GAS-INSULATED MEDIUM-OR HIGH-VOLTAGE ELECTRICAL APPARATUS INCLUDING HEPTAFLUOROISOBUTYRONITRILE AND TETRAFLUOROMETHANE

Abstract

The present invention relates to medium- or high-voltage equipment comprising a leaktight enclosure in which there are located electrical components and a gas mixture for providing electrical insulation and/or for extinguishing electric arcs that are likely to occur in the enclosure, the gas mixture comprising heptafluoroisobutyronitrile and tetrafluoromethane. Electrical components covered in solid dielectric layers of varying thickness are located inside the leaktight enclosure of the equipment of the invention.

Claims

1. Medium- or high-voltage equipment comprising a leaktight enclosure in which there are located electrical components and a gas mixture for providing electrical insulation and/or for extinguishing electric arcs that are likely to occur in said enclosure, the gas mixture comprising heptafluoroisobutyronitrile and tetrafluoromethane.

2. Equipment according to claim 1, characterized in that said gas mixture further comprises a dilution gas.

3. Equipment according to claim 2, characterized in that said dilution gas is selected from carbon dioxide, nitrogen, oxygen, air, and mixtures thereof.

4. Equipment according to claim 1, characterized in that heptafluoroisobutyronitrile is present in said equipment at a partial pressure selected as a function of the saturated vapor pressure presented by heptafluoroisobutyronitrile at the minimum utilization temperature of said equipment.

5. Equipment according to claim 1, characterized in that heptafluoroisobutyronitrile is present in said equipment at a partial pressure that lies in the range 95% to 100% of the pressure corresponding, at the filling temperature of said equipment, to the saturated vapor pressure presented by heptafluoroisobutyronitrile at the minimum utilization temperature of the equipment.

6. Equipment according to claim 4, characterized in that said minimum utilization temperature of said equipment 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, selected from 0 C., 5 C., 10 C., 15 C., 20 C., 25 C., 30 C., 35 C., and 40 C.

7. Equipment according to claim 1, characterized in that said gas mixture is a ternary mixture consisting of 1 mol % to 20 mol % of i-C.sub.3F.sub.7CN; 1 mol % to 40 mol % of CF.sub.4; and 40 mol % to 98 mol % of dilution gas and in particular CO.sub.2.

8. Equipment according to claim 1, characterized in that electrical components covered in solid dielectric layers of varying thicknesses are located inside said leaktight enclosure.

9. Equipment according to claim 8, characterized in that, the thickness of said solid dielectric layer is a function of the electric field utilization factor, , defined as the ratio of the mean electric field (U/d) divided by the maximum electric field Emax (=U/(Emax*d)), and said solid dielectric layer is a thick layer presenting a thickness that is greater than 1 mm and less than 10 mm for utilization factors lying in the range 0.2 to 0.4.

10. Equipment according to claim 9, characterized in that the material(s) selected for making said thick solid dielectric layer present(s) relative permittivity that is less than or equal to 6, in particular less than or equal to 4 and in particular less than or equal to 3.

11. Equipment according to claim 8, characterized in that the thickness of said solid dielectric layer is a function of the electric field utilization factor, , defined as the ratio of the mean electric field (U/d) divided by the maximum electric field, Emax (=U/(Emax*d)), and said solid dielectric layer is a thin layer presenting a thickness that is less than 1 mm, advantageously less than 500 m, in particular lying in the range 60 m to 100 m for utilization factors greater than 0.5, and in particular greater than 0.6.

12. Equipment according to claim 11, characterized in that the material(s) selected for making said thin solid dielectric layer present(s) relative permittivity lying in the range 2 to 4 and in particular in the range 2.5 to 3.5.

13. Equipment according to claim 1, characterized in that said equipment is a gas-insulated electrical transformer, a gas-insulated line for transporting or distributing electricity, an element for connecting to other pieces of equipment in the network, or a connector/disconnector.

14. A use of a gas mixture comprising heptafluoroisobutyronitrile and tetrafluoromethane as a gas for electrical insulation and/or for electric arc extinction in medium- or high-voltage equipment, having electrical components that are possibly covered with a solid insulating layer of varying thickness.

Description

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0092] The invention is based on the use of a particular gas mixture having a low environmental impact and improved breaking ability combining heptafluoroisobutyronitrile and tetrafluoromethane as defined above, with or without dilution gas.

[0093] In the present invention, the expressions dilution gas, neutral gas, or buffer gas are equivalent and may be used interchangeably.

[0094] Advantageously, heptafluoroisobutyronitrile and tetrafluoromethane are present in the equipment exclusively or almost exclusively in gaseous form over the entire range of utilization temperatures for said equipment. It is therefore advisable for the partial pressure of the heptafluoroisobutyronitrile in the equipment to be selected as a function of the saturated vapor pressure (PVS) presented by this compound at the lowest utilization temperature of said equipment.

[0095] However, since equipment is usually filled with gas at ambient temperature, the pressure to which reference is made in order to fill the equipment with heptafluoroisobutyronitrile is the pressure P.sub.Tfill that corresponds, at the filling temperature, e.g. 20 C., to the PVS presented by said compound at the lowest utilization temperature T.sub.min of said equipment. This correspondence is given, for each compound, by the formula:

P.sub.Tfill=PVS.sub.Tmin293)/T.sub.min

with T.sub.min expressed in kelvins.

[0096] By way of example, the Table II below gives the saturated vapor pressures, referenced PVS.sub.i-C3F7CN and expressed in hectopascals, presented by heptafluoroisobutyronitrile at temperatures of 0 C., 5 C., 10 C., 15 C., 20 C., 25 C., 30 C., 35 C., and 40 C., as well as the pressures, referenced P and expressed in hectopascals, which correspond to those saturated vapor pressures raised to 20 C.

TABLE-US-00003 TABLE III saturated vapor pressures of i-C.sub.3F.sub.7CN PVS.sub.i-C3F7CN P.sub.i-C3F7CN Temperatures (hPa) (hPa) 0 C. 1177 1264 5 C. 968 1058 10 C. 788 877 15 C. 634 720 20 C. 504 583 25 C. 395 466 30 C. 305 368 35 C. 232 286 40 C. 173 218

[0097] Tetrafluoromethane, with a boiling point of the order of 128 C., is always in the gaseaous state at the usual maximum pressure and minimum temperatures for medium- and high-voltage equipment. As a result, the saturated vapor pressures are not given for this compound since they are never reached.

[0098] Thus, for example, equipment designed for being used at a minimum temperature of 30 C. will be filled, at the temperature of 20 C., with a partial pressure of heptafluoroisobutyronitrile that does not exceed 368 hPa at 20 C. if it is desired to maintain this compound in the gaseous state in said equipment over the entire range of utilization temperatures for said equipment.

[0099] Depending on the equipment, the recommended total filling pressure for filling with the gaseous medium varies. However, said pressure is typically of several bars, i.e. several hundreds of kilopascals (kPa).

[0100] Also, although in theory heptafluoroisobutyronitrile and tetrafluoromethane may represent the only components of the gaseous medium, they usually have a dilution gas (or vector gas or buffer gas) added thereto, making it possible to obtain the recommended level of filling pressure.

[0101] Preferably, the dilution gas is selected from gases presenting, firstly, a very low boiling temperature, less than or equal to the minimum utilization temperature of the equipment, and, secondly, a dielectric strength that is greater than or equal to that of carbon dioxide under test conditions (same equipment, same geometrical configuration, same operating parameters, . . . ) that are identical to those used in order to measure the dielectric strength of the carbon dioxide.

[0102] In addition, it is preferred for the dilution gas to be non-toxic and for it to present a GWP that is low, or zero, in such a manner that dilution of the tetrafluoromethane by said gas also has the effect of reducing the environmental impact of said compound since the GWP of a gaseous mixture is proportional to the partial pressures of each of its components.

[0103] Also, the dilution gas is preferably: carbon dioxide having a GWP that is equal to 1; nitrogen, oxygen, or air, advantageously dry air, having a GWP that is equal to 0; or mixtures thereof.

[0104] Since heptafluoroisobutyronitrile and tetrafluoromethane have dielectric strengths that are greater than those of the gases likely to be used as a dilution gas, it is desirable to optimize filling of the equipment with heptafluoroisobutyronitrile and tetrafluoromethane. The equipment should therefore be filled with heptafluoroisobutyronitrile at a partial pressure that advantageously lies in the range 95% to 100% and, more preferably in the range 98% to 100% of the pressure corresponding, at the filling temperature, to the saturated vapor pressure presented by the compound at the minimum utilization temperature of the equipment.

[0105] In other words, heptafluoroisobutyronitrile is preferably present in the gaseous medium at a molar percentage lying in the range 95 mol % to 100 mol % and, more preferably, in the range 98 mol % to 100 mol %, where the molar percentage M is given, for each compound, by the formula:

M=(P.sub.Tfill/P.sub.medium)100, in which: [0106] P.sub.Tfill represents the pressure that corresponds, at the filling temperature and for heptafluoroisobutyronitrile, to the saturated vapor pressure presented by said compound at the minimum utilization temperature of the equipment; and [0107] P.sub.medium represents the total pressure of the gaseous medium (i-C.sub.3F.sub.7CN+CF.sub.4+dilution gas) at the filling temperature.

[0108] A first particular example of a ternary gas mixture for use in the invention at a minimum temperature of 30 C. consists of: [0109] 4.1 mol % of i-C.sub.3F.sub.7CN; [0110] 20 mol % of CF.sub.4; and [0111] 75.9 mol % of CO.sub.2.

[0112] Such a mixture makes it possible to obtain a reduction of the order of 90.2% of the carbon equivalent for pure SF.sub.6 (Table V).

TABLE-US-00004 TABLE IV Mass Molar mol % fraction Gas mass GWP (% P) (w %) i-C.sub.3F.sub.7CN 195 2210 4.10% 13.55% CF.sub.4 88 6500 20.00% 29.84% CO.sub.2 44 1 75.90% 56.61% GWP mixture = 2239 Reduction/SF.sub.6 = 90.2%

[0113] A second particular example of a ternary gas mixture for use in the invention at a minimum temperature of 25 C. consists of: [0114] 6.3 mol % of i-C.sub.3F.sub.7CN; [0115] 20 mol % of CF.sub.4; and [0116] 73.7 mol % of CO.sub.2.

[0117] Such a mixture makes it possible to obtain a reduction of the order of 90.0% of the carbon equivalent for pure SF.sub.6 (Table VI).

TABLE-US-00005 TABLE V Mass Molar mol % fraction Gas mass GWP (% P) (w %) i-C.sub.3F.sub.7CN 195 2210 6.30% 19.71% CF.sub.4 88 6500 20.00% 28.24% CO.sub.2 44 1 73.70% 52.04% GWP mixture = 2272 Reduction/SF.sub.6 = 90.0%

[0118] From a practical point of view, after creating a vacuum by means of an oil vacuum pump, commercial equipment at 5 bar (500 kPa) for use at 30 C. may be filled by means of a gas mixer making it possible to control the ratio between the pressures of the heptafluoroisobutyronitrile and of the tetrafluoromethane, and the pressure of the dilution gas, said ratio being kept constant and equal to 6.3 mol % for heptafluoroisobutyronitrile, and to 20 mol % for tetrafluoromethane throughout filling by using a precision mass flowmeter. The vacuum (0 kPa to 0.1 kPa) is preferably prepared beforehand inside the equipment.

[0119] In addition, it should be observed that future equipment will be fitted with molecular sieves of the anhydrous calcium sulfate (CaSO.sub.4) type, which adsorb the humidity of the gas and therefore reduce the toxicity and the acidity of the gaseous medium after a partial discharge, as caused by potentially toxic molecules, typically HF.

[0120] In addition, at the end of its life or after circuit-breaking tests, the gaseous medium can be recovered by conventional recovery techniques using a compressor and a vacuum pump. The heptafluoroisobutyronitrile and the tetrafluoromethane may then be separated from the dilution gas by using a zeolite capable of trapping only the smaller-sized dilution gas; alternatively, it is possible to use a selective separation membrane that allows the dilution gas to escape and retains the heptafluoroisobutyronitrile and the tetrafluoromethane, since said heptafluoroisobutyronitrile and tetrafluoromethane have greater molar masses than the dilution gas. Naturally, any other option may be envisaged.

[0121] Thus, the present invention proposes gas mixtures having a low environmental impact with reduction factors of the CO.sub.2 equivalent that are very substantial (of the order of 90%), that are compatible with the minimum utilization temperatures of the equipment, and that have dielectric properties that are improved relative to typical gases such as CO.sub.2, air, or nitrogen, and close to those of pure SF.sub.6 while improving its breaking abilities. This gaseous medium may advantageously replace the SF.sub.6 currently used in equipment, with the design of the equipment being modified little or not at all: the same production lines can be used, while changing only the gaseous medium used for filling.

[0122] So as to obtain dielectric equivalence with SF.sub.6, (reaching 100% of the strength of SF.sub.6), without reducing its performance at low temperature or increasing the total amount of pressure, the gas mixture presented above is used in combination with solid insulation having low dielectric permittivity that is applied on those conductive parts that are subjected to respective electric fields that are greater than the breakdown field of the system without solid insulation.

[0123] The solid insulation implemented in the context of the present invention is in the form of a layer of thickness that varies for a given piece of equipment. The implemented insulating layer may present low thickness (thin or fine layer), or high thickness (thick layer).

[0124] Since the thickness of the insulating layer is a function of the electric field factor, , defined as the ratio of the mean electric field (U/d) divided by the maximum electric field Emax (=U/(Emax*d)), the layer is thick for utilization factors close to 0.3, and the layer is thin for utilization factors approaching 0.9.

[0125] This solution therefore makes it possible to reduce the maximum electric field on the gaseous phase and thus to increase the dielectric strength of the mixed total insulation that is made up in series of solid insulation and of gas insulation. This phenomenon of reducing the electric field acting on the gaseous phase is more pronounced when the dielectric permittivity of the solid layer is low.

REFERENCES

[0126] [1] European patent application, in the name of Mitsubishi Denki Kabushiki Kaisha, published under number 0 131 922 on Jan. 23, 1985. [0127] [2] U.S. Pat. No. 4,547,316, in the name of Mitsubishi Denki Kabushiki Kaisha, published on Oct. 15, 1985. [0128] [3] International application WO 2008/073790, in the name of Honeywell International Inc., published on Jun. 19, 2008. [0129] [4] International application WO 2012/080246, in the name of ABB Technology AG., published on Jun. 21, 2012. [0130] [5] European patent application, in the name of Mitsubishi Denki Kabushiki Kaisha, published under number 1 724 802 on Nov. 22, 2006. [0131] [6] International application WO 2014/037566, in the name of Alstom Technology Ltd, published on Mar. 13, 2014.