Analysis of on-site detection method for the vacuu

With the development of circuit breaker technology, vacuum circuit breaker has been widely used in the medium voltage field with its own superior characteristics, especially in recent years, with the implementation of the "oil-free" transformation project of circuit breaker, the occupation rate of vacuum circuit breaker in the medium voltage field of power system is getting higher and higher

the core component of the vacuum circuit breaker is the vacuum arc extinguishing chamber (vacuum bubble). During the operation of the vacuum circuit breaker, its vacuum arc extinguishing chamber will leak to varying degrees, and some may leak to the point that they cannot work normally even within the expected service life. In this case, operation will cause serious consequences, and vacuum circuit breaker accidents are mostly caused by this reason. Therefore, it has become a very important work to regularly detect the quality of vacuum interrupters

2. Method for on-site detection of vacuum interrupters

the common method for on-site detection of vacuum interrupters is the power frequency withstand voltage method. With the development of testing technology, the vacuum degree testing method is gradually widely used on site

2.1 power frequency withstand voltage method

the power frequency withstand voltage method is to apply a certain power frequency voltage between the moving and static contacts when the vacuum circuit breaker is in the disconnected state (the voltage value is according to the manufacturer's technical standard, generally 10kV level plus 42KV), detect the size of the leakage current and observe the discharge phenomenon in the arc extinguishing chamber. There should be no continuous discharge phenomenon in the vacuum arc extinguishing chamber, Otherwise, it is necessary to replace the vacuum interrupter. The power frequency withstand voltage method is actually an indirect method to judge whether the vacuum degree of the vacuum interrupter meets the requirements. If there is leakage in the vacuum arc extinguishing chamber, the gas pressure in the vacuum arc extinguishing chamber will rise to a certain extent, and the relatively short gap between the contacts will not be able to withstand the test voltage, and even electrical breakdown will occur during the pressure rise process. The power frequency withstand voltage method has the advantages of simple principle and convenient operation, and can qualitatively detect the quality of the vacuum interrupter. During the handover test, it should be noted that before the power frequency withstand voltage, the mechanical stroke of the contact of the vacuum circuit breaker should be adjusted to the required value before the pressure test

2.2 vacuum degree test method

(1) vacuum degree test principle

vacuum degree test method directly tests the vacuum degree of the vacuum interrupter (in this paper, the vacuum degree represents the absolute pressure of the gas in the interrupter, in PA as the unit, which is different from the general vacuum degree) to judge the quality of the vacuum interrupter. Most of the vacuum testers used on site are non disassembly products, and the instruments adopt advanced synchronous pulse magnetron discharge and single chip microcomputer technology. The brief mechanism is as follows: the thin gas molecules are ionized by the collision of electrons excited by a strong electric field, thus forming a large measurable ion current. Since the ion current is basically linear with the logarithm of vacuum degree, the vacuum degree value can be obtained as long as the ion current is measured. The purpose of adding a magnetic field is to lengthen the travel of electrons, increase the collision probability with gas molecules, and increase the ionic current, so as to reduce the strength requirements of the external electric field. The measurement range and value of ionic current are related to the applied electric field, magnetic field strength and contact opening distance, but mainly depends on the strength of the applied electric field and magnetic field and the direction of the magnetic field. It should be noted that the vacuum circuit breaker has suction and discharge effects when applying electric and magnetic fields. In order to measure accurately, the method of applying pulse magnetic field and electric field synchronously is often used. This is the basic principle of pulse magnetic control discharge method

(2) test method

separate the moving and static contacts of the vacuum arc extinguishing chamber, apply pulsed high voltage, wrap the electromagnetic coil around the outside of the arc extinguishing chamber, and apply large current to the coil, so as to generate a pulsed magnetic field synchronized with the high-voltage electric field in the arc extinguishing chamber. In this way, under the action of the pulsed magnetic field, the electrons in the arc extinguishing chamber make a spiral motion, and collide and ionize with the residual gas molecules, The ion current generated is approximately proportional to the residual gas density, that is, the vacuum degree. For vacuum bubbles with different diameters, under the same vacuum degree, the flow of ionic electricity, including astronaut materials and scientific experimental supplies, is also different. Through experiments, the corresponding relationship curve between vacuum degree and ionic current of various tube types can be calibrated. When the ionic current is measured, the vacuum degree of the tube type can be obtained by querying the ionic current vacuum degree relationship curve of the tube type. This process is automatically completed by the single chip microcomputer

(3) standard of vacuum degree of vacuum interrupter

the performance of vacuum interrupter has an important impact on the safe operation of vacuum circuit breaker, and vacuum degree is the key parameter of vacuum interrupter, for which specific requirements are made in relevant regulations and standards. The industry standard "technical conditions for ordering 10~35kv indoor high voltage vacuum circuit breakers (DL)" stipulates that the vacuum degree of vacuum circuit breaker arc extinguishing chamber shall not be greater than 6.6 at the end of the validity period of 15 ~ 20 years × Pa[l] the industry standard "3.6~405kv indoor AC high voltage vacuum circuit breaker (JB)" stipulates that the gas pressure in the vacuum arc extinguishing chamber used to assemble the vacuum circuit breaker should be lower than 1.33 × Pa[2]。

3. Advantages and disadvantages of power frequency voltage withstand method and vacuum degree test method

3.1 air leakage fault of vacuum arc extinguishing chamber

air leakage fault of vacuum arc extinguishing chamber can be divided into two types, one is "hard fault", that is, the external light is broken, the bellows is broken and the air is fed, so that the arc extinguishing chamber loses its vacuum degree and is connected with the surrounding atmosphere; The other is "soft fault", that is, the arc extinguishing chamber is not connected with the atmosphere, but the gas pressure in the arc extinguishing chamber is higher than the allowable value due to manufacturing process, transportation, installation, use and maintenance, etc. the arc extinguishing chamber cannot meet the normal breaking capacity, and there is a latent fault. The traditional detection method, power frequency withstand voltage method, is more effective in customer service for the "hard fault" of the vacuum interrupter, and can qualitatively distinguish the quality of the vacuum interrupter; But there is nothing we can do about "soft failure". Because when the true space is 1 × 104Pa～1 × PA, the power frequency withstand voltage test can pass, and the difference cannot be distinguished. The vacuum tester can test 1 × 10-lpa～1 × Accurately measure the vacuum degree within the range of PA, master the change of vacuum degree of the vacuum interrupter, understand the leakage development of the vacuum interrupter, make the detection of the vacuum interrupter rise from qualitative to quantitative stage, and calculate the service life of the vacuum interrupter according to the change of vacuum degree within a certain interval, which provides a technical means to ensure the reliability of the operation of the vacuum circuit breaker. For example, in the same batch of 10kV vacuum circuit breakers in a substation, the vacuum degree pressure of most vacuum arc extinguishing chambers is lower than 6.60 × PA, but one vacuum pressure is 3.12 × PA, although it is also within the qualified range, and the power frequency withstand voltage test has also passed, but compared with similar arc extinguishing chambers, its vacuum degree is worse. This type of vacuum arc extinguishing chamber should be mainly monitored to grasp the transformation trend of its vacuum degree

3.2 vacuum test

the vacuum tester is limited by the test range (1 × pa～l × PA), which is beyond the test range, the relationship between the ion current relied on by the vacuum tester and the residual gas density, that is, the vacuum degree, has changed, and the data display cannot guarantee the accuracy of the test results. Especially for the "hard fault" with full leakage (connected with the atmosphere), the test value is just close to the value when the vacuum is good, which is easy to cause wrong judgment. The reason can be explained by the theory of gas collision and the principle of instrument testing vacuum degree: the gas pressure in the vacuum arc extinguishing chamber increases, and only a small ionic current flows between the fractures of the arc extinguishing chamber as the gas pressure is very small, because when the gas pressure is high, the gas density increases, and the average free travel of electrons decreases. At this time, although the number of collisions increases, However, due to the small kinetic energy accumulated by electrons, the possibility of ionization of gas molecules is reduced. In the process of use, this standard experimental condition can also be used to check the system, which leads to the false indication of the vacuum tester that the vacuum degree is good (low pressure). Before testing the vacuum degree, we cannot accurately judge whether the vacuum degree of the arc extinguishing chamber is within the test range, and we cannot guarantee the accuracy of the vacuum degree measured by the vacuum degree tester, Therefore, the power frequency withstand voltage test cannot be omitted in the field vacuum interrupter inspection. If the vacuum interrupter passes the power frequency voltage withstand test, it can ensure that the vacuum degree of the vacuum interrupter is within the test range of the vacuum degree tester. Then the vacuum degree test can ensure the accuracy of the vacuum degree test. In short, vacuum degree test and power frequency withstand voltage test should complement each other in order to make an accurate diagnosis of vacuum interrupter. In the field work, we have encountered three examples of Vacuum Interrupters with defects but good vacuum degree test: #1 the glass cover of the interrupter has been broken, but the vacuum degree test value is normal, the local altitude is 1700m, and the atmospheric pressure is 8.4 × 104pa，；# 3. When the voltage between the fractures of the arc extinguishing chamber is increased to 10kV, the breakdown occurs, and the vacuum test value is normal# 4. The insulation between the fractures of the arc extinguishing chamber is only 2Mo, the voltage between the fractures has just been increased, the current surges, and the vacuum test value is normal. It can be seen that it is not reliable to judge the quality of vacuum interrupter only by vacuum degree test

3.3 difficulties in conducting power frequency voltage withstand test of vacuum interrupter on site

if the bus is not powered off, the fracture voltage withstand test of vacuum interrupter is not easy to implement. Taking 10kV vacuum circuit breaker as an example, the manufacturer requires that 42KV frequency voltage be processed on both sides of the fracture of the vacuum arc extinguishing chamber. Because the 10kV bus is charged, it cannot be pressurized from the upper wiring board of the circuit breaker (the lower wiring board is grounded). Because the fracture distance of the disconnector on the upper side of the circuit breaker can not meet the test voltage requirements, although the measures of adding insulating diaphragms at the fracture of the disconnector can be taken to solve this problem, the risk is too great to be used by field testers; The pressurization method of pressurizing from the lower wiring board of the circuit breaker (the upper wiring board is grounded) is adopted, and because the lower wiring board is connected with dry-type CT and circuit breaker insulation pull rod and other equipment, their withstand voltage standard is 27KV, so the fracture of the vacuum arc extinguishing chamber can only be increased by 27KV. The low test voltage will reduce the sensitivity of this test to detect the defects of arc extinguishing chamber

3.4 difficulties in vacuum degree detection of vacuum interrupter on site

it is convenient to test the vacuum degree of handcart vacuum circuit breaker; It is troublesome and time-consuming to test the vacuum degree of the fixed cabinet vacuum circuit breaker on site. The cable room of the high-voltage switchgear is narrow and connected with high-voltage cables. Some circuit breakers are also equipped with arc barriers between phases. The tester has a small space in the cabinet, which makes the wiring inconvenient. During the test, the high-voltage outgoing cable must be disconnected. If the outgoing end of the circuit breaker is equipped with lightning arrester or resistance capacitance absorber, it must be disconnected, which is time-consuming and laborious

4. Conclusion

(1) the development of vacuum degree test makes our detection of vacuum interrupter rise from qualitative to quantitative stage, so that we can grasp the development and changes of vacuum interrupter leakage and estimate its service life. It is convenient to test the handcart vacuum circuit breaker, but it is not very convenient to test the fixed cabinet vacuum circuit breaker

(2) vacuum degree test cannot replace the traditional power frequency withstand voltage test. The vacuum test is limited by the test range (1 × Pa × 104pa), it is necessary to cooperate with the power frequency withstand voltage test to make an accurate diagnosis of the vacuum interrupter

(3) before the test, carefully check the appearance of the vacuum interrupter. If there is damage, it is not necessary to do other tests, and it should be replaced immediately; Although the vacuum degree test is good, the arc extinguishing chamber that fails to pass the power frequency withstand voltage must be replaced; The arc extinguishing chamber that passes the power frequency withstand voltage but the vacuum degree is unqualified must also be replaced: if the withstand voltage passes, the vacuum degree is qualified, but the vacuum degree drops rapidly, the detection should be strengthened. (end)