Insulation Coordination for Gas Insulated Substations

This paper presents the development of BIL (Basic Lightning Impulse Insulation Level) ratings for SF6 gas insulated substations from 72.5 kV to 765 kV. The unique properties of the gas insulated substation are analyzed and insulation coordination design techniques are utilized to determine BIL ratings for the substation equipment, other than the transformer, with a lightning arrester located at the entrance terminals. The resulting analysis shows that, in general, the required BIL level of apparatus for gas insulated systems can be lower than for conventional equipment because of lower surge impedance and shorter lengths of the gas insulated bus.