Particles and breakdown in SF6¿ insulated apparatus

An investigation has been made of the movement of conducting particles, under the action of the applied electric field, on the surface of an insulating spacer immersed in SF6 between 0.47 and 1.5 MPa. Spheres moved fairly readily but particles of an irregular geometry, such as filings, did not. Movement was more pronounced with the higher stresses obtainable from impulsive fields than a.c. There was no effect of pressure. The incorporation of a particle trap showed that while it was effective in keeping a spacer clean if contamination could be induced to enter it, the trap could not guarantee spacer cleanliness because of the lack of particle movement. However, a further investigation of the prebreakdown discharge behaviour of a short length of SF6 insulated cable showed that the range of contamination which caused low impulse flashovers can be detected on the laboratory scale at least. Using a balanced discharge detection system, it was observed that, for a given degree of contamination, discharge inception and the maximum discharge magnitude before 50 Hz flashover were both functions of pressure. The largest discharges (2pC) occurred at 0.3¿0.4 MPa, and there was a correlation coefficient of 0.93 between the negative-polarity standard impulse breakdown voltage at 1.1 MPa and the discharge inception voltage at 0.4 MPa.