Compared with other isolating switches, the principle of vacuum circuit breakers is different from that of magnetic blowing substances. There is no dielectric in a vacuum, which makes the arc extinguish quickly. Thus, the dynamic and static data contact points of the disconnect switch are not very spaced apart. Isolation switches are generally used for power engineering equipment in processing plants with relatively low rated voltages! With the rapid development trend of power supply system, 10kV vacuum circuit breakers have been mass-produced and applied in China. For maintenance personnel, it has become an urgent problem to improve the mastery of vacuum circuit breakers, strengthen maintenance, and make them operate safely and reliably. Taking ZW27-12 as an example, the paper briefly introduces the basic principle and maintenance of vacuum circuit breaker.
1. Insulation properties of vacuum.
Vacuum has strong insulating properties. In the vacuum circuit breaker, the vapor is very thin, and the arbitrary stroke arrangement of the molecular structure of the vapor is relatively large, and the probability of collision with each other is small. Therefore, random impact is not the main reason for the penetration of the vacuum gap, but under the effect of the high toughness electrostatic field, the electrode-deposited metal material particles are the main factor of insulation damage.
The dielectric compressive strength in a vacuum gap is not only related to the size of the gap and the balance of the electromagnetic field, but also greatly affected by the characteristics of the metal electrode and the standard of the surface layer. At a small distance gap (2-3mm), the vacuum gap has the insulating properties of high-pressure gas and SF6 gas, which is why the contact point opening distance of the vacuum circuit breaker is generally small.
The direct influence of the metal electrode on the breakdown voltage is specifically reflected in the impact toughness (compressive strength) of the raw material and the melting point of the metal material. The higher the compressive strength and melting point, the higher the dielectric compressive strength of the electric stage under vacuum.
Experiments show that the higher the vacuum value, the higher the breakdown voltage of the gas gap, but basically unchanged above 10-4 Torr. Therefore, in order to better maintain the insulation compressive strength of the vacuum magnetic blowing chamber, the vacuum degree should not be lower than 10-4 Torr.
2. The establishment and extinguishing of the arc in the vacuum.
The vacuum arc is quite different from the charging and discharging conditions of the vapor arc that you have learned before. The random condition of the vapor is not the primary factor causing arcing. Vacuum arc charging and discharging are generated in the vapor of metal material volatilized by touching the electrode. At the same time, the size of the breaking current and the arc characteristics also vary. We usually divide it into low-current vacuum arc and high-current vacuum arc.
1. Small current vacuum arc.
When the contact point is opened in a vacuum, it will cause a negative electrode color spot where the current and kinetic energy are very concentrated, and a lot of metal material vapor will volatilize from the negative electrode color spot. ignited. At the same time, the metal material vapor and electrified particles in the arc column continue to spread, and the electric stage also continues to volatilize new particles to fill up. When the current crosses zero, the kinetic energy of the arc decreases, the temperature of the electrode decreases, the actual effect of volatilization decreases, and the mass density in the arc column decreases. Finally, the negative electrode spot subsides and the arc is extinguished.
Sometimes volatilization cannot maintain the propagation rate of the arc column, and the arc is suddenly extinguished, resulting in trapping.
Post time: Apr-25-2022