First, the basic concept of vacuum
Vacuum technology, the “vacuum” refers to a given space, the gas pressure is below atmospheric pressure gas state, that is to say, compared with the normal atmospheric pressure, a gas is relatively lean state.
The degree of vacuum is an objective measure of the degree of lean gas. According to the theory of vacuum technology, the vacuum level is usually represented by gas pressure. In the International System of Units, the pressure is Pa (Pa) as a unit 1Pa = 1N / m2. Another common unit as well as child care (Torr), millimeters of mercury (mmHg), millibar (mbar), engineering atmospheric pressure (kg / cm 2).
Divided vacuum region is no uniform requirement, it usually is divided country:
Rough vacuum: (760 to 10) Torr
Low vacuum: (10 to 10-3) Torr
High vacuum: (10-3 to 10-8) Torr
Ultrahigh vacuum: (10-8 ~ 10-12) care
XHV: 10-12 Torr
Relations care and Pa: 1 Torr = 1 mm Hg (mmHg) = 133.322Pa, 1 Pa = 7.5 × 10-3 Torr.
Features vacuum region of their different applications are different, such as vacuum cleaners work in rough area, thermos bottle, light bulbs and other work in the low vacuum region, while the vacuum switch and some other vacuum devices are working in a high vacuum region.All of the vacuum pressure need the vacuum pumps,or vacuum system to reach,such as rotary vane vacuum pump,liquid ring vacuum pump,rotary piston vacuum pump,dry scroll vacuum pump and so on.
Second, the insulating properties of the vacuum gap
A pair of electrodes placed in a vacuum, when high voltage is applied, at a certain voltage will produce an electrical breakdown between the electrodes. Its breakdown and electrical breakdown in the air is very different. Air breakdown is due to a small amount of free electron gas in high-speed motion, collide with gas molecules in the electric field produced more electrons and ions, electrons and ions newborn again collide with neutral atoms to produce more electrons and ions. This avalanche ionization process in the discharge channel is formed between the electrodes, resulting in an arc. The vacuum or vacuum pump , due to the low pressure, gas molecules rarely, in such an environment, even if the electrode gap exists in electronic, they fly from one electrode while the other electrode, also rarely have the opportunity to collide with gas molecules. Thus there can be caused by the collision of electrons and gas molecules avalanche electrical breakdown. It is very rare because of the gas molecules, electrical breakdown required vacuum gap formation is possible at very high voltages out of the scene emission and other phenomena. Theoretically speculated that the electric field strength required to cause electrical breakdown will be reached when the 108V / cm or more, in fact, due to a series of negative factors such as the impact strength of the vacuum insulation gap electrode surface roughness, cleanliness, etc., will be lower than the theoretical calculation value several orders of magnitude.
A high degree of vacuum in the vacuum interrupter, typically 10-3 ~ 10-6 Pa, when the dielectric strength of the vacuum gap is much higher than an atmospheric pressure of air and the dielectric strength of SF6, the ratio of the transformer oil dielectric strength even higher. Because of the high dielectric strength vacuum, vacuum interrupter all clearances can be made very small. For example 12kV vacuum interrupter contacts open only from 8 ~ 12mm, 40.5kV vacuum interrupter contact opening distance as long as 18 ~ 25mm, vacuum interrupter other clearances are in this range of scales.
Third, the main factors affecting the level of vacuum insulation
Vacuum insulation is a very complex physical process and the mechanism is so far no clear conclusion. From a practical point of view, mainly in the following aspects:
1, the geometry of the electrodes
Geometry of the electrodes has a great influence on the distribution of the electric field, are often not appropriate due to the geometry, causing localized excessive concentration of the electric field caused by the breakdown, which is particularly prominent in the high voltage vacuum products.
Radius of curvature of the edges of the electrodes is an important factor. In general, large radius of curvature the capacity electrode is smaller than the breakdown voltage of the large radius of curvature.
In addition, the breakdown voltage is also inversely proportional to the electrode area, and that with the increase of the electrode area is reduced. The main area is increased resulting in a reduced withstand voltage because the discharge probability increases.
2, the gap distance
Vacuum breakdown voltage and the gap distance has a relatively clear relationship. Tests showed that when the gap distance is small (≤5mm), the breakdown voltage of the gap distance increases linearly increase, but with the further increase in the gap distance, the breakdown voltage of the slowdown, i.e. vacuum gap breakdown electric field strength increases gap distance decreases. When the gap reached a certain length (≥20mm), increasing the gap distance alone raise the pressure level has been very difficult, but this time using more favorable than single fracture fracture.
Generally believed that electrical short gap breakdown under field emission is mainly caused by the long gap between the lower and the electrical breakdown of the particle effects are mainly due.
3, the electrode material
Vacuum switches operate in a high vacuum over 10-2Pa, because this time is very rare gas molecules, gas molecules collide against the free breakdown has no effect, so the breakdown voltage and electrode materials exhibit a strong correlation.
Breakdown voltage of the vacuum gap varies with different electrode materials, the researchers found that the breakdown voltage and the hardness and mechanical strength of the material. In general, the hardness and high mechanical strength material, tend to have a higher dielectric strength. For example, steel electrodes hardness after quenching, before hardening than the breakdown voltage can be increased by 80%.
In addition, the breakdown voltage and also a cathode material physical constants such as melting point, specific heat and density are related, i.e., a material whose melting point higher breakdown voltage is higher. Specific heat and density in terms versa. The essence of the problem is in the same action of thermal energy, the greater the probability of occurrence of the molten material, the lower the breakdown voltage.
4, the degree of vacuum
Figure 1 shows the relationship between the breakdown voltage and the gas pressure in the gap between. Can be seen from FIG vacuum higher than 10-2Pa (10-4 Torr), the breakdown voltage is substantially longer with decreasing gas pressure increases, the gas molecules collide because the phenomenon is no longer free to function. When the gas pressure is gradually increased from l0-2Pa (vacuum reduction), the breakdown strength is gradually decreased, while close to 1 Torr (approximately 102Pa) minimum, and later with increased pressure and increased. As can be seen from the curve when the degree of vacuum higher than 10-2Pa its compressive strength is substantially unchanged. This indicates that the degree of vacuum interrupter when 10-2Pa more fully able to meet the needs of normal use.
5, the electrode surface condition
Condition of the electrode surface breakdown voltage greater impact on the vacuum gap. Oxide electrode surface, impurities and metal particles will cause the breakdown voltage of the vacuum gap decreased.
In addition, regardless of the electrode surface in a vacuum interrupter manufacturing process was how large current breaking will involve electrode surface becomes uneven, which will also make the breakdown voltage decreases.
6. Aging effect
Aging electrode voltage and current Burn Burn two kinds.
When a new test vacuum gap, the breakdown voltage tends to lower the first few times. With the increase of the number of trials of the breakdown voltage is gradually increased, eventually stabilize at a certain value. This breakdown voltage with frequency increasing breakdown voltage phenomenon is the role of the old mill.
Aging is the voltage on the electrode surface by microscopic projections discharge elimination of impurities and defects. So that a small current through the discharge point of the surface of microscopic projections seizure, evaporation, smooth surface of the electrode, a local electric field enhancement effect is reduced, improving the breakdown voltage. Aging purification effect on the electrode surface is also very important. Since the electron emission electrode surface prone to lower the work function of the place where the impurities, also makes the discharge breakdown melting and volatile impurities, can also increase the breakdown voltage of the gap. Aging process, if at the same time the vacuum pump pumping, the evaporating gaseous substance pumped, the better. Aging voltage only suitable for use in a vacuum gap breakdown voltage increase, the vacuum interrupter contacts gap breakdown voltage increase will not have much effect. Arc on the contact surface of the burning effect will make the voltage experienced all fail.