Positive corona inception in air at elevated temperatures

The conditions for the inception of non-uniform field corona in air are discussed. A model, that requires simple assumptions, is used to define the characteristics of the critical avalanches needed to initiate streamer corona as the temperature increases and the air density decreases. An iteration procedure that combines measured values of the inception voltages with simple criteria and published data on basic processes is used to define such avalanche characteristics as length, avalanche number and the positive ion space-charge field at criticality. The Meek criterion is invoked to equate the space-charge field to the local applied field, at which replication of the avalanche and streamer initiation is assumed to occur. The procedure is repeated using trial inception voltages to simulate the range measured experimentally at temperatures up to similar to 530 K, which corresponds to a relative air density of similar to 0.55. It is shown that for relative air densities < similar to 0.8, the formation of critical avalanches follows a different law from that operating in the range between 1.0 and 0.8. The assumptions made in the calculations result in a good agreement between the calculated and experimental properties.