Uniform and non-uniform breakdown in compressed air under lightningand switching impulse voltages

摘要

The authors describe measurements of breakdown potentials (V₅₀) in air over the pressure range of 0.09 to 0.6 MPa(absolute) under lightning impulse (1.2/50 μs) and switching surge(100/2500 μs and 250/2500 μs) voltages for both positive andnegative polarities in uniform fields (d=4 to 50 mm), and in nonuniformfields (5-mm-dia hemispherically ended rod-plane geometry) for gaplengths of 30, 40 and 50 mm. It is shown that the breakdown strength ofair exhibits a slight saturation at pressures above 0.6 MPa under bothtypes of voltages and polarities. Under uniform fields the V₅₀ for the switching surge voltage is generally lower thanthe corresponding lightning impulse data. Under nonuniform fields it isobserved that, for a given gap, the critical pressure effect is seen forcertain risetimes only. The breakdown voltage at any given gap alsovaries with the risetime of the waveform and appears to reach a maximumfor about 100 μs