The main measures to ensure the accuracy of SF6 gas humidity measurement in the field- SF6在线监测系统_SF6检漏仪_SF6传感器_SF6密度继电器

韦弗斯新闻动态

最新动态

SF6安全保障专家

专注于SF6气体检测的专业化

135 85763546

The main measures to ensure the accuracy of SF6 gas humidity measurement in the field

发布时间：2018-11-27 来源：ydlcb 浏览次数：32次

Abstract: The moisture content of SF6 gas plays an important role in the safe operation of electrical equipment. According to the main factors that affect SF6 gas humidity measurement results, such as measuring instruments, connected gas paths, and ambient temperature, the causes are analyzed and corresponding solutions are put forward to provide reference for SF6 gas humidity measurement.

With the development of power grid and the renovation of high voltage equipment, SF6 electrical equipment has been widely used, and SF6 gas humidity is one of the important indicators of measurement at present. There are many methods to measure SF6 gas humidity, including gratings, electrolysis, resistance capacitance and dew point, etc. At present, resistance capacitance method and condensation dew point method (mirror method) are generally used to measure SF6 gas humidity in substations. However, in actual operation, the measurement data is often inaccurate due to external interference or other factors, resulting in misjudgment of measurement results. Only by fully understanding the main influencing factors of SF6 gas humidity measurement, analyzing them and taking measures to avoid them, can we obtain more accurate analytical data. This paper focuses on the application of condensation dew point method (mirror method) to measure the humidity of SF6 gas in the field.

1 Influence of the instrument itself and countermeasures

At present, there are many kinds of instruments used to measure the humidity of SF6 gas in high pressure and ultra-high pressure electrical equipment, and each has its own characteristics. Dew-point meter is widely used in substations because of its short measuring time, simple operation and maintenance, high measuring accuracy and low gas consumption. Due to the great influence of external interference and other factors in the field, the dew point meter itself has become one of the main factors affecting the measurement results.

1.1 When measuring low humidity SF6 gas, it is difficult to stabilize the dew point display value. The smaller the humidity of the gas measured by the dew point meter, the longer the measurement time. Under normal circustances, when measuring the humidity of SF6 gas, the dew point display value is damped and oscillates and eventually stabilizes. However, when measuring low humidity SF6 gas, the dew point display value is sometimes difficult to stabilize. The main reasons are: ① In the measurement of low humidity gas, the instrument needs a larger cooling power and a longer cooling time. Since the water vapor content of low-humidity gases is less, and the cold mirror of the instrument needs to collect enough water molecules to establish a stable dew layer of gas phase equilibrium, this process needs enough time and the measured gas. In this case, the instrument indicates that dew has formed, and the dew point indicates that the value rises at a very slow rate, sometimes the surveyor will mistake the reading for having reached a stable reading, resulting in a large measurement error.

The low humidity measurement and rapid stabilization device "ORIS" on the back panel of the dew point meter can solve the problem that the dew point display value is difficult to stabilize when the humidity is low. The basic principle is that when the mirror temperature drops to a preset value without condensation, the instrument automatically injects a small amount of moisture into the measuring chamber to make the mirror condensation quickly, thereby shortening the time to establish balance and thus shortening the measurement time. The preset temperature can be determined by estimating the humidity of the gas to be measured, and the SF6 gas can be selected at -40 ~ -50 ° C. Usually, the device can effectively shorten the measurement time, but in some cases, it will produce shock and cannot be measured, which should be withdrawn from the device.

1.2 When the ambient temperature is high, the instrument is prone to abnormal reading. The dew point meter makes the water vapor in the gas in the mirror by cooling the mirror

Condensation is used to measure the humidity of the gas, and the ambient temperature will affect the refrigeration effect of the instrument. In the summer, when the ambient temperature is very high (above 35 ° C), the gas with small humidity may not meet the requirements of the instrument cooling capacity, that is, the mirror temperature can not fall, but the mirror has always no condensation. In addition, in a high temperature environment, the display value of the instrument may suddenly rise, and eventually stabilize at a value significantly higher than the true dew point, that is, get a false reading.

Therefore, it is necessary to avoid the high temperature weather as much as possible, such as the measurement in the early and late weather when the weather is cooler, or the instrument is placed in the air-conditioned car, or other types of instruments are used for measurement.

1.3 Instrument type and sensitivity

The measurement results of different measuring instruments vary greatly. The same principle of different types of measuring instruments, there are also differences in manufacturing quality and performance. Therefore, each SF6 electrical equipment uses the same model or the same instrument for measurement, so that historical data comparability can be found when the measurement data is abnormal. If there is any doubt about the measurement results, the measuring instrument of other principles should be replaced. The instrument should also be sent regularly to a provincial test center with a standard humidity device for calibration, and the validity of the calibration is one year.

2. Influence of connecting gas path and countermeasures

The filling and discharging joint of the SF6 equipment's gas chamber, the length of the gas extraction pipe used for measurement, and the residual moisture and impurities on the inner wall of the pipe will affect the measurement results. In the measurement, it was found that the water content measured by using the oxygen pressure reducing valve and rubber tube as the gas path was 4 to 5 times higher than that measured by using the small regulating valve and stainless steel tube as the connecting gas path. The reason is that the internal cavity of the oxygen pressure reducing valve is large, there are more dead angles, the surface of the material inside the rubber tube is relatively loose, the molecular gap is large, and the moisture absorption is strong, and it is not easy to dry thoroughly when using nitrogen, which interferes with the measurement of water content.

Countermeasures:

(1) Use the special all-metal joint equipped by the equipment manufacturer, or use the special joint provided by the instrument manufacturer. At present, whether it is different manufacturers or different models of SF6 equipment from the same manufacturer, its gas intake interface is very different, and gas leakage may occur during gas intake, which brings a lot of inconvenience to field measurement. Therefore, it is recommended to use the joint provided by the equipment manufacturer and store the joint in the dryer.

(2) Use polytetrafluoroethylene pipe with an inner diameter of 3 ~ 6mm or stainless steel pipe as a sampling pipe, and its length is as short as possible. It is not suitable to use rubber pipes with loose material and high moisture absorption. The tube should be clean, free of grease and solder. The sampling tube should be placed in a closed, dry storage box, and be careful to avoid contact with moisture and other impurities.

(3) Before measurement, wipe the air inlet with a clean large piece of cotton cloth to make it dry and clean. The raw material tape is wrapped and sealed to prevent air leakage during measurement. The obvious phenomenon of air leakage is that when the humidity is reduced to a certain extent, it begins to rebound and rise, significantly exceeding the allowable value stipulated by the national standard. At this time, the sealing condition of the gas path should be checked and re-measured after taking measures.

3. Influence of ambient temperature and corresponding measures

The moisture in SF6 electrical equipment is not only present in the gas, but also adheres to the insulating surface. The distribution of water between the two depends on the change of temperature, and the measurement results of winter and summer even differ by several times. The main reasons that the ambient temperature affects the measurement result are as follows:

(1) The material inside the equipment The solid insulation material inside the equipment and the wall will adhere to a trace amount of water, with the increase of temperature, gradually release the water permeated in these materials, so that the water vapor in SF6 gas increases.

(2) A large part of the water in the adsorbent (molecular sieve) equipment exists in the adsorbent, the temperature change will make the adsorption capacity of the adsorbent change, resulting in the phenomenon of water transfer from the adsorbent to SF6 gas when the temperature rises, and water transfer from SF6 gas to the adsorbent when the temperature drops.

(3) The water content of gas molecules is proportional to the number of molecules and partial pressure. Changes in temperature and partial pressure will result in changes in measurement results.

(4) Environmental humidity The moisture in the atmosphere will penetrate into the equipment through the weak link of the equipment. When the ambient temperature is high and the external humidity is relatively large, the external water vapor intrusion is large, and on the contrary, the amount of entry is small.

Countermeasures:

(1) Avoid measuring at high or low temperatures as much as possible. If the temperature is too high during the test, when the measured value of SF6 gas humidity exceeds the standard is not serious, choose a time when the temperature is close to 20℃ to test again, and then decide whether to treat; If the temperature is lower than 20℃ for a long time, if the measured value of SF6 gas humidity is not exceeded but close to the standard value, it should also be considered to arrange a retest at about 20℃.

(2) Try to choose the period of low ambient humidity for measurement.

(3) Correct the measurement results according to the curves and charts provided by the manufacturer. In the absence of manufacturer information, the following empirical formula can also be used to correct:

In the formula, the water volume ratio at H2 -- 20 ℃;

H1 -- real-time measurement of moisture μl/L;

P1 -- the pressure of SF6 gas at the time of measurement;

P2 - SF6 gas pressure converted to 20 ℃ according to the density measurement principle;

P1S - Measures the pressure of saturated water at temperature;

P2S -- pressure of saturated water at 20 ° C.

4. Effects of various impurities and countermeasures

4.1 Impurities in the form of steam

A dew point meter measures everything that can condense on a mirror surface, not just water. If the gas contains impurities in the form of steam (such as hydrocarbons), it will condense on the mirror before the steam, causing the illusion that the humidity of the gas sample is too high, or the gas contains substances that can condense on the mirror with water (such as methanol), which seriously interferes with the measurement results of the dew point meter. If there is any doubt about the measurement results, you should replace the measuring instrument with other principles (such as electrolysis or resistance and capacitance humidity measuring instrument), and perform gas sampling analysis if necessary to determine whether the measurement results are correct.

4.2 Dust and other solid impurities

SF6 electrical equipment, especially the SF6 gas in the switch equipment with a long running time, often carries solid impurities such as dust, which are the chloride powder generated by the reaction of SF6 with the metal under the action of the arc and the adsorbent powder inside the equipment. If these impurities enter the dew point meter and adhere to the mirror, they will affect the measurement of the dew point of the photoelectric system, causing measurement errors. Filter the dust impurities of the gas to eliminate their impact on the measurement results. It is worth noting that the filter device used cannot absorb water, so as not to affect the normal measurement of water.

4.3 Other external impurities

When the dew point meter has measured SF6 gas with high humidity or gas containing other chemical components, if it is not thoroughly brushed, it is easy to pollute the instrument and affect the accuracy of the measurement data. Other solid particles or dust should also be avoided from entering the inside of the instrument.

5 Closing remarks

To sum up, measuring the gas humidity in SF6 electrical equipment in substation is an important work for SF6 gas supervision and operation management. There are many factors affecting the field measurement results of SF6 gas humidity. During the measurement, it is necessary to analyze the main influencing factors such as measuring instruments, connected gas paths, field ambient temperature, and impurities in gas samples, and take measures to avoid them as much as possible to ensure the smooth development of the measurement work, so as to save the measured gas and obtain more real results.