Compressed-Gas Insulation in Large Coaxial Systems

The factors which affect the insulation strength of large compressed-gas-insulated systems were investigated. A coaxial electrode geometry of 10-inch outer diameter (OD) and either 3-or 4¿inch inner diameter (ID) was employed because of its applicability to large high-voltage apparatus currently under consideration, particularly gas-insulated cables. The power supply is a Van de Graaff generator with an 1800-kV dc capability. The study showed that the insulation strength of an annular gap can be improved by placing a thin layer of dielectric material on the inner conductor. A further improvement occurred when free conducting particles (FCPs) or metallic dirt was electrostatically precipitated from the system. With these improvements on the least favorable polarity, 1100 kV was insulated in 150 psi of SF6 and 850 kV in 300 psi of N2. With bare electrodes under similar conditions only 600 kV was insulated in SF6 and 450 kV in N2.