[Key points of miniaturized vacuum interrupter design and manufacturing]: Abstract: This paper introduces the characteristics of miniaturized vacuum interrupter with small size, high requirements for electrical properties and mechanical properties and low cost, and discusses in detail the key points that need to be considered in the design and manufacture of miniaturized vacuum interrupter around these characteristics, which has a certain reference effect on the research and development of miniaturized vacuum interrupter. 0...
Abstract: This paper introduces the characteristics of small size, high electrical properties and mechanical properties and low cost of miniaturized vacuum interrupter, and discusses in detail the key points that need to be considered in the design and manufacture of miniaturized vacuum interrupter around these characteristics, which has a certain reference role for the research and development of miniaturized vacuum interrupter.
0 Introduction
With the development of vacuum switch technology, the design and manufacturing level of the switch is increasing, especially the mechanical characteristics of the switch are becoming more and more scientific and stable, so that the arc extinguishing chamber design does not need to give too much margin to meet the performance requirements of the switch, and the arc extinguishing chamber can also develop in the direction of miniaturization in structure. At the same time, due to the emergence of solid-sealed poles, the volume of the switch is decreasing, so the miniaturization of the arc extinguishing chamber is even more urgent. In addition, with the emergence of Siemens, Eaton and other joint ventures, the market competition of the arc extinguishing chamber has intensified, then the price competition is also very critical, and the material cost usually accounts for 50% ~ 60% of the total cost of the arc extinguishing chamber, miniaturized products can greatly reduce the material cost of the arc extinguishing chamber, so that it has a good market competition qualification, so miniaturized vacuum interrupter is an inevitable trend in the development of the arc extinguishing chamber.
1 Features of miniaturized vacuum interrupter
Usually the miniaturized arc extinguishing chamber is used in the solid seal circuit breaker, so compared with the ordinary arc extinguishing chamber with the same parameters, the miniaturized vacuum interrupter has the following characteristics.
(1) Small size.
(2) High requirements for electrical properties and mechanical properties
1) Small electrode and large breaking current.
2) Small insulation gap and high insulation level.
3) Low loop resistance.
4) Long mechanical life.
(3) Low cost
It can be seen from the above characteristics that the design and manufacture of miniaturized vacuum interrupters should fully consider the control ability of the electrode to the arc during the breaking process, the conduction and thermal conductivity of the electrode, the design of the electrode structure, the selection of contact materials, the design of the insulation system, the design of the bellows and the manufacturing process of parts and the whole tube. At the same time, the circuit breaker equipped with the miniaturized vacuum interrupter is relatively small, so the distance between phases is limited, and the electrodynamic force between phases has a greater impact on the arc, so the shielded external magnetic field should be fully considered when designing the arc extinguishing chamber. In addition, the miniaturized arc extinguishing chamber considers its design and manufacturing costs to ensure that it has good market competitiveness and create high economic benefits for the company.
2 Key points of miniaturized vacuum interrupter electrode design
2.1 Selection of electrode structure
The electrode structure currently used generally has a cup-shaped transverse magnetic field electrode, a plate transverse magnetic field, a cup-shaped longitudinal magnetic field, a coil longitudinal magnetic field, a longitudinal magnetic field electrode with a horseshoe and a plate electrode. These electrode structures have their own advantages, and we can choose different electrode structures for the occasions used in the arc extinguishing chamber and different electrical parameters.
(1) Selection of transverse magnetic field electrode structure: relevant research data believe that the arc extinguishing chamber with parameters below 12kV/31.5kA, the breaking capacity of the transverse magnetic field electrode is better than the cup-shaped longitudinal magnetic field electrode of the same size (see Figure 1), and the loop resistance is significantly better than the cup or coil longitudinal magnetic structure electrode, but the ablation of the contact and shielding cylinder by the arc in the breaking process is relatively large, and the number of short circuit breaks of the current arc extinguishing chamber is more, and after multiple short circuit breaks, The shielding barrel of the transverse magnetic interrupter will be severely burned by the arc thrown on it, thereby destroying its insulation system, so the transverse magnetic electrode structure can be applied to miniaturized products below 12kV/25kA.
(2) Selection of longitudinal magnetic field electrode structure: longitudinal magnetic electrode has cup-shaped longitudinal magnetic field, coil longitudinal magnetic field, longitudinal magnetic field electrode with horseshoe, longitudinal magnetic electrode has low arc voltage when breaking, small arc energy, strong arc control ability of longitudinal magnetic field, small ablation amount of contacts, and relatively stable mechanical parameters in the process of closing and closing. Among them, the cup-shaped longitudinal magnetic electrode is relatively simple to process, and the conduction ability and short-circuit breaking ability can be effectively controlled by adjusting the inclination angle and rotation angle of the chute, so that both can reach a better level, so the cup-shaped longitudinal magnetic structure is preferred in the design of medium-voltage large-capacity miniaturized arc extinguishing chamber. Longitudinal magnetic field electrodes with horseshoes, which have a limited breaking capacity, are currently used in low-voltage or 12 kV load switches and interrupters for contactors. Coil longitudinal magnetic field electrode, the structure is difficult to process, long electrode path, although the magnetic field is strong but the resistance is large is not easy to be used in miniaturized arc extinguishing chamber, for low rated working current arc extinguishing chamber can be used.
2.2 Miniaturization design of cup-shaped longitudinal magnetic electrode
If the diameter of the electrode is small, the diameter of the arc extinguishing chamber can be small, so reasonable optimization of the electrode can better realize the miniaturization of the arc extinguishing chamber. The following mainly discusses the miniaturization of cup-shaped longitudinal magnetic electrodes.
A large number of magnetic field calculations and tube loading actual tests have proved that the introduction of ferromagnetic substances in the cup-shaped longitudinal magnetic electrode redistributes the magnetic field between the electrodes, so that the largest possible contact surface is within the control range of the magnetic field, so as to improve the contact breaking utilization area and effectively reduce the contact diameter. However, the introduction of the iron core will increase the longitudinal magnetic field lag time and have a negative impact on the breaking, so when introducing the iron core, not only should we consider strengthening the longitudinal magnetic field, but also fully consider reducing the longitudinal magnetic field lag time and try to eliminate the negative impact of the introduction of the iron core. The structural design of the iron core is very important, and the core structure commonly used in our company is very good. In order to reduce the hysteresis of the core, the magnetic field hysteresis can be reduced by blocking the magnetic conduction circuit. The well-structured core ensures the strength of the magnetic field and low hysteresis. The following table compares the breaking capacity of longitudinal field electrodes using iron cores with conventional longitudinal field electrodes.
The data in the above table is sufficient to show that the use of iron core can effectively reduce the contact diameter, which is conducive to the miniaturization design of the arc extinguishing chamber.
2.3 Selection of contact materials
The miniaturized vacuum interrupter breaking current is much harsher and the resistance requirements are lower than the ordinary arc extinguishing chamber with the same parameters, so we prefer CuCr contact material, which is superior in breaking ability, conductivity, and voltage resistance. At present, CuCr contact materials are manufactured by a variety of processes, including, vacuum hot carbon reduction fusion method (dissolution method) production, powder mixing and sintering, vacuum induction melting rapid solidification method (melting and casting method) and vacuum arc melting method used by Siemens, so what kind of process is good?
Through the previous experience, we can know that the contact material manufactured by the melt penetration method is difficult to avoid the phenomenon of enrichment of the same material, which is extremely unfavorable to breakage, easy to produce capacitive welding, and is not easy to be used in miniaturized vacuum interrupters.
The intergranular bonding of the metal particles of the mixed and sintered contact material is poor, and it is easy to pull off the material on the surface of the contact to form defects when breaking, so it is not conducive to breaking, and the performance of its cutting capacitance is relatively poor, so it is not suitable for use.
Fast vacuum induction melting [Key points of miniaturized vacuum interrupter design and manufacturing]: Abstract: This paper introduces the characteristics of miniaturized vacuum interrupter with small size, high requirements for electrical properties and mechanical properties and low cost, and discusses in detail the key points that need to be considered in the design and manufacture of miniaturized vacuum interrupter around these characteristics, which has a certain reference effect on the research and development of miniaturized vacuum interrupter. 0...
The contact material of the quick-solidification method (melting and casting method) overcomes the shortcomings of poor intergranular bonding of the dissolution contact material and copper enrichment of the dissolution contact material, and more impurities can be removed during the manufacturing process, so this contact material can be regarded as the first choice. Of course, in the selection of the arc extinguishing chamber electrical parameters to choose different grades of copper-chromium alloy contact materials, the current commonly used cast copper-chromium contact materials are CuCr30 and CuCr40, from the theoretical analysis of CuCr30 performance indicators are better than CuCr40 and CuCr50, but in actual use, contact fusion welding often occurs, therefore, we generally choose CuCr40. Perhaps as the processing level of the contact material increases, the performance of CuCr30 will be improved, and the contact material will be used in large quantities in the future, because its price is lower than that of CuCr40.
The contact material of the vacuum arc melting method shows very good performance on the relevant data, which is also the contact material widely used by Siemens.
2.3 Consideration of electric field structure
Compared with the ordinary arc extinguishing chamber, the internal insulation gap of the miniaturized vacuum interrupter is much smaller, so it must be reasonably designed when designing, and the following points need to be considered.
(1) The insulation gap should be relatively uniform, to avoid the formation of field strength points due to small local gaps, and breakdown is easy to occur here.
(2) Shielding design of the intersection of air, insulation materials and vacuum.
(3) The corners of the parts of the internal insulation system adopt arc transition as much as possible, so that the electric field transitions uniformly, and the electric field distortion is avoided and the overall insulation level of the arc extinguishing chamber is destroyed.
(4) The surface roughness requirements of parts should be relatively strict, and the surface of electrodes, shielding barrels, equalizing hoods and other parts must be smooth, which can avoid field to emission and increase the breakdown voltage of the arc extinguishing chamber.
3 Consideration of shielding external magnetic fields
The volume of circuit breaker used in miniaturized vacuum interrupter is relatively small, so the distance between phases is limited, and the electrodynamic force between phases has a greater impact on the arc, so the shielded external magnetic field should be fully considered when designing the arc extinguishing chamber.
At present, we mainly use ferromagnetic shielding barrels to shield the external lateral magnetic field, so that the arc does not appear biased when breaking, and the entire area of the contact is fully utilized to ensure the breaking ability of the electrode. This ferromagnetic shielding cylinder is for a larger capacity miniaturized vacuum interrupter, and can not be used for the arc extinguishing chamber with relatively low electrical parameters. Of course, ferromagnetic materials are relatively cheap, and no negative effect on the performance of the arc extinguishing chamber has been found, so from the perspective of cost, we can also consider increasing the amount of use. However, because the iron shielding cylinder has the problem of heating, its structure and avoid serious heating should be considered when using the iron shielding cylinder.
4 Low cost considerations
4.1 Low cost considerations by design
Try to design parts with simple and easy to process structure, and the assembly structure should be as simple and easy to operate as possible. In terms of materials, use materials that are as low as possible and can ensure the performance of the product. For example, shielding cylinders, protective covers, dynamic and static cover plates can all use stainless steel materials, avoiding the use of expensive oxygen-free copper materials, and the dynamic and static cover plate is sealed on the porcelain shell with oxygen-free copper. But for a very small volume of arc extinguishing chamber is not like this, the dynamic and static cover plate of the arc extinguishing chamber can consider the use of oxygen-free copper cover plate directly sealed on the porcelain shell, although the cost of materials has increased, in fact, due to one less to the airtight weld, one less solder ring, the total cost may not increase. When the part is relatively small, its cost is not mainly based on material costs, but more on processing costs. For example, if a very small oxygen-free copper part is made into a stainless steel cover plate and an oxygen-free copper sealing ring, the cost may not be reduced.
For some arc extinguishing chambers that cannot use oxygen-free copper and stainless steel sealing, such as exhaust arc extinguishing chambers, in addition to changing it to a primary sealing pipe, we can also introduce nickel-copper materials or iron-nickel materials as replacement parts for sealing parts of removable materials and porcelain, which will also reduce the cost a lot.
4.2 Reduce manufacturing costs
For parts, it is mainly according to the actual requirements of the arc extinguishing chamber, and the requirements are relaxed as much as possible for some sizes that do not affect the performance of the product, that is, to reduce the difficulty of processing, so as to improve production efficiency and yield to reduce the manufacturing cost of parts.
For the whole pipe, it is mainly to improve production efficiency and product yield through perfect processing technology to reduce manufacturing costs.
5 Key points of the final assembly process
In the assembly process of parts, the surface of the parts will definitely have different degrees of pollution, dust particles will be hidden in the gaps, and the surface of the contact will also be oxidized, which will affect the performance of the arc extinguishing chamber, especially the negative effect of the miniaturized arc extinguishing chamber is greater, so the following matters should be paid attention to when processing the whole pipe:
5.1 Cleaning of parts
In addition to ensuring vacuum hygiene in the assembly space, it is necessary to strictly implement the cleaning process and strictly control the cleaning of the cleaning solvent.
5.2 Guarantee of geometric tolerances
Since the internal insulation gap of the miniaturized arc extinguishing chamber is relatively small, its shape and position tolerance must be ensured during the assembly process. The large shape and position tolerance is very unfavorable to the insulation performance of the arc extinguishing chamber, so that the insulation performance of the arc extinguishing chamber is very unstable, some performance is very good, and some cannot reach the rated value, so to ensure the insulation of the miniaturized arc extinguishing chamber, it will greatly improve the quality of all our products.
5.3 Aging of the whole pipe
The whole tube old refining includes voltage old refining, current old refining and capacitor old refining, its purpose is mainly through various old refining burrs and oxides inside the arc extinguishing chamber, so as to improve the insulation level of the arc extinguishing chamber and ensure reliable breaking ability.
(1) The role of current aging: remove the oxide on the surface of the contact and the surface gas through the arc between the contacts. In the current aging process, a molten layer is formed on the surface of the contact, and when the arc is extinguished, the molten surface layer is re-solidified, forming a denser metal thin layer of 5~10μm, which is more conducive to breaking. At the same time, when the metal gas that maintains the arc diffuses and condenses on the surface of other parts, the formation of a fresh chromium layer has a suction effect, which can maintain the vacuum state in the tube, so the vacuum degree of the tube aged by current is usually improved.
(2) The role of capacitor refining: For some type test arc extinguishing chambers, it is also necessary to do capacitive current refining, it has little capacity, will not cause damage to the contact surface, and can also remove the burr generated on the contact during static refining, and the surface of the old refining contact will be covered with bright spots, which is the result of old refining.
(3) The role of voltage refining: through continuous discharge in the aging process, remove the tiny burrs between the dynamic and static contacts, between the contacts and the shielding cylinder, and improve the insulation level of the arc extinguishing chamber.
The effects and effects of the above kinds of old refining are different, and the different orders of old refining have different effects on the arc extinguishing chamber. Through long-term practice and exploration, we believe that the following old refining sequence is the most conducive to improving the quality of the arc extinguishing chamber: current old refining, capacitor refining old refining, and voltage refining old refining.
About author:Bi Dongli, an engineer, engaged in the design of vacuum interrupters
