Breakdown characteristics of particle-contaminated HVDC GIL under superimposed voltage of DC and impulse

Contamination of free conducting particles is a major threat to gas insulated systems. In HVDC GIL, free conducting particles can be even more harmful due to the fact that particles can easily cross the gas gap under DC voltage and distort the electric field. In HVDC systems, impulse overvoltage can be generated and superimposed on the DC operating voltage. As a result, GIL will experience superimposed DC and impulse voltage. In this paper, the behavior of free conducting particles in HVDC GIL is observed and breakdown characteristics of particle contaminated GIL under superimposed DC and impulse voltages are studied. Two motion patterns of free conducting particles in HVDC GIL are mainly found: bouncing and firefly motions. Particles tend to firefly motion near the enclosure under positive DC voltage and near the high voltage bus bar under negative DC voltage. With increased DC voltage amplitude, the probability of firefly motion increases while the probability of bouncing motion decreases. It is found that the insulation strength of GIL is greatly reduced by free conducting particles under superimposed DC and impulse voltages. With increasing particle length, the breakdown voltage of GIL under superimposed voltage decreases sharply. Breakdown voltage decreases slightly with the increase of particle radius. Breakdown voltage of GIL is lower under superimposed voltage of negative DC and positive impulse than under negative DC and negative impulse with the same size of particle. The results indicate that superimposition of DC operating voltage and impulse overvoltage can be a critical condition for HVDC GIL if free conducting particles exist. Therefore, a superimposed DC and impulse voltage on-site test can be considered for HVDC GIL to check for insulation performance.