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GIS (Picture source: Internet)
1. Basic characteristics of SF6 gas
SF6 gas is currently the best insulating medium and arc-extinguishing medium, and its insulation and arc-extinguishing properties are much better than insulating oil.
Its molecular structure is: six fluorine atoms at the vertex, sulfur atoms at the center, combined with covalent bonds , a completely symmetrical regular octahedral structure. Molecular structure of
SF6 gas (picture source: Internet)
Physical properties : colorless, tasteless, odorless, non-toxic, non-flammable, the critical temperature is 45.6°C; difficult to dissolve in water, solubility of in some organic solvents relatively high.
Chemical properties : Very stable. Chemical reactions generally do not occur at room temperature or even within the maximum allowable temperature of electrical equipment of 150°C. Above 150°C, a weak chemical reaction occurs with plastic, above 200°C it slowly reacts with copper and aluminum, and at 500°C it begins to decompose itself.
Thermal conductivity : The thermal conductivity is poor, the thermal conductivity is only 2/3 of air, but the surface heat transfer coefficient is 15, which is 2.5 times that of air. Generally speaking, its heat dissipation is better than that of air.
The following table shows some performance parameters of SF6:
Some parameters of SF6 gas (picture source: Internet)
Negativity : Molecules easily absorb free electrons to form negative ions. Because the outermost shell of F has 7 electrons, it is easy to absorb one electron and become a stable 8 electrons.
Arc extinguishing principle : SF6 is electronegative to form negative ions, which combine with positive ions in the arc to rapidly reduce the point ions in the arc, increase the breakdown voltage of the arc, and make the arc recover quickly.
Manufacturing method : S+3F2→SF6+Q (exothermic). The synthetic crude product undergoes a series of purification treatments such as water washing, alkali washing, pyrolysis, and adsorbent suction, and is packed into steel bottles for liquid transportation.
After the GIS equipment is installed, various impurities and moisture must be removed. Its standards are formulated by International Electrotechnical Commission (IEC) and various countries.
The following table shows the electron affinity of certain elements:
The electron affinity of certain elements (picture source: Internet)
2. State parameters of SF6 gas
Due to the large molecular mass of SF6 gas, the formula P=RT is used To calculate the three state parameters of temperature, pressure, and density, the deviation is very large.
But you can calculate the three parameters S (sublimation point), T (melting point), and K (critical point) based on the curve chart below.
SF6 gas curve chart 1 (Picture source: Internet)
SF6 gas liquefaction temperature is related to the density value. As long as the density of SF6 gas remains unchanged, its liquefaction temperature will also remain unchanged. The intersection of each density line and the dividing line is the liquefaction point of SF6 gas, so the liquefaction temperature at this density can be obtained.
T remains unchanged, P↑, liquefaction temperature ↑, so the pressure cannot be too high.
The temperature of SF6 gas is too low and it is easy to liquefy, so in cold areas, GIS must take insulation measures.
SF6 gas state parameter characteristic curve chart (picture source: Internet)
3. Toxicity and other properties of SF6 gas decomposition products
Note: SF6 gas itself is non-toxic. But the molecular weight is large, and it has the effect of suffocating .
The following is the chemical equation of SF6 self-decomposition and arc extinguishing:
Chemical equation of SF6 self-decomposition and arc extinguishing (picture source: Internet)
The main factors affecting the SF6 gas decomposition process are :
The influence of arc energy : increased energy, The content of SF6 gas decomposition products also increases;
The influence of electrode materials: The content of decomposition products depends on the metal evaporation of the electrode material, and copper and silver have less influence;
The influence of moisture: The amount of water content affects the composition of arc decomposition products content has a great impact.
Influence of insulation materials : The ideal material is polytetrafluoroethylene . In fact, more epoxy resins are used, and moisture and oxygen are released during operation.
The influence of oxygen : As shown in the figure below:
The influence of oxygen on the decomposition of SF6 gas (Picture source: Internet)
The properties of SF6 decomposition products under the action of arc are as follows :
SF4 Properties : colorless, pungent smell, easily soluble in Benzene, toxic; stable performance, reacts with water and alkali;
S2F4 Properties : colorless, bromine smell, toxic; reacts with alkali, sodium and water, can vulcanize rubber ;
SF2 Properties : toxic; extremely chemical properties Unstable, can be absorbed by alkali, activated alumina ;
S2F10 Properties : colorless, volatile, toxic; stable performance, insoluble in water, alkali ;
SOF2 Properties : colorless, irritating, toxic; Stable performance, high electrical breakdown stability;
SO2F2 Properties : odorless, toxic; stable performance, easily soluble in toluene and carbon tetrachloride ;
SOF4 Properties : colorless, irritating, Toxic; reacts with water.
Molecular structural formula of sulfur tetrafluoride (Picture source: Internet)
4. Moisture in GIS equipment
4.1 Causes
(1) GIS equipment may be exposed to moisture during the manufacturing, transportation, installation, and maintenance processes;
(2) The insulating parts of GIS equipment contain a small amount of moisture;
(3) The GIS adsorbent itself contains moisture;
(4) SF6 gas contains moisture;
(5) The diameter of water molecules is much smaller than the diameter of SF6 gas molecules, and water molecules can Get in there.
4.2 The unit of water content in SF6 gas and its conversion
(1) Volume concentration unit: the ratio of the volume of gas to the moisture in the gas (PPMV);
(2) Mass concentration unit: 1g of gas contains 1μg of water vapor, which is called 1PPMW;
(3) Conversion of volume ratio and mass ratio in SF6 gas: volume ratio/(146/18) = mass ratio;
(4) As the ambient temperature increases, the moisture in SF6 gas also increases; the ambient humidity increases , the moisture in SF6 gas also increases.
The following figure shows the relationship between moisture content and ambient temperature in different media:
The relationship between moisture content and ambient temperature in different media (Picture source: Internet)
4.3 The harm of moisture to GIS equipment
Moisture can cause Chemical corrosion of equipment. On GIS equipment, in addition to corrosion, condensation will also form on the surface of the insulating parts and adhere to the surface of the insulating parts, causing flashover along the surface. But when the pressure and temperature reach a certain value, the water vapor turns into ice, which greatly reduces the harm to the insulation parts.
4.4 Control of moisture in GIS
Strictly control the moisture content of new SF6 gas;
Improve the quality of GIS equipment sealing materials and strictly comply with the process regulations for installing sealing rings ;
Place adsorbents in GIS equipment ;
GIS equipment should be arranged indoors as much as possible, which can control indoor humidity and temperature, reduce the chance of moisture generation, and avoid the intrusion of dust and other impurities.
SF6 gas equipment (Picture source: Internet)
5. Adsorbents in GIS equipment
The moisture and toxic gases in SF6 gas can be absorbed by adsorbents.
has the following adsorbents: activated carbon, activated alumina, caustic soda, lime, and molecular sieves.
activated carbon: strong adsorption capacity, but poor selectivity, can adsorb SF6, so it cannot be used in GIS;
activated alumina: does not absorb SF6, can absorb most decomposition products, has good selectivity, and is ideal for GIS equipment Adsorbent;
activated carbon can adsorb SF6 gas (picture source: Photo New Vision)
caustic soda: can adsorb most decomposition products, has good adsorption capacity, similar to alumina ;
lime: has good adsorption capacity of SO2F2, but has good adsorption capacity. S2F10 is poor;
molecular sieve: good adsorption capacity for SO2, but not an ideal adsorbent.
Caustic soda has a good ability to adsorb SF6 gas decomposition products (Picture source: Internet)
6. Detection of SF6 gas impurities
The content of impurities and moisture in SF6 gas is very small, but it is very harmful, and it is enough to change SF6 gas performance. Therefore, strict testing is required. The detection methods are as follows:
1, Moisture determination methods : gravimetric method, electrolysis method , dew point method.
2, Determination method of air and carbon tetrafluoride : Measured by gas chromatography.
3, Determination method of mineral oil : Use infrared spectrophotometer to measure.
4, Determination method of free acid : Use standard alkali to absorb the acid in SF6 gas, and then use standard acid to back-titrate the excess alkali by acid-base neutralization titration. Determination method of
5, hydrolyzable fluoride : using two methods including colorimetric spectrophotometric analysis and fluoride ion selective electrode.
SF6 gas bottle (Picture source: Internet)
7. Insulating properties of SF6 gas
1. Breakdown properties of gas under uniform electric field:
free : Under the irradiation of cosmic rays, atoms absorb energy, and the outermost electrons break away from the nucleus. is bound and becomes free electrons, and neutral atoms become positive ions.
composite : Positive ions encounter free electrons and adsorb them to the outermost layer to become neutral atoms, releasing energy.
The gap is broken down : As the voltage increases, electron collisions generate more free electrons, which eventually become plasma in the gap, causing the gap to completely lose its insulating properties and become a conductor.
Self-sustained discharge : Once the gap breaks down, the external dissociated factors can be eliminated, and the dissociated energy itself can be maintained.
2. Breakdown characteristics of gas under extremely uneven electric field (rod - plate level):
Corona discharge : As the voltage increases, the gas around the rod electrode reaches self-sustaining discharge, and corona occurs around it.
The gap is broken down: As the voltage increases, the corona discharge continues to expand until the gap is broken down.
Polarity effect : positive rod - negative plate, high corona starting voltage, low breakdown voltage;
negative rod - positive plate, low corona starting voltage, high breakdown voltage.
Relationship diagram between current and voltage in gas medium (picture source: Internet)
3. Breakdown characteristics of gas under a slightly uneven electric field:
The breakdown field strength of a slightly uneven electric field is between a uniform electric field and an uneven electric field. time, the more uniform the electric field, the higher the breakdown field strength. Therefore, when installing GIS equipment, do not destroy the electric field uniformity.
4. The breakdown characteristics of SF6 gas:
are similar to the breakdown characteristics of air, but SF6 gas has a strong electronegative property and can easily absorb electrons to become negative ions, greatly reducing its dissociation ability and hindering the formation of discharge. The diameter of
SF6 gas molecules is much larger than that of oxygen and nitrogen . The average free path is shortened and the collision dissociation ability is weakened. The movement speed of is small, and recombination is more likely to occur, which reduces the number of charged particles.
Therefore, SF6 is a gas with very high electrical strength, about three times that of air. The electrical strength is equivalent to that of insulating oil at 3 atmospheres. Electrical equipment made of SF6 gas has small insulation distance and small volume.
Comparison chart of breakdown voltage of SF6 gas, air and transformer oil under power frequency voltage (picture source: Internet)
8. Arc extinguishing performance of SF6 gas
If you want to know this answer, you must first know the definition of arc.
The definition of arc: When the temperature is very high, gas molecules are dissociated, and neutral molecules are decomposed into free electrons and positive ions, which move in a certain direction under the action of an electric field, causing gas discharge. ion flow and electron flow together form a discharge current, called an arc.
characteristics: has a very high temperature , 10000-20000°C; has a high density , with an average density of 1000A/cm², and has negative volt-ampere characteristics and negative resistance temperature characteristics.
The basic process of arc extinguishing: the characteristics of the arc are determined by the characteristics of the plasma, and the decisive factor is the temperature of the arc. Therefore, if the arc is extinguished, the temperature of the arc must be lowered and strong cooling means must be used. As the arc temperature decreases, its conductivity also decreases accordingly. This causes the point particles in the gap to recombine into neutral molecules, causing the gas to return to an insulating state.
Arc-extinguishing sulfur hexafluoride circuit breaker (Picture source: Internet)
When the temperature reaches above 5000K, SF6 gas will become ionized. generates free electrons e-, positive ions S+, and negative fluorine ions F- in the air gap , thereby charging the gas, and the arc has a high conductivity . The reason why
SF6 gas arc extinguishing is good is that: during the decomposition process of
SF6 gas, the free electrons are very fast and difficult to recombine with positive ions; SF6 is inactive and easily combines with positive ions to form neutral atoms, which reduces the gas conductivity and restores insulation. .
The following figure is a schematic diagram of the decomposition and ionization of SF6 gas:
A schematic diagram of the decomposition and ionization of SF6 gas (picture source: Internet)
Regarding the heat conduction of the SF6 gas arc , the SF6 gas arc has a lot of energy at the junction of the arc core and the arc flame. With high thermal conductivity, the temperature drops quickly, the arc core temperature is high, and the voltage gradient of the arc is low, so it is not easy to re-ignite.
SF6 gas maintains a high-temperature structure in the center of the arc core. This feature is very beneficial to arc extinguishing, because when the current crosses zero, the volume of the arc is already very small, and the energy stored is also very small, which is conducive to the recovery of insulation and No more rekindles.
Arc voltage gradient comparison chart of sulfur hexafluoride gas and nitrogen (picture source: Internet)
Regarding the radiation of SF6 gas arc , the pressure is proportional to the radiation. The greater the air pressure, the greater the radiation energy.
When the radiation energy passes through the 5mm arc, it is reduced by an order of magnitude under the same pressure.
Pay attention to the following three points:
a. The radiant energy of SF6 gas without metal vapor is an order of magnitude greater than that with metal vapor;
b. The radiant energy of pure SF6 gas is 100 times greater than the lower temperature of the arc outer layer. 99% is absorbed by the arc itself;
c, only pure SF6 gas has a slightly higher radiation energy than nitrogen.
Curve diagram of the relationship between radiation energy and temperature in the arc core of sulfur hexafluoride gas (picture source: Internet)
Regarding the convection heat dissipation of the arc , it refers to the energy taken away by the thermal movement of the gas, which can be caused by the local pressure difference. Causes gas flow. Here we look at three situations: the center of the arc, the position 2mm from the center of the arc, and the position 2mm from the center of the arc.
a, arc center: conduction is the main heat dissipation;
b, 2mm away from the arc center: radiation heat dissipation, convection heat dissipation, and conduction heat dissipation are all available, but radiation is the main heat dissipation.
c, 2mm in the center of the arc: radiation heat dissipation, convection heat dissipation, and conduction heat dissipation are all available, but convection heat dissipation accounts for 66.6%, radiation heat dissipation accounts for 20%, and conduction heat dissipation only accounts for 13.4%.
Diagram of the effect of different heat dissipation methods on arc heat dissipation (picture source: Internet)
Finally, let’s take a look at the excellent arc extinguishing performance of SF6 gas, which is reflected in what points.
a, has excellent thermal properties
SF6 gas arc has high temperature, high electrical conductivity and low thermal conductivity in the arc core area, and the low temperature area also has high thermal conductivity. Therefore, the temperature of the arc decreases quickly, resulting in low arc voltage and low power, that is, the energy of arc extinguishing is small. Since the voltage gradient of the arc in SF6 gas is two times smaller than that in air, the input power of the arc in SF6 gas is less. Under the same heat dissipation conditions as in air, the arc in SF6 gas is easily extinguished, the insulation recovers quickly, and it is not easy to reset. Burn.
b, the insulation performance of SF6 gas recovers quickly
The arc column of SF6 gas can maintain a high-temperature structure when the current is close to zero. This allows the arc column to maintain a small cross-section and the energy of the arc is small. When the current crosses zero, the arc will not cause the current to break. When breaking the inductive small current, it will not produce a high cut-off overvoltage, so that the dielectric insulation strength recovers quickly after the current crosses zero.
Container containing sulfur hexafluoride gas (Picture source: Internet)
c, electronegative
SF6 gas is electronegative, so the moving speed of negative ions SR- is much lower than that of free electrons. When ion collision occurs, it recombines with positive ions The probability of becoming a neutral molecule is higher than that of free electrons. Therefore, the conductivity of the arc can be reduced, making the arc extinguishing performance of SF6 gas better than that of air.
d, the arc time constant of SF6 gas is small. The excellent breaking performance of GIS equipment is inseparable from the small arc time constant of SF6 gas. Tests have shown that the time constant of the arc is only one percent of that of air, and its arc extinguishing ability is 100 times that of air. The above are the results of small current disconnection. In the test of breaking large currents, the breaking capacity of SF6 gas is about 2 to 3 times that of air. When breaking the same current, the pressure required for SR gas arc blowing is only one-half to one-third that of air. After the current crosses zero, the time constant of the medium recovery process is, during the thermal recovery stage, SF6 gas is twice as fast as air.
e, SF6 gas has excellent insulation properties
Under a slightly uneven electric field, the insulation performance of SR gas is three times stronger than that of air.
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