1、 The basic concept of vacuum
In vacuum technology, “vacuum” generally refers to the gas state in which the gas pressure is lower than one atmospheric pressure in a given space, that is to say, compared with the normal atmospheric pressure, it is a relatively thin gas state.
Vacuum is an objective measure of the rarefaction of gases. According to the theory of vacuum technology, the vacuum degree is usually expressed by the pressure of gas. In the international system of units, the pressure is 1pA = 1n / m2 in Pascal (PA). In addition, commonly used units include Torr, mmHg, mbar, engineering atmospheric pressure (kg / cm), etc.
There is no uniform stipulation for the division of vacuum area, which is usually divided in China as follows:
Coarse vacuum: (760-10) Torr
Low vacuum: (10-10-3) Torr
High vacuum: (10-3 ~ 10-8) Torr
Ultra high vacuum: (10-8 ～ 10-12) Torr
Very high vacuum: 10-12 Torr
The relationship between Torr and PA: 1 Torr = 1 mmHg = 133.322pa, 1 pa = 7.5 × 10-3 Torr.
For example, vacuum cleaner works in the rough vacuum area, thermos bottle and bulb work in the low vacuum area, while vacuum switch tube and other electric vacuum devices work in the high vacuum area.
2、 Insulation characteristics of vacuum gap
When a pair of electrodes are placed in vacuum and high voltage is applied, the electric breakdown between electrodes will also occur under a certain voltage. Its breakdown is very different from that in air. The breakdown in air is due to the high-speed movement of a small number of free electrons in the gas under the action of electric field. More electrons and ions are produced when they collide with gas molecules, and new electrons and ions collide with neutral atoms to produce more electrons and ions. In this avalanche ionization process, a discharge channel is formed between the electrodes and an arc is generated. In vacuum, because of the low pressure, there are very few gas molecules. In such an environment, even if there are electrons in the electrode gap, they have little chance to collide with gas molecules when flying from one electrode to another. Therefore, it is impossible for electrons and gas molecules to collide to cause avalanche type breakdown. It is precisely because of the rarity of gas molecules that the breakdown of vacuum gap is possible only when the field emission and other phenomena occur under very high voltage. Theoretically, it can be inferred that the electric field strength needs to be above 108V / cm to cause electric breakdown. In fact, due to a series of adverse factors such as electrode surface roughness, cleanliness and so on, the insulation strength of vacuum gap will be lower than the theoretical calculation value by several orders of magnitude.
The vacuum degree in the vacuum interrupter is very high, generally 10-3 ~ 10-6 Pa. at this time, the insulation strength of the vacuum gap is much higher than that of the air and SF6 of one atmosphere, and higher than that of the transformer oil. Because of the high insulation strength of vacuum, all electrical gaps in the vacuum interrupter can be made very small.
3、 Main factors affecting vacuum insulation level
Vacuum insulation is a very complex physical process, and its mechanism has not been clearly concluded so far. From the perspective of practical application, it mainly includes the following aspects:
1. Electrode geometry
The geometry of the electrode has a great influence on the distribution of the electric field, which often leads to the breakdown of the electric field due to the improper geometry, especially in high voltage vacuum products.
The curvature radius of electrode edge is an important factor. Generally speaking, the electrode with larger radius of curvature can withstand breakdown voltage more than the electrode with smaller radius of curvature.
In addition, the breakdown voltage is also inversely proportional to the size of the electrode area, that is, it decreases with the increase of the electrode area. The main reason for the decrease of withstand voltage is the increase of discharge probability.
2. Gap distance
The breakdown voltage of vacuum has a clear relationship with the gap distance. The results show that when the gap distance is less than 5mm, the breakdown voltage increases linearly with the increase of gap distance, but with the further increase of gap distance, the increase of breakdown voltage slows down, that is to say, the electric field intensity of real space gap breakdown decreases with the increase of gap distance. When the gap reaches a certain length (≥ 20 mm), it is very difficult to increase the gap distance alone to improve the withstand voltage level. At this time, it is better to use multiple fractures than single fractures.
It is generally believed that the short gap electrical breakdown is mainly caused by field emission, while the long gap electrical breakdown is mainly caused by particle effect.
3. Electrode material
When the vacuum switch works in high vacuum above 10-2Pa, the breakdown voltage has a strong correlation with the electrode material, because the gas molecules are very rare at this time, and the collision free of gas molecules has no effect on the breakdown.
The breakdown voltage of the vacuum gap varies with the electrode material. The researchers found that the breakdown voltage and the hardness of the material are related to the mechanical strength. Generally speaking, materials with high hardness and mechanical strength tend to have high insulation strength. For example, the breakdown voltage of steel electrode can be increased by 80% after quenching compared with that before quenching.
In addition, the breakdown voltage i s also positively related to the physical constants of cathode materials, such as melting point, specific heat and density, which means that the breakdown voltage of materials with higher melting point is also higher. The same is true for heat and density. The essence of this problem is that under the same heat energy, the higher the probability of material melting, the lower the breakdown voltage.
4. Vacuum degree
The relationship between gap breakdown voltage and gas pressure. It can be seen that when the vacuum degree is higher than 10-2Pa (10-4 Torr), the breakdown voltage will not increase with the decrease of gas pressure, because the phenomenon of gas molecules colliding and dissociating no longer works. When the gas pressure increases from l0-2pa (the vacuum degree decreases), the breakdown strength decreases gradually, 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 the compressive strength is basically unchanged when the vacuum degree is higher than 10-2Pa. This shows that when the vacuum degree of the vacuum interrupter is above 10-2Pa, it can completely meet the normal use demand.
5. Surface condition of electrode
The breakdown voltage of vacuum gap is greatly affected by the surface condition of electrode. Oxides, impurities and metal particles on the surface of the electrode can significantly reduce the breakdown voltage of the vacuum gap.
In addition, no matter how the electrode surface of the vacuum interrupter is processed in the manufacturing process, high current breaking will make the electrode surface uneven, which will also reduce the breakdown voltage.
6. Aging effect
There are two kinds of electrode aging: voltage aging and current aging.
When a new vacuum gap is tested, the breakdown voltage of the first few times is often low. With the increase of the number of tests, the breakdown voltage increases gradually, and finally it will stabilize at a certain value. The phenomenon that the breakdown voltage increases with the breakdown times is the effect of voltage aging.
Voltage aging is the elimination of micro bumps, impurities and defects on the electrode surface by discharge. After small current discharge, the micro convex point of the surface is burned and evaporated, the electrode surface is smooth and flat, the enhancement effect of local electric field is reduced, and the breakdown voltage is increased. Aging is also very important for the purification of electrode surface. Because the electron emission from the electrode surface is easy to occur at the place where the impurity with low work can escape, the breakdown discharge can also make the impurity melt and volatilize, and also can improve the breakdown voltage of the gap. If the gas can be extracted at the same time in the aging process, the effect will be better. Voltage aging is only suitable for the increase of true space gap breakdown voltage, which will not have much effect on the increase of contact gap breakdown voltage of vacuum interrupter. The burn of electric arc on the contact surface will make the effect of voltage sophistication all invalid.