passage through the intermediate chambers 30, 52 and exhaust volumes 40, 62, heat radiation emitted from the outflowing hot switching gas is efficiently absorbed by the porous layer 72, resulting in an overall increase in the cooling of the switching gas during its passage from the arcing region 22 into the tank volume 13. Due to the porous material 72 being made of a metal material, a particularly efficient heat absorption is achieved which is required for cooling the hot switching gas under the required threshold value .

In the embodiment shown in Fig. 2, the inner surface of the tank wall is covered by a porous layer 72, 72', particularly by a ceramic porous material 72 ' .

The embodiment shown in Fig. 2 is specifically designed for augmenting absorption of heat generated under normal conditions by ohmic heating under nominal current. This is achieved by the porous ceramic layer 72' of relatively great area applied on the tank wall 11. Specifically, the heat of the gas in the tank volume 13 is efficiently absorbed and transferred to the tank wall 11, from which it is emitted to the surrounding. A further embodiment designed for augmenting absorption of heat generated under normal conditions by ohmic heating under nominal current is shown in Fig. 3, according to which the outer surface of the nominal contacts 12, specifically the first nominal contact 121 and the second nominal contact 122, is covered by a ceramic porous layer 72' . This embodiment allows ohmic heating of the nominal contacts 12 to be efficiently absorbed and, thus, to be dissipated. Of course, the layer arrangement according to Figs. 1 and/or 2 and/or 3 can be combined in order to achieve a particularly efficient dissipation of heat generated.

The concept of the present invention illustrated by Fig. 1 to 3 has further been evaluated by means of a test device. In the test device, the temperature of the switching gas as well as the pressure present in the respective compartment has been determined by numerical experiments using a simulation model and by running a test in which the temperature and pressure were actually measured after a switching operation. The test device used encompasses a test device nozzle arrangement directly connected to a test device hollow tube opening into a test device intermediate chamber. In the downstream direction, the test device intermediate chamber is fluidically connected to a test device exhaust volume which opens into a test device tank volume. The interior of the test device nozzle arrangement, the test device hollow tube, the test device intermediate chamber, the test device exhaust volume and the test device tank volume is in each case subdivided into two compartments, of which a first compartment of the test device intermediate chamber, the test device exhaust volume and the test device tank volume comprises a porous layer according to the present invention, whereas the second compartment of said components is devoid of the porous layer.

According to Fig. 4a-d, the temperature measurement shows that whereas the temperature measured in the nozzle arrange^¬ ment (Fig. 4a) is identical, there is a substantial temperature drop of the switching gas of about 22% in the test device intermediate chamber (Fig. 4b) comprising the porous layer in comparison to the one devoid of it. The effect is even more pronounced in the test device exhaust volume (Fig. 4c) and the test device tank volume (Fig. 4d) , in which the temperature drops by 67% and 55%, respectively. As further shown in Fig. 5, the concept of the present invention is further confirmed by the pressure development measured in the intermediate chamber:

According to Fig. 5, a significantly reduced pressure is mea^¬ sured for the case where the inner surface of the exhaust volume is covered by a porous layer (continuous line) accord^¬ ing to the present invention compared to the case where there is no porous layer (dashed line) . Specifically, a 40% lower maximum pressure increase when the exhaust volume is covered with the porous layer confirms a substantial enhancement of heat absorption by application of the porous layer (s) .

List of reference numerals

10 switching chamber

11 tank wall

12 nominal contacts

121; 122 first nominal contact (main contact);

second nominal contact (contact cylinder)

13 tank volume

14 nozzle arrangement

16 nozzle

18 conducting portion

17 self-blast volume

19 arcing contacts

191; 192 hollow tubular (arcing) contact; pin

(arcing) contact

20 arc

22 arcing region

24 actuating rod

26 angular lever

28 nozzle channel, nozzle throat and diffusor 30 first intermediate chamber

32 openings in wall of tubular hollow contact

34 wall of tubular hollow contact

36 first intermediate chamber wall

361, 362, 363 proximal side wall, distal side wall, circumferential wall (of first inter^¬ mediate chamber wall)

38 openings in the first intermediate chamber wall first exhaust volume

first exhaust volume wall

; 422; 423; 424 proximal sidewall; distal sidewall; outer circumferential wall; inner circum^¬ ferential wall (of first exhaust volume wall )

gap, first gap volume

annular channel

second intermediate chamber distal end of pin contact

openings in second intermediate chamber wall

second intermediate chamber wall

; 603 distal side wall; circumferential wall of second intermediate chamber second exhaust volume

second exhaust volume wall

; 642; 643; 644 proximal sidewall; distal sidewall; outer circumferential wall; inner circumferen^¬ tial wall of second exhaust volume wall gap, second gap volume

annular channel formed by circumferential walls of second exhaust volume

openings from nozzle channel into second exhaust volume

72' porous layer ( s ) .

Claims

1. Electrical switching device comprising at least one switching chamber (10), which comprises at least two arcing contacts (191, 192) movable in relation to each other and defining an arcing region (22) in which an arc (20) is formed during a current breaking operation, at least a portion of said switching chamber (10) being filled with a switching medium for quenching the arc (20) and for providing dielectrical insulation, the switching chamber (10) further comprising an exhaust volume (40, 62), which is fluidically connected to the arcing region (22), to allow the switching medium heated by the arc (20) to flow out of the arcing region (22) in direction to the exhaust volume (40, 62), thereby transferring heat to a metal component of the switching chamber (10), characterized in that at least a portion of a surface contained in the switching chamber (10) is covered with a porous layer (72, 72 ' ) .

2. Electrical switching device according to claim 1, characterized in that the porous layer (72, 72') has a characteristic property selected from the group consisting of: a porosity of at least 45%, a pore density ranging from 15 ppi (pores per inch) to 70 ppi, a mean pore diameter in the range from 0.7 mm to 2.0 mm; a thickness of more than 1 mm; a thickness of less than 50 mm; and combinations thereof.

3. Electrical switching device according any one of the preceding claims, characterized in that at least a portion of the metal component is covered with the porous layer (72, 72').

4. Electrical switching device according to any one of the preceding claims, characterized in that it is a circuit breaker or a generator circuit breaker.

5. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') is in contact with at least a portion of the heated switching medium.

6. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') contains or essentially consists of a porous insulating or porous metal material, particularly a metal foam, and/or a ceramic porous material.

7. Electrical switching device according to claim 6, characterized in that the metal foam is made of aluminum, and/or the metal foam has gas-filled pores that are sealed to form a closed-cell metal foam or that are interconnected to form an open-cell metal foam.

8. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer, in particular the metal foam, has been produced on the surface contained in the switching chamber (10) by metallic sintering, by electrodeposition, by chemical vapor deposition, or by metal deposition through evaporation .

9. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') has a pore density ranging from 15 ppi to 70 ppi, preferably from 30 ppi to 60 ppi.

10. Electrical switching device according to any one of the preceding claims, characterized in that the heated switching medium has a temperature of 2000 K at most and the porous layer has a pore density of 50 ppi at most, or the heated switching medium has a temperature of 1500 K at most and the porous layer has a pore density of 30 ppi at most.

11. Electrical switching device according to any one of the preceding claims, characterized in that the heated switching medium has a temperature of higher than 2000 K and the porous layer has a pore density of higher than 50 ppi.

12. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') has a porosity of at least 45%, preferably at least 65%, more preferably at least 85%, and most preferably at least 95%.

13. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') has a mean pore diameter in the range from 0.7 mm to 2.0 mm, preferably from 1 mm to 1.5 mm, most preferably from 1.1 mm to 1.3 mm.

14. Electrical switching device according to any one of the preceding claims, characterized in that the surface covered by the porous layer (72, 72') relates to an inner surface of a hollow body designed to be passed through by at least a portion of the heated switching medium.

15. Electrical switching device according to any one of the preceding claims, characterized in that the exhaust volume (40, 62) is delimited by an exhaust volume wall (42, 64), at least a portion of the inner surface of the exhaust volume wall (42, 64) being covered with the porous layer (72, 72').

16. Electrical switching device according to any one of the preceding claims, characterized in that in the exhaust volume (40, 62) an exhaust volume baffle is arranged, at least a portion of the surface of the exhaust volume baffle being covered with the porous layer (72, 72 ' ) .

17. Electrical switching device according to any one of the preceding claims, in particular a circuit breaker, characterized in that it further comprises an inter^¬ mediate chamber (30, 52), which, in direction of the outflow of the heated switching medium, is arranged between the arcing region (22) and the exhaust volume (40, 62), said intermediate chamber (30, 52) being delimited by an intermediate chamber wall (36, 60), at least a portion of the inner surface of the intermediate chamber wall (36, 60) being covered with the porous layer (72, 72 ' ) .

18. Electrical switching device according to claim 17, characterized in that in the intermediate chamber

(30, 52) an intermediate chamber baffle is arranged, at least a portion of the surface of said intermediate chamber baffle being covered with the porous layer

(72, 72 ' ) .

19. Electrical switching device according to any one of the preceding claims, characterized in that at least one baffle that is covered with the porous layer (72, 72'), in particular the intermediate chamber baffle and/or the exhaust volume baffle, is arranged such that it functions as a filter for removing dust particles from the outflowing switching medium, specifically the switching gas.

20. Electrical switching device according to any one of the preceding claims, characterized in that the surface, which is covered with the porous layer (72, 72'), forms part of or corresponds to the surface of the switching chamber (10), and in particular forms part of or corresponds to a surface selected from the group consisting of: the inner surface of the exhaust volume wall (42, 64), the surface of the exhaust volume baffle, the inner surface of the intermediate chamber wall (36, 60), the surface of the intermediate chamber baffle, any portion thereof, and any combination thereof .

21. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') contains or essentially consists of a porous insulating or porous metal material, in particular a porous metal material containing or essentially consisting of a metal, in particular selected from the group consisting of: copper and aluminum, a metal alloy, an iron/carbon alloy, a steel, a copper/zinc alloy, a brass, a nickel alloy, and any combination thereof; with all these materials being in porous form.

22. Electrical switching device according to any of the preceding claims, characterized in that the exhaust volume (40, 62) opens out into a tank volume (13) delimited by a tank wall (11), at least a portion of the inner surface of the tank wall (11) is covered with the porous layer (72, 72').

23. Electrical switching device according to any one of the preceding claims, characterized in that it further comprises a self-blast volume (18) for building up pressure of the switching medium, at least a portion of the inner wall of said self-blast volume being covered with the porous layer.

24. Electrical switching device according to any of the preceding claims, characterized in that the porous layer (72') contains or essentially consists of a ceramic porous material containing or consisting of alumina ceramic in porous form, in particular porous alumina ceramic having a porosity of at least 45%, preferably at least 65%, more preferably at least 85%, and most preferably at least 95%.

25. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') has a thickness of more than 1 mm, preferably more than 2 mm, more preferably more than 3 mm, and most preferably more than 4 mm.

26. Electrical switching device according to any one of the preceding claims, characterized in that the porous layer (72, 72') has a thickness of less than 50 mm, preferably less than 40 mm, more preferably less than 20 mm, most preferably less than 10 mm, and specifically of about 5 mm.

27. Electrical switching device according to any one of the preceding claims, characterized in that the switching medium is a switching gas.

28. Electrical switching device according to any one of the preceding claims, characterized in that the switching medium comprises or essentially consists of an organo- fluorine compound.

29. Electrical switching device according to any one of the preceding claims, characterized in that the switching medium comprises or essentially consists of an organo- fluorine compound selected from the group consisting of: fluoroethers , in particular hydrofluoromonoethers , fluoroketones , in particular perfluoroketones , fluoro- olefins, in particular hydrofluoroolefins , and fluoro- nitriles, in particular perfluoronitriles , and mixtures thereof .

Electrical switching device according to any one of the preceding claims, characterized in that the switching medium comprises or essentially consists of a fluoro- ketone containing from four to twelve carbon atoms, preferably containing exactly five carbon atoms or exactly six carbon atoms or mixtures thereof.

Electrical switching device according to any one of the preceding claims, characterized in that the switching medium comprises or essentially consists of a hydro- fluoro monoether containing at least three carbon atoms.

Electrical switching device according to any one of the preceding claims, characterized in that the switching medium comprises sulphur hexafluoride (SF₆) , air and/or at least one air component, in particular selected from the group consisting of: oxygen (0₂) , nitrogen (N₂) , carbon dioxide (C0₂) , and mixtures thereof.

Electrical switching device according to any one of the preceding claims, wherein the switching medium comprises a mixture of carbon dioxide and oxygen.

Electrical switching device according to claim 31 or 32, wherein the ratio of the amount of carbon dioxide to the amount of oxygen ranges from 50:50 to 100:1, preferably from 80:20 to 95:5, more preferably from 85:15 to 92:8, even more preferably from 87:13 to less than 90:10, and in particular is about 89:11.

35. Electrical switching device according to any of the preceding claims, the surface covered by the porous layer (72, 72') is a surface other than the surface of a nozzle arranged in the switching device.

36. Process for cooling a switching medium in an electrical switching device of any one of the preceding claims, specifically a circuit breaker or a generator circuit breaker, whereby the switching medium after being heated by an arc (20) generated during a current breaking operation in an arcing region (22) flows out of the arcing region (22) in direction to an exhaust volume (40, 62), characterized in that during flowing out, the switching medium transfers heat to a porous layer (72, 72') applied on a metal component of the switching device.

37. Process according to claim 35, characterized in that heat is transferred to the porous layer (72, 72') at least partially by heat radiation.

38. Process according to any one of the claims 36 to 37; characterized in that the porous layer (72, 72') has a characteristic property selected from the group consisting of: a porosity of at least 45%, a pore density ranging from 15 ppi to 70 ppi, a mean pore diameter in the range from 0.7 mm to 2.0 mm; a thickness of more than 1 mm; a thickness of less than 50 mm; and combinations thereof.