Vacuum circuit breaker has been operating in China for nearly 30 years, and the development of vacuum circuit breaker has two big technological leaps, one is in the 70s of the 20th century, when the Chinese generation vacuum circuit breaker appeared. It adopts the contact structure of the Archimedes spiral groove, the contact material is copper, bismuth silver and copper-bismuth aluminum, limited by its material properties, the breaking capacity can only reach 20 kA. The other time is in the early 80s of the 20th century, the groove is carved on the cup-shaped contact, so that it generates a transverse magnetic field when it is broken, so that the arc rotates on the contact, reduces the contact burning, and improves the contact life. In the mid-to-late 80s of the 20th century, the introduction of German Siemens 3AF by Beijing Switchgear Factory, the introduction of Japanese Toshiba VK10J by Guangzhou Nanyang Electric Appliance Factory, the Belgian EIB Company VB5 and its ABB VD4 as representatives, the use of cup-shaped disc contacts to carve grooves, in the breaking of the longitudinal magnetic field, so that the arc is diffuse, to minimize the burning. The contact materials are all made of copper-chromium (CuCr) new material, which has a very low interception value, generally only 3~5 A. Therefore, it can limit the generation of overvoltage in the inductive circuit, and the heavy breakdown of the interrupting capacitive load almost does not occur, and the power frequency withstand voltage after the arc is basically not reduced, which overcomes the three major defects of the vacuum switch in the 70s of the 20th century and the early 80s of the 20th century: (1) the power frequency withstand voltage is particularly low after the arc of the interrupting fault; (2) Heavy breakdown often occurs when breaking capacitive loads; (3) The operating overvoltage is particularly high in the inductive circuit. In this way, the vacuum switch fever has been set off in China, and the manufacturers of vacuum circuit breakers have exceeded 350, and the models of vacuum circuit breakers have reached more than 50 kinds, which can be said to rank first in the world.
1. Types and differences of domestic vacuum circuit breakers at this stage
1.1Species
At present, the vacuum circuit breakers produced in China can be roughly divided into three categories:
It is a split structure, which is designed according to the imitation of the original oil breaker SN10 type, mainly for the transformation of the old oil breaker switchgear, and is also installed on the new switchgear, such as ZN7-10X, ZN13-10X, ZN19-10X, ZN28-10A, etc.; The second type is a patchwork structure with an independent model mechanism, which is made up of CD10, CD17, CT8, CT17, CT19 and other mechanisms and vacuum interrupters and rotating shafts, springs, etc., to form a circuit breaker, such as ZN7-10, ZN13-10, ZN19-10, ZN28-10, etc.; The third type is the integral structure, which is characterized by no independent mechanism, less transmission loss of vacuum circuit breaker, such circuit breaker mainly to the introduction of technology accounted for the main body, such as ZN12-10 vacuum circuit breaker is the introduction of Siemens 3AF vacuum circuit breaker represented by Beijing switch factory, ZN18-10 vacuum circuit breaker for Guangzhou Nanyang Electric Appliance Factory introduced by Toshiba VK10J vacuum circuit breaker, Xiamen ABB Switch Co., Ltd. VD4 vacuum circuit breaker, VM1 permanent magnet vacuum circuit breaker and VS1 vacuum circuit breaker designed by Senyuan company, etc.
1.2 The third type is different from the first and second types of vacuum circuit breakers
1) The circuit breaker with integral structure is generally one set of four-link transmission system; The split structure and the patchwork structure are both two sets of four-link connected transmission systems or one set of five-link and one set of four-link connected transmission systems.
2) The contact compression stroke of the circuit breaker with integral structure is generally 3~4 mm; The contact compression stroke of the circuit breaker with split structure and patchwork structure is generally 6~10 mm.
3) The circuit breaker with integral structure is designed from the overall requirements of the circuit breaker in the control loop; The circuit breakers with split structure and patchwork structure are designed by imitation (split imitation SN10-10) and improvisation (combining split and operating mechanism).
2 Optimized design of vacuum circuit breaker
2.1 Handle the reliability of the circuit breaker
The reliability of the vacuum circuit breaker is of vital interest to the user. From the day of the advent of the vacuum circuit breaker, its mechanical life has jumped from 2 000 times to 10 000 times of the traditional circuit breaker, and there have been 20 000 times and 30 000 times of products in recent years, and the foreign Siemens company has 60 000 times and 120 000 times of long-life vacuum circuit breakers, which is mainly due to the unparalleled characteristics of the low-voltage arc in the vacuum state and the increase in service life. Therefore, the mechanical life and reliability of the operating mechanism have become very problematic. The reliability of a product is mainly guaranteed by the manufacturing quality and design quality, and the former requires the manufacturer to have a strict quality assurance system, which involves management, personnel quality and training, the use of new equipment and other factors. Therefore, in China, 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 itself. The reliability of the product rests on the shoulders of the designer. A high-reliability product design must be a simple design structure, that is, to achieve the necessary functions of the product with fewer parts. In addition, the optimized design of the product should be mass production, which is conducive to the improvement of quality and the reduction of production cost. Foreign vacuum circuit breakers are implementing this principle, which can improve reliability from two aspects.
2.1.1 Simplify the design of the linkage system
As mentioned above, domestic vacuum circuit breakers generally use the original oil-less switch operating mechanism CD10, CT8 and their improved CD17, CT17, CT19, etc. They are all five-link structural systems, and their design intentions are to meet the complex structure with free tripping function. Some of these limiting links are fragile. Such as the dead band connecting rod of CD10, the half-shaft structure of CD17, and various half-shafts and gussets of CT17 and CT19." "Free tripping" is proposed in a certain technical and historical background, which not only cannot perform normal opening operations, but also causes personal safety accidents to operators. For the electromagnetic mechanism, the DC power supply is controlled by the operation of the old CZO-40C DC contactor, and its "opening time" (the time from the time when the power supply of the control coil is removed to the separation of the main contact) is about 150~200 ms. Because the high-quality closing core is too late to reset, it will block the movement route of the conductive rod, reduce the opening and switching speed, and affect the breaking performance. Vacuum circuit breakers have no such concerns. The development of technology makes it inevitable that certain restrictions will die out. The GB1984 standard and the technical conditions of the Ministry of Electric Power have cancelled the restriction of free tripping on the electromagnetic mechanism with vacuum switch. Therefore, the newly designed CD17 retains the function of free tripping, which indicates that the designer is not open-minded enough. The CT17 and CT19 spring mechanism to engage in free tripping is tantamount to adding to the snake, resulting in unnecessary complexity of the structure, reducing reliability, in foreign countries (such as Japan, Germany) there are also reports on canceling the free tripper 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 continuation of the concept of oil less switch. Because this method is used in the field of vacuum circuit breakers, whether it is harmful to mechanical properties or electrical properties. CD10, CD17, CT8, CT17 and CT19 are composed of a five-link system. Its output shaft is not the main shaft of the circuit breaker, and the output shaft and the main shaft of the circuit breaker must form another set of four-link to transmit the closing force, so that the structure is complex and the transmission loss is large. Compared with the integrated circuit breaker (such as ZN12, ZN18, VD4, VM1, VS1, where ZN12, ZN18, VD4, VS1 are spring force operation; VM1 is a permanent magnet operation) is composed of a set of four connecting rods, one of which is the main shaft of the circuit breaker, the structure is simple, the transmission loss is small, and the reliability is greatly improved. Therefore, such vacuum circuit breakers should be vigorously developed.
2.2 Handle the opening process of the circuit breaker
The opening process of vacuum circuit breaker is not as simple as people think, and how to deal with these processes is an important indicator of the quality of vacuum circuit breaker design.
2.2.1 Handle the initial stage of opening (arc initiation stage)
Modern theory proves that the initial stage (0~3mm) of vacuum circuit breaker is very important for the breaking performance. The arc current of the vacuum circuit breaker always changes from the agglomeration type to the diffusion type at the beginning of the opening, and the faster the transformation process, the better. At present, all technicians engaged in this industry should implement this concept into the design.
There are three measures to speed up the transformation of the arc current at the beginning of the opening from the agglomeration type to the diffusion type.
(1) Reduce the quality of moving parts: In the process of developing vacuum circuit breakers, reduce the quality of moving parts by reducing the conductive clamps, and the results are that the initial division speed has been improved to different degrees.
(2) Increase the elastic force of the opening spring, and make it play a role in the initial stage of 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 dynamic and static contact contact method is plug-in. When a short-circuit current occurs, the electrodynamic force makes the plum blossom touch finger to hold the conductive rod, and the force component in the direction of movement of the moving conductive rod is zero. The dynamic and static contact contact mode of vacuum circuit breaker is plane contact, and its strong electrodynamic force is repulsive force to the contact movement when the short-circuit current occurs. In this way, the separation of the contact does not have to wait for the release of the contact compression spring to be pulled by the opening spring, and its separation and spindle movement time does not lag (or the lag is very small), if the travel of the compression spring is small, the opening spring can participate in the movement as soon as possible. In order to increase the speed of the initial score. Therefore, the compression of the contact compression spring of the vacuum circuit breaker is as small as possible (2~3 mm), since the prime mover of the initial stage is the repulsion force of the electrodynamic force, the motion mass range to be reduced is the whole moving parts. It can be seen that the split structure and patchwork structure are transplanted to the design of the vacuum circuit breaker, because the connecting rod is too long and too many, which is not conducive to improving the initial division speed of the vacuum circuit breaker.
2.2.2 Handle the second stage of opening (arc extinguishing stage 3~8 mm)
When the contact is separated to 3~4 mm, the transition from arc to diffusion type has been completed, and it is a good time to extinguish the arc (the opening distance of the arc extinguishing is confirmed to be 3~4 mm from a large number of tests). L is the opening distance of electricity), the insulation strength between the fractures is quickly restored, and the breaking is successful, in order to make X faster than L, the second stage speed of the moving contact shall prevail (the driving force of the second stage of the movement speed is mainly based on the opening spring). The average opening speed should be 0.8~1.1 m/s, which is converted into the average opening speed of 6mm, which is widely used today, is about 11.0~1.3 ms, which 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 at no load. When breaking a large current, the opening speed will greatly exceed this value. This is because the repulsive force of electrodynamic forces is involved in motion. Therefore, in the same time, the moving contact will run to 6~8 mm, the L value is too large, X can not exceed L in most cases, and the arc will be extended by 3.3 or 6.6 mm to extinguish, in order to shorten the arcing time, buffering measures should be taken in the second stage after the opening, and the speed of the conductive rod movement should be greatly reduced in time. The early rubber buffer (can only be regarded as the later buffer) is definitely not good, and the oil buffer (mid-buffer) is too late and the effect is not good. Near Westinghouse's pneumatic cushioning (full buffering) in the second stage of the cushioning force is not. Therefore, the existing buffer methods can not 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, many types of vacuum circuit breakers in China use the opening spring force is designed to be very large (much larger than similar foreign products).
2.2.3 Deal with the third stage of opening (8~11 mm in the shock stage)
Due to the small opening distance of the vacuum circuit breaker, the short opening process time, the fast-moving contact should stop in such a short time, no matter what way is used, the final velocity change rate is still very large, and strong vibration is inevitable, therefore, the aftershock will generally continue for 30 ms. Nowadays, the vacuum circuit breaker at home and abroad is opened, and the moving contact generally lasts about 10~12 ms from separation to entering the earthquake area, and the arcing time is mostly 12~15 ms. Obviously, The local surface of the arc melted contact begins to cool and solidify after entering the earthquake zone. Strong aftershocks inevitably cause the liquid metal to fly shallow, forming sharp objects on the surface of the contact and suspended metal particles between the contacts. This is one of the external causes of heavy breakdown. However, this design deficiency is often not fully reflected in the limited type test. Therefore, people have not been fully aware of this for a long time.
In short, as the designer of vacuum circuit breaker, the opening process should be paid attention to. Reduce the mass of moving parts and increase the initial division speed. Reduce the opening speed of the second stage in time, shorten the arcing time, make the arc extinguish before entering the earthquake area, give a certain cooling time to the contact surface, and also require to weaken the vibration intensity, so that the whole opening process conforms to the above mechanism, which is conducive to improving the mechanical life and electrical life.
2.3 Deal with the closing process of the vacuum circuit breaker The closing process of the vacuum circuit breaker is much simpler than the switching process, and its breakdown distance is very small (about 1 mm), and it will not vaporize and expand the medium after breaking down the arc like oil and SF6 circuit breakers. Therefore, its closing speed is much lower than that of other circuit breakers. The electromagnetic mechanism is 0.35~0.50 m/s, and the spring mechanism is 0.8~1.0 m/s. So the rebound must be controlled within a certain range." The definition of "closing bounce time" is shown in Figure 1 (schematic diagram of closing operation waveform).Closing bounce time T=T1+T2.where T1 is the contact stagnation time between the moving contact and the static contact before springing up, and T2 is the time when the moving contact bounces up and takes off. 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, the greater T1. Therefore, the T value of the spring mechanism is smaller than that of the electromagnetic mechanism. A large number of tests show that where the closing bounce only occurs once, T=1.8~2.6ms; When people try to eliminate the bouncing and try to reduce the T value to less than 2 ms, they ignore that this is only a mechanical operation when there is no load.> When the dynamic and static contacts enter the breakdown distance when closing, they cause breakdown, produce a strong electric repulsion force, and slow down the impact of the contact. Will there still be a measured bounce when no-load at this time? After consulting the waveform of the size short-circuit current test of Xi'an High Voltage Electrical Apparatus Research Institute and Beijing Electric Power Research Institute, it is not difficult to see that no bounce has been found, which fully proves that the electric repulsion force after breakdown is an effective means to eliminate bounce.
3 Select a vacuum interrupter
At present, the varieties of domestic vacuum interrupters are no worse than those of foreign countries, the vacuum interrupters using longitudinal and transverse magnetic field technology, the vacuum interrupters using the central ignition contact technology, the performance of Cu-Cr alloy contact materials has reached the world level, and the contacts made of Cu-Cr alloy materials have successfully broken 50 kA and 63 kA. However, the glass tube still has its vitality, because of the cheap price, stable sealing technology, the air leakage rate is lower than that of the ceramic tube, but the embrittlement resistance of the ceramic tube is high, which is convenient for mass production. China's vacuum interrupters have reached a high level. The vacuum circuit breaker can choose the domestic vacuum interrupter.
4. Vacuum circuit breaker type test and its various parameters
The parameters and type test items of the vacuum circuit breaker often represent its level and value. Therefore, in today's fierce market competition, manufacturers do not hesitate to invest a lot of money to surpass their opponents in the type test. Mutual improvement of the content of the experiment, but the product itself is rarely improved, causing some users who do not know the truth to cause misdirection. In fact, there are more disadvantages than benefits and should be treated correctly.
4.1 The number of short-circuit current interruptions at full capacity This is an indication of the electrical life of a certain type of vacuum interrupter with the cooperation of a circuit breaker in the form of a certain technical level structure to break the rated short-circuit current. It is not considered that a circuit breaker with a large number of breaks is a good circuit breaker, because when the circuit breaker is running on the power grid, it is impossible to have so many short-circuit faults, and too many breaking times are not beneficial to users.
4.2 Fit capacitor bank test
The vacuum circuit breaker has strong breaking ability, and is suitable for frequent operation and is widely used in the capacitor bank, and 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 the foreign vacuum circuit breaker is 8 mm, and the domestic stroke is 11 mm; The vacuum circuit breaker contact spring compression overtravel should be selected within 3~4 mm through the above description.
4.4 Opening speed
At present, the opening speed of vacuum circuit breaker is mostly 1.0~1.2 m/s, from the above elaboration and analysis, the initial speed of opening should be checked (0~3mm).
4.5 Closing bounce time The bounce of closing is harmless, but the bounce is generally not more than 2 ms to 3 ms.
4.6 The three phases of opening and closing are the same
As far as the circuit breaker is concerned, the requirements of the same period are put forward on the basis of the traditional circuit breaker, because the breaking capacity of the oil circuit breaker and SF6 circuit breaker is in a certain interval after the contact is separated, and the breaking capacity will weaken or disappear after this range, so the three-phase is required to have a higher period. The vacuum circuit breaker has a strong self-extinguishing arc ability, and has the arc extinguishing ability in any interval, and the synchronicity is not important. Even if the performance of the vacuum circuit breaker is as high as 5 mm, when the current crosses the zero coincides with the backward phase, the consequence is only 3.3 ms more arcing per phase, and when the current crosses the zero is in the advanced phase, there is no adverse consequence.
There is no adverse effect on the stability of the power grid in different phases of the switch, because the arc extinguishing time of the three-phase system is always in different phases, and the first phase and the last phase are always separated by 5 ms, so there is no impact on the power grid in different periods.
The opening is the same, and there is no need to worry about the different periods of closing.
In short, the different phases of opening and closing do not cause much damage to the power grid and its circuit breaker itself, and the technical workers of vacuum circuit breaker design should have enough understanding in order to grasp what is particularly critical and what is insignificant.
5 Concluding remarks
To sum up, after more than 30 years of development, China's vacuum circuit breaker has 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 weaknesses in design and craftsmanship, and overcoming these weaknesses is a top priority.
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