The main factors affecting the vacuum insulation level
Vacuum insulation is a very complicated physical process, and the mechanism has not yet been clearly defined. From the actual application situation, there are the following aspects:
1.The geometry of the electrode
The geometry of the electrode has a great influence on the distribution of the electric field. Often, due to the insufficient geometry, the electric field is locally concentrated and the breakdown is caused. This is particularly prominent in high-voltage vacuum products.
The radius of curvature of the electrode edges is an important factor. Generally, an electrode with a large radius of curvature can withstand a breakdown voltage greater than a small radius of curvature.
In addition, the breakdown voltage is also inversely proportional to the size of the electrode area, that is, it decreases as the electrode area increases. The increase in area leads to a decrease in withstand voltage mainly due to an increase in discharge probability.
The breakdown voltage of vacuum has a clear relationship with the gap distance. The test shows that when the gap distance is small (≤5mm), the breakdown voltage increases linearly with the increase of the gap distance, but with the further increase of the gap distance, the growth of the breakdown voltage slows down, that is, the electric field where the vacuum gap breakdown The intensity decreases as the gap distance increases. When the gap reaches a certain length (≥20mm), it is already very difficult to increase the pressure resistance level by increasing the gap distance alone. At this time, using multiple fractures is more advantageous than single fractures.
It is generally believed that electrical breakdown in short gaps is mainly caused by field emission, while electrical breakdown in long gaps is mainly caused by particle effects.
The vacuum switch works at a high vacuum of more than 10-2Pa. Since the gas molecules are very scarce at this time, the collision and separation of gas molecules has no effect on the breakdown, so the breakdown voltage shows a strong correlation with the electrode material.
The breakdown voltage of the vacuum gap varies with the electrode material. Researchers have found that breakdown voltage and material hardness are related to mechanical strength. Generally speaking, materials with higher hardness and mechanical strength often have higher insulation strength. For example, the hardness of a steel electrode increases after quenching, and its breakdown voltage can be increased by 80% compared to that before quenching.
In addition, the breakdown voltage is also positively related to the physical constants of the cathode material, such as melting point, specific heat, and density. That is, materials with higher melting points also have higher breakdown voltages. The same is true for heat and density. The essence of this problem is that the greater the probability of melting of the material under the same thermal energy, the lower the breakdown voltage.
Figure 1 shows the relationship between gap breakdown voltage and gas pressure. It can be seen from the figure that when the vacuum is higher than 10-2Pa (10-4 Torr), the breakdown voltage basically no longer increases with the decrease in gas pressure, because the phenomenon of gas molecules colliding and freeing no longer works. When the gas pressure gradually increases from 10-2Pa (the degree of vacuum decreases), the breakdown strength gradually decreases, and it is the lowest near 1 Torr (about 102Pa), and then increases with the increase of air pressure. It can be seen from the curve that when the vacuum degree is higher than 10-2Pa, its compressive strength remains basically unchanged. This shows that when the vacuum degree of the vacuum interrupter is above 10-2Pa, it can fully meet the normal use requirements.
5, the surface condition of the electrode
The surface condition of the electrode has a large effect on the breakdown voltage of the vacuum gap. Oxide, impurities and metal particles on the electrode surface will significantly reduce the breakdown voltage of the vacuum gap.
In addition, no matter how the electrode surface of the vacuum interrupter is processed in manufacturing, the large current breaking will make the electrode surface uneven, which will also reduce the breakdown voltage.
6, the ageing effect
There are two types of electrode ageing: voltage ageing and current ageing.
When a new vacuum gap is tested, the breakdown voltage of the first few times is often lower. As the number of tests increases, the breakdown voltage also gradually increases, and will eventually stabilize at a certain value. This phenomenon that the breakdown voltage increases with the number of breakdowns is the effect of voltage aging.
Voltage aging is to remove microscopic bumps, impurities and defects on the electrode surface through discharge. After the small current discharge, the micro-bumps on the surface are melted and evaporated, so that the electrode surface is smooth and flat, the enhancement effect of the local electric field is reduced, and the breakdown voltage is increased. Ageing is also important for the purification of electrode surfaces. Because the electron emission on the electrode surface is easy to occur at the impurity with a lower work function, the breakdown discharge can also melt and volatilize the impurity, and also increase the breakdown voltage of the gap.
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