Observation of conducting particles in VCB with temporal and spatial resolution

Since conducting particles are considered to be the reason for the late discharge phenomenon in Vacuum Circuit Breaker (VCB) after the interruption of large current, such as some non-sustained disruptive discharge (NSDD) and some re-strike. This paper constructs the synthetic experiment system using the duplicate circuit to research the expected particles induced late discharge phenomenon. This experiment system consists of the electrical circuit system and the optical observation system. The main electrical circuit is the Skeats circuit with high voltage source and large current source. The observation system is the laser diffraction system. Through computation of the diffraction pattern, the real size of the particle can be derived from the radius of the Airy disk. The computation results show the size of the particle is in the range from several microns to more than one hundred micron. And the distribution of the particle size is near normal distribution with mean value as 32 micron, and the standard derivation is 8.8 micron. Almost all particles are spherical, and ellipsoidal and filamentous particles are barely observed. The result shows the life time of the particle seems to be Exponential distribution. More than 90% of the particles have the lifetime less than 5ms. This time can be considered to be 1/4 cycle of the AC source. Particles are tending to appear on the positive peak value of the high voltage that is 90 degree. Some particles are tending to appear at voltage across zero. These phenomena can be explained as welded particles detached at the voltage peak under strong e-field, while the loosely contacted particles are lifted from the electrode surface by weak e-field while voltage across zero.