Abstract
High-voltage direct current gas-insulated switchgear (HVDC GIS) using pressurized synthetic air is being developed for 550 kV DC power transmission, as a low global warming potential alternative to conventional HVDC GIS with SF 6-gas. According to CIGRE reliability studies and field experience with AC GIS, free moving metallic particles can reduce the electric performance of the insulation system and lead to dielectric failures. Early detection of free metallic particles moving in the GIS compartment is crucial for reducing the risk of outages. This can be achieved using partial discharge (PD) detection techniques, as the particles typically produce recognizable PD pulse signatures. It is, however, more difficult to identify specific defects based on PD patterns under DC than under AC, as the phase information is missing. In the literature, pulse sequence analysis (PSA) is widely employed as an alternative to phaseresolved partial discharge analysis (PRPDA). The PSA technique is, however, not as mature as PRPDA, and there are few PSA studies on SF6-free GIS in the literature. This work reports on a study of free-moving metallic particles under ≤65 kVDC in a 48 mm sphere-plane gap in air at 5 bar and 10 bar abs. A high-speed camera was used to record the particle motion, while a UHF sensor picked up the PD signals from the particles. Steel shards (5 mm to 6 mm) were used as the metallic particles. The type of particle motion was found to strongly influence the PSA patterns. Three forms of particle motion were observed at 5 bar (standing, dancing, and flying) cobserved at 5 bar. Both of these pressure dependencies are attributed to the PD inception voltage.