Source: Dianlian Intelligent Manufacturing
Recommended unit: Liaoning Qinghe Power Generation Co., Ltd
Authors of this article: Wu Wensheng, Che Yujia, Lv Chunyu, Ming Xiaomin
summary
Sulfur hexafluoride is used as the insulation and arc extinguishing medium of GIS equipment, and its water content directly affects the insulation and arc extinguishing performance of GIS equipment.
Preface
With the continuous development of China's power industry, sulfur hexafluoride gas (SF6) has received more and more attention in high-voltage switchgear applications as the most ideal insulation and arc extinguishing medium for power equipment today. The control of the water content of sulfur hexafluoride is an important test item in the application of sulfur hexafluoride gas, and the amount of water content determines the operational safety of high-voltage switchgear. This paper first introduces the application of sulfur hexafluoride in high-pressure equipment, and the source of gas moisture in sulfur hexafluoride equipment. The hazards of moisture to high-voltage switchgear and the importance of moisture content control are summarized, and the current monitoring methods of SF6 gas micro-water in domestic and foreign measuring instruments, the treatment of SF6 gas moisture exceeding the standard in circuit breakers, and some measures to control the water content of SF6 gas in circuit breakers are reviewed.
PART 1 Application of sulfur hexafluoride in high-pressure equipment and source of gas moisture
SF6 gas is widely used in high-pressure equipment due to its good chemical stability. At temperatures below 180 degrees, it is similar to nitrogen in nature, both are inert gases, colorless, non-toxic, tasteless, non-flammable and non-corrosive. When the temperature reaches 500°C, the pure SF6 gas does not decompose. It is only when the arc is discharged at high temperatures that the SF6 gas decomposes. When the arc is extinguished, the temperature also drops, and SF6 has good recovery, so the originally decomposed SF6 gas will be recombined into SF6 molecules, recycled, and the life of the circuit breaker is extended.
However, during each arc extinguishing process, SF6 decomposes sulfides and other products. If there is a frequent breaking process, the continuous high temperature generated by the arc will cause a chemical reaction between the SF6 gas decomposition product and the metal vapor, and combine to form solid particles such as ammonia and sulfate with strong corrosion and insulation, which will be adsorbed on the contact surface, seriously corrode the contact material, resulting in a sharp increase in contact resistance on the contact surface, so that the contact contact of the SF6 circuit breaker can not work reliably. Therefore, the gas is suitable as an insulating and arc extinguishing medium for infrequent high-voltage electrical appliances. In addition, the corrosive gas produced by the decomposition of SF6 gas in the high-temperature environment during arc discharge has a serious corrosive effect on aluminum alloys, and even causes damage to phenolic resin laminates and porcelain insulation.
The moisture in SF6 electrical equipment mainly comes from four aspects. One is the moisture brought in by SF6 fresh air. The mainland stipulates that the moisture in SF6 fresh gas is 8 μg/g, which is equivalent to 65 μL/L. Second, the water mixed in during the installation of the equipment, the large equipment is installed and inflated on site, and after the installation is completed, although the vacuum and drying process is carried out, it is impossible to completely remove the moisture on the inner wall of the equipment. Third, solid insulation (such as epoxy resin) itself contains a certain amount of water, which is slowly released with the extension of running time and temperature changes. Fourth, the SF6 equipment leaks and enters the moisture.
In the SF6 circuit breaker, the moisture will decompose with the SF4 in the SF6 gas. Hydrofluoric acid (H2O+SF4→SOF2+2HF) is produced to corrode the contact material, so the moisture content of SF6 gas must be strictly controlled so that it does not exceed the specified standard value.
When the moisture content is high, the SF6 gas may undergo a hydrolysis reaction at temperatures above 200°C, resulting in S02 and HF. S02 can further form sulfurous acid with H20. Both hydrofluoric acid and sulfurous acid are corrosive and can severely corrode the internal components of circuit breakers. Moisture can exacerbate the hydrolysis of low fluoride. SF6 can be decomposed into atomic states S and F under the action of high-temperature arc above 5000°C. After the arc is extinguished, the S and F atoms are recombined to form SF6, but some of them are still incompletely bound to form hypofluoride. While SF6 is arced into atomic states of S and F, a large amount of metal copper and tungsten vapor evaporates from the circuit breaker contact, which reacts with SF6 at high temperature to form metal fluoride CuF2 and WO3 and low fluoride SF4. Due to the presence of moisture, metal fluoride is hydrolyzed, and low fluoride can be further hydrolyzed to form highly toxic sulfurous fluoride.
In severe cases, the moisture in SF6 gas will also liquefy on the surface of the insulating layer, such as contacts and insulators, which will significantly reduce the dielectric strength and even cause discharge along the surface.
PART 2 The current monitoring methods of SF6 gas micro-water by domestic and foreign measuring instruments
At present, the measurement of SF6 gas micro-water content in high-voltage circuit breaker equipment at home and abroad mainly uses micro-water meter, and an appropriate amount of SF6 gas is extracted from the SF6 circuit breaker, and then the standard measurement is carried out offline. There are many principles of micro-water meter, mainly including, gravimetric method, electrolysis method, resistance and capacitance method, dew point method, infrared spectral absorption method, etc. Among them, the electrolysis method and the dew point method are used more.
(1) Gravimetric method
The gravimetric method is the arbitration method for the moisture measurement of SF6 gases in the power industry. It absorbs a certain volume of SF6 gas with constant-weight anhydrous magnesium perchlorate, and the trace amount of SF6 is known by judging the weight. The advantage is direct measurement and intuitive results. The disadvantages are that the experimental conditions are strictly controlled, the operation requirements are high and cumbersome, and the test time is long, which is only suitable for laboratory measurements. The measuring device is shown in Figure 1
Fig.1 Schematic diagram of gravimetric moisture determination device
1-nitrogen cylinder; 2-sulfur hexafluoride cylinder; 3-pressure reducing valve;
4-drying tower; 5-four-way valve; 6-flowmeter;
7, 8, 9, 10-absorbent tubes; 11-drying oven; 12-Wet gas flow meter
(2) Electrolysis
The electrolysis method, also known as the Coulomb method, was commonly used in the early power industry. SF6 gas is passed through an electrolysis cell, and the water in it is absorbed by a hygroscopic film coated with phosphorus pentoxide. The water content is directly proportional to the electrolytic current. The advantages of this method are low cost, accurate current measurement, and suitable for measuring pure gases. The disadvantage is that the gas path of the electrolytic cell needs to be dried for a long time before the measurement, and the measurement balance is slow and the air consumption is large. Moreover, the detection gas is required to be clean, otherwise it will cause a short circuit in the electrolytic cell. The measuring device is shown in Figure 2
Figure 2: Schematic diagram of the electrolytic moisture measurement device
1-bypass flowmeter; 2-communicating tube; 3-electrolytic cell;
4-measuring flowmeter; 5-dryer; 6-Control valve
(3) Resistance and capacitance method
The resistor-capacitance method uses the dielectric coefficient (moisture content is inversely proportional to the dielectric coefficient) of the hygroscopic insulating medium in the capacitor to determine the moisture content in SF6. The advantage is that the measurement is fast, and the disadvantage is that the alumina film is affected by the corrosive medium in the gas, and the accuracy is poor, and it is currently being replaced by polymer thin film sensors. The structural device is shown in Figure 3
Figure 3 Capacitor structure and schematic diagram
(4) Dew point method
The dew point method is the use of mirror cooling technology, when the condensation or frost formed by moisture on the mirror surface, the temperature indicated on the instrument is the gas dew point temperature. There are two ways to identify whether the gas is condensation (frost), one is to use the principle of incident light and reflected light offset, but dust and salt deposits on the mirror surface will produce errors. The second is surface acoustic wave identification, which has little impact on the detection of impurities on the mirror surface, and the detection accuracy is greatly improved. Its advantages are fast measurement speed and stable measured data; However, the disadvantage is that it is not suitable for on-site reading, because the gas in the operating equipment often contains corrosive components and impurities such as dust, and in the high temperature environment in summer, the air cooling effect is poor, and it is impossible to test the moisture in the gas with low content. The two assay principles are shown in Figure 4 and Figure 5
Figure 4: Schematic diagram of the principle of optical dew point detection
Figure 5: Schematic diagram of the dew point detection principle of surface acoustic wave method
(5) Infrared spectroscopic absorptiometry
According to the spectral theory of gas molecules, infrared spectroscopic absorption method studies the near-infrared spectral absorption characteristics of water molecules, and gives a theoretical basis for detecting water content. Different wavelengths of light are different, and SF6 gas also has a specific spectrum, when the wavelength of the two lights is the same, resonance can occur, and the degree of intensity attenuation of the light measurement can be used to determine the amount of SF6 gas microfluid. The advantages are a wide range of applications, strong characteristics, and a large amount of information. The disadvantage is that it is not suitable for on-site in-line testing, and the instrument is expensive.
At present, the method of micro-water detection of SF6 circuit breaker in our factory is mainly to use the dew point meter on site on a regular basis for testing, and the main defects are:
(1) The measurement is limited by the ambient temperature
The dew point instrument cannot be measured in a low temperature environment, and the storage and working environment temperature is about -10°C~+50°C. In order to ensure that the ambient temperature has little influence on the test results, the on-site test was carried out in April and October every year when the ambient temperature was about 20°C.
(2) Time-consuming, trouble-consuming, and laborious
The test time of the dew point meter is long, and SF6 gas needs to be emitted for a long time for testing. Due to the moisture content of the ventilation duct, it takes at least 10 minutes to dry the ventilation duct before measurement. A complete and accurate test process takes about 25~35min, calculated according to the flow rate of the standard sampled gas, that is, 30~40L/h, and a valve test needs to extract and discharge about 15 liters of SF6 gas, which will affect the health of on-site staff and pollute the atmosphere.
(3) Offline detection
The regular detection of the dew point meter is an offline detection method, which cannot reflect the dynamic changes of the equipment operation status and various parameters in real time. Moreover, the dew point meter can not accurately monitor the density, temperature and humidity of SF6 gas in the gas chamber of SF6 circuit breaker in real time, and predict the change trend of each parameter in advance, so that real-time monitoring can not be achieved, and the communication transmission and early warning prompts of the detected data cannot be realized, and the design requirements of comprehensive automation of substation cannot be realized.
(4) The measurement is limited by ambient humidity:
Table 1 is a recent cylinder gas humidity analysis in our factory. The cylinder is located in the GIS room, and the measuring instrument is a dew point meter, and the ordinary pressure reducing valve is used to connect the instrument with its own pipeline during measurement. Among them, on May 28, 2021, the data of bottle #2 was abnormal, #2钢瓶是当天测量的第一瓶气体, the surveyor carried instruments and pipelines to measure in rainy weather, and the reason was suspected to be that the inner wall of the pipeline adhered to a large amount of water in high humidity weather, although there was a long period of gas washing, but the data was only barely balanced at a large number. Several of the cylinders measured after the day had lower moisture than the #2 bottle, but the moisture content was still large compared to the good weather measurements. And the data equilibration time is too long, and a lot of gas is wasted. The data shows that the ambient humidity has a great influence on the determination of the dew point meter, and the measurement should be sunny and the humidity should not be more than 60%.
Table 1 A recent analysis of SF6 gas humidity in a cylinder
PART 3 TEST SF6 gas moisture and moisture exceeding the standard handling precautions
For the determination of SF6 gas, special pipes with low moisture absorption should be used, which should be kept clean and dry, and the connections should be sealed without leakage.
When doing the gas moisture acceptance test in the SF6 equipment, the time interval from the SF6 gas filling to the test is not less than 24h, so as to avoid the error caused by the imbalance of gas-solid dynamic mass transfer in the equipment.
The moisture in the SF6 circuit breaker should be measured when the ambient temperature is about 20°C, and the ambient temperature should be indicated in the report. The SF6 tester should be calibrated regularly. When measuring with a dew point meter, if the result is expressed as a volume ratio or mass ratio, the saturated vapor pressure at the dew point should be found and converted according to the actual pressure of the operating equipment. Note the effect of volatile organic solvents in the internal insulation on the test results. When the moisture in the SF6 circuit breaker exceeds the standard, the cause is immediately analyzed and repaired in time to avoid greater losses:
Precautions for handling excessive moisture
(1) The SF6 moisture treatment process should be carried out under the condition of sunny days and < 80% relative humidity of the air. When recovering gas, it is necessary to pay attention to ventilation and air humidity, and take protective measures to avoid poisoning of personnel.
(2) During the filling process, it is necessary to determine the pressure value of SF6 gas according to the on-site ambient temperature and the temperature curve of the equipment. When the pressure of the circuit breaker body is close to the rated value, adjust the pressure reducing valve to slow down the inflation speed: close the inflation valve when the pressure indicates to the rated value, observe for 5~10min, and only end the inflation when the pressure does not change.
(3) Update the sealing ring. When replacing the sealing ring, it is necessary to check and confirm that the sealing ring is free of defects such as hardening, deformation, scratches, cracks and fluffing, and clean the sealing ring with anhydrous alcohol and wipe it clean. The size of the "0" ring should be checked to ensure that they fit well with the sealing groove. Apply sealing grease to the gap between the sealing ring and the sealing groove on the side in contact with the atmosphere (do not apply it to the side in contact with SF6 gas). A thin layer of sealing grease is applied to the entire flange surface on the outside air side to prevent moisture penetration and rust.
(4) When replacing the adsorbent, the drying temperature of the adsorbent is about 200C (the temperature used by different types of adsorbents is different), and the drying time is about 12h. The dried adsorbent should be placed in a sealed dry container and cooled to room temperature before being loaded into the appliance immediately. Minimize the exposure time of the adsorbent to the atmosphere. The time from installing the adsorbent to vacuuming the equipment should be < 10min, and the maximum time should not exceed 30min.
PART 4 MEASURES TO CONTROL THE MOISTURE CONTENT OF SF6 GAS IN CIRCUIT BREAKERS
When the moisture content of SF6 exceeds the index, the performance of the circuit breaker will be seriously affected, and the moisture content in the SF6 gas should be strictly controlled, with the following measures:
(1) The parts of the SF6 circuit breaker should be cleaned one by one and dried before assembly in the manufacturing plant. The assembly environment should maintain a certain temperature and humidity, during on-site installation and commissioning, the ambient temperature should not be lower than 10 °C, and the ambient humidity should not be higher than 80%, before filling with SF6 gas, through vacuuming, cleaning with high-purity nitrogen, as far as possible to reduce the moisture trapped in the switchgear.
(2) An adsorbent should be placed in each gas suffocation of the circuit breaker, which can not only adsorb the moisture in the SF6 gas, but also effectively remove the arc decomposition products. After the circuit breaker is installed, the new adsorbent should be replaced before vacuuming, or the original adsorbent of the product should be regenerated according to the temperature, time and other procedures required by the manufacturer.
(3) The moisture inside the circuit breaker must be treated before the circuit breaker is filled with SF6 gas. Each SF6 circuit breaker contains the same amount of moisture in the air as it does in the air during production, commissioning and repair in the field. In order for the circuit breaker to meet the requirements after filling with SF6 gas, the water content in the circuit breaker after installation must be reduced to a few tenths of the original level. To achieve this goal, it is usually to vacuum the switchgear, fill it with high-purity nitrogen (99.999%), clean it, release nitrogen, and then vacuum it for treatment, the lower the vacuum degree when vacuuming, the better, and the vacuum degree is generally required to be less than 40Pa. As long as the procedure is operated properly, the water content of SF6 gas in the circuit breaker can meet the operating requirements of the product.
(4) Ensure that the SF6 gas charged into the circuit breaker is qualified, which requires that the water content of the SF6 gas to be filled is not more than 65ppm, and the gas in the SF6 gas cylinder should be detected before filling, and the unqualified gas cannot be charged into the switchgear.
(5) The switchgear should be reliably sealed, so that on the one hand, the leakage of SF6 gas can be reduced, and on the other hand, the external moisture can be reduced to invade the inside of the circuit breaker. Leak detection should be carried out after the circuit breaker is installed.
(6) When filling SF6 gas, it is necessary to ensure that the inflatable pipeline has good air tightness, it is best to use PVC plastic pipe or pipeline that does not absorb water, and the pipeline should be dried or vacuumed before inflation to minimize the water brought in by the gas.
PART 5 CONCLUSION
Moisture monitoring is of great significance for finding the early fault of SF6 circuit breaker, but there are various reasons for the growth of moisture in electrical equipment, so it is necessary to correctly judge the fault, adopt a variety of methods for detection, and combine historical data analysis. By monitoring the moisture content of SF6 gas, problems can be detected early and the safe and stable operation of the equipment can be ensured.
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