Modeling of Channel Heating Breakdown in Dry Air

In dry air with a relatively short gap length under non-uniform electric field, the channel heating breakdown mechanism becomes critical to the electrical insulation design of power apparatus. We have experimentally clarified that the input energy density into the discharge channel almost decides the occurrence of channel heating breakdown in dry air. This implies that the rise of the channel temperature due to Joule heating causes the rise of the channel conductivity and the current, leading to the breakdown. In this paper, we construct the discharge channel heating model by the resistance circuits in consideration of the temperature dependence of conductivity and the shape of discharge channel, and investigate the process leading to the channel heating breakdown in dry air. The electrode system in this model is needle-plane electrodes with the gap length of 15 mm based on our experiments. At first, we verified that the process of channel heating breakdown can be explained by the temperature dependence of conductivity by using the preliminary zero-dimensional model with uniform temperature distribution in the discharge channel. With the given applied voltage and initial temperature, the temporal evolution of current and conductivity was calculated. Calculation results revealed that the temporal evolution of conductivity and the time to breakdown were determined by not only the conductivity itself but also the temperature dependence of conductivity. Subsequently, we constructed the discharge channel heating model in consideration of the radial and axial temperature distribution. With the given initial temperature distribution, the calculated temporal evolution of current agreed with the measured current waveform just before breakdown. This result revealed that the temperature rise in the center column of discharge channel is the most important factor for the breakdown process. The calculated temporal evolution of the temperature distribution along the center axis revealed that the high temperature region would develop from the tip of the needle electrode toward the center of the gap. In addition, the calculated temporal evolution of the radial temperature distribution suggested that the temperature at the center column of the discharge channel would rise sharply just before breakdown. In conclusion, in the channel heating breakdown in dry air, we clarified the process of the current concentration on the center axis of the discharge channel, as well as the progress of high temperature region or high conductivity region in the axial direction from the tip of the needle electrode.