Sulfur hexafluoride gas is a colorless, odorless, non-toxic and non-flammable gas at normal temperature and pressure, and its chemical properties are very stable. It is 5 times the density of air, and the critical temperature of sulfur hexafluoride gas is 45.6 ℃.
The electrical insulating properties and arc extinguishing properties of sulfur hexafluoride gas are very strong. The molecular weight of sulfur hexafluoride is five times that of air, so the running speed of sulfur hexafluoride ions in the electric field is much smaller than that of nitrogen and oxygen in the air, and recombination is more likely to occur. Greatly improve the insulation level of gas, about 3 times that of air. Fluorine has the strongest affinity for electrons among all elements, so sulfur hexafluoride has strong electronegativity, great ability to attract electrons, and it is easy to form negative ions, so the arc-extinguishing performance of sulfur hexafluoride gas 100 times that of air. Therefore, sulfur hexafluoride gas is widely used in electrical equipment, and it is the substance with the best insulating arc extinguishing performance that has been found so far.
Pure sulfur hexafluoride is an inert gas, and the discharge in the equipment will cause the decomposition of sulfur hexafluoride gas, and its decomposition products are incompatible with the materials of construction. Sulfur hexafluoride gas is decomposed under the action of the arc, and most of the decomposed products are single atoms of sulfur and fluorine. The metal atoms and water react chemically to produce metal fluorides and HF toxic and corrosive substances.
Through the analysis of the relationship between the pressure and temperature of sulfur hexafluoride, it can be seen that on the right side of the liquefaction curve, the density of the gas remains unchanged when the temperature changes, and only the pressure changes, that is, the dielectric strength and arc extinguishing performance remain unchanged, but when the gas density When the temperature drops to the liquefaction temperature and continues to drop, the gas will liquefy, and its pressure and density will drop rapidly. At this time, the arc-extinguishing insulation performance of the gas will decrease rapidly. Therefore, the sulfur hexafluoride equipment is not allowed to operate at a temperature lower than the liquefaction temperature.
In addition, sulfur hexafluoride is a chemically extremely stable gas, and its lifetime in the atmosphere is about 3200 years. In particular, SF6 has a strong ability to absorb infrared radiation, that is to say, sulfur hexafluoride is a gas with a strong greenhouse effect. times. Combined with the current emissions of sulfur hexafluoride gas into the atmosphere, it is increasing at a rate of 8.7%. It should be pointed out that the greenhouse effect of sulfur hexafluoride has not been discovered in the past, but because the concentration of sulfur hexafluoride gas existing in the earth's atmosphere is very low, it is considered that its impact is small and no serious consideration has been given. The emission into the atmosphere mentioned here does not refer to the natural leakage of GIS and GIT equipment, which is less than 1/1000 of each year and is completely negligible. The leakage volume referred to here mainly refers to the discharge volume of the product during manufacture, installation, on-site debugging and maintenance.
1 Regularly test the micro-water content of sulfur hexafluoride gas
If it is found that its content exceeds the allowable value, effective measures should be taken, including gas purification treatment, replacement of adsorbent and sulfur hexafluoride gas, and equipment disassembly and maintenance. The main sources of moisture in sulfur hexafluoride equipment are: ①Moisture contained in the fresh air of sulfur hexafluoride; ②Moisture entering when the equipment is assembled; ③Moisture released from solid insulating objects; ④Infiltration through seals during operation ⑤Moisture entering during repeated gas supply during operation and testing; ⑥The adsorbent in the gas chamber fails.
There are many methods for testing the micro-water content in sulfur hexafluoride gas. At present, there are three types of instruments in China: electrolytic moisture meter, resistance-capacitance dew point meter and mirror dew point meter. Among them, the mirror dew point meter has the highest accuracy, the resistance-capacitance dew point meter has the widest measurement range, and the resistance-capacitance dew point meter is the most convenient for on-site operation. At present, there are only electrolytic moisture meters in China, which are cheap; mirror dew point meters and resistance-capacitance dew point meters rely on imports, which are expensive, about 30 times that of domestic products, but they are easy and accurate to use.
2 Regularly conduct leak detection of sulfur hexafluoride gas
Sulfur hexafluoride gas leak inspection is divided into qualitative and quantitative inspection. Qualitative inspection is to directly detect the aluminum castings of the sealing points of each joint of the equipment, and the location of each leakage point of the equipment can be found out. Quantitative inspection is to find out the amount of leakage through bandaging detection, bottle hanging method or pressure conversion, so as to obtain the annual leakage rate.
There are two methods of qualitative leak detection: vacuum leak detection and leak detector detection:
① The vacuum leak detection method is to vacuum the equipment to 40Pa, stop the pump for 0.5h, read the number A on the vacuum gauge, stop for 5h, and read the number B, if B - A≤133Pa, it is considered that the seal is good;
② The leak detection of the leak detector is to move the probe of the leak detector along the surface of each connection port of the equipment and the surface of the aluminum casting, and judge the leakage of the gas according to the reading of the leak detector.
Attention should be paid to the leak detection of the leak detector: the moving speed of the probe should be slow to prevent the probe from moving too fast and miss the leak point; the leak detection should not be carried out under the condition of high wind speed to avoid the leakage gas being blown away by the wind and affecting the leak detection; The leak detector should choose a leak detector with high sensitivity and low response speed. Generally, the minimum detection amount of the leak detector is less than 1ppm, and the response speed is less than 5s.
Quantitative leak detection usually adopts methods such as the button cover method, the bottle hanging method, the local bandaging method, and the pressure drop method. The buckle cover method is suitable for high-voltage circuit breakers and small equipment suitable for cover occasions. The bottle hanging method is suitable for occasions where there are double sealing grooves on the flange surface. The partial wrapping method is generally used for assembly units and large products. The pressure drop method is suitable for The leakage rate is determined when the leakage of the equipment compartment is large or during operation. Usually, in the handover acceptance test of sulfur hexafluoride equipment, the leak detection work uses the local wrapping method and the buckle method for leak detection.
3 Pay attention to ventilation
Qualified sulfur hexafluoride gas is non-toxic, but there is a danger of suffocation. Sulfur hexafluoride gas should be stored in a well-ventilated place, and should be protected from sun and moisture. Before entering the low-level area, the staff should check the oxygen content in the area. If the oxygen content is found to be lower than 18%, they cannot enter the area.
4 Wear protective equipment
Sulfur hexafluoride gas sampling operation and treatment of general leakage should be carried out in a ventilated condition wearing a gas mask.