Suppression of Conditioning Effect in Vacuum by Micro-protrusions from Anode

Spark conditioning is an effective method to improve electrical insulation strength of vacuum circuit breakers. We discuss the spark conditioning process in consideration of the generation of micro-protrusions on cathode surface by breakdown. We conducted the spark conditioning on three pairs of rod-plane electrodes made of OFHC Cu. For one pair, the voltages were applied until conditioning saturation, namely 600 times voltage applications. For the others, a smaller number of voltage applications (100 times and 300 times) before conditioning saturation were given. After these conditioning experiments, we observed the surface of rod cathodes with a microscope. After the conditioning by about 600 times voltage applications, many micro-protrusions appeared on the cathode surface. Some of them had a diameter larger than 20 μm. They would be caused by adhesion of metallic particles from anode and remained through the conditioning process. On the other hand, few micro-protrusions were found on the cathodes with 100 and 300 times voltage applications. This means that the micro-protrusions were generated at the last period of the conditioning process. In other words, the generation of micro-protrusions can be a dominant factor of suppression and saturation of conditioning effect.