Compressed-gas insulation for concentric power lines

This paper will describe two methods of significantly improving the reliable insulating ability of concentric-gap gas-filled systems. The first of these makes use of a thin dielectric coating bonded on one or both concentric conductors. The second utilizes the field of the concentric line to remove free conducting particles electrostatically from the system. This work was done using dc voltages with both polarities applied. These studies can predict the ac performance as well since the formation time of a gas discharge is less than a microsecond and is thereby short compared to the 8 millisecond 60 Hz half period. On the least favorable polarity in 13 atmospheres of SFe, 1100 kV was insulated whereas with nitrogen at 20 atmospheres 850 kV was insulated. The paper closes with a discussion of the operating parameters of ac compressed-gas-insulated cables in the 138 kV to 1000 kV range. These indicate that the power rating of compressed-gas-insulated cables can exceed that of overhead transmission lines at the same voltage. Critical lengths greater than 100 miles and lower loss as a percentage of power transmitted are also obtained. These results are due to the unity dielectric constant, low dielectric loss, and good convective heat transfer properties of compressed gas.