What are the specific electrical characteristics of vacuum circuit breakers?

There is no doubt that vacuum circuit breaker is a typical electrical product in essence, but it is realized by complex mechanical transmission, so the secondary characteristics of mechanical products are more prominent in people's impressions. No wonder many switch enterprises always put mechanical engineers who are proficient in CAD technology in the first place when recruiting talents. Therefore, there is an abnormal phenomenon in our industry that all electrical products are developed by mechanical engineers. They naturally do mechanical design according to the parameters listed in the interrupter. If all goes well, nothing will happen However, the open-circuit type test failed to live up to expectations, and there has been a failure rate of 12- 15% for many years. The responsibility and reason are hard to find, and the pipe factory claims to operate strictly according to the process, and there is no suspicion of cutting corners. The whole machine factory has also completely pushed the responsibility, and the stroke, overtravel and opening and closing speed are not bad at all. In the end, it is often suspected that contact materials are installed and a "charge" is installed casually. It would be nice to have pores. Few people doubt whether the parameters provided by the pipe factory (especially the opening motion parameters) are correct. If not, how to be correct. This is the topic to be discussed in this article.

The mechanical characteristics of vacuum switch have the greatest influence on electrical performance. This article takes this as an example. The opening motion characteristics of vacuum switches are all expressed by "opening speed". The opening speed of the first generation vacuum switches in China (from 1970s to early 1980s) is defined by the average speed of the whole process. The second generation (from the mid-1980s to the present) was changed to the average of the first half, that is, the average of the first 6mm. This change has its positive significance, and it begins to smell of initial speed, but the problem has not been completely solved. In the process of the transition from less oil to vacuum in the mid-1990s, a phenomenon has attracted people's attention: the failure rate of split vacuum switch imitating the structure of less oil switch is much higher than that of other vacuum switches. What is the truth? The opening speed is also the first half1.20.2m/s. When we analyzed the spring force of the mechanical device, we found the problem. At that time, the opening speed of 10kV vacuum switch was basically the same, but the opening spring force was quite different. ZN 12 is 1 100N, ZN65 is 900N, and this split switch is 1800N, which is much higher than similar switches. This shows that the "equivalent mass" of the latter is much higher than that of the former. Although the average speed of 6mm is equal, the starting speed is very slow. This starting speed is not conducive to the condition of the first phase disconnection. Therefore, we think that the current definition of opening speed is not accurate enough, and meeting it cannot guarantee its electrical performance. 6mm is too far for the first opening stage and too close for the second opening stage. Therefore, it is necessary to define the breaking motion of vacuum switch scientifically, and to do this, we must have a preliminary understanding of vacuum arc, especially its dynamic characteristics.

When we study vacuum switch, we will be more or less influenced by the traditional concept of circuit breaker, especially the arc characteristics. It is very important to know the difference between the two. There are two major differences between vacuum arc and atmospheric arc: first, the medium of atmospheric arc is insulator, which becomes conductor after ionization. The medium of vacuum switch is metal vapor, and neither atomic state nor ionic state is an insulator. Therefore, the success of the former depends on ion recombination; The latter can only rely on the diffusion of metal vapor. When the density of metal vapor decays at the moment when the current crosses zero, the free stroke of metal particle collision is greater than the arc extinguishing distance (λ >; Ti0), establish the insulation strength of cracks. This is the basic property of vacuum gap insulation performance. Secondly, the dielectric gas molecules of atmospheric arc exist objectively, which makes the disordered thermal motion (Brownian motion) static and evenly distributed on the macro level. However, the vacuum arc medium does not exist originally, which is the result of evaporation of metal vapor from cathode spots. Once generated, it forms a pressure difference in a vacuum state, which has huge potential energy and spreads rapidly in a cone shape. At this time, the distribution gradient of metal vapor density is very large, and only at the tip of cathode spot cone can collision and dissociation (cathode plasma) occur in the region with high metal vapor density. Most areas of the arc basically do not collide and dissociate. We call it "diffusion" arc.

The linear volt-ampere characteristic endows it with the characteristics of being able to exist in parallel (small contact burning loss), and because of its large space λ >; Therefore, the necessary condition for successful switching of current zero crossing must be that the arc must spread before current zero crossing. Meet λ > Ti0 inequality.

With a preliminary understanding of vacuum arc, we can discuss how to set the opening characteristics of circuit breaker. Since it is a three-phase alternating current, it is also a zero-crossing interruption. There must be a first open phase and a second open phase (neutral ungrounded system).

The arc extinguishing environment required by the first phase opening and the second phase opening is different, and different environments must be created to meet their respective needs. There are both favorable and unfavorable conditions for the opening and closing of the first stage. The advantage is that the current is in the descending stage, and the actual current it interrupts is far less than the rated value. Disadvantages: the first phase opening recovery voltage is high (1.5 times the phase voltage). At this time, the contact has just separated, and the metal vapor in the narrow space is bound to have a high density and a small gradient, and the arc is in a high voltage state, just like the atmospheric arc, which seriously damages the contact. Any delay at this moment will fail the opening ceremony. The most effective measures to rapidly attenuate the metal vapor arc from high pressure to low pressure are to increase the initial velocity (0 ~ 2mm), expand the space where the metal vapor exists (the density is inversely proportional to the volume), and expand the diffusion channels of the surrounding metal vapor. Therefore, the design of vacuum circuit breaker must reduce the equivalent mass (ZN65 separates the contact pressure spring and its related components from the opening movement) and reduce the transmission link of the opening spring force. Of course, the most ideal thing is to learn non-vacuum circuit breaker to realize v 0 >；; 0。

After the first phase was successful, it was the second phase. Its advantages are: because the neutral point is not grounded, after one phase current is interrupted, the other two-phase current becomes a loop, one fracture arc goes out, and the other fracture current ends naturally. At this time, the recovery voltage of each fracture is half of the line voltage (phase voltage 0.866), which is much lower than the first phase recovery voltage 1.5 times the phase voltage. Moreover, the insulation strength of two vacuum gaps is much higher than that of a single fracture of a gap with twice the length. The disadvantage is that the arcing time in the later breaking stage is 5ms longer than that in the first breaking stage, and the contact burning is naturally more serious. There is another disadvantage: when the arcing time of the first opening phase is 4-6ms (high probability), the arcing time of the later opening phase will be 9- 1 1ms, so the arcing zone of the later opening phase will enter the mechanical vibration zone where the brake is turned off to the end, and the arcing environment is extremely bad.

After analyzing the advantages and disadvantages of the later opening stage, the second stage setting of vacuum switch opening characteristics has a theoretical basis. In order to create a favorable environment for arc extinguishing after the first phase of interruption, the interruption speed should be greatly reduced after the first phase of interruption, and the ideal arc extinguishing is 5-6mm in the middle. The reasons are as follows: First, it is not necessary to run for 5ms after startup than when it was first started. Secondly, the necessary condition for successful breaking is λ >; Ti0, the inequality with shorter Ti0 is easier to hold. Thirdly, short-distance arc extinguishing can reduce the breaking value when breaking small inductive current. Experiments show that the interception value in the later opening stage is 4.5 times that in the former opening stage. The reason is simple: for the same amount of metal vapor, the density of small space is higher than that of large space, which is equivalent to adding low melting point metal into contact material without affecting the ability of breaking large current. Fourth, the arc extinguishing environment should be as calm as possible, and it should be completed at a uniform or near uniform speed. At this point, the arc is completely extinguished, but the whole breaking process is not over yet, and the moving conductive rod is still moving. At this time, the switch designer does not handle the mechanical motion characteristics properly (most of them are not ideal at present), which will still leave the hidden danger of breaking failure. Therefore, the opening speed of the third stage crushing ratio should be further reduced to achieve two purposes: (1) reducing the mechanical vibration intensity. (2) Keep the mechanical vibration zone away from the back arc extinguishing zone. This important indicator is often ignored, which makes decision-making very troublesome.

All circuit breakers in the world obtain electrical performance through their mechanical characteristics, and vacuum switches are no exception. Moreover, it is different from traditional circuit breakers in some characteristics, such as small opening distance, small contact movement space, high insulation strength, extremely sensitive to electric field, mutual transformation of high and low voltage arcs, and uncertain arc volt-ampere characteristics. Its mechanical characteristics are more exquisite and important. It is no exaggeration to say that every millimeter of the vacuum switch opening process is learned. Therefore, designers are required to have a preliminary understanding of the micro-world relationship among mechanical motion, arc motion and thermal motion in the process of vacuum circuit breaker breaking, so as to break seemingly opposite things on the surface as much as possible: large current and small current; Cut off inductor current and capacitor current; High voltage arc and low voltage arc; Zero-crossing continuous opening (failure), zero-crossing opening (interception); Fast and slow opening movements; A new generation of vacuum circuit breaker with excellent performance and independent innovation is designed by dialectical unity of static and dynamic.