1. Overview
With the development of circuit breaker technology, vacuum circuit breakers have been widely used in the field of medium voltage with their own superior characteristics, especially in recent years, the implementation of the "oil-free" transformation project of circuit breakers, vacuum circuit breakers occupy an increasing share in the field of voltage in the power system.
The core component of the vacuum circuit breaker is the vacuum interrupter (vacuum bubble), the vacuum circuit breaker will have different degrees of leakage during the operation of its vacuum interrupter, and some may leak to the point of not working normally even within the expected service life, in this case the operation will cause serious consequences, vacuum circuit breaker accidents are mostly caused by this reason. Therefore, regularly detecting the quality of the vacuum interrupter has become a very important work.
2. The method of on-site detection of vacuum arc extinguishing chamber
The common method used to detect the vacuum interrupter on site is the power frequency withstand voltage method, and with the development of testing technology, the vacuum degree test method has gradually been widely used in the field.
2.1 Power frequency withstand voltage method
The power frequency withstand voltage method is to make the vacuum circuit breaker in the broken state, apply a certain power frequency voltage between the dynamic and static contacts (the voltage value is according to the manufacturer's technical standards, 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 inside the vacuum interrupter chamber, otherwise the power frequency withstand voltage method at the mouth of the vacuum interrupter should be replaced is actually an indirect method to judge whether the vacuum degree of the vacuum extinguishing chamber meets the requirements. If there is a leakage in the vacuum interrupter, the gas pressure in the vacuum interrupter rises to a certain extent, and the relatively short gap between the contacts cannot withstand the test voltage, and even the discharge breakdown phenomenon will occur during the boost process. The advantage of the power frequency withstand voltage method is that the principle is simple and easy to operate, and it can qualitatively detect the quality of the vacuum interrupter. During the handover test, it should be noted that the mechanical stroke of the vacuum circuit breaker contact should be adjusted to the required value before the power frequency withstand voltage is adjusted and then pressurized test.
2.2 Vacuum degree test method
(1) Vacuum degree test principle
The vacuum degree test method directly tests the vacuum degree of the vacuum extinguishing chamber (the vacuum degree in this article actually represents the absolute pressure of the gas in the arc extinguishing chamber, in Pa, which is different from the general vacuum degree meaning) to judge the quality of the vacuum interrupter. Most of the vacuum testers used in the field are disassembly-free products, and the instruments adopt advanced synchronous pulse magnetron discharge and single-chip microcomputer technology. The brief mechanism is as follows: rarefied gas molecules are ionized by electron collisions excited by a strong electric field, resulting in a large measurable ion current. Since this ion current is basically linear with the logarithmic value of the vacuum degree, the vacuum degree value can be obtained by simply measuring the ion current. The purpose of adding a magnetic field is to lengthen the travel of electrons, increase the probability of collision with gas molecules, increase ion current, and reduce the strength requirements of the applied electric field. The measurement range of ion current and the measured value are related to the applied electric field, magnetic field strength, and contact opening distance, but mainly depend 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 in the application of electric field and magnetic field, there is a suction and gassing effect, in order to accurately measure the port often use the method of synchronous application of pulse magnetic field and electric field, which is the basic principle of pulse magnetron discharge method.
(2) Test method
The vacuum arc extinguishing chamber dynamic and static contacts are separated, the pulse high voltage is applied, the electromagnetic coil is wound outside the arc extinguishing room, and a large current is passed to the coil, so as to generate a pulse magnetic field synchronized with the high-voltage electric field in the arc extinguishing chamber, so that under the action of the pulse magnetic field, the electrons in the arc extinguishing chamber move spirally, and collide with the residual gas molecules to ionize, and 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 conditions, the size of ion current is not the same, through experiments can calibrate the vacuum degree of various tube types and ion current correspondence curve, when the ion current is measured, you can obtain the vacuum degree of the tube type by querying the ion current - vacuum relationship curve of the tube type, this process is automatically completed by a single-chip microcomputer.
(3) The standard of vacuum degree of vacuum interrupter
The performance of the vacuum interrupter has an important impact on the safe operation of the vacuum circuit breaker, and the vacuum degree is the key parameter of the vacuum interrupter, and the relevant regulations and standards are clearly required. The industry standard "10~35kV indoor high-voltage vacuum circuit breaker ordering technical conditions (DL403-91)" stipulates that the vacuum interrupter arc extinguishing chamber is valid for 15~20 years, and the vacuum degree of the vacuum interrupter shall not be greater than 6.6×10-2Pa[l] The industry standard "3.6~405KV indoor AC high-voltage vacuum circuit breaker (JB3855-1996)" stipulates that the gas pressure in the vacuum interrupter used to assemble the vacuum interrupter should be lower than 1.33×10-3Pa [2].
3. Advantages and disadvantages of power frequency withstand voltage method and vacuum degree test method
3.1 Vacuum arc extinguishing chamber air leakage failure
The fault of vacuum interrupter leakage can be divided into two types, one is "hard fault", that is, the external bright is cracked, the bellows is broken and the air is intake, so that the arc extinguishing chamber loses the vacuum degree and communicates with the surrounding atmosphere; The other is a "soft fault", that is, the arc extinguishing chamber is not connected with the atmosphere, but due to manufacturing technology, transportation, installation and maintenance, etc., the gas pressure in the arc extinguishing chamber is higher than the allowable value, and the arc extinguishing chamber cannot meet the normal breaking capacity, and there is a latent fault. The traditional detection method, a power frequency withstand voltage method, is more effective for the "hard fault" of the vacuum interrupter, and can qualitatively distinguish the quality of the vacuum interrupter; But nothing can be done about "soft failures". Because when the vacuum degree is between 1×104Pa~1×10-3Pa, the power frequency withstand voltage test can pass, can not distinguish the difference, vacuum tester can accurately measure the vacuum degree in the range of 1×10-lpa~1×10-5pa, grasp the change of vacuum degree of the arc extinguishing chamber, understand the leakage development of the vacuum interrupter, so that the detection of the vacuum interrupter rises from qualitative to quantitative stage, And the service life of the vacuum interrupter can be calculated according to the change of vacuum degree in a certain interval period, which provides a technical means to ensure the reliability of the operation of the vacuum circuit breaker. For example, a substation of the same batch of 10KV vacuum circuit breakers, the vast majority of vacuum interrupters vacuum pressure is lower than 6.60×10-4pa, but there is a vacuum pressure of 3.12×10-3pa, although it is also within the qualified range, the power frequency withstand voltage test is also passed, but compared with similar arc extinguishing chambers, its vacuum degree is worse, this type of vacuum interrupter should be focused on monitoring, grasp the vacuum degree transformation trend.
3.2 Vacuum degree test
The vacuum tester is limited by the test range (1×10-1pa~l×10-5Pa), beyond the test range, the ion current relied on by the vacuum tester and the residual gas density, that is, the vacuum degree approximately proportional relationship changes, can not guarantee the accuracy of the test results. Especially for the "hard fault" of total leakage (communicating with the atmosphere), the test value is just close to the value when the vacuum degree is good, which is easy to cause wrong judgment. The reason can be explained by gas collision theory and the principle of instrument testing vacuum: the gas pressure of the vacuum extinguishing chamber increases, and it can also flow only a small ion current between the fractures of the arc extinguishing chamber like the gas pressure is very small, because when the gas pressure is large, the gas density becomes larger, and the average free travel of electrons decreases, at this time, although the number of collisions increases, but the possibility of ionization of gas molecules caused by the small kinetic energy accumulated by electrons is reduced, which leads to the wrong indication of the vacuum tester as the vacuum degree is better (small 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 the accuracy of the vacuum degree measured by the vacuum degree tester cannot be guaranteed, so the power frequency withstand voltage test cannot be omitted in the on-site vacuum interrupter testing work. If the vacuum interrupter passes the power frequency withstand voltage test, it can ensure that the vacuum degree of the vacuum extinguishing chamber is within the test range of the vacuum tester, and 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, the two methods should complement each other to make an accurate diagnosis of the vacuum interrupter. In field work, we have encountered three examples of vacuum interrupters with defects but good vacuum tests: #1灭弧室玻璃罩己破碎, but the vacuum test value is normal, the local altitude is 1700m, atmospheric pressure 8.4×104pa,; #3灭弧室断口间电压加至10KV时击穿, the vacuum degree test value is normal; #4灭弧室断口间绝缘仅2Mo, the voltage has just been boosted between the breaks, the current surges, and the vacuum test value is normal. It can be seen that it is unreliable to judge the quality of the vacuum interrupter only by the vacuum degree test.
3.3 Difficulty in carrying out the power frequency withstand voltage test of vacuum arc extinguishing chamber on site
If the bus bar is not interrupted, the vacuum arc extinguishing chamber fracture withstand voltage test is not easy to implement. Taking the 10KV vacuum circuit breaker as an example, the manufacturer requires the processing frequency voltage of 42kV on both sides of the vacuum interrupter fracture, because the 10kV bus bar is charged, can not be pressurized from the circuit breaker upper terminal board (the lower terminal board is grounded), because the upper side of the circuit breaker isolation switch break distance can not meet the test voltage requirements, although the measures of adding an insulating partition at the break of the isolation knife gate can be taken to solve it, but the risk is too large and it is not easy for field test personnel to use; The pressurization method of pressurizing from the lower terminal board of the circuit breaker (grounding of the upper terminal board) is adopted, and because the lower terminal board is connected with dry CT and circuit breaker insulation tie rod and other equipment, their withstand voltage standard is 27kV, so the vacuum interrupter fracture can only add 27kV. The low test voltage will reduce the sensitivity of the test to detect arc extinguishing chamber defects.
3.4 Difficulty in detecting the vacuum degree of vacuum interrupter on site
It is more convenient to do vacuum degree test for handcart vacuum circuit breaker; The vacuum degree test of the fixed cabinet vacuum circuit breaker is more troublesome and time-consuming to disassemble and wire. High-voltage switchgear cable room space is small, and connected with high-voltage cable, some circuit breakers are also equipped with arc insulation plates, testers in the cabinet activity space is small, wiring is inconvenient, test to undo the high-voltage outlet cable, circuit breaker outlet end equipped with lightning arrester or resistance absorber also have to be disconnected, more time-consuming and laborious.
4. Concluding remarks
(1) The development of vacuum degree test enables us to detect the vacuum interrupter from qualitative to quantitative stage, grasp the development and change of vacuum interrupter leakage, and estimate its service life. The test is more convenient for the car type vacuum circuit breaker, but the test is not very convenient for the fixed cabinet type vacuum circuit breaker.
(2) The vacuum degree test cannot replace the traditional power frequency withstand voltage test. Due to the limitation of the test range (1×10-1Pa×104Pa), the vacuum degree test must be combined with the power frequency withstand voltage test to make an accurate diagnosis of the vacuum interrupter.
(3) Before the test, the appearance of the vacuum interrupter should be carefully checked, and if it is damaged, no other tests are necessary, and it should be replaced immediately; Although the vacuum degree test is good, the arc extinguishing chamber that has not passed the power frequency withstand pressure must be replaced; The arc extinguishing chamber that is passed by the power frequency withstand pressure but the vacuum degree is unqualified must also be replaced: the pressure resistance passes, the vacuum degree is qualified, but the vacuum degree decreases quickly to strengthen the detection.
