167 / 2024-04-15 14:42:18

Electrode Material Dependence of Dielectric Strength Improvement in High Frequency AC Conditioning in Vacuum

vacuum interrupters,vacuum circuit breakers,spark conditioning,breakdown strength,AC

To increase the dielectric strength of vacuum interrupters (VIs), spark conditioning with repetitive breakdowns (BDs) by AC or impulse voltage application is one of the effective methods, which can remove the micro-protrusions on the electrode surface, namely weak points in insulation. AC conditioning with the commercial frequency has been widely used owing to the advantage of lower cost for conditioning completion. However, in AC conditioning process, high voltages continue to be applied after a BD, so that the subsequent BDs (multiple BDs) in the same half cycle might be induced by the collision of metal particles from the opposite electrode due to the previous BD. Such multiple BDs would not be induced by the micro-protrusions on the electrode surface, and would not contribute or would be harmful to dielectric strength improvement in AC conditioning. In previous research, we have confirmed the suppression of the multiple BDs and dielectric strength improvement by increasing frequency of AC conditioning from 60 Hz to 550 Hz for the Cu-Cu electrode system. However, the improvement in high frequency AC conditioning may be affected by the electrode material properties. In this paper, we conducted high frequency AC conditioning for the SUS-SUS electrode system and compared it with that for the Cu-Cu electrode system.

Experimental results have revealed that the high frequency AC conditioning improved the dielectric strength by 16% for the Cu-Cu electrode system, but did not almost improve (<2%) for the SUS-SUS electrode system. On the other hand, the suppression ratio of the multiple BDs in the SUS-SUS electrode system was similar to that in the Cu-Cu one. These results suggest that the harmful effect of the multiple BDs depends on the electrode material properties, i.e. the harmful effect for the SUS-SUS electrode system is lower than that for the Cu-Cu electrode system.

There are two possible harmful effects of the multiple BDs: (a) damage on the anode and (b) the generation of new micro-protrusions on the cathode. The multiple BDs do not contribute to the melting of micro-protrusions on the cathode. In addition, in the harmful effect (a), the multiple BDs can create BD craters could damage the anode surface. For electrode materials with high thermal conductivity, such as Cu, the harmful effect (a) is considered to be more significant due to the larger melting area on the anode surface than those with SUS. On the other hand, the harmful effect (b) can cause the adhesion of new micro-protrusions to the cathode, and it may remain as the weak points on the cathode, which cannot be re-melted. For electrode materials with high electrical conductivity, such as Cu, where the Joule heat of BDs is small, the harmful effect (b) is considered to be more significant due to the low ability to re-melt the new micro-protrusions on the cathode.

Therefore, the harmful effects of the multiple BDs are significant for electrode materials with high electrical and thermal conductivities such as Cu, and the dielectric strength improvement by the suppression of the multiple BDs due to high frequency AC conditioning is expected to be useful for such electrode materials.