Gas insulated electrical instrument comprising heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone

Abstract

The invention relates to a medium or high voltage device comprising a sealed housing in which are arranged electrical components and a gas mixture for providing electrical insulation and / or for extinguishing arcs that may occur in the housing, the gas mixture comprising heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone mixed with a diluent gas.

Description

Field of the Invention

[0001] The present invention relates to the field of electrical insulation and arc quenching in medium-voltage or high-voltage equipment (and especially in high-voltage equipment).

[0002] The present invention relates to a medium-voltage or high-voltage equipment, in which electrical insulation and / or arc quenching is carried out by means of a gaseous medium, which gaseous medium comprises heptafluoroisobutyronitrile and heptafluoro(trifluoromethyl)ketone mixed with a diluent gas, which diluent gas especially comprises carbon dioxide and molecular oxygen or nitrogen and molecular oxygen.

[0003] The present invention also relates to the use of a gaseous medium comprising heptafluoroisobutyronitrile and heptafluoro(trifluoromethyl)ketone mixed with a diluent gas as a gas for electrical insulation and / or arc quenching in medium-voltage or high-voltage equipment, which diluent gas especially comprises carbon dioxide and molecular oxygen or nitrogen and molecular oxygen.

[0004] More particularly, the present invention relates to the use of an insulation with low environmental impact based on a gaseous medium as a gas for electrical insulation and / or arc quenching in medium-voltage or high-voltage equipment, which gaseous medium comprises

[0005] - heptafluoroisobutyronitrile, heptafluoroisopropyl(trifluoromethyl)ketone, carbon dioxide and molecular oxygen

[0006] - or heptafluoroisobutyronitrile, heptafluoroisopropyl(trifluoromethyl)ketone, nitrogen and molecular oxygen.

[0007] Such an insulation based on such a gaseous medium can optionally be combined with a solid insulation with a low dielectric constant. Background Art

[0008] In medium-voltage or high-voltage substation equipment, electrical insulation and, if necessary, arc quenching are usually carried out by a gas confined within the enclosure of said equipment.

[0009] In this context, the expression "medium voltage" is used in a conventionally accepted manner, i.e. the term "medium voltage" means a voltage greater than 1000 volts (V) for alternating current (AC) or greater than 1500 volts for direct current (DC), but not exceeding 52,000 V for AC or 75,000 V for DC.

[0010] Furthermore, the expression "high voltage" is used in a conventionally accepted manner, i.e. the expression "high voltage" means a voltage strictly greater than 52,000 V for AC and strictly greater than 75,000 V for DC.

[0011] Currently, the gas most commonly used in this type of equipment is sulfur hexafluoride (SF6). This gas exhibits a relatively high dielectric strength, good thermal conductivity, and low dielectric losses. It is chemically inert, non-toxic to humans and animals, and upon dissociation by an electric arc, it recombines quickly and almost completely. Additionally, it is non-flammable and its price remains moderate.

[0012] However, the main drawback of SF6 is that it has a global warming potential (GWP) of 25,200 (relative to CO2, over 100 years) and remains in the atmosphere for a period of 3200 years, which places it among the gases with strong global warming capabilities. Therefore, SF6 is included in the list of gases whose emissions need to be restricted in the Kyoto Protocol (1997).

[0013] The best way to limit SF6 emissions lies in restricting the use of the gas, which has led manufacturers to search for alternatives to SF6.

[0014] "Natural" gases have no negative impact on the environment (carbon dioxide (CO2) with a GWP equal to 1, or gases with a GWP of zero, such as nitrogen (N2) or air), but they exhibit a dielectric strength much lower than that of SF6. Thus, for example, the alternating current (AC) (50 hertz (Hz)) dielectric strength of air and nitrogen is essentially one-third of the dielectric strength of SF6. As a result, using these "natural" gases for electrical insulation and / or arc quenching in medium-voltage or high-voltage equipment would require a significant increase in the volume and / or filling pressure of the equipment, which goes against the efforts made in the past few decades to develop equipment with a low life cycle assessment (LCA) (i.e., developing compact, safe for personnel, and increasingly smaller equipment).

[0015] International Application WO 2012 / 080246 describes the use of one (or more) fluoroketones mixed with air as an electrical insulating and / or arc quenching device with low environmental impact [1]. Due to the high boiling points of the proposed fluids, namely 49 °C for fluoroketone C6 and 26.9 °C for heptafluoroisopropyl (trifluoromethyl) ketone (fluoroketone C5), it was found that these fluids are liquid at the usually lowest pressures and operating temperatures of medium and high voltage equipment, thus forcing the present inventors to add a system for evaporating the liquid phase or for heating outside the equipment in order to keep the temperature of the equipment above the liquefaction temperature of the fluoroketone. The external evaporation system (and especially the heating system) complicates the design of the equipment, reduces its reliability in the case of cutting off its power supply, and results in additional power consumption, which can reach one megawatt-hour (MWh) during the lifetime of the equipment. This is also contrary to the goal of low LCA, i.e., reducing the environmental impact of the equipment and especially reducing carbon emissions. From the viewpoint of low-temperature reliability, in the case of cutting off the power supply at low temperature, the gas phase of one or more fluoroketones liquefies, thus significantly reducing the concentration of one or more fluoroketones in the gas mixture and therefore reducing the insulating capacity of the equipment, and then the equipment cannot withstand the voltage when the power supply is restored.

[0016] In addition, a new gas has been developed that exhibits electrical insulating properties sufficient for applications in the field of high or medium voltage equipment. More precisely, the gas is a mixture of two molecules: one is present in the vast majority, while the second is heptafluoroisobutyronitrile of formula (I): (CF3)2CF-CN (I) and CAS number: 42532-60-5, and is present in a smaller amount. The advantage of this gas mixture is that, based on an SF6 alternative, it exhibits a GWP lower than that of SF6 in a solution of a main or diluent gas with a very low GWP, said diluent gas such as carbon dioxide (CO2) with a GWP equal to 1, or a GWP of zero, such as nitrogen (N2) or air. The heptafluoroisobutyronitrile gas used in these mixtures is sold by 3M TM Company under the trade name 3M TM Novec TM 4710.

[0017] International Application WO 2014 / 037566 describes the use of such mixtures as insulating gases in high or medium voltage equipment, in combination with solid insulation [2].

[0018] A specific insulating gas is described in international application WO 2015 / 040069, namely a gas medium containing heptafluoroisobutyronitrile, CO2 and molecular oxygen (O2), with the molecular oxygen present in the gas medium in a molar percentage in the range of 1% to 25% [3]. In fact, it has been determined that a few percent of oxygen added to a mixture containing heptafluoroisobutyronitrile and CO2 enables a synergistic effect to be obtained in the insulating properties of the gas mixture as a whole.

[0019] General Electric Company sells an electric insulating gas mixture (for "green gas for power grids") named g 3 whose GWP impact is 98% less than that of SF6, and which contains the following and is in particular composed of:

[0020] - 70 mole percent (mol%) to 97 mole percent of CO2,

[0021] - 3 mol% to 10 mol% of (CF3)2CF-CN, and

[0022] - 0 mol% to 20 mol% of O2.

[0023] The inventors have sought to discover an insulating system that contains at least one gas or gas mixture that exhibits electrical insulating or arc extinguishing properties sufficient for application in the field of medium-voltage or high-voltage equipment and is in particular comparable to SF6 equipment, and also has a low environmental impact in terms of GWP and LCA.

[0024] They have also sought to provide an insulating system and in particular the gas or gas mixture included in the system that is non-toxic to humans and the environment.

[0025] They have further sought to provide an insulating system and in particular the gas or gas mixture that has a manufacturing or purchase cost compatible with industrial-scale use.

[0026] They have further sought to provide medium-voltage or high-voltage equipment based on the insulating system and in particular the gas or gas mixture, where the size and pressure of the gas or gas mixture are close to those of an equivalent equipment insulated with SF6 and the gas or gas mixture does not exhibit liquefaction at the lowest operating temperature without adding an external heat source. Summary of the Invention

[0027] These and other objects are achieved by the present invention, which proposes the use of a specific gas mixture that enables medium-voltage or high-voltage equipment with a low environmental impact in terms of GWP and LCA to be obtained. In fact, this specific gas mixture does not require the equipment insulated therewith to have a larger size and / or provide an external heat source than an equivalent equipment insulated with SF6.

[0028] Thus, the insulation system implemented in the context of the present invention is based on a gaseous medium comprising heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl) ketone mixed with a diluent gas, which is used as a gas for electrical insulation and / or arc quenching in medium-voltage or high-voltage equipment.

[0029] In the present invention, the expressions "gas mixture", "gaseous mixture", "gas medium" and "gaseous medium" are equivalent and can be used interchangeably.

[0030] Generally, the present invention provides a medium-voltage or high-voltage equipment comprising a sealed housing in which electrical components are provided and a gas medium for providing electrical insulation and / or for quenching an arc that may occur in the housing, the gas medium comprising heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl) ketone mixed with a diluent gas.

[0031] In the equipment of the present invention, gas insulation is carried out using a gas medium comprising heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl) ketone.

[0032] As already explained, heptafluoroisobutyronitrile of formula (I): (CF3)2CFCN (I), hereinafter written as i-C3F7CN, corresponds to 2,3,3,3-tetrafluoro-2-trifluoromethylpropionitrile, CAS number: 42532-60-5.

[0033] This compound exhibits

[0034] (i) a boiling point of -4.7 °C at 1013 hPa (boiling point measured according to ASTM D1120-94 "Standard Test Method for Boiling Point of Engine Coolants");

[0035] (ii) a molar mass of 195 g·mol -1 ;

[0036] (iii) a GWP of 2750 (calculated according to the IPCC method 2022, over 100 years); and

[0037] (iv) an ozone depletion potential (ODP) of 0.

[0038] Table I below gives the relative dielectric strength of heptafluoroisobutyronitrile of formula (I), normalized with respect to the gas to be desirably replaced (i.e., SF6), and compared with the relative dielectric strength of N2, the dielectric strength being measured at atmospheric pressure, under DC voltage, between two steel electrodes with a diameter of 2.54 cm and a spacing of 0.1 cm.

[0039] <![CDATA[SF6]]> <![CDATA[N2]]> <![CDATA[i-C3F7CN]]> 1.0 0.35-0.4 2.6

[0040] Table I

[0041] Heptafluoroisopropyl(trifluoromethyl) ketone of formula (II): CF3C(O)CF(CF3)2 (II), corresponding to 1,1,1,3,4,4,4-heptafluoro-3-(trifluoromethyl)butan-2-one, CAS No.: 756-12-7. This compound exhibits:

[0042] (i) a boiling point of 26.9 °C at 1013 hPa (boiling point measured according to ASTM D1120-94 "Standard Test Method for Boiling Point of Engine Coolants");

[0043] (ii) a molar mass of 266 g·mol -1 ;

[0044] (iii) GWP < 1 (calculated according to the IPCC method 2022, over 100 years); and

[0045] (iv) an ozone depletion potential (ODP) of 0.

[0046] Table II below gives the relative dielectric strength of heptafluoroisopropyl(trifluoromethyl) ketone of formula (II), normalized with respect to the gas to be replaced as desired (i.e., SF6), and the dielectric strength is measured according to ASTM D877 using disc electrodes with a 2.5 mm gap.

[0047] <![CDATA[SF6]]> <![CDATA[CF3C(O)CF(CF3)2]]> 1.0 1.5

[0048] Table II

[0049] The heptafluoroisopropyl(trifluoromethyl) ketone gas is sold by 3M TM Company under the trade name 3M TM Novec TM 5110.

[0050] Therefore, the above-mentioned heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl) ketone are given electrical insulation and arc quenching properties, which are neither toxic, nor corrosive, nor flammable, and exhibit a GWP significantly less than that of SF6, and are suitable for mixing them with a diluent gas to replace SF6 as the gas for electrical insulation and / or arc quenching in medium-voltage or high-voltage equipment.

[0051] However, even though the GWP of heptafluoroisobutyronitrile is lower than that of SF6, the GWP of heptafluoroisobutyronitrile is higher than that of heptafluoroisopropyl(trifluoromethyl) ketone. Therefore, it is appropriate to minimize the presence of this heptafluoroisobutyronitrile in the gas mixture and determine its amount according to the target GWP of the gas mixture.

[0052] To this end, it should be noted that the inventors have determined the synergy factor of heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl)ketone in the gaseous medium according to the invention, which makes it possible to improve the dielectric and quenching properties while using an amount of heptafluoroisobutyronitrile lower than the amount implemented in the prior art gas mixtures.

[0053] In fact, adding an amount of heptafluoroisopropyl(trifluoromethyl)ketone less than or equal to 5 mol% to the g 3 type mixture makes it possible to increase the dielectric tolerance of the resulting mixture by about 20%. In addition, if the amount of heptafluoroisobutyronitrile in the mixture implemented in the present invention is reduced to an amount less than or equal to 0.9 mol%, adding heptafluoroisopropyl(trifluoromethyl)ketone in the above-determined amount allows at least partial compensation for the loss of dielectric performance.

[0054] In a specific embodiment, the gaseous medium implemented in the device according to the invention contains an amount of heptafluoroisopropyl(trifluoromethyl)ketone less than or equal to 5 mol%. Advantageously, in the gas mixture implemented in the device according to the invention, the amount of heptafluoroisopropyl(trifluoromethyl)ketone ranges between 0.1 mol% and 5 mol%, particularly between 0.5 mol% and 3 mol%, and more particularly between 1 mol% and 2 mol%. In a more specific embodiment, the amount of heptafluoroisopropyl(trifluoromethyl)ketone is about 1.5 mol% (i.e., 1.5 mol% ± 0.3 mol%).

[0055] In another specific embodiment, the gaseous medium implemented in the device according to the invention contains an amount of heptafluoroisobutyronitrile less than or equal to 0.9 mol%. Advantageously, in the gas mixture implemented in the device according to the invention, the amount of heptafluoroisobutyronitrile ranges between 0.001 mol% and 0.9 mol%, particularly between 0.01 mol% and 0.5 mol%, and more particularly between 0.05 mol% and 0.2 mol%. In a more specific embodiment, the amount of heptafluoroisobutyronitrile is about 0.1 mol% (i.e., 0.1 mol% ± 0.03 mol%).

[0056] More particularly, the present invention provides gas insulation with a low environmental impact, which combines a gas mixture with a low environmental impact (low GWP relative to SF6 or even relative to prior art gas mixtures containing heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl)ketone), which is compatible with the lowest operating temperature of the device, and which has better dielectric, quenching, and heat dissipation properties than conventional gases (such as CO2, air, or nitrogen).

[0057] In the context of the present invention, heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl)ketone exist exclusively or almost exclusively in gaseous form under all temperature conditions in medium-pressure or high-pressure equipment, where the gaseous medium, once confined within the equipment, will be subject to said temperature conditions. For this purpose, heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl)ketone should be present in the equipment at a partial pressure, which is selected based on the respective saturation vapor pressures presented by these compounds at the lowest operating temperature of the equipment. The term "lowest operating temperature" for the equipment refers to the lowest temperature at which the equipment is designed to be used.

[0058] Taking into account the usually recommended filling pressure levels for medium-pressure and high-pressure equipment, which are typically a few bar, and taking into account, firstly, the liquefaction temperature of heptafluoroisopropyl(trifluoromethyl)ketone at normal atmospheric pressure (1013.25 hPa), and secondly, the GWP of heptafluoroisobutyronitrile, which is most often used in a diluted form in at least one other gas in order to obtain the recommended filling pressure level for the equipment under consideration, while ensuring that heptafluoroisopropyl(trifluoromethyl)ketone remains in gaseous form throughout the operating temperature range of the equipment.

[0059] According to the present invention, when there is said other gas (referred to as the diluent gas or carrier gas or buffer gas), it is selected from gases that meet the following four criteria:

[0060] (1) Exhibits a very low boiling temperature, below the lowest operating temperature of the equipment; the boiling temperature is typically equal to or less than -50 °C at standard pressure;

[0061] (2) Exhibits a dielectric strength greater than or equal to that of carbon dioxide under the same test conditions as those used to measure the dielectric strength of said carbon dioxide (i.e., the same equipment, the same geometry, the same operating parameters, etc.);

[0062] (3) Is non-toxic to humans and the environment; and

[0063] (4) Exhibits a GWP lower than that of the mixture of heptafluoroisobutyronitrile and heptafluoroisopropyl(trifluoromethyl)ketone, such that diluting the mixture with the diluent gas also serves to reduce the environmental impact of the mixture, since the GWP of a gas mixture is the weighted average obtained from the sum of the weight fractions of each compound in the mixture multiplied by its respective GWP.

[0064] The diluent gas that is usually used is a GWP-neutral gas with a very low GWP (usually equal to or less than 1).

[0065] Gases exhibiting this set of properties are, for example, air, and advantageously dry air (GWP = 0), nitrogen (GWP = 0), helium (GWP = 0), carbon dioxide (GWP = 1), and molecular oxygen (GWP = 0). Thus, any one of these gases or a mixture thereof can be used as the diluent gas in the present invention. In particular, the diluent gas implemented in the present invention comprises or consists of a mixture of CO2 and O2 or a mixture of N2 and O2.

[0066] In the context of the present invention, in order to avoid the liquefaction of heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone at the lowest operating temperature of the device, the partial pressures of the different components of the gas mixture according to the present invention are selected to satisfy the following inequality (III):

[0067] A + B + C < 1 (III)

[0068] where

[0069] A = P i-C3F7CN / PVS i-C3F7CN

[0070] B = P CF3C(O)CF(CF3)2 / PVS CF3C(O)CF(CF3)2

[0071] C = P 稀释 / PVS 稀释

[0072] P i-C3F7CN = the partial pressure of heptafluoroisobutyronitrile at the lowest operating temperature,

[0073] P CF3C(O)CF(CF3)2 = the partial pressure of heptafluoroisopropyl (trifluoromethyl) ketone at the lowest operating temperature, and

[0074] P 稀释 = the partial pressure of the diluent gas at the lowest operating temperature,

[0075] PVS i-C3F7CN = the saturated vapor pressure of heptafluoroisobutyronitrile at the lowest operating temperature,

[0076] PVS CF3C(O)CF(CF3)2 = the saturated vapor pressure of heptafluoroisopropyl (trifluoromethyl) ketone at the lowest operating temperature, and

[0077] PVS 稀释 = the saturated vapor pressure of the diluent gas at the lowest operating temperature.

[0078] In a specific embodiment, when the gas diluent is a mixture of CO2 and O2, C = P CO2 / PVS CO2 + P O2 / PVS O2 。

[0079] In another specific embodiment, when the gas diluent is a mixture of N2 and O2, C = P N2 / PVS N2 +P O2 / PVS O2 。

[0080] Advantageously, in the context of the present invention, the minimum use temperature T min is selected from 0 °C, -5 °C, -10 °C, -15 °C, -20 °C, -25 °C, -30 °C, -35 °C, -40 °C, -45 °C and -50 °C, and in particular from 0 °C, -5 °C, -10 °C, -15 °C, -20 °C, -25 °C, -30 °C, -35 °C and -40 °C.

[0081] A specific example of the gas mixture for use in the present invention comprises or consists of:

[0082] - 0.001 mol% to 0.9 mol% of i-C3F7CN;

[0083] - 0.1 mol% to 5 mol% of CF3C(O)CF(CF3)2; and

[0084] - 94.1 mol% to 99.899 mol% of the diluent gas as defined above.

[0085] A more specific example of the gas mixture for use in the present invention comprises or consists of:

[0086] - 0.001 mol% to 0.9 mol% of i-C3F7CN;

[0087] - 0.1 mol% to 5 mol% of CF3C(O)CF(CF3)2; and

[0088] - 94.1 mol% to 99.899 mol% of a diluent gas which is a mixture of CO2 and O2 or a mixture of N2 and O2.

[0089] An even more specific example of the gas mixture for use in the present invention comprises or consists of:

[0090] - 0.001 mol% to 0.9 mol% of i-C3F7CN,

[0091] - 0.1 mol% to 5 mol% of CF3C(O)CF(CF3)2;

[0092] - 1 mol% to 25 mol% of O2, and

[0093] - 69.1 mol% to 98.899 mol% of CO2 or N2.

[0094] Thus, the gas mixture implemented in the present invention can be a quaternary mixture (i-C3F7CN + CF3C(O)CF(CF3)2 + O2 + CO2 or i-C3F7CN + CF3C(O)CF(CF3)2 + O2 + N2).

[0095] To increase the total dielectric strength, in a hybrid insulation system, a gas mixture comprising i-C3F7CN, CF3C(O)CF(CF3)2, and a dilution gas as defined above can be used in combination with a solid insulation, in particular a solid insulation with a low dielectric constant, which is applied as a solid insulation layer on those conductive components that are subjected to a corresponding electric field greater than the breakdown field of a medium-voltage or high-voltage device without solid insulation. In other words, a part of the electrical components arranged within the enclosure of a medium-voltage or high-voltage device is covered by a solid insulation layer.

[0096] These solid insulation layers can have different thicknesses, such as those disclosed in international application WO 2014 / 037566 [2]. Alternatively, the insulation layer in a medium-voltage or high-voltage device must have a thickness of less than 1 mm, such as those disclosed in international application WO2017 / 114862 [4].

[0097] According to the present invention, the device can firstly be a gas-insulated electrical transformer, such as a power transformer or a measuring transformer.

[0098] It can also be an overhead or buried gas-insulated line, or a set of busbars for transmitting or distributing electric power.

[0099] There can also be elements for connecting to other devices in a network, such as overhead lines or separating bushings.

[0100] Finally, the device can also be a connector / isolator (also known as a switchgear), such as a circuit breaker, such as a "dead tank, live tank or GIS" type circuit breaker, a "puffer" or "self-exploding" type circuit breaker, a puffer circuit breaker with double-acting arc contacts, a puffer circuit breaker with thermal effect and single-acting arc contacts, a puffer circuit breaker with thermal effect and partially moving contact pins, a switch, an isolator, such as an air-insulated switchgear (AIS) or a gas-insulated switchgear (GIS), a unit combining a switch and a fuse, a grounding switch or a contactor.

[0101] As already explained, the present invention can be applied to low-pressure or high-pressure, medium-voltage or high-voltage devices. In particular, the present invention can be applied to high-voltage devices.

[0102] The present invention also provides the use of a gas medium comprising i-C3F7CN, CF3C(O)CF(CF3)2 and a diluent gas as defined above as a gas for electrical insulation and / or arc quenching in medium-voltage or high-voltage equipment, wherein a part of the electrical component may further be covered with a solid insulating layer as defined below.

[0103] Other features and advantages of the present invention will become more apparent from the additional description given below by way of illustrative and non-limiting examples. Detailed Description

[0104] The present invention is based on the use of a specific gas mixture having a low environmental impact and improved breakdown capability, which combines heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone as defined above with a diluent gas.

[0105] In the present invention, the expressions "diluent gas", "neutral gas" or "buffer gas" are equivalent and may be used interchangeably.

[0106] Advantageously, heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone are present exclusively or almost exclusively in gaseous form in the equipment over the entire operating temperature range of the equipment. Therefore, it is recommended that the partial pressures of heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone in the equipment be selected according to the saturated vapor pressures (PVS) presented by these compounds at the lowest operating temperature of the equipment.

[0107] However, since the equipment is usually filled with gas at ambient temperature (e.g., 20 °C), it is first necessary to verify the inequality (III) as defined above at the lowest operating temperature, and then it is necessary to bring the partial pressures of each gas implemented in the mixture at the lowest operating temperature to the filling pressure of all the gases implemented in the mixture at 20 °C by using the equation of state of each gas.

[0108] Depending on the equipment, the recommended total filling pressure for filling with the gaseous medium varies. However, the pressure is typically a few bar, i.e., several hundred kilopascals (kPa).

[0109] In the context of the present invention, heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone are used together with the diluent gas (or carrier gas or buffer gas) added thereto such that a recommended level of filling pressure can be obtained.

[0110] Preferably, the diluent gas is selected from gases which firstly exhibit a very low boiling temperature, lower than or equal to the lowest operating temperature of the equipment, and secondly have a dielectric strength greater than or equal to that of carbon dioxide under the same test conditions (same equipment, same geometry, same operating parameters,...) as those used for measuring the dielectric strength of carbon dioxide.

[0111] Furthermore, it is preferred that the diluent gas is non-toxic and has a low or zero GWP, such that the dilution of heptafluoroisobutyronitrile by said gas also serves to reduce the environmental impact of the compound, since the GWP of the gas mixture is proportional to the partial pressure of each of its components.

[0112] Furthermore, the diluent gas implemented in the present invention is preferably: carbon dioxide with a GWP equal to 1; nitrogen, oxygen or air with a GWP equal to 0, advantageously dry air; or a mixture thereof. In a specific embodiment, the diluent gas implemented in the present invention comprises or consists of a mixture of CO2 and O2. In another specific embodiment, the diluent gas implemented in the present invention comprises or consists of a mixture of N2 and O2.

[0113] Since the dielectric strength of heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone is greater than that of the gases that may be used as diluent gases, it is desirable to optimize the filling of the equipment with heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone. Therefore, the equipment should be filled with heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone at a partial pressure in the range of 95% to 100%, and more preferably in the range of 98% to 100%, of the pressure corresponding to the saturated vapor pressure presented by these compounds at the lowest operating temperature of the equipment, advantageously at the filling temperature.

[0114] Table III below provides different examples of gas mixtures that can be implemented in the present invention (where the amounts are given in mol%):

[0115] Example 1 2 3 <![CDATA[i-C3F7CN]]> 0.01-0.5 0.05-0.2 0.1±0.03 <![CDATA[CF3C(O)CF(CF3)2]]> 0.5-3 1-2 1.5±0.3 <![CDATA[O2]]> 1-25 1-25 1-25 <![CDATA[CO2 or N2]]> <![CDATA 7 1.5 - 98.49]]> 72.8-97.95 73.07-97.73

[0116] Table III

[0117] A first specific example of a quaternary gas mixture for use in the present invention at a minimum temperature of -30 °C consists of:

[0118] - 0.075 mol% of i-C3F7CN;

[0119] - 1.2 mol% of CF3C(O)CF(CF3)2;

[0120] - 13 mol% of O2, and

[0121] - 85.73 mol% of CO2.

[0122] Such a mixture makes it possible to obtain a reduction on the order of at least 99.95% of the carbon equivalent of pure SF6 (Table IV).

[0123]

[0124]

[0125] Table IV

[0126] A second specific example of the quaternary gas mixture for use in the present invention at a minimum temperature of -25 °C consists of the following:

[0127] - 0.075 mol% of i-C3F7CN;

[0128] - 1.75 mol% of CF3C(O)CF(CF3)2;

[0129] - 13 mol% of O2, and

[0130] - 85.18 mol% of CO2.

[0131] Such a mixture allows for a reduction of at least 99.95% of the carbon equivalent of pure SF6 to be obtained (Table V).

[0132]

[0133] Table V

[0134] From a practical point of view, after creating a vacuum with an oil vacuum pump, a commercial device operating at 5 bar (500 kPa) at -30 °C can be filled via a gas mixer such that the ratio between the pressures of heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone and the pressure of the dilution gas can be controlled, the ratio remaining constant, and during the entire filling process, by using a precision mass flow meter, the ratio of heptafluoroisobutyronitrile is equal to 0.075% and the ratio of heptafluoroisopropyl (trifluoromethyl) ketone is equal to 1.2%. It is preferable to prepare a vacuum (0 kPa - 0.1 kPa) inside the device in advance.

[0135] Furthermore, at the end of its life or after a circuit interruption test, the gaseous medium can be recovered by means of a compressor and a vacuum pump using conventional recovery techniques. Then, heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone can be separated from the dilution gas by using zeolites that are only capable of capturing dilution gases of smaller size; alternatively, a selective separation membrane can be used, which allows the dilution gas to escape and retains heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone since the latter have a larger molar mass than the dilution gas. Naturally, any other options can be envisaged.

[0136] Therefore, the present invention proposes a gas mixture which has a low environmental impact, a very large reduction factor of CO2 equivalent (on the order of 99.95%), is compatible with the lowest operating temperature of the equipment, and has improved dielectric properties with respect to typical gases (such as CO2, air or nitrogen) and approaches the dielectric properties of pure SF6, while improving its breakdown capacity. This gaseous medium can advantageously replace the SF6 currently used in equipment, where the design of the equipment is modified little or not at all: the same production line can be used, while only changing the gaseous medium used for filling. In addition, due to the higher dielectric strength, the equipment implemented in the present invention can have a reduced size and thus reduced costs and LCA.

[0137] In order to obtain dielectric equivalence with SF6 (reaching 100% of the strength of SF6) without degrading its low-temperature performance or increasing the total pressure, the above gas mixture is used in combination with a solid insulation having a low dielectric constant, the solid insulation being applied to those conductive components which are subjected to a corresponding electric field greater than the breakdown field of the system without solid insulation.

[0138] The solid insulation implemented in the context of the present invention can be the solid insulation disclosed in international application WO 2014 / 037566 [2] or in international application WO 2017 / 114862 [4].

[0139] Bibliography

[0140] [1] International application WO 2012 / 080246 A1

[0141] [2] International application WO 2014 / 037566 A1

[0142] [3] International application WO 2015 / 040069 A1

[0143] [4] International application WO 2017 / 114862 A1.

Claims

1. A medium-voltage or high-voltage device, comprising a sealed housing in which electrical components are provided and a gas medium for providing electrical insulation and / or for extinguishing an arc that may occur in the housing, the gas medium comprising heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone mixed with a diluent gas, Among them, In the gas medium, the amount of heptafluoroisobutyronitrile is less than or equal to 0.9 mol%.

2. The medium-voltage or high-voltage device according to claim 1, wherein, In the gas medium, the amount of heptafluoroisopropyl (trifluoromethyl) ketone is less than or equal to 5 mol%.

3. The medium-voltage or high-voltage device according to claim 1 or 2, wherein, In the gas medium, the amount of heptafluoroisopropyl (trifluoromethyl) ketone ranges from 0.1 mol% to 5 mol%.

4. The medium-voltage or high-voltage device according to any one of claims 1 to 3, wherein, In the gas medium, the amount of heptafluoroisobutyronitrile ranges from 0.001 mol% to 0.9 mol%.

5. The medium-voltage or high-voltage device according to any one of claims 1-4, wherein the diluent gas comprises or consists of a mixture of CO2 and O2.

6. The medium-voltage or high-voltage device according to any one of claims 1-4, wherein the diluent gas comprises or consists of a mixture of N2 and O2.

7. The medium-voltage or high-voltage device according to any one of claims 1-6, wherein the gas medium comprises or consists of the following: - 0.001 mol% to 0.9 mol% of heptafluoroisobutyronitrile, - 0.1 mol% to 5 mol% of heptafluoroisopropyl (trifluoromethyl) ketone; - 1 mol% to 25 mol% of O2, and - 69.1 mol% to 98.899 mol% of CO2 or N2.

8. The medium-voltage or high-voltage device according to any one of claims 1-7, wherein a part of the electrical components arranged within the housing of the device is covered by a solid insulation layer.

9. The medium-voltage or high-voltage device according to any one of claims 1-8, wherein the device is a gas-insulated electrical transformer, an overhead or underground gas-insulated line, a set of busbars for power transmission or distribution, an element for connection to other devices in a network, or a connector / isolator.

10. Use of a gas medium as a gas for electrical insulation and / or arc extinguishing in a medium-voltage or high-voltage device, the gas medium comprising heptafluoroisobutyronitrile and heptafluoroisopropyl (trifluoromethyl) ketone mixed with a diluent gas, wherein a part of the electrical components may be further covered by a solid insulation layer, Among them, In the gas medium, the amount of heptafluoroisobutyronitrile is less than or equal to 0.9 mol%.

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