Discussion on evacuation process of SF6 switchgear for electric power construction equipment and materials Zhuang Xiaofeng (Xi'an Xi'an Xigao Electric Power Co., Ltd., Xi'an 71(1)77) I. SF6 Switchgear The purpose of vacuuming during installation and maintenance is to reduce the water inside the SF6 switchgear. Steam content and qualitative leak detection of SF6 switchgear. Vacuum testing sequence is: the vacuum degree reaches 133Pa. After maintaining the vacuum pump operation for at least 30min, stop the pump and isolate it from the pump. After waiting 30min, read the vacuum value; wait for more than 5h to read the vacuum value, and ask for the 2 readings. The difference is not greater than 67Pa to be qualified. To prevent the five kinds of misoperations often encountered when vacuuming.
I 丨 SF6 Switchgear Vacuum Detection Due to the superior performance of SF6 switchgear in terms of electrical performance, reliability, economy, and floor space, the product is gradually recognized by the majority of power users, especially in the 725kV or higher voltage class area. Become a product of urban and rural power grids. However, this equipment is very different from the air circuit breaker and the oil circuit breaker when it is installed and overhauled. The entire vacuum pumping process of the switchgear, namely the purpose of vacuuming; vacuum, vacuum unit and vacuum zone division; insulation strength in vacuum; vacuum program and technical requirements; discussion of technical requirements and processes, etc. 6 parts to explore.
1 The purpose of evacuation of SF6 switchgear - GIS, SFf, BUS, SFf, CT, etc. Vacuum pumping is a necessary procedure before they inject SFf and gas. The purpose of the SFf, the vacuum of the switchgear, is to: (1) reduce the SFf, switch the humidity inside the equipment (vapor content), ie remove the trace moisture inside the equipment; (2) as a qualitative leak detection method. In actual operation, due to various reasons, there have been some misoperations that have caused the product to be damaged or returned to repair.
2 vacuum, vacuum unit and vacuum zone division The so-called vacuum, refers to the state of lean gas. Any state where the absolute pressure is lower than the normal atmospheric pressure can be called a vacuum state. The space where absolute pressure equals 0 is called absolute vacuum.
The degree of vacuum is characterized by the absolute pressure of the gas. The lower the pressure, the higher the degree of vacuum. The unit of vacuum measurement is: In the international system of units, the absolute pressure is in Pa (ie, 1 Nm2 force).
Our country divides the vacuum area into five areas: (1) rough vacuum.
Vacuum pressure range: 101X105~133X102Pa(2) Low vacuum. Vacuum pressure range: 1 high vacuum. Vacuum pressure range: 1 line (4) ultra-high vacuum. Vacuum pressure range: 133\106~133X10-10Pa(5) Extremely high vacuum. The vacuum pressure range is a small SFf, and the vacuum pressure of the switchgear is in the range of 1 01X105 33X102Pa in the rough vacuum category, while the vacuum circuit breaker pressure range is in the 1 vacuum category. The insulation strength of the two is very different.
3 Insulation strength in vacuum The dielectric strength in vacuum is as shown. The curve is the measured curve at a distance of 20mm between the electrodes of a vacuum level vacuum tube. It is not difficult to see from the trend of the curve that when the absolute pressure drops below 133 32×10-4 Pa, the insulation breakdown voltage remains basically unchanged and has a higher value. This is because the collision of gas molecules has no effect, and the breakdown voltage is determined by other conditions such as the surface condition of the electrode. It can be seen that if the vacuum circuit breaker is evacuated to a 133 32 Pa container, this range is used to obtain high dielectric strength. It can be completely removed. When the pressure is around 133 10-3Pa, the dielectric strength begins to drop. The inflection point occurred between 13332X10-1~13332Pa, and the breakdown voltage reached the lowest value. Since then, with the increase of pressure, the dielectric strength has shown an upward trend, and the variation of the curve is in accordance with Paschen's law. 4 Procedures and technical requirements for vacuum pumping 1 Procedures for vacuum pumping 1989 and Rules for the Operation and Maintenance of Gas-insulated Metal-Enclosed Switchgears DL/T603-1996 There are specific requirements for the vacuum sequence and technical requirements. The latter stipulates: air pump operation is at least 30mh or more; (2) stop the pump and isolate it from the pump, wait for 30 minutes before reading the vacuum A(3) and wait for more than 5 hours, read the vacuum B request B 67Pa (limit allowable The value of 133Pa) is acceptable, otherwise, the leakage point must be detected first; (4) There must be a person responsible for vacuuming, and it is necessary to prevent accidental vacuum pump oil backflow accident.
2 Discussion on the technical requirements for vacuum pumping The test device for vacuum pumping under temperature. After the test, the relationship between the water content in the container and the degree of vacuum is as follows: (1) When water is about 13332 Pa, the water boils; (2) At about 53328 Pa, the vacuum degree is not changed and the water is gradually reduced: 3 is about 266 64Pa When the water has been removed I It will continue to vacuum when the test, the vacuum reaches 13 332 ~ 19998Pa when in saturation, the saturation point is reached 133 32Pa pressure, then the starting point of vacuum 30min. The time elapsed to reach the specified vacuum.
As described above, the residual moisture can be removed by vacuuming the inside of the device. However, if the moisture that has been precipitated from the organic insulating material is also removed by vacuum, a long period of vacuum must be applied. Must be removed with an adsorbent.
The above introduction can understand the process of removing water by vacuum and test results; also can understand the role of 13332Pa in the standard and continue vacuuming for 30min.
5 Discussion on the vacuuming process The vacuuming process often encounters the following 5 kinds of misoperations, and precautions must be taken.
1 The voltage to be applied when evacuating and vacuum is maintained SF6 The evacuation of the switchgear is in the rough vacuum category; the relationship between the insulation breakdown voltage and the vacuum degree is on the right side; when it is around 133Pa, the breakdown voltage is at the lowest point. Therefore, the voltage applied during evacuation and vacuum holding, even at low voltage, is also ingHouse.Allrightsreserved, http:// extremely dangerous. Here are the typical examples below.
During the vacuum holding period, a loop resistance test was performed in order to shorten the construction period for the crossover operation. Although the applied voltage is only 6V, it has constituted a discharge condition, causing the basin insulator to flash over.
Example 2 A project 550 IV tank SFf, circuit breaker (with closing resistor) water content exceeding the standard, the installation unit first with high-purity nitrogen (gas), after baking with infrared light, failed to fully remove the water. Later, they tried to wind coils on the outside of the tank and heated them with eddy currents. The voltage was applied to the closing resistors to drive off the water. At the same time, the vacuum was applied to remove the water. Obviously, this plan will have serious consequences if it is implemented.
In summary, it is recommended that: (1) Do not apply voltage during vacuuming and vacuum maintenance, even if it is low voltage. (2) For the expansion project, if the newly installed product is in a high-voltage coverage area, such as under the high-voltage bus, the induced voltage should be protected from damage when vacuuming. It is recommended that the product inlet and outlet terminals be directly grounded.
52 Pumping and not detaching from the pump when reading the vacuum The standard specifies that the pump should be stopped and isolated from the pump. However, during the operation, there are cases where it is convenient or incomprehensible to read the vacuum without stopping the pump and without being isolated from the pump. The consequence is often that the read value is a false value.
A common method of isolating the pump is to close the stop valve (middle shutoff valve 1) located on the suction side of the vacuum pump. The purpose is to ensure that the reading correctly reflects the absolute pressure inside the product. Conversely, the reading value is most likely the value of the vacuum pump inlet. This value is usually greater than the vacuum that the product actually reaches.
53 Oil return phenomenon Back oil refers to the accident that the vacuum pump is in vacuum operation and the pump oil is poured into the product due to power loss. Currently, vacuum pumps are mostly equipped with vane or piston pumps. Combine it with SF6 cylinders and other originals to form an "inflator". Combine it with components such as compressors and gas tanks to form a "charge and bleed device." Regular "devices" are equipped with anti-return devices.
There are several ways to prevent oil return. It is common to install a solenoid valve at the inlet of the vacuum pump (on the left side of the stop valve 1). When the power is lost, the solenoid valve opens and communicates with the atmosphere, so that the pressure at the inlet and outlet of the vacuum pump can be quickly reached, which can effectively prevent the return of oil. Conversely, the vacuum pump that is running will force the pump oil from the high pressure zone into the low pressure zone due to the pressure difference between the air inlet and the air outlet, and enter the product through the pipeline.
Once oil spills occur, they should be dealt with promptly and repairs should not be delayed, as returning oil can compromise product performance and safe operation.
Therefore, it is recommended that: (1) Before using the "inflator" or "inflator and bleeder", the operational reliability of the solenoid valve should be included in the inspection program. (2) When the vacuum pump is used alone, it should be known whether the pump has an anti-return device or measure. If so, its reliability should be examined; if not, corresponding measures should be taken.
(3) The “installation†that has been purchased but has no measures to prevent oil return should be reformed as soon as possible; (4) Special personnel should be responsible. It is very beneficial for equipment maintenance, proficiency in business, and supervision of "device" operations.
4 Vacuum holding time issues The vacuum retention time specified in the above two standards is more than 5h, that is, the rest of the vacuum is read more than 5h to read the vacuum B. This is to hide the moisture in the insulation, castings, metal parts and metal welds, etc. Gradually vaporize under vacuum. The BA value is used to determine the dry state before assembly of the internal components of the product and whether the next program can be performed.
In this regard, I believe that 5h should be the lower limit. The vacuum holding time should vary depending on the product (especially GE). It may be advantageous for some foreign manufacturers to set the 24h limit as 24h.
5 One of the ways to leak the vacuum leak detection problem. The standard stipulates that the sample shall be evacuated to a vacuum level of 113X10-6MPa and then maintained for 30mh after the vacuum pump is operated. The pump shall be stopped. After 30 minutes, the vacuum degree shall be read after A5h and the vacuum degree shall be read. If the BA value is less than 133X10-6MPa, the sealing performance is considered to be good.
In the implementation of this standard, it is often considered that: When the value of -A exceeds 133X10-6MPa, the sample is surely sealed and has a leak. The author believes that it should not be absolute, and the specific circumstances should be specifically analyzed. If the BA value is far greater than the 133Pa, there is a high possibility of a poor seal; if the difference is small, it may be due to the vaporization of the moisture inside the sample component during vacuum holding. In order to confirm this, it is recommended to repeat the test specified in the above standard again. If the BA value drops below 133 Pa, the conclusion that the seal is bad can be eliminated.
6 Conclusions 1 to 53 are misoperations. If you can understand and implement the standards, the misoperations of 52 and 53 are easily avoided. 5 1 is due to insufficient attention to the effect of the vacuum condition on the SF6 switchgear. 54 and 55 are the operations provided by the author. It is not inconsistent with the standard. There is only an understanding problem. If the vacuum retention time is more than 5h as specified in the standard in 5.4, the upper limit is not mentioned. The author suggests that the upper limit be 55. The standard stipulates that the BA value is less than 133Pa. IJ thinks that the seal is good, but no clear opposite conclusion has been made. Based on my own experience, the author proposes to conduct another test. 54 and 55 can be used as opinions, but they can be used without prejudice to the standards.
(Editor: Wang Pingzhi)
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