XNUMX. The basic concept of vacuum

In vacuum technology, "vacuum" generally refers to a gas state where the gas pressure is lower than one atmosphere in a given space, that is, it is a rarer gas state compared with normal atmospheric pressure.

The degree of vacuum is an objective measure of the thinness of gas.According to the theory of vacuum technology, the degree of vacuum is usually expressed by the pressure of gas.In the International System of Units, the pressure is 1Pa=1N/m2 in Pascals (Pa).In addition, the commonly used units include Torr, millimeters of mercury (mmHg), millibars (mbar), engineering atmospheric pressure (kg/cm2) and so on.

There is no uniform regulation on the division of vacuum areas. Our country usually divides it like this:

Rough 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 millimeter of mercury (mmHg) = 133.322 Pa, 1 Pa = 7.5×10-3 Torr.

The characteristics of the vacuum area are different, and its applications are different. For example, vacuum cleaners work in rough vacuum areas, thermos bottles, light bulbs, etc. work in low vacuum areas, while vacuum switch tubes and other electric vacuum devices work in high vacuum areas.

Second, the insulation characteristics of the vacuum gap

A pair of electrodes are placed in a vacuum, and when high voltage is applied, electrical breakdown between the electrodes will also occur under a certain voltage.Its breakdown is very different from electric breakdown in air.Breakdown in the air is due to a small amount of free electrons in the gas moving at high speed under the action of an electric field, colliding with gas molecules to produce more electrons and ions, and the new electrons and ions collide with neutral atoms to produce more electrons. And ions.This avalanche-like ionization process forms a discharge channel between the electrodes and generates an electric arc.In vacuum, due to the low pressure, there are very few gas molecules. In such an environment, even if there are electrons in the gap between electrodes, when they fly from one electrode to another, they have little chance to collide with gas molecules.Therefore, it is impossible for electrons and gas molecules to collide to cause an avalanche electric breakdown.It is precisely because of the scarcity of gas molecules that the electrical breakdown of the vacuum gap is possible when other phenomena such as field emission occur at a very high voltage.It is theoretically speculated that the electric breakdown will only occur when the electric field strength is above 108V/cm. In fact, the insulation strength of the vacuum gap will be lower than the theoretical calculation due to a series of unfavorable factors such as electrode surface roughness and cleanliness. Value of several orders of magnitude. 

真空灭弧室中的真空度很高，一般为10-3～10-6 帕，此时真空间隙的绝缘强度远远高于1 个大气压的空气和SF6 的绝缘强度，比变压器油的绝缘强度还要高。正因为真空的绝缘强度很高，真空灭弧室中的所有电气间隙都可以做得很小。例如12kV 真空灭弧室的触头开距只有8～12mm，40.5kV 真空灭弧室的触头开距也只要18～25mm，真空灭弧室中的其它电气间隙也在此尺度范围。

Three, the main factors affecting the level of vacuum insulation

Vacuum insulation is a very complex physical process, and its mechanism has not been clearly concluded so far.From the actual application situation, there are mainly 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 improper geometry, the electric field is too concentrated locally and lead to breakdown. This is especially prominent in high-voltage vacuum products.

The radius of curvature of the electrode edge is an important factor.Generally speaking, an electrode with a larger radius of curvature is more capable of withstanding a breakdown voltage than with a smaller 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 main reason for the decrease in the withstand voltage caused by the increase in the area is the increase in the probability of discharge.

2. Gap distance

The breakdown voltage of the vacuum has a clear relationship with the gap distance.Experiments show that when the gap distance is small (≤5mm), the breakdown voltage increases linearly with the increase of the gap distance, but as the gap distance further increases, the growth of the breakdown voltage slows down, that is, the electric field where the vacuum gap breaks down. The intensity decreases as the gap distance increases.When the gap reaches a certain length (≥20mm), it is already very difficult to increase the gap distance to improve the pressure level. In this case, using multiple fractures is more advantageous than single fractures.

It is generally believed that the electrical breakdown under the short gap is mainly caused by field emission, while the electrical breakdown under the long gap is mainly caused by the particle effect.

3. Electrode material

The vacuum switch works in a high vacuum above 10-2Pa. Because the gas molecules are very rare at this time, the collision and dissociation of the 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 the breakdown voltage and the hardness of the material are related to the mechanical strength.Generally speaking, materials with higher hardness and mechanical strength tend to 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 with that before quenching.

In addition, the breakdown voltage is also positively correlated with the physical constants of the cathode material, such as melting point, specific heat, and density. That is, materials with higher melting points have higher breakdown voltages.The same is true for contrast heat and density.The essence of this problem is that under the same thermal energy, the greater the probability of the material melting, the lower the breakdown voltage.

4. Vacuum

图一显示了间隙击穿电压和气体压强之间的关系。由图可以看到真空度高于10-2Pa（10-4托）时，击穿电压基本上不再随着气体压力的下降而增大，因为气体分子碰撞游离现象已不再起作用。当气体压力从l0-2Pa逐步升高时(真空度下降)，击穿强度逐渐下降，而在接近1托(102Pa左右)最低，以后又随气压的增高而增高。从曲线上可以看出真空度高于10-2Pa时其耐压强度基本上保持不变。这就表明，真空灭弧室的真空度在10-2Pa以上时完全能够满足正常的使用需求。

5. The surface condition of the electrode

The surface condition of the electrode has a great influence on the breakdown voltage of the vacuum gap.Oxides, 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 the manufacturing process, the large current interruption will cause the electrode surface to become uneven, which will also reduce the breakdown voltage.

6. Aging 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.With the increase of the number of tests, the breakdown voltage gradually increases, and finally stabilizes 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 the elimination of microscopic bumps, impurities and defects on the electrode surface through electrical discharge.After a small current discharge, the microscopic convex points on the surface are melted and evaporated, the electrode surface is smooth and flat, the enhancement effect of the local electric field is reduced, and the breakdown voltage is increased.Aging is also very important for the purification of the electrode surface.Since the electron emission on the electrode surface is likely to occur at the location of impurities with lower work function, breakdown discharge can also melt and volatilize impurities, and can also increase the breakdown voltage of the gap.If the air can be pumped at the same time during the aging process to remove the vaporized gaseous matter, the effect will be better.Voltage aging is only suitable for increasing the breakdown voltage of the vacuum gap, and will not have much effect on the increase of the breakdown voltage of the contact gap of the vacuum interrupter.Burning damage to the surface of the contact by the arc will completely invalidate the effect of voltage aging.