SF6 gas is a colorless, odorless, non-toxic, and non flammable gas at room 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 insulation 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 speed of sulfur hexafluoride ions in an electric field is much slower than that of nitrogen and oxygen in air, making them more prone to recombination. Greatly improve the insulation level of gas, about three times that of air. Fluorine has the strongest affinity for electrons among all elements, so sulfur hexafluoride has strong electronegativity, strong ability to attract electrons, and is easy to form negative ions. Therefore, the arc extinguishing performance of sulfur hexafluoride gas is 100 times that of air. Therefore, sulfur hexafluoride gas is widely used in electrical equipment.
Pure sulfur hexafluoride is an inert gas that, when emitted from equipment, can cause the decomposition of sulfur hexafluoride gas, and its decomposition products are incompatible with building materials. Sulfur hexafluoride gas decomposes under the action of an electric arc, and the decomposition products are mostly single atom sulfur and fluorine. Metal atoms react chemically with water to produce toxic and corrosive substances such as metal fluorides and HF.
By analyzing the relationship between 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 constant with temperature changes, and only the pressure changes, that is, the dielectric strength and arc extinguishing performance remain unchanged. However, when the gas density temperature drops to the liquefaction temperature and continues to decrease, the gas will liquefy, and its pressure and density will rapidly decrease. At this point, the arc extinguishing insulation performance of the gas will rapidly decrease. Therefore, sulfur hexafluoride equipment is not allowed to operate at temperatures lower than the liquefaction temperature.
In addition, sulfur hexafluoride is an extremely chemically stable gas with a lifespan of approximately 3200 years in the atmosphere. Especially SF6 has a strong ability to absorb infrared radiation, which means that sulfur hexafluoride is a gas with a strong greenhouse effect. Next time. Combined with the current emissions of sulfur hexafluoride gas into the atmosphere, it is growing 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 due to the low concentration of sulfur hexafluoride gas in the Earth's atmosphere, its impact is considered to be minimal and has not been seriously considered. The emissions into the atmosphere mentioned here do not refer to natural leaks from GIS and GIT equipment, which are less than 1/1000 per year and can be completely ignored. The leakage amount referred to here mainly refers to the emissions during the manufacturing, installation, on-site debugging, and maintenance processes of the product.
1. Regularly check the trace moisture content of sulfur hexafluoride gas
If its content exceeds the allowable value, effective measures should be taken, including gas purification treatment, replacement of adsorbents and sulfur hexafluoride gas, equipment disassembly and maintenance, etc. The main sources of moisture in sulfur hexafluoride equipment are: ① moisture contained in fresh air of sulfur hexafluoride; ② Water ingress during equipment assembly; ③ Moisture released from solid insulating objects; ④ During operation, water seeps in through seals. During repeated gas supply during operation and testing, moisture enters The gas chamber adsorbent has failed.
There are many methods for detecting trace moisture content in sulfur hexafluoride gas. At present, there are three main 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 is only an electrolytic moisture meter in China, which is cheap in price; Mirror dew point meters and resistive capacitive dew point meters rely on imports and are expensive, about 30 times more expensive than domestic products, but they are easy to use and accurate.
2. Regularly conduct leak detection on sulfur hexafluoride gas
The detection of sulfur hexafluoride gas leakage is divided into qualitative detection and quantitative detection. Qualitative inspection is the direct detection of aluminum castings at the sealing points of each joint of the equipment, which can identify the location of each leakage point of the equipment. Quantitative testing is the process of determining the annual leakage rate by bandaging, bottle hanging, or pressure conversion to determine the amount of leakage.
There are two methods for qualitative leak detection: vacuum leak detection and leak detector detection
① The vacuum leak detection method involves evacuating the equipment to 40Pa, stopping the pump for 0.5 hours, reading the number A on the vacuum gauge, stopping for 5 hours, and reading the number B. If B-A ≤ 133Pa, it is considered that the seal is in good condition;
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 determine whether the gas leaks based on the reading of the leak detector
Attention should be paid to leak detection with a leak detector: the probe should move slowly to prevent it from moving too fast and missing the leak point; Leakage detection should not be carried out under high wind speed conditions to avoid the leakage gas being blown away by the wind and affecting the leakage detection; Leak detectors should be selected with high sensitivity and slow response speed. The minimum detection capacity of a general leak detector is less than 1ppm, and the response speed is less than 5s.
Quantitative leak detection usually uses methods such as capping, bottle hanging, local wrapping, pressure drop, etc. The buckle cover method is suitable for high-voltage circuit breakers and small equipment suitable for cover cover applications. The hanging bottle method is suitable for situations where there are double sealing grooves on the flange surface. Partial wrapping method is generally used for assembling units and large products. The pressure drop method is suitable for determining the leakage rate in equipment compartments with large leakage or during operation. Usually, in the handover acceptance test of sulfur hexafluoride equipment, leak detection is carried out using local wrapping method and buckle method.
3. Pay attention to ventilation
Qualified sulfur hexafluoride gas is non-toxic, but there is a risk of suffocation. Sulfur hexafluoride gas should be stored in a well ventilated area and protected from direct sunlight and moisture. Before entering the low altitude area, the staff should check the oxygen content in the area. If the oxygen content is found to be below 18%, they cannot enter the area.
4. Wear protective equipment
The sampling operation and general leakage treatment of sulfur hexafluoride gas should be carried out under ventilated conditions, and a gas mask should be worn.