Effect of solid impurities on power frequency breakdown strength in compressed air in uniform, non-uniform and co-axial fields

Solid impurities such as micronised metallic particles in a Compressed Gas Insulated System (GIS) are detrimental to system performance. Detailed studies were conducted to quantify the extent of reduction in 50 Hz power frequency breakdown strength due to the presence of free and fixed conducting particles (0.4 mm dia × 4 mm length) and variation of particle length (4 mm, 5 mm and 6 mm) in compressed air in uniform, non-uniform and co-axial fields. The stud1es were conducted for pressures varying from 100 to 500 kPa and electrode gap varying from 10 to 50 mm. The experimental results indicate that the reduction in breakdown strength (BDS) due to the presence of fixed conducting particle is predominant (> 50%) in uniform and co-axial fields, whereas, it is very nominal (<10%) in non-uniform field. The reduction is found to increase with the increase in length of the fixed conducting particle. The reduction, in general, decreases with increase in gap length and pressure, but the variation is random in non-uniform field. In case of free conducting particle the reduction is more than the fixed one in the electrode system.