Gas-insulated medium or high-voltage electrical apparatus including carbon dioxide, oxygen, and heptafluoro-isobutyronitrile

1. Medium- or high-voltage equipment including 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, wherein the gas mixture consists of heptafluoroisobutyronitrile, carbon dioxide, and oxygen, the oxygen being present in said gaseous medium at a molar percentage lying in the range 1% to 25%.

2. Equipment according to claim 1 , wherein the oxygen is present in said gas mixture at a molar percentage lying in the range 2% to 15% and, in particular, in the range 2% to 10%.

3. Equipment according to claim 1 , wherein said heptafluoroisobutyronitrile is present in said gas mixture at a molar percentage (M he ) that is not less than 80% of the molar percentage M, determined by formula (II):

M =( P he /P mixture )×100  (II)

in which P mixture represents the total pressure of the mixture at 20° C. in the equipment and P he represents the partial pressure, expressed in the same units, that is equivalent at 20° C. to the saturated vapor pressure presented by heptafluoro-isobutyronitrile as defined above at the minimum utilization temperature of the equipment.

4. Equipment according to claim 1 , wherein said heptafluoroisobutyronitrile is present in said gas mixture at a molar percentage (M he ) that lies in the range 95% to 130%, better still in the range 97% to 120%, ideally in the range 99% to 110% of the molar percentage M, determined by formula (II):

M =( P he /P mixture )×100  (II)

in which P mixture represents the total pressure of the mixture at 20° C. in the equipment and P 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.

5. Equipment according to claim 1 , wherein said heptafluoroisobutyronitrile is present in said gas mixture at a molar percentage (M he ) that lies in the range 95% to 100%, in particular in the range 98% to 100% of the molar percentage M, determined by formula (II):

M =( P he /P mixture )×100  (II)

in which P mixture represents the total pressure of the mixture at 20° C. in the equipment and P 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.

6. Equipment according to claim 1 , wherein electrical components covered in a solid dielectric layer of varying thickness are located inside said leaktight enclosure.

7. Equipment according to claim 6 , wherein, the thickness of said solid dielectric layer is a function of the utilization factor of the electric field, η, defined as the ratio of the mean electric field (U/d) divided by the maximum electric field Emax (η==U/(Emax*d)), and 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.

8. Equipment according to claim 7 , wherein the material(s) selected for making said thick solid dielectric layer present(s) relative permittivity that is less than or equal to 6.

9. Equipment according to claim 7 , wherein the material(s) selected for making said thick solid dielectric layer present(s) relative permittivity that is less than or equal to 4 and in particular less than or equal to 3.

10. Equipment according to claim 8 , wherein said material(s) are selected from: polytetrafluoroethylene, polyimide, polyethylene, polypropylene, polystyrene, polycarbonate, polymethyl methacrylate, polysulfone, polyetherimide, polyether ether ketone, parylene N™, Nuflon™, silicone, and epoxy resin.

11. Equipment according to claim 6 , wherein the thickness of said solid dielectric layer is a function of the utilization factor of the electric field, η, defined as the ratio of the mean electric field (U/d) divided by the maximum electric field, Emax (η=U/(Emax*d)), and said solid dielectric layer is a thin layer presenting a thickness less than 1 mm, advantageously less than 500 μm, in particular lying in the range 60 to 100 μm for utilization factors greater than 0.5, and in particular greater than 0.6.

12. Equipment according to claim 11 , wherein the material(s) selected for making said thin solid dielectric layer present 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 11 , wherein said material(s) is/are selected from polytetrafluoroethylene, polyimide, polyethylene, polypropylene, polystyrene, polyamide, ethylene-monochlorotrifluoroethylene, parylene N™, Nuflon™, HALAR™, and HALAR C™.

14. Equipment according to claim 1 , wherein 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.

15. A method for electrical insulation and/or for electric arc extinction comprising a step of utilizing equipment according to claim 6 .