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

The authors describe measurements of breakdown potentials (V ₅₀) 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 and negative polarities in uniform fields (d=4 to 50 mm), and in nonuniform fields (5-mm-dia hemispherically ended rod-plane geometry) for gap lengths of 30, 40 and 50 mm. It is shown that the breakdown strength of air exhibits a slight saturation at pressures above 0.6 MPa under both types of voltages and polarities. Under uniform fields the V ₅₀ for the switching surge voltage is generally lower than the corresponding lightning impulse data. Under nonuniform fields it is observed that, for a given gap, the critical pressure effect is seen for certain risetimes only. The breakdown voltage at any given gap also varies with the risetime of the waveform and appears to reach a maximum for about 100 μs