GIS Steel Enclosures: There are two basic tasks of the enclosure: to keep the gas pressure inside and to be gas tight so that SF6 will not be released to the atmosphere. Both tasks can be well covered by using sheet steel materials. Cast steel has porosities that cannot guarantee such high gas tightness values as required for GIS. Steel is relative easy to weld and to form. Therefore, the steel enclosures have been welded by steel sheets that have been mechanically formed to cylinders and then were welded.
GIS Aluminum Enclosures: Used for the conductors as an extruded pipe or in cast technology. For enclosures aluminum is used in cast technology or with welded sheet materials using longitudinal welds or spiral welding processes. Extruded pipes can be used as conductors in a straight bus bar section. When conductors are used inside disconnecting or ground switches, the required shape and design requires a casting technology. The disadvantage of cast aluminum in the early years was that the insulation gas was released to the atmosphere due to its porosity. Today, most enclosures are manufactured in cast aluminum as the top performer for GIS In pic shown Straight conductor graphic that manufacture by aluminium.
Metal used in GIS Enclosures Enclosures are normally cast or welded aluminum, although steel is also commonly used. Steel enclosures require painting on the interior and exterior to prevent rusting. Aluminum enclosures don’t require painting, but may be painted to improve cleaning, provide a better appearance, or to optimize heat transfer to the ambient atmosphere. Deciding between aluminum and steel is based on the cost differences between the two metals and also on the continuous current. With continuous currents exceeding roughly 2000 amps, steel enclosures will require non-magnetic inserts of stainless steel. Otherwise the enclosure material should be changed to all stainless steel or aluminum.
Grounding Individual Enclosures The individual metal enclosure sections of the GIS modules are electrically connected. This is accomplished through either of two means: 1.) The flange’s enclosure joint being a good electrical contact in itself. 2.) With external shunts bolted to the flanges or to grounding pads on the enclosure. Grounding of Single and Three Phase Enclosures Earlier single-phase GIS enclosures - were single-point grounded to prevent circulating currents from flowing in the enclosures. Presently, the practice is to use ‘‘multi-point grounding’’ even though this leads to some electrical losses in the enclosures due to circulating currents. Three enclosures of a single-phase GIS - should be bonded together at the ends of the GIS to encourage circulating currents to flow. These circulating enclosure currents act to cancel the magnetic field that would otherwise exist outside the enclosure due to the conductor current. Three-phase GIS enclosures - do not have circulating currents, but do have eddy currents in the enclosure, and should also be multi-point grounded. With multipoint grounding and the many resulting parallel paths for the current from an internal fault to flow to the substation ground grid, it is easy to keep the touch and step voltages for a GIS to the safe levels prescribed in IEEE 80.
Pressure Vessel Specifications Specifications for pressurized vessels in GIS enclosures are established within GIS standards. The actual design, manufacture, and testing procedures follow the established pressure vessel standard for the country where the equipment is manufactured. Due to the modest pressures involved in GIS enclosures, and the classifying of GIS as electrical equipment, third party inspections and code stamping of the GIS enclosures are not mandatory requirements.
Protection of Outdoor Enclosures GIS equipment will not show the effects of aging, whether installed indoors or out. For outdoor GIS systems, special measures must be taken to ensure adequate protection from corrosion, effects of low and high ambient temperatures, and solar UV radiation.
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