To connect GIS modules that are not directly connected to each other, an SF6 bus consisting of an inner conductor and outer enclosure is used. Support insulators, sliding electrical contacts, and flanged enclosure joints are usually the same as for the GIS modules.
SF6-to-air bushings are made by attaching a hollow insulating cylinder to a flange on the end of a GIS enclosure. The insulating cylinder contains pressurized SF6 on the inside and is suitable for exposure to atmospheric air on the outside. The conductor continues up through the center of the insulating cylinder to a metal end plate. The outside of the end plate has provisions for bolting to an air insulated conductor. The insulating cylinder has a smooth interior.
Sheds on the outside improve the performance in air under wet and/or contaminated conditions. Electric field distribution is controlled by internal metal shields. Higher voltage SF6-to-air bushings also use external shields. The SF6 gas inside the bushing is usually the same pressure as the rest of the GIS. The insulating cylinder has most often been porcelain in the past, but today many are a composite consisting of a fiberglass epoxy inner cylinder with an external weather shed of silicone rubber. The composite bushing has better contamination resistance and is inherently safer because it will not fracture as will porcelain.
A cable connecting to a GIS is provided with a cable termination kit that is installed on the cable to provide a physical barrier between the cable dielectric and the SF6 gas in the GIS. The cable termination kit also provides a suitable electric field distribution at the end of the cable. Because the cable termination will be in SF6 gas, the length is short and sheds are not needed.
The cable conductor is connected with bolted or compression connectors to the end plate or cylinder of the cable termination kit. On the GIS side, a removable link or plug in contact transfers current from the cable to the GIS conductor. For high voltage testing of the GIS or the cable, the cable is disconnected from the GIS by removing the conductor link or plug-in contact. The GIS enclosure around the cable termination usually has an access port. This port can also be used for attaching a test bushing.
Direct Transformer Connections:
To connect a GIS directly to a transformer, a special SF6-to-oil bushing that mounts on the transformer is used. The bushing is connected under oil on one end to the transformer’s high voltage leads. The other end is SF6 and has a removable link or sliding contact for connection to the GIS conductor. The bushing may be an oil-paper condenser type or more commonly today, a solid insulation type.
Because leakage of SF6 into the transformer oil must be prevented, most SF6 -to-oil bushings have a center section that allows any SF6 leakage to go to the atmosphere rather than into the transformer. For testing, the SF6 end of the bushing is disconnected from the GIS conductor after gaining access through an opening in the GIS enclosure. The GIS enclosure of the transformer can also be used for attaching a test bushing.
Related: GIS – Gas Insulated Substations
Source: Electric Power Substations Engineering