CN112467599A - Medium-high pressure equipment shell filled with mixed gas of heptafluoroisobutyronitrile and difluoromethane - Google Patents

Abstract

The medium-high voltage equipment shell filled with mixed gas of heptafluoroisobutyronitrile and difluoromethane comprises a shell, wherein the shell is filled with the mixed gas and electrical components, the outer side of the shell is covered with a solid dielectric layer, the mixed gas comprises heptafluoroisobutyronitrile, difluoromethane and rare gas, and the rare gas is carbon dioxide, nitrogen, oxygen, air and a mixture thereof. The beneficial effects are as follows: the insulation capacity is comparable to that of the SF6 equipment, and does not significantly increase equipment size and pressure requirements; the closed equipment is harmless to the environment and human body.

Description

Medium-high pressure equipment shell filled with mixed gas of heptafluoroisobutyronitrile and difluoromethane

Technical Field

The invention relates to the field of medium-pressure or high-pressure equipment, in particular to a medium-pressure or high-pressure equipment shell filled with mixed gas of heptafluoroisobutyronitrile and difluoromethane.

Background

In the field of electrical insulation equipment, gas insulation has been widely used, and Gas Insulated Switchgear (GIS), gas insulated switchgear (c-GIS), and gas insulated power transmission lines (GIL) which are hot in recent years have played an important role in each field. This is due to the unique advantages of gas insulation over other insulation technologies. Firstly, the gas insulation equipment is light in weight and small in size, so that more transportation, installation and space costs can be saved; secondly, the gas insulation equipment hardly ages and deteriorates under normal use conditions and has self-recovery characteristics under the influence of the characteristics of the gas medium, so that the maintenance period and the service life of the equipment can be greatly prolonged. In addition, the gas insulation technology has the advantages of mature and simple process, small influence of the use condition by the outside, and high safety factor of non-combustion and non-explosion, so that the gas insulation equipment has a very wide application range, and is particularly suitable for being applied to urban core population dense areas, power plants, electrified railway lines, industrial and mining metallurgy operation areas, and other places with strict requirements on space, safety and maintenance.

Gas-insulated devices typically use sulfur hexafluoride (SF6) as the gas-insulating medium. SF6 has excellent electrical insulation properties and can meet the insulation requirements in equipment. SF6 is non-toxic and non-combustible, so that the safety of gas insulation equipment is guaranteed, and SF6 is stable in chemical property and compatible with most materials, so that decomposition or deterioration is less likely to occur in the operation process of the equipment, and the operation stability of the equipment is guaranteed. SF6 is an important industrial gas, the annual domestic SF6 yield is already over 5000 tons, the global annual yield is over 20000 tons, and more than 80% of the annual yield is applied to the field of electrical equipment. With the continuous increase of the power demand in China and the enlargement of the scale of the power industry, the demand of insulating gas will also continuously increase.

Although the properties of SF6 are satisfactory for electrical insulation equipment, its environmental impact and hazard has received increasing attention in recent years. SF6 is a strong greenhouse gas, which causes serious harm to the environment, and its Global Warming Potential (GWP) is about 23900 times higher than that of CO2, which means that under the condition of 100 years as a calculation period, the environmental impact of SF6 per unit emission will far exceed CO2, and with the stricter and stricter limit of carbon emission nowadays, the emission of SF6 will be calculated with a correspondingly high weight. More seriously, due to the very stable chemical nature of SF6, which is difficult to degrade after diffusion into the environment, the existence time can reach 3200 years, and the environmental impact and greenhouse effect caused by the SF6 are accumulated continuously.

Under the influence of climate change, in recent years, more and more cooperation is developed internationally to reduce the emission of greenhouse gases, thereby inhibiting global climate change and maintaining the sustainable development of environment and resources. In the kyoto protocol of the united nations climate change framework convention of kyoto, signed by kyoto in 1997, SF6 has been clearly regulated among 6 classes of greenhouse gases and requires developed countries to freeze and reduce the total amount of greenhouse gas emissions. With the signing of Paris 'agreement', China also has increased efforts to reduce carbon emission in recent years, which means that SF6 is subject to more and more restrictions and pressure in industrial fields. For this reason, it is an urgent task to develop a new gas insulation scheme, replacing SF 6.

Since the 80 s of the 20 th century, various researchers have studied substitute gases for SF6 in an attempt to find a suitable solution to reduce or avoid the amount of SF6 and to reduce the greenhouse effect caused by insulating gases. The new gas insulation scheme needs to meet the conditions of high insulation strength, stable physicochemical properties, good material compatibility, no toxicity or slight toxicity, low liquefaction temperature, environmental friendliness and the like, so as to realize the replacement of SF 6. The current research direction can be roughly divided into 3 categories, including SF6 mixed gas, high-pressure buffer gas, and novel environment-friendly gas.

The first solution is to replace pure SF6 with a mixed gas of SF 6. The proposal is that buffer gas, such as air, nitrogen (N2), carbon dioxide (CO2) and the like, is mixed with SF6 gas in a certain proportion, and the mixed gas is used as an insulating medium in gas insulating equipment, thereby reducing the using amount of the SF6 gas. The insulating strength of the mixed gas is related to the ratio of the mixed gas containing SF6 and the type of the buffer gas, and recently, many researchers have studied such a measure in more detail. According to the existing research results, the mixed gas of SF6, N2, SF6 and air has higher insulation strength under the same conditions, when the mixing proportion of SF6 is 50%, the insulation strength of the mixed gas under a uniform electric field can reach 85% of that of pure SF6, the two have good positive synergistic effect, and meanwhile, the cost is lower, and the application feasibility is realized. Some electrical equipment manufacturers in europe have already tried to use SF6 mixed gas as a gas insulating medium for use in switchgear. The scheme is a scheme capable of directly reducing the dosage of SF6, and on one hand, the comprehensive GWP of the mixed gas is reduced; on the other hand, because the usage amount of the SF6 gas is reduced, the cost and the liquefaction temperature of the mixed gas are also reduced, so that the mixed gas can further meet the requirements of gas insulation application in a high-pressure or low-temperature environment. However, in the solutions, SF6 is still used as a main insulating medium, so that the use and dependence on SF6 in a long period cannot be avoided, the influence of SF6 on the environmental greenhouse effect can only be alleviated, and the insulation strength of the mixed gas is reduced, the inflation pressure of the equipment needs to be increased, or the equipment needs to be improved in design, and additional cost in industrial production is caused, so that the solutions are not obviously approved and popularized as long-term solutions.

The second solution is to use high-pressure buffer gas, such as compressed air, compressed N2 or compressed CO2, as an environment-friendly gas insulation medium to increase the insulation strength of the gas by increasing the gas pressure. The gas hardly causes any negative influence on the environment, only CO2 has extremely low GWP value, and the gas is mostly non-artificial gas with low cost. However, due to the nature of the gas, under the same conditions, the insulating strength of the gas is only about 30% of that of SF6 gas, and the arc extinguishing capability is weak. If the high-voltage insulation device needs to be applied to gas insulation equipment, the insulation requirement of the equipment can be met only by extremely high inflation pressure, so that the material strength and the manufacturing process of the equipment are greatly tested, the structural strength and the air tightness of the equipment must be greatly improved, the electrode in the existing high-voltage equipment is changed, and the scheme additionally causes use problems and cost problems. On the other hand, the dielectric strength of the buffer gas increases with the increase of the gas pressure, but when the gas pressure increases to a certain degree, the dielectric strength of the gas tends to be saturated and does not continue to increase. Therefore, at present, only partial low-voltage equipment at home and abroad adopts air insulation or N2 insulation, and the solution is difficult to be applied to high-voltage equipment.

The third category is to find a new environment-friendly insulating gas, which has similar insulating properties and other physical and chemical properties to SF6, thereby realizing replacement of SF 6. SF6 belongs to inorganic fluorinated gases, and its molecular structure is a regular octahedral structure with 6 fluorine atoms (F) around the central sulfur atom (S). The fluorine element belongs to halogen elements, and the electron layer at the periphery of the atom is occupied by 7 electrons, and 1 electron is lacked to form a stable structure, so the fluorine element has strong electron absorption tendency. In the molecule of SF6, the F atom and the S atom form a stable covalent chemical bond by sharing electrons, but the F atom still has a certain tendency to absorb electrons and shows a tendency to adsorb electrons in the whole molecule, so that the molecule has a good insulating property. Although the gas characteristics expressed by the gas macro-elements cannot accurately represent the insulation strength of the gas, even though there is a counter example, the gas characteristics are emphasized by researchers, and the research on the substitute gas is focused on the halogenated gas. In 1997, a plurality of potential substitute gases were introduced in the insulation and arc extinguishing characteristic research report of SF6 substitute gas written by the national institute of standards, and part of the fluoro-organic gases were listed as the important research objects, and the relative breakdown voltage of each gas under a DC uniform electric field compared with SF6 is shown in Table 1-1. The reported results show that most of the fluoro gases have better adsorption effect on free electrons, which is related to the addition of fluorine element, but not all fluoro organic gases have good insulation performance, the judgment of gas insulation characteristics by only the elements forming the gases is not comprehensive and accurate, and specific analysis needs to be carried out on the characteristics of different gases. Because the physical and chemical properties of C-C4F8 gas are close to those of SF6 in all aspects, the cost is low, and the GWP value is lower than that of SF6, the report also particularly proposes that C-C4F8 and mixed gas thereof can be listed as long-term research objects, and the gas is also widely paid attention and researched by researchers in recent years.

In addition to the fact that C-C4F8 is receiving much attention, in recent years, the halogenated organic gas CF3I containing fluorine element (F) and iodine element (I) is also receiving attention from researchers because of its extremely low GWP value and good insulating properties. Meanwhile, g3, which is an electrically insulating mixed gas mainly composed of a fluoronitrile gas having a trade name of Novec 4710, C4F7N, which is introduced by alstonia corporation and 3M corporation, usa, and an electrically insulating mixed gas mainly composed of a fluoroketone compound, such as C5F10O and C6F12O, which is introduced by ABB corporation, have respective characteristics.

TABLE 1-1 relative breakdown voltage of some fluorinated organic gases under DC uniform electric field

Table.1-1 Relative DC breakdown voltages of some fluorination gases

Alston France (ALSTOM) and 3M have also jointly developed substitute gas for SF6, which is a gas with a trade name of Novec 4710, namely an organic compound containing 4C atoms and 7F atoms, selected from a plurality of organic fluoro gases, which is also used as a refrigerant substitute material, and has a chemical formula of C4F7N, wherein a fluorine atom is substituted by a cyano group (-C.ident.N) on the basis of fluoro-hydrocarbon organic gases to form fluoro-nitrile gas, and the cyano group containing carbon-nitrogen triple bond has a unique chemical bond structure, so that the C4F7N gas has good insulating property which can reach about 2 times of SF 6. The C4F7N gas has a relative molecular mass of 195.0 and also has a high liquefaction temperature, the liquefaction temperature is-4.7 ℃, SF6 cannot be replaced by a single gas, and the gas needs to be used together with other gases with lower liquefaction temperature and buffer gas to form a mixed gas.

Disclosure of Invention

The invention aims to solve the problems and designs a medium-high pressure equipment shell filled with mixed gas of heptafluoroisobutyronitrile and difluoromethane. The specific design scheme is as follows:

the utility model provides a pack with middle and high voltage equipment shell of heptafluoroisobutyronitrile and difluoromethane mist, includes casing, sealed lid, equalizer ring, flange ring, support frame and fixed station, the casing is filled with mist, conducting rod, insulator, electrical apparatus part, the outside of casing covers has solid-state dielectric layer, mist includes heptafluoroisobutyronitrile, difluoromethane, noble gas, the noble gas is carbon dioxide, nitrogen gas, oxygen, air and mixture thereof.

The partial pressure of heptafluoroisobutyronitrile present in the apparatus is selected such that the partial pressure of heptafluoroisobutyronitrile present in the apparatus at the minimum use temperature of the apparatus is between 95% and 100% of the partial pressure of heptafluoroisobutyronitrile present at the minimum use temperature of the apparatus at the filling temperature of the apparatus, said minimum temperature being-50 ℃ to 0 ℃.

The mole percentages of the heptafluoroisobutyronitrile, the difluoromethane and the rare gas are respectively 1 mol% to 20 mol% of heptafluoroisobutyronitrile, 1 mol% to 40 mol% of difluoromethane and 40 mol% to 98 mol% of the rare gas.

The thickness of the solid dielectric layer is a function of an electric field utilization factor, nThe factor n is defined as the average electric field strength (U/d) divided by the maximum electric field strength E_max(η＝U/(E_maxThe ratio obtained by multiplying d is,

when the utilization factor is between 0.2 and 0.4, the solid dielectric layer is a thick layer with a thickness greater than 1mm and less than 10mm,

when the utilization factor is greater than 0.5, in particular greater than 0.6, the solid dielectric layer is less than 1mm thick, the relative dielectric constant of the material selected for preparing the thick solid dielectric layer being between 2 and 6.

The electrical components comprise a gas insulated metal enclosed switchgear (GIS), a gas insulated switchgear (c-GIS), a gas insulated transmission line (GIL) and a Gas Insulated Transformer (GIT). The gas insulation is achieved by a gas mixture comprising heptafluoroisobutyronitrile and difluoromethane.

Medium voltage refers to alternating current voltages greater than 1000 volts and not more than 52000 volts or direct current voltages greater than 1500 volts and not more than 75000 volts, and high voltage refers to alternating current voltages strictly greater than 52000 volts and direct current voltages strictly greater than 75000 volts. Heptafluoroisobutyronitrile, a compound of formula C3F7CN, exhibits the following characteristics: boiling point of-4.7 ℃ at 1013 hectopascal (hPa) (boiling point according to ASTM D1120-94 "Standard test method for boiling points of Engine coolants; molar mass 195 g.mol-1; GWP 2210; ozone depletion potential 0.

The relative dielectric strength values of heptafluoroisobutyronitrile (normalized with respect to the gas SF6 to be replaced, and the value of N2 given as a comparison, were measured between two steel electrodes of 25mm diameter and 1mm apart at atmospheric pressure and direct voltage.

SF6	N2	C3F7CN
			1.0	0.35-0.4	2.6

Chemical formula is CH₂F₂Difluoromethane exhibits the following properties:

the boiling point is-51.7 ℃ under 1013 hectopascal;

the molar mass is 52g.mol < -1 >;

GWP is 550;

(iv) the ozone depletion potential is 0.

Chemical formula is CH₂F₂The relative dielectric strength values of difluoromethane, as above, are:

SF6	CH2F2
		1.0	0.3-0.4

the above gases thus exhibit a GWP much lower than that of SF6, have electrical insulation properties and arc extinguishing properties, can be mixed with a diluent gas, and are suitable for use as electrical insulation and/or arc extinguishing gas in medium and or high voltage equipment in place of SF 6.

The invention provides gaseous insulation with little environmental impact (GWP vs. SF)₆Low in terms of temperature) and compatible with the minimum use temperature of the equipment, possessing dielectric, arc-extinguishing and heat-dissipating characteristics such as those of carbon dioxide, air orThe buffer gas of nitrogen is good.

The buffer gas meets the following four standards:

the boiling point is low, and the normal atmospheric pressure is lower than-40 ℃; dielectric strength of greater than or equal to CO under the same measurement conditions₂The influence on human bodies and the environment is small, and the toxicity is low or no harm is caused; (ii) a GWP lower than that of a mixture of heptafluoroisobutyronitrile and difluoromethane, ODP being 0; the electrical components comprise a gas insulated metal enclosed switchgear (GIS), a gas insulated switchgear (c-GIS), a gas insulated transmission line (GIL) and a Gas Insulated Transformer (GIT).

The equipment for preparing the mixed gas consisting of the heptafluoroisobutyronitrile, the difluoromethane and the buffer gas, which is obtained by the technical scheme of the invention, has the beneficial effects that:

the insulation capacity is comparable to that of the SF6 equipment, and does not significantly increase equipment size and pressure requirements;

the closed equipment is harmless to the environment and human body;

drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a view showing an apparatus case using a mixed gas composed of heptafluoroisobutyronitrile, difluoromethane and a buffer gas, in which heptafluoroisobutyronitrile, difluoromethane and the buffer gas are filled as an insulating gas, and the case is composed of a 1-grading ring, a 2-flange ring, a 3-conductive rod, a 4-bushing, a 5-insulator, a 6-conductive rod terminal, a 7-sealing cover, an 8-aluminum case, a 9-support frame, a 10 and a fixing table.

Detailed Description

The invention is based on the use of a specific gas mixture comprising heptafluoroisobutyronitrile and difluoromethane as defined above, mixed with an optional buffer gas, which has a low environmental impact and improved breakdown characteristics. The structure of the shell is schematically shown in figure 1.

Both heptafluoroisobutyronitrile and difluoromethane are present in the apparatus completely or almost completely in the gaseous state over the entire temperature range of use of the apparatus. For the problem of liquefaction temperature, the total pressure of the mixed gases is equal to the sum of the individual pressures of each of the gaseous components, according to the law of dalton partial pressure under ideal gas conditions. The partial pressure of a specific gas in the gas mixture, which is equal to the pressure of the gas when it fills the same volume of container independently at the same temperature, can be expressed as the following formula:

P＝∑P_i (1-2)

wherein P represents the total gas pressure of the mixed gas, P_iRepresenting the partial pressure of each gas component. Since the ideal gas has the same volume of the same substance under the same temperature and pressure conditions, for example, the standard molar volume of the gas, the volume of any one mole (mol) of the gas is 22.4L under the conditions of 0 ℃ and one standard atmospheric pressure, and the total volume of the gas under the same pressure and temperature is the sum of the volumes of the gases under the same conditions, and is not changed by mixing of the gases, it is considered that the pressure of each gas in the mixed gas can be distributed according to the mixing ratio of each gas, which is equivalent to the total pressure of the mixed gas. If so

The total gas pressure P can also be used by taking the ratio of the amount of each gas in the mixed gas, i.e. the content of the gas in the mixed gas

Expressed as:

similar partial pressures Pi of the respective gases can also be expressed as:

the saturation vapor pressure of a gas varies with temperature and pressure, and can be determined by the gas state parameters and Van Der Waals equation (Van Der Waals equation) for the saturation vapor pressure of a specific gas under different pressure and temperature conditions, the state equation being as in formulas 1 to 5:

where P is gas pressure, n is the amount of gaseous species, V is volume, R is the universal gas constant, T is temperature, and a and b are both Van der Waals constants related to the critical gas pressure and critical temperature. When the gas is in the critical state of liquefaction, the relationship between the gas pressure P and the temperature T still satisfies the equation, and it can be seen that for a specific gas, the lower the gas pressure, the lower the liquefaction temperature, which means that for the mixed gas, when the mixing ratio of a certain gas component is reduced, the gas pressure Pi is linearly changed along with the change of the ratio, and the liquefaction temperature is reduced along with the relevant parameters of the gas state equation.

For the mixed gas, the advantages are that the liquefaction temperature of the gas can be reduced by reducing the partial pressure of the gas through reasonable component proportion collocation, the cost of the mixed gas is reduced by reducing the using amount of the gas, and the defects of partial insulating gas are avoided. However, the mixed gas has its own disadvantages in practical application, the mixing ratio of the insulating gas is reduced, the insulating strength will be reduced accordingly, and the reduction range is not linear to the change of the gas content, but related to the gas type, the gas content ratio, the gas pressure, the electric field condition, and other factors.

The dielectric strength of the mixed gas is affected not only by the dielectric properties of the components themselves but also by the interaction between the components, and thus the properties show different and different changing trends. The variation trend can be divided into three categories, one is that the variation of the insulation strength and the mixing ratio of the insulating gas are basically in a linear variation relationship, a curve of the variation of the gas breakdown voltage due to the gas mixing ratio in an image approaches to a straight line, which indicates that the insulating gas is influenced by other gas components in the mixed gas, and the insulation strength of the mixed gas can be calculated according to the insulation strength of each pure gas and the proportion of the pure gas in the mixed gas. This is the theoretical situation, and most of the gas components do not exhibit a linear change in dielectric strength after mixing. The second trend is a positive synergistic effect, and the mixing ratio of the main component of the insulating gas is reduced, and the reduction range of the insulating strength of the insulating gas is smaller than the reduction range of the gas content, that is, the gas content is reduced, but the insulating strength is not reduced too much, which means that the components of the mixed gas exhibit a positive synergistic effect, often exhibiting a downward opening dome shape in an image in which the gas breakdown voltage is changed by the gas mixing ratio. Such a gas component having a positive synergistic effect is a mixed component most suitable for application to an electrical insulating apparatus, and can significantly reduce the liquefaction temperature of the insulating gas, the gas cost, and at the same time, does not cause a very significant decrease in the insulating properties of the gas. The third trend is a negative synergistic effect, also called penning effect in a part of mixed gases, which means that after two or more gases are mixed, the breakdown voltage of the mixed gases is lower than that of the respective gases, so that the gases are more easily broken down to form discharge, and the curve of the mixed gases in the image of the change of the gas breakdown voltage due to the gas mixing ratio is an arc with an upward opening. The origin of the penning effect, and the most prominent example, is the use of mixed gases of mercury vapor (Hg) and argon (Ar) in neon tubes. Because the argon is excited in the discharging process and can jump to a metastable state, and the metastable state excitation energy of the argon is slightly higher than the ionization energy of mercury vapor, the argon is equivalent to a catalyst of the mercury vapor, and when the excited metastable state argon collides with mercury vapor molecules, the metastable state excitation energy of the argon can be transferred to the mercury vapor molecules, so that the mercury vapor is ionized by utilizing the accumulated energy, the whole mixed gas has the characteristic of being easier to be punctured and ionized, and macroscopically shows that the mixed gas has lower breakdown voltage. This effect can be applied to the plasma related art, can more easily realize the breakdown of plasma, and forms a stable discharge process, which is contrary to the requirement of gas insulation, so the tendency of the breakdown characteristic of such mixed gas is not suitable for the application in the field of gas insulation equipment. The penning effect has very strict limits on the types of the gases and the mixing ratio of the gases, and needs the microscopic characteristics of the two gases and the energy change between different states to have an extremely accurate relationship, one component of the mixed gases has higher excitation energy and ionization energy than the other components, the excited state has a higher occurrence probability, and the resonance intensity is high enough to have enough action time with molecules of the other gases. Since the penning effect is the result of the interaction of various gas molecules from the microscopic view, it is very important to avoid the generation of the process of exciting the ionization promotion by reasonably collocating and combining the mixed insulating gas based on the analysis of the microscopic energy of the gas molecules of each component when the components are selected.

The saturated vapor pressures of heptafluoroisobutyronitrile at temperatures from 0 ℃ to-40 ℃ and the pressures corresponding to these saturated vapor pressures rising to 20 ℃ are given in Table II below

Temperature of	PVSi-C3F7CN(hPa)	Pi-C3F7CN(hPa)
			0℃	1177	1264
-5℃	968	1058
			-10℃	788	877
-15℃	634	720
			-20℃	504	583
-25℃	395	466
			-30℃	305	368
-35℃	232	286
			-40℃	173	218

Difluoromethane, which has a boiling point around-51.6 c, is normally always in the gaseous state at the maximum pressure and minimum temperature of medium-high pressure equipment. Therefore, the saturated vapor pressure of difluoromethane is not given here because it does not reach.

The composition of the first embodiment of the ternary gas mixture used in the present invention at a temperature of-30 ℃ is:

4 mol% C3F7CN, 20 mol% CH2F2 and 76 mol% CO 2. Such a gas mixture may reduce the carbon equivalent of pure SF6 by about 90%.

A second specific embodiment of a ternary gas mixture for use in the present invention has a composition at a temperature of 25 c of:

6.3 mol% C3F7CN, 20 mol% CH2F2 and 74 mol% C02.

Such a gas mixture may reduce the 89.2% carbon equivalent of pure SF 6.

The scheme can effectively replace the sulfur hexafluoride used in the equipment at present, can be produced by using the same production line, only needs to change the components of the insulating gas, well reduces the cost and reduces the influence on the environment and the human body.

Claims (5)

1. The medium-high voltage equipment shell filled with mixed gas of heptafluoroisobutyronitrile and difluoromethane comprises a shell, wherein the shell is filled with the mixed gas and electrical components, and the outer side of the shell is covered with a solid dielectric layer.

2. The medium-high pressure equipment enclosure filled with a mixed gas of heptafluoroisobutyronitrile and difluoromethane according to claim 1, wherein said mixed gas is capable of fully exerting its insulating ability, whereby the partial pressure of heptafluoroisobutyronitrile present in said equipment is selected from the saturated vapor pressure of heptafluoroisobutyronitrile present at the lowest use temperature of said equipment, and the partial pressure of heptafluoroisobutyronitrile present in said equipment is between 95% and 100% of the saturated vapor pressure of heptafluoroisobutyronitrile present at the lowest use temperature of said equipment, said lowest temperature being-50 ℃ to 0 ℃.

3. The medium-high pressure equipment enclosure filled with a mixed gas of heptafluoroisobutyronitrile and difluoromethane according to claim 1, wherein the molar percentages of heptafluoroisobutyronitrile, difluoromethane, and rare gas are 1 mol% to 20 mol% of heptafluoroisobutyronitrile, 1 mol% to 40 mol% of difluoromethane, and 40 mol% to 98 mol% of rare gas, respectively.

4. The medium-high voltage equipment enclosure filled with a mixed gas of heptafluoroisobutyronitrile and difluoromethane according to claim 1, wherein the thickness of the solid dielectric layer is a function of an electric field utilization factor n, which is defined as an average electric field strength (U/d) divided by a maximum electric field strength E_max(η＝U/(E_maxX d) is obtained as a ratio of,

when the utilization factor is between 0.2 and 0.4, the solid dielectric layer is a thick layer with a thickness greater than 1mm and less than 10mm,

when the utilization factor is greater than 0.5, in particular greater than 0.6, the solid dielectric layer is less than 1mm thick,

the relative dielectric constant of the material selected for making the thick solid dielectric layer is between 2-6.

5. The medium and high voltage equipment enclosure filled with a mixed gas of heptafluoroisobutyronitrile and difluoromethane according to claim 1, wherein said electrical parts comprise gas insulated metal enclosed switchgear (GIS), gas insulated switchgear (c-GIS), gas insulated transmission line (GIL), Gas Insulated Transformer (GIT).

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