Techniques for the protection of gas-insulated substation to cable interfaces

Line-to-ground faults in gas-insulated substations (GIS) generate fast nanosecond risetime transients that cause sparkovers across the insulated flange of high-pressure oil-filled cable-GIS interface. The ionized path formed by the sparkovers creates a low-impedance path for power frequency fault current, resulting in flange damage with potentially serious consequences. Various technique for protecting the insulating flange from such damage are investigated and discussed, in terms of both new designs and retrofits for existing installations. To be effective, each protection scheme must adequately deal with the fast transients generated by breakdown in the GIS. As the frequencies of these transients are one to two orders of magnitude higher than for usual power system transients, special considerations are necessary in the protection scheme chosen