Medium- or high-voltage electrical appliance having a low environmental impact and hybrid insulation

Abstract

The present invention relates to medium- or high-voltage equipment having low environmental impact, including a leaktight enclosure in which there are located electrical components covered with a solid dielectric layer of varying thickness and a gaseous medium for providing electrical insulation and/or for extinguishing electric arcs that are likely to occur in said enclosure, and that comprises heptafluoroisobutyronitrile in a mixture with a dilution gas.

Claims

1. Medium- or high-voltage equipment including a leak tight enclosure in which there are located electrical components covered with a solid dielectric layer of varying thickness and a gaseous medium for providing electrical insulation and/or for extinguishing electric arcs that are likely to occur in said enclosure, the equipment being characterized in that the gaseous medium comprises heptafluoroisobutyronitrile in a mixture with carbon dioxide, wherein, the thickness of said solid dielectric layer being a function of the utilization factor of the electric field, , defined as the ratio of the mean electric field (U/d) divided by the maximum electric field Emax (=U/(Emax*d)), wherein said solid dielectric layer is a thick layer presenting a thickness greater than 1 mm and less than 10 mm for utilization factors lying in the range 0.2 to 0.4 and the material(s) selected for making said thick solid dielectric layer present relative permittivity that is less than or equal to 6, and wherein said solid dielectric layer is a thin layer presenting a thickness lying in the range 60 m to 500 m for utilization factors greater than 0.5 and the material(s) selected for making said thin solid dielectric layer present relative permittivity lying in the range 2 to 4.

2. Equipment according to claim 1 wherein said heptafluoroisobutyronitrile is present in the heptafluoroisobutyronitrile/carbon dioxide mixture at a molar percentage (M.sub.he) that is not less than 80% of the molar percentage M, determined by the formula (II):
M=(P.sub.he/P.sub.mixture)100(II) in which P.sub.mixture represents the total pressure of the mixture at 20 C. in the equipment and P.sub.he represents the partial pressure, expressed in the same units, that is equivalent at 20 C. to the saturated vapor pressure presented by heptafluoroisobutyronitrile as defined above at the minimum utilization temperature of the equipment.

3. Equipment according to claim 1, wherein said heptafluoroisobutyronitrile is present in the heptafluoroisobutyronitrile/carbon dioxide mixture at a molar percentage (M.sub.he) that lies in the range 95% to 130% of the molar percentage M determined by the formula (II):
M=(P.sub.he/P.sub.mixture)100(II) in which P.sub.mixture represents the total pressure of the mixture at 20 C. in the equipment and P.sub.he represents the partial pressure, expressed in the same units, that is equivalent at 20 C. to the saturated vapor pressure presented by heptafluoroisobutyronitrile as defined above at the minimum utilization temperature of the equipment, said equipment being medium-voltage or high-voltage equipment in which having some of the mixture in the liquid state does not reduce insulation.

4. Equipment according to claim 1, wherein said heptafluoroisobutyronitrile is present in the heptafluoroisobutyronitrile/carbon dioxide mixture at a molar percentage (M.sub.he) that lies in the range 95% to 100% of the molar percentage M determined by the formula (II):
M=(P.sub.he/P.sub.mixture)100(II) in which P.sub.mixture represents the total pressure of the mixture at 20 C. in the equipment and P.sub.he represents the partial pressure, expressed in the same units, that is equivalent at 20 C. to the saturated vapor pressure presented by heptafluoroisobutyronitrile as defined above at the minimum utilization temperature of the equipment, said equipment being medium-voltage or high-voltage equipment in which insulation may be affected by the presence of a liquid phase.

5. Equipment according to claim 1, wherein the material(s) selected for making said thick solid dielectric layer present relative permittivity that is less than or equal to 4.

6. Equipment according to claim 1, wherein said material(s) presenting relative permittivity that is less than or equal to 6 are selected from polytetrafluoroethylene, polyimide, polyethylene, polypropylene, polystyrene, polycarbonate, polymethyl methacrylate, polysulfone, polyetherimide, polyether ether ketone, parylene N, Nuflon, silicone, and epoxy resin.

7. Equipment according to claim 1, characterized in that said material(s) presenting relative permittivity lying in the range 2 to 4 are selected from polytetrafluoroethylene, polyimide, polyethylene, polypropylene, polystyrene, polyamide, ethylene-monochlorotrifluoroethylene, parylene N, Nuflon, HALAR, and HALAR C.

8. Equipment according to claim 1, wherein said equipment is a gas-insulated electrical transformer, a gas-insulated line for transporting or distributing electricity, or a connector/disconnector.

9. Equipment according to claim 1, wherein said heptafluoroisobutyronitrile is present in the heptafluoroisobutyronitrile/carbon dioxide mixture at a molar percentage (M.sub.he) that lies in the range 97% to 120 of the molar percentage M determined by the formula (II):
M=(P.sub.he/P.sub.mixture)100(II) in which P.sub.mixture represents the total pressure of the mixture at 20 C. in the equipment and P.sub.he represents the partial pressure, expressed in the same units, that is equivalent at 20 C. to the saturated vapor pressure presented by heptafluoroisobutyronitrile as defined above at the minimum utilization temperature of the equipment, said equipment being medium-voltage or high-voltage equipment in which having some of the mixture in the liquid state does not reduce insulation.

10. Equipment according to claim 1, wherein said heptafluoroisobutyronitrile is present in the heptafluoroisobutyronitrile/carbon dioxide mixture at a molar percentage (M.sub.he) that lies in the range 99% to 110% of the molar percentage M determined by the formula (II):
M=(P.sub.he/P.sub.mixture)100(II) in which P.sub.mixture represents the total pressure of the mixture at 20 C. in the equipment and P.sub.he represents the partial pressure, expressed in the same units, that is equivalent at 20 C. to the saturated vapor pressure presented by heptafluoroisobutyronitrile as defined above at the minimum utilization temperature of the equipment, said equipment being medium-voltage or high-voltage equipment in which having some of the mixture in the liquid state does not reduce insulation.

11. Equipment according to claim 1, wherein said heptafluoroisobutyronitrile is present in the heptafluoroisobutyronitrile/carbon dioxide mixture at a molar percentage (M.sub.he) that lies in the range 98% to 100% of the molar percentage M determined by the formula (II):
M=(P.sub.he/P.sub.mixture)100(II) in which P.sub.mixture represents the total pressure of the mixture at 20 C. in the equipment and P.sub.he represents the partial pressure, expressed in the same units, that is equivalent at 20 C. to the saturated vapor pressure presented by heptafluoroisobutyronitrile as defined above at the minimum utilization temperature of the equipment, said equipment being medium-voltage or high-voltage equipment in which insulation may be affected by the presence of a liquid phase.

12. Equipment according to claim 1, wherein the material(s) selected for making said thick solid dielectric layer present relative permittivity that is less than or equal to 3.

13. Equipment according to claim 1 wherein, the thickness of said solid dielectric layer being a function of the utilization factor of the electric field, , defined as the ratio of the mean electric field (U/d) divided by the maximum electric field Emax (=U/(Emax*d)), said solid dielectric layer is a thin layer presenting a thickness lying in the range 60 m to 100 m for utilization factors greater than 0.5.

14. Equipment according to claim 1, wherein the material(s) selected for making said thin solid dielectric layer present relative permittivity lying in the range between 2.5 to 3.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a diagram used for performing the dielectric calculation.

(2) FIG. 2 shows the profile of the electric field in the solid insulation layer and the gaseous phase for a relative permittivity of 2.9 and 5.3.

(3) FIG. 3 is a diagrammatic view of a portion of equipment of the present invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(4) The invention is based on the use of a hybrid insulation system having low environmental impact, combining heptafluoroisobutyronitrile as described above, and used for the comparative examples given below with at least one dilution gas, also called neutral gas or buffer gas, of the carbon dioxide, air, nitrogen, or oxygen type, or a mixture thereof, and with solid insulation of low dielectric permittivity applied in a layer of small or large thickness on conductive parts that are subjected to an electric field that is greater than the breakdown field of the system without solid insulation.

(5) In the present invention, the expressions dilution gas, neutral gas, or buffer gas are equivalent and may be used interchangeably.

(6) Advantageously, heptafluoroisobutyronitrile is present in the equipment in entirely gaseous form whatever the utilization temperature of the equipment. It is therefore advisable for the pressure of the heptafluoroisobutyronitrile inside the equipment to be selected as a function of the saturated vapor pressure (SVP) presented by heptafluoroisobutyronitrile at the lowest utilization temperature of said equipment.

(7) However, since the equipment is usually filled with gas at ambient temperature, the pressure of the heptafluoroisobutyronitrile to which reference is made in order to fill the equipment is the pressure that corresponds, at the filling temperature, e.g. 20 C., to the SVP presented by heptafluoroisobutyronitrile at the lowest utilization temperature of said equipment.

(8) By way of example, the Table II below gives the saturated vapor pressures, referenced SVP.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., and 40 C., as well as the pressures, referenced P.sub.i-C3F7CN and expressed in hectopascals, which correspond at 20 C. to those saturated vapor pressures.

(9) TABLE-US-00002 TABLE II saturated vapor pressures of i-C.sub.3F.sub.7CN SVP.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
Example of Application and Filling

(10) Depending on the equipment, the recommended pressure for filling in a medium for electric insulation and/or for electric arc extinction varies. However, it is typically of several bars (i.e. a few thousand hectopascals).

(11) Heptafluoroisobutyronitrile is used in a mixture with a dilution gas in order to be able to obtain the recommended filling pressure level.

(12) Thus, for example, equipment designed firstly for being used at a minimum temperature of 30 C., and secondly for being filled to 5 bars (i.e. 5000 hPa), should be filled with 0.368 bars (i.e. 368 hPa) of heptafluoroisobutyronitrile and 4.632 bars (i.e. 4632 hPa) of a dilution gas.

(13) Such equipment is in particular high-voltage equipment of the 145 kV (kilovolt) gas-insulated equipment (GIS) type sold by Alstom under reference B65, designed for an application at 30 C. filled with a dilution gas/i-C.sub.3F.sub.7CN. For this equipment having a minimum application temperature of 30 C., heptafluoroisobutyronitrile should be at a pressure of 0.368 bars absolute measured at 20 C. Buffer gas should be added in order to obtain the final properties of the gas mixture. Since the partial pressure of heptafluoroisobutyronitrile is 0.368 bars absolute measured at 20 C. and the total pressure of the gas is 5 bars absolute, the molar ratio of i-C.sub.3F.sub.7CN is thus 0.368/5, i.e. about 7.4%.

(14) Typically, the dilution gas is selected from among gases that present, firstly, a very low boiling temperature, that is less than or equal to the minimum utilization temperature for the equipment, and secondly, a dielectric strength that is greater than or equal to that of carbon dioxide under test conditions that are identical to those used for measuring the dielectric strength of carbon dioxide (same equipment, same geometrical configuration, same operating parameters, . . . ).

(15) Advantageously, the dilution gas is selected from carbon dioxide having a GWP that is equal to 1, nitrogen, oxygen, air, advantageously dry air, having a GWP that is equal to 0, and mixtures thereof. In particular, the dilution gas is selected from carbon dioxide, nitrogen, air, advantageously dry air, and mixtures thereof.

(16) In order to determine the composition of the gaseous mixture during filling, a molar percentage M is determined for heptafluoroisobutyronitrile at the recommended filling pressure of the equipment, that represents the maximum proportion of heptafluoroisobutyronitrile that the heptafluoroisobutyronitrile/dilution gas mixture should include in order for there not to be any liquid in the enclosure of the equipment. The molar percentage M is given by the formula M=(P.sub.he/P.sub.mixture)100, with P.sub.he representing the pressure, at the filling temperature (typically of the order of 20 C.), equivalent to the saturated vapor pressure SVP of heptafluoroisobutyronitrile at the minimum utilization temperature T.sub.min of the equipment (P.sub.he=(SVP.sub.he293)/(273+T.sub.min)).

(17) Then, the molar percentage M.sub.he for filling is chosen as a function of M. In some circumstances, it is imperative that M.sub.he does not exceed M in order to avoid any presence of liquid.

(18) However, it is sometimes possible, for example at medium voltage or for some high-voltage equipment for which insulation is not affected by the presence of a liquid phase, to have a little liquid at low or very low temperature, and then M.sub.he may reach 110% or even 130% of M. In addition, as heptafluoroisobutyronitrile has a better dielectric strength than neutral gases, it is desirable to optimize filling using heptafluoroisobutyronitrile. Therefore, preferably, M.sub.he is selected so that it is not less than 80% of M, better still not less than 95% of M, better still not less than 98% of M, e.g. equal to 99% of M.

(19) The equipment is filled by means of a gas mixer making it possible to control the ratio between heptafluoroisobutyronitrile and dilution gas, this ratio being held constant and equal to about 7.4% by pressure for the duration of filling by using a precision mass flowmeter.

(20) Dielectric Results: Strength Tests at Power Frequency and Under a Lightning Impact at High Voltage

(21) These tests were carried out on a set of busbars of a B65 shielded substation from ALSTOM having a rated voltage of 145 kV, in accordance with IEC standard 62271-1 relating to high-voltage equipment.

(22) The Table III below shows the results obtained for a gaseous medium consisting in a mixture of heptafluoroisobutyronitrile and of CO.sub.2 (i-C.sub.3F.sub.7CN/CO.sub.2) or of heptafluoroisobutyronitrile and of air (i-C.sub.3F.sub.7CN/Air) in a molar ratio of 7.4/92.6, compared to the results obtained for a gaseous medium containing only CO.sub.2 (CO.sub.2), only air (Air), or only SF.sub.6 for an identical total pressure, i.e. 4 bars relative.

(23) TABLE-US-00003 TABLE III Power Positive Negative frequency lightning lightning Gaseous medium (kV) impact (kVc) impact (kVc) i-C.sub.3F.sub.7CN/CO.sub.2 362 694 653 i-C.sub.3F.sub.7CN/Air 380 695 646 CO.sub.2 176 366 310 Air 211 334 369 SF.sub.6 456 890 889

(24) On the basis of the results in Table III, Tables IV, V, and VI below show the relative dielectric strengths relative to the CO.sub.2 and air buffer gas and relative to SF.sub.6, respectively.

(25) TABLE-US-00004 TABLE IV Power Positive Negative frequency lightning lightning Gaseous medium (kV) impact (kVc) impact (kVc) i-C.sub.3F.sub.7CN/CO.sub.2 2.1 1.9 2.1 CO.sub.2 1 1 1

(26) TABLE-US-00005 TABLE V Power Positive Negative frequency lightning lightning Gaseous medium (kV) impact (kVc) impact (kVc) i-C.sub.3F.sub.7CN/Air 1.8 2.1 1.75 Air 1 1 1

(27) TABLE-US-00006 TABLE VI Power Positive Negative frequency lightning lightning Gaseous medium (kV) impact (kVc) impact (kVc) i-C.sub.3F.sub.7CN/CO.sub.2 0.79 0.78 0.73 i-C.sub.3F.sub.7CN/Air 0.83 0.78 0.73 SF.sub.6 1 1 1

(28) Tables IV and V show that the gaseous media consisting in mixtures of heptafluoroisobutyronitrile and of a gas endowed with dielectric properties such as carbon dioxide or dry air withstand lightning impacts at high voltage much better than said gases when they are used alone.

(29) Table VI shows that the gaseous media consisting in mixtures of heptafluoroisobutyronitrile and of a gas endowed with dielectric properties such as carbon dioxide or dry air withstand lightning impacts at high voltage nearly as well as SF.sub.6 used on its own, making it possible to replace SF.sub.6 in high-voltage equipment.

(30) Thus, for minimum service temperatures of 30 C., i.e. an absolute pressure for heptafluoroisobutyronitrile of 0.368 bars, additional tests performed on the same set of busbars of a B65 shielded substation from ALSTOM having a rated voltage of 145 kV, and in accordance with IEC standard 62271-1 relating to high-voltage equipment show that mixtures of heptafluoroisobutyronitrile and CO.sub.2 achieve dielectric equivalence with SF.sub.6 at 4 bars relative for a mixture at a total pressure of 6 bars, i.e. an i-C.sub.3F.sub.7CN/CO.sub.2 mixture ratio of 0.368/7=5.25%.

(31) Toxicity

(32) Heptafluoroisobutyronitrile presents no specific toxicity for humans and has an LC.sub.50 (lethal concentration, 50%) that is greater than 15000 ppm. In addition, by diluting it by about 5% (5.25% precisely) in CO.sub.2 or in air, toxicity is further reduced in the volume ratio of the mixture in order to reach an LC.sub.50 of the order of 78000 ppm for the mixture and which classifies it in the field of gases considered to be practically non-toxic (toxicity classification 5, according to the Hodge and Sterner toxicity scale).

(33) Flammability

(34) Pure heptafluoroisobutyronitrile, as well as the i-C.sub.3F.sub.7CN/CO.sub.2 and i-C.sub.3F.sub.7CN/Air mixtures are non-flammable.

(35) Environmental Impact/GWP

(36) The global warming potential or GWP of heptafluoroisobutyronitrile is of the order of 2400, i.e. 9.5 times lower than that of SF.sub.6 and more than 3.1 times lower than that of a mixture of SF.sub.6 and nitrogen at 10% by volume of SF.sub.6.

(37) Heptafluoroisobutyronitrile presents a molar mass of 195 grams per mole (g/mol).

(38) The GWP of the gaseous mixture is calculated according to the May 17, 2006 Regulation (EC) No 842/2006 of the European Parliament and of the Council on certain fluorinated greenhouse gases, Part 2 Method of calculating the total global warming potential (GWP) for a preparation. According to that text, the GWP factor of a gaseous mixture is an average weighted by the fraction by weight of each substance multiplied by its GWP factor.

(39) In use in a mixture at 5.25 molar percent in CO.sub.2 (44 g/mol), the fraction by weight of heptafluoroisobutyronitrile is 19.7%, therefore the GWP of the mixture is of the order of 474, which represents a reduction of the order of 98% compared with the carbon equivalent for pure SF.sub.6 (Table VII).

(40) TABLE-US-00007 TABLE VII molar % Weight fraction Gas Molar mass GWP (% P) (% w) i-C.sub.3F.sub.7CN 195 2400 5.3% 19.7% CO.sub.2 44 1 94.7% 80.3% GWP of mixture = 474 Reduction/SF.sub.6 = 97.9%

(41) In use in a mixture at 5.25 molar percent in air (28.8% g/mol), the fraction by weight of heptafluoroisobutyronitrile is 27%, therefore the GWP of the mixture is of the order of 655, which represents a reduction of the order of 97% compared with the carbon equivalent for pure SF.sub.6 (Table VIII).

(42) TABLE-US-00008 TABLE VIII molar % Weight fraction Gas Molar mass GWP (% P) (% w) i-C.sub.3F.sub.7CN 195 2400 5.3% 27.3% Air 28.8 0 94.7% 72.7% GWP of mixture = 655 Reduction/SF.sub.6 = 97.1%
End of Life

(43) At the end of its life or after circuit-breaking tests, the gas can be recovered by means of conventional recovery techniques using a compressor and a vacuum pump. The heptafluoroisobutyronitrile is then separated from the buffer gas using a zeolite capable of trapping only the smaller-sized buffer gas; alternatively, a selective separation membrane allows the buffer gas such as nitrogen, CO.sub.2, or air to escape and retains the heptafluoroisobutyronitrile, which has a greater size and molar mass; any other option may be envisaged.

(44) Association with Solid Insulation

(45) So as to obtain dielectric equivalence with SF.sub.6, without reducing its performance at low temperature or increasing the total amount of pressure, the gaseous 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 an electric field that is greater than the breakdown field of the system without solid insulation.

(46) 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).

(47) Since the thickness of the insulating layer is a function of the utilization factor of the electrical field, , 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.

(48) The calculations presented in FIG. 1 call attention to the reduction of the maximum electric field to which the insulation gas is subjected for mixed insulation combining solid insulation applied in a layer on the parts subjected to high electric fields, typically at the electrodes.

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

(50) In the example presented, hybrid insulation is composed of solid spherical insulation of a thickness of 10 mm in combination with gas insulation of a thickness of 15 mm, the total insulation being of a thickness of 25 mm. Electric field calculation was performed for two different configurations of solid insulation presenting significantly different relative permittivities, typically of 5.3 and 2.9.

(51) In this precise example, the factor by which the electric field on the gaseous phase is reduced is of the order of 15% for solid insulation having dielectric permittivity of 5.3 and of the order of 30% for solid insulation having dielectric permittivity of 2.9 (FIG. 2). In the context of the invention, a material presenting a relative permittivity of the order of 3, or less, is preferred for making the thick layers on the electrodes.

(52) These dielectric calculations have been confirmed by measurements performed on equipment presenting an improvement factor of the order of 20% in dielectric strength (relative to a non-covered electrode) for a thick layer made of epoxy resin presenting relative permittivity of the order of 5 and an improvement factor of the order of 30% (relative to a non-covered electrode) in dielectric strength for a thick layer made of silicone presenting relative permittivity of the order of 3.

(53) For the thin layers made on the electrical parts subjected to weaker electric fields, the materials used present dielectric permittivities of the order of 3 and are applied in the form of thin layers having thickness that is typically of the order of 60 m to 100 m. The results obtained using equipment with thin layer deposits of the order of 60 m to 100 m of Nuflon (relative permittivity of 2.7) or parylene N (relative permittivity of 2.65) deposited on electrodes showing improvement factors for dielectric strength of the order of 8% relative to a non-covered electrode.

(54) In the context of the present invention, the equipment shown in part in a diagram in FIG. 3 has a metal enclosure (3) with an insulator (2) and electrical components including a conductor (1) and electrodes (5). In said equipment, the hybrid insulation is constituted both by gas insulation consisting of a gaseous mixture (4) under pressure of heptafluoroisobutyronitrile and of a dilution gas as defined above and by solid insulation present in the form of a thick dielectric layer (6) or of a thin dielectric layer (7) as defined above.

REFERENCES

(55) [1] European patent application, in the name of Mitsubishi Denki Kabushiki Kaisha, having publication number 0 131 922 on Jan. 23, 1985. [2] U.S. Pat. No. 4,547,316, in the name of Mitsubishi Denki Kabushiki Kaisha, published on Oct. 15, 1985. [3] International application WO 2008/073790, in the name of Honeywell International Inc., published on Jun. 19, 2008. [4] International application WO 2012/080246, in the name of ABB Technology AG., published on Jun. 21, 2012. [5] European patent application, in the name of Mitsubishi Denki Kabushiki Kaisha, having publication number 1 724 802 on Nov. 22, 2006.