Brief Analysis Of Vacuum Circuit Breaker
Vacuum circuit breakers have been in operation in China for nearly 30 years. The development of High Voltage Disconnect Switch has two major technological leaps. The first was in the 1970s, when China's first generation vacuum circuit breakers appeared. It uses an Archimedean spiral groove contact structure, and the contact materials are copper-bismuth-silver and copper-bismuth-aluminum. Due to the limitations of its material properties, the breaking capacity can only reach 20 kA. Another time was in the early 1980s, when grooves were cut on the cup-shaped contacts to generate a transverse magnetic field when breaking, allowing the arc to rotate on the contacts, reducing contact burnout and increasing contact life. At the same time, copper-bismuth-antimony, copper-bismuth-aluminum and copper-chromium materials were developed for contact materials, making a big leap in the electrical life and mechanical life of the circuit breaker. In the mid-to-late 1980s, the Beijing Switch Factory introduced the German Siemens 3AF, the Guangzhou Nanyang Electric Factory introduced the Japanese Toshiba VK10J, the Belgian EIB VB5 and the ABB VD4, etc., which were represented by grooves cut on the cup-shaped disc contacts to generate a longitudinal magnetic field when breaking, making the arc diffuse and minimizing burnout. The contact materials are all made of new copper-chromium (CuCr) materials, which have a very low cutoff value, generally only 3~5 A. Therefore, the operating overvoltage can be limited in the inductive circuit, and the heavy breakdown of the capacitive load is almost non-existent, and the power frequency withstand voltage after the arc is basically not reduced, which overcomes the three major defects of vacuum switches in the 1970s and early 1980s: ① The power frequency withstand voltage is particularly low after the fault arc is broken; ② Heavy breakdown often occurs when breaking the capacitive load; ③ The operating overvoltage is particularly high in the inductive circuit. This has set off a vacuum switch fever in China. At present, there are more than 350 manufacturers producing vacuum circuit breakers, and there are more than 50 types of vacuum circuit breakers, which can be said to be the first in the world.
1 Types and differences of domestic vacuum circuit breakers at this stage
1.1 Types
The vacuum circuit breakers currently produced in China can be roughly divided into three categories:
The first type is a split structure, which is designed based on the original low-oil circuit breaker SN10 type. It is mainly for the transformation of old low-oil circuit breaker switchgear, and is also installed on new switchgear, such as ZN7-10X, ZN13-10X, ZN19-10X, ZN28-10A, etc.; The second type is a patchwork structure with independent model mechanisms, which is composed of CD10, CD17, CT8, CT17, CT19 and other mechanisms and vacuum interrupter and shaft, spring, etc. to form a circuit breaker, such as ZN7-10, ZN13-10X, ZN19-10X, ZN28-10A, etc. -10, ZN19-10, ZN28-10, etc.; The third type is an integral structure, which is characterized by the absence of independent mechanisms and low transmission loss of vacuum circuit breakers. This type of circuit breaker is mainly based on imported technology, such as the ZN12-10 vacuum circuit breaker, which is a Siemens 3AF vacuum circuit breaker introduced by Beijing Switch Factory, the ZN18-10 vacuum circuit breaker, which is a Toshiba VK10J vacuum circuit breaker introduced by Guangzhou Nanyang Electric Factory, the VD4 vacuum circuit breaker of Xiamen ABB Switch Co., Ltd., the VM1 permanent magnet vacuum circuit breaker, and the VS1 vacuum circuit breaker designed by Senyuan Company.
1.2 The difference between the third type and the first and second types of vacuum circuit breakers
1) The circuit breaker of the integral structure is generally a set of four-link transmission system; while the split structure and the patchwork structure are both two sets of four-link transmission systems or a set of five-link and a set of four-link transmission systems.
2) The contact compression stroke of the circuit breaker of the integral structure is generally 3~4 mm; while the contact compression stroke of the circuit breaker of the split structure and the patchwork structure is generally 6~10 mm.
3) The circuit breaker of the integral structure is designed according to the overall requirements of the circuit breaker in the control circuit; while the circuit breakers of the split structure and the patchwork structure are designed by imitation (the split imitates SN10-10) and makeshift (the split and the operating mechanism are combined).
2 Optimization design of vacuum circuit breakers
2.1 Dealing with the reliability of circuit breakers
The reliability of vacuum circuit breakers is of vital interest to users. From the day the vacuum circuit breaker came into being, its mechanical life has increased from 2 to 10 times that of traditional circuit breakers. In recent years, there are products with the capacity of 20,000 and 30,000 times. Siemens has products with the capacity of 60,000 and 120,000 times. 000 times of long-life vacuum circuit breakers, which is mainly due to the unparalleled characteristics of low-voltage arcs in vacuum state and the increased service life. Therefore, the mechanical life and reliability of the operating mechanism that cooperates with it become a very important issue. The reliability of a product is mainly guaranteed by manufacturing quality and design quality. The former requires the manufacturer to have a strict quality assurance system, which involves management, personnel quality and training, the adoption of new equipment and other factors. Therefore, in China, where the quality of personnel is not very high, the sense of responsibility is not very strong, the management is relatively backward, and the equipment is outdated, it is unrealistic to rely too much on the reliability of the product. The reliability of the product falls on the shoulders of the designer. A high-reliability product design must be a simple design structure, that is, using fewer parts to achieve the necessary functions of the product. In addition, the optimized design of the product should be mass-produced, which is conducive to improving quality and reducing production costs. Foreign vacuum circuit breakers all implement this principle, and reliability can be improved from two aspects.
2.1.1 Simplified connecting rod system design
As explained above, domestic vacuum circuit breakers generally use the original low-oil switch operating mechanism CD10, CT8 and its improved CD17, CT17, CT19, etc. They are all five-link structure systems, and their design intention is to meet the free tripping function, and the structure is complex. Some of the limit connecting rods are fragile. For example, the dead zone connecting rod of CD10, the half-axis structure of CD17, and the various half-axes and buckle plates of CT17 and CT19. "Free tripping" was proposed under a certain technical historical background. Not only can it not perform normal opening operations, but it will also cause personal safety accidents to operators. For electromagnetic mechanisms, the DC power supply is controlled by the old CZO-40C DC contactor, and its "opening time" (the time from cutting off the control coil power supply to the separation of the main contacts) is about 150~200 ms. If there is no free tripping function, the circuit breaker equipped with the electromagnetic mechanism cannot perform normal opening operations, even if the CZO-40D DC contactor designed specifically for circuit breakers (the action time is about 70 ms), when the oil-less switch is closed or opened, the high-quality closing iron core will not be reset in time, which will block the moving conductive rod movement route, reduce the opening switch speed, and affect the breaking performance. Vacuum circuit breakers do not have such concerns. The development of technology has made it inevitable that certain restrictions will disappear. The GB1984 standard and the Ministry of Electric Power's ordering technical conditions have both cancelled the restrictions on electromagnetic mechanisms with vacuum switches by free tripping. Therefore, the newly designed CD17 retains the free tripping function, which shows that the designer's thinking is not open enough. The free tripping of the CT17 and CT19 spring mechanisms is nothing but superfluous, causing unnecessary complexity in the structure and reducing reliability. There are also reports in foreign countries (such as Japan and Germany) that the free tripping device is cancelled to improve reliability.
2.1.2 Integrated design
The idea of using a vacuum interrupter unit with an independent operating mechanism (electromagnetic or spring) unit to form a circuit breaker is a concept inherited from the oil-less switch. Because this method is harmful to both mechanical and electrical performance in the field of vacuum circuit breakers. Regardless of CD10, CD17, CT8, CT17 and CT19, they are composed of a five-link system. Its output shaft is not the main shaft of the circuit breaker, and its output shaft and the main shaft of the circuit breaker must form another set of four-links to transmit the closing force, which makes the structure complex and the transmission loss large. In contrast, the integrated circuit breaker (such as ZN12, ZN18, VD4, VM1, VS1, among which ZN12, ZN18, VD4, VS1 are spring force operated; VM1 is permanent magnetic force operated) is composed of a set of four-links, one of which is the main shaft of the circuit breaker. It has a simple structure, low transmission loss, and greatly improved reliability. Therefore, this type of vacuum circuit breaker should be vigorously developed.
2.2 Handle the opening process of the circuit breaker well
The opening process of the vacuum circuit breaker is not as simple as people imagine. How to handle these processes is an important indicator of the quality of the vacuum circuit breaker design.
2.2.1 Handle the initial opening stage (arc striking stage) well
Modern theory proves that the initial opening stage (0~3mm) of the vacuum circuit breaker is crucial to the breaking performance. The arc current of the vacuum circuit breaker at the beginning of the opening always changes from the concentrated type to the diffuse type. The faster this process changes, the better. At present, all technicians in this field should implement this concept in the design.
There are three measures to speed up the transformation of the arc current from the concentrated type to the diffuse type at the beginning of the opening.
① Reduce the mass of moving parts: In the process of developing vacuum circuit breakers, the conductive clip is reduced to reduce the mass of moving parts. After comparison, the result is that the initial opening speed is improved to varying degrees.
② Increase the elastic force of the opening spring, and make it work in the initial opening (0~3 mm).
③ The contact compression stroke must be as small as possible (2~3 mm), so that the opening spring can participate in the opening movement as soon as possible. Because the traditional circuit breaker moving and static contact mode is plug-in type. When a short circuit current occurs, the electric force makes the plum blossom contact fingers hold the conductive rod tightly, and the component force in the moving direction of the moving conductive rod is zero. The moving and static contact mode of the vacuum circuit breaker is plane contact. When a short circuit current occurs, its strong electric force is a repulsive force on the contact movement. In this way, the separation of the contacts does not have to wait for the release of the contact compression spring to be pulled by the opening spring. There is no lag (or very small lag) between its separation and the main shaft movement time. If the stroke of the compression spring is very small, the opening spring can participate in the movement as soon as possible. In order to increase the initial separation speed. Therefore, the compression of the contact compression spring of the vacuum circuit breaker currently developed is as small as possible (2~3 mm), since the driving force of the initial separation stage is the repulsive force of the electromotive force, the range of moving mass to be reduced is all moving parts. It can be seen that the transplantation of the split structure and the patchwork structure to the design of the vacuum circuit breaker is not conducive to improving the initial separation speed of the vacuum circuit breaker due to the long and too many connecting rods.
2.2.2 Handle the second stage of the separation (arc extinguishing stage 3~8 mm)
When the contacts are separated to 3~4 mm, the transformation of the arc to the diffusion type has been completed, and this is a good time to extinguish the arc (a large number of tests have confirmed that the arc extinguishing distance is 3~4 mm). If the current passes through zero at this time, the limited metal vapor density decays quickly. When the XL inequality holds true (X is the collision free travel of the metal particles; L is the electrical opening distance), the insulation strength between the fractures recovers quickly and the breaking is successful. In order to make X greater than L faster, what is the second stage speed of the moving contact? (The original power of the second stage movement speed is mainly based on the opening spring). In a three-phase system, if the arc is to be extinguished at zero point, it will take 3ms (the contacts are separated between the two zero points, so the opening distance is large enough at this time). Therefore, the arc should be extinguished at an opening distance of 3~4 mm. The average opening speed during this period should be 0.8~1.1 m/s. Converted to the 6mm average opening speed widely used today, it is about 11.0~1.3 ms. This data is almost adopted by vacuum circuit breakers at home and abroad. However, this is the data measured for the mechanical operation of the circuit breaker when it is no-load. When breaking a large current, the opening speed will greatly exceed this value. This is because the repulsive force of the electromotive force participates in the movement. Therefore, in the same time, the moving contact will run to 6~8 mm, L value is too large, X cannot exceed L in most cases, and the arc will be extinguished for 3.3 or 6.6 mm. In order to shorten the arc burning time, buffering measures should be taken in the second stage after the opening of the gate to reduce the speed of the conductive rod movement in time and significantly. The early rubber buffer (can only be regarded as a late buffer) is definitely not good, and the oil buffer (mid-term buffer) is too late to act, and the effect is poor. Recently, the pneumatic buffer (full-course buffer) of Westinghouse Company in the United States has insufficient buffering force in the second stage. Therefore, the existing buffering methods cannot meet the technical requirements of modern vacuum circuit breakers. A simple buffer suitable for this requirement should be designed as soon as possible. At present, the spring force of the opening of many types of vacuum circuit breakers in China is designed to be very large (much larger than similar foreign products). The contact separation is fast in the opening stage, and it has no time to participate. The second stage is slow, but it is very powerful, and the arc burning time is prolonged, which makes it difficult to handle the third stage.
2.2.3 Handle the third stage of the opening gate (oscillation stage 8~11 mm)
Since the vacuum circuit breaker has a small opening distance and a short opening process, the fast-moving contacts must stop in such a short time. No matter what method is used, the final speed change rate is still very large, and strong vibration is inevitable. Therefore, aftershocks will generally continue for 30 ms. At present, the opening of vacuum circuit breakers at home and abroad generally takes about 10~12 ms for the moving contacts to separate and enter the earthquake zone, and the arcing time is mostly 12~15 ms. Obviously, the local surface of the contact melted by the arc begins to cool and solidify after entering the earthquake zone. Strong aftershocks will inevitably cause the liquid metal to fly shallowly and form sharp objects on the contact surface and suspended metal particles between the contacts. This is one of the external factors causing heavy breakdown. However, this design deficiency is often not fully reflected in limited type tests. Therefore, people have not fully understood this for a long time.
In short, as a designer of a vacuum circuit breaker, attention should be paid to the opening process. Reduce the mass of moving parts and increase the initial opening speed. Timely reduce the opening speed of the second stage, shorten the arcing time, and extinguish the arc before entering the earthquake zone. Give the contact surface a certain cooling time. At the same time, it is also required to reduce the vibration intensity so that the entire opening process conforms to the above mechanism, which is conducive to improving the mechanical life and electrical life.
2.3 Handle the closing process of the vacuum circuit breaker The closing process of the vacuum circuit breaker is much simpler than the opening process, and its breakdown distance is very small (about 1 mm), and it will not cause the medium to vaporize and expand after breaking down and striking the arc like oil and SF6 circuit breakers. Therefore, its closing speed is much lower than other circuit breakers. The electromagnetic mechanism is 0.35~0.50 m/s, and the spring mechanism is 0.8~1.0 m/s. In fact, vacuum circuit breakers do not have strict requirements on closing speed, as long as they can complete the closing action (including closing capacity). When talking about the closing process, we must talk about the closing bounce problem, because the contacts of vacuum circuit breakers are flat contacts, and there will be rebound under collision. Therefore, the rebound must be controlled within a certain range. The definition of "closing bounce time" is shown in Figure 1 (closing operation waveform diagram). Closing bounce time T=T1+T2. Among them, T1 is the stagnation time of the moving contact and the static contact before bouncing, and T2 is the time when the moving contact bounces and flies. T1 in the closing bounce time is related to the closing speed. The faster the closing speed, the smaller the value of T1, and vice versa. Therefore, the T value of the spring mechanism is smaller than that of the electromagnetic mechanism. A large number of tests have shown that for those who only have one closing bounce, T=1.8~2.6ms; for those who have two closing bounces, T>5 ms. When people try to eliminate the bounce and strive to reduce the T value to less than 2 ms, but ignores that this is only a mechanical operation performed when no load is applied. When closing the short-circuit fault point and closing the load current, the situation is completely different.
When the distance is crossed, it causes breakdown, generates strong electric repulsion, and slows down the impact of the contact. At this time, will there be the bounce measured when no-load? After consulting the waveforms recorded by the oscilloscope of the short-circuit current test of Xi'an High Voltage Electrical Equipment Research Institute and Beijing Electric Power Research Institute, it is not difficult to see that no bounce was found, which fully proves that the electric repulsion after breakdown is an effective means to eliminate bounce. In addition, adding an external buffer to achieve no-load bounce is redundant, which will also prolong the breakdown arcing time and cause unnecessary burning.
3 Selecting vacuum interrupter
At present, the varieties of vacuum interrupters in China are not worse than those in foreign countries. The vacuum interrupters using longitudinal and transverse magnetic field technology and the vacuum interrupters using central ignition contact technology, and the performance of Cu-Cr alloy contact materials have reached the world level. Contacts made of Cu-Cr alloy have successfully interrupted 50 kA and 63 kA. Ceramic tubes have an absolute advantage in the shell material of vacuum interrupter. However, glass tubes still have vitality because of their low price, stable sealing technology, and lower leakage rate than ceramic tubes. However, ceramic tubes have high brittleness resistance and are easy to mass produce. China's vacuum interrupter has reached a relatively high level. Vacuum circuit breakers can use domestic vacuum interrupters.
4 Type test and various parameters of vacuum circuit breakers
The parameters and type test items of vacuum circuit breakers often represent its level and value. Therefore, in today's fierce market competition, various manufacturers have invested a lot of money to surpass their opponents in type tests. They improve the test content mutually, but the product itself is rarely improved, which leads to wrong guidance for some users who do not know the truth. In fact, it has more disadvantages than advantages and should be treated correctly.
4.1 Full-capacity short-circuit current interruption times This is a test that shows the electrical life of a certain type of vacuum interrupter when breaking the rated short-circuit current in cooperation with a circuit breaker of a certain technical level and structure. It is not considered that a circuit breaker with a large number of breaking times is a good circuit breaker, because when the circuit breaker is running on the power grid, it is impossible for so many short-circuit faults to occur, and too many breaking times are not beneficial to users.
4.2 Test of cutting capacitor bank
The vacuum circuit breaker has a strong breaking capacity and is suitable for frequent operation and is widely used in cutting capacitor banks. The relevant standards stipulate that if the overvoltage caused by heavy breakdown is less than 2.5, it is judged to be qualified.
4.3 Stroke and contact spring compression overtravel
At present, the stroke of foreign vacuum circuit breakers is 8 mm, and the stroke of domestic ones is 11 mm; the compression overtravel of the contact spring of the vacuum circuit breaker should be selected within 3~4 mm according to the above explanation.
4.4 Disconnection speed
At present, the disconnection speed of vacuum circuit breakers is mostly 1.0~1.2 m/s. From the above analysis, the initial speed of the opening should be strictly controlled (0~3mm).
4.5 Closing bounce time The closing bounce is harmless, but the bounce is generally not more than 2 ms to 3 ms.
4.6 Three-phase synchronization of opening and closing
As far as circuit breakers are concerned, the requirement of three-phase synchronization is proposed on the basis of traditional circuit breakers, because the breaking capacity of oil circuit breakers and SF6 circuit breakers is in a certain interval after the contact separation. The breaking capacity is weakened or disappears outside this interval, so the three-phase synchronization is required to be higher. The vacuum circuit breaker has a strong self-extinguishing ability and has the ability to extinguish arcs in any interval, so synchronization is not important. Even if the vacuum circuit breaker has a synchronization of up to 5 mm, when the current zero crossing coincides with the lagging phase, the consequence is only 3.3 more arcs per phase. ms, and when the current zero-crosses in the leading phase, there will be no adverse consequences.
Different opening and closing phases have no adverse effect on the stability of the power grid, because the arc extinguishing time of the three-phase system is always different, and the first opening phase and the second opening phase are always 5 ms apart, so the different phases have no effect on the power grid.
This is the case for opening and closing, so there is nothing to worry about.
In short, different opening and closing phases do not cause much damage to the power grid and its circuit breaker itself. Technical workers who design vacuum circuit breakers should have enough understanding to understand what is particularly critical and what is insignificant.
5 Conclusion
In summary, after more than 30 years of development, China's vacuum circuit breakers have accumulated rich design and production experience, and the production means are also moving towards modernization at a high speed, and have a considerable production scale. However, there are still many weak links in design and technology, and overcoming these weaknesses is a top priority.
