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.
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