Working Principle and Function of Vacuum Circuit Breaker

When the vacuum circuit breaker is in the closed position, its ground insulation is borne by the supporting insulator. Once a permanent ground fault occurs in the line connected to the vacuum circuit breaker, after the circuit breaker trips, the ground fault point is not cleared, and there is an electrical busbar. The ground insulation must also be borne by the vacuum gap of the breaker fracture; when various faults are interrupted, the vacuum insulation gap between a pair of contacts of the fracture must withstand the action of various recovery voltages without breakdown.

Representation of vacuum degree

A space where the absolute pressure is lower than the atmospheric pressure is called a vacuum space. The higher the vacuum degree, the lower the gas pressure in the space. The unit of vacuum is expressed in three ways: Torr (i.e. 1 mm mercury column height), millibar (103bar) or Pascal (Pascal: Pa). (1 Torr = 131. 6Pa, 1 mbar = 100Pa) We usually say that the vacuum degree inside the vacuum interrupter should reach 10-4 Torr, which means that the gas pressure in the arc interrupter is only "1/10,000 mm of mercury column" High", which is 1.31x10-2Pa.

"Parson's Theorem" is also translated as "Baschen's Law", which refers to the relationship between the gap voltage withstand strength and gas pressure. Figure 1 shows that the relationship curve of Paison's theorem is "V" shaped, that is, the increase or decrease of the inflation pressure can improve the dielectric strength of the inter-pole gap. Its breakdown mechanism is still unclear, because the internal vacuum degree of the vacuum interrupter is higher than 10-4 Torr. In such a thin air space, the free travel of gas molecules is 103mm. In the volume of the vacuum interrupter, the occurrence of chance of a collision is almost zero. Therefore, there will be no collision release to break down the vacuum gap. The "V"-shaped curve of Parson's theorem is obtained experimentally, provided that in the case of a uniform electric field, the gap breakdown voltage Uj can be expressed as: Uj=KLa L------gap distance; a-- ---- Gap coefficient (a=1 when the gap is <5mm, a=0.5 when the gap is >5mm)

It can be seen from the "V"-shaped relationship curve of Pyson's theorem that when the vacuum degree reaches 103 Torr, an inflection point occurs, the curve around the inflection point becomes flat, and the breakdown voltage has almost no change.

When the vacuum degree and the gap distance are the same, the breakdown voltage varies with the contact electrode material. When the electrode material has high mechanical strength and high melting point, the breakdown voltage of the vacuum gap also increases.

Destruction mechanism of vacuum insulation

As mentioned above, in a space with a high degree of vacuum such as a vacuum interrupter, the free travel of gas molecules is very large, and there will be no collision and separation, so that the vacuum gap will break down under the action of high-voltage electricity, which is an objective existence. There is a mechanism to explain the breakdown of vacuum insulation, breakdown by field emission, breakdown by microblock and breakdown by microdischarge.

Conclusion

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