Tips for selecting between air-insulated and gas-insulated switchgear

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Tips for selecting between air-insulated and gas-insulated switchgear
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Aligning equipment with application considerations

Aligning equipment with application considerations Switchgear provides essential functions: control, protection, and isolation of electric equipment and circuits. Power distribution systems across every application and market use switchgear to support power reliability and enhance safety. Typically, Switchgear is installed and expected to work for 25 years or longer. Despite Switchgear’s longevity and critical function, equipment selected for a specific job site can be based on past experiences. This method may not always align with the current application or job-site requirements. In medium-voltage applications, selecting between air-insulated Switchgear (AIS) and gas-insulated Switchgear (GIS) can be driven objectively by understanding the differences between the equipment. Today, the bulk of installed equipment is air-insulated Switchgear. Yet gas-insulated equipment can provide advantages in specific applications. As we’ll see on the next page, one main advantage of GIS is its footprint. To determine the right equipment to apply, it is best to understand how it is designed, operated, and maintained. This paper identifies critical considerations for selecting the type of medium-voltage Switchgear used in industrial, oil & gas, utility, data centre and other commercial applications.

Difference between AIS and GIS equipment?

AIS uses air as the primary insulating medium. AIS is further classified as metal-clad or metal-enclosed. Metal-clad Switchgear has extensive barriers, shutters, primary bussing covered with insulation and draws out vacuum circuit breakers to interrupt faults. Metal-clad Switchgear is designed, manufactured, and tested to applicable American National Standards Institute (ANSI), Institute of Electrical and Electronics Engineers (IEEE) and National Electrical Manufacturers Association (NEMA) standards. GIS has sealed enclosures filled with insulating gas sulfur hexafluoride (SF6) or a mixture of SF6 and other insulating gases that have been recently released to the market) for insulation and uses fixed-mounted circuit breakers with three-position disconnect switches. GIS switchgear is designed, manufactured, and tested to International Electrotechnical Commission (IEC) performance standards. IEEE standards for metal-enclosed GIS are expected to be released soon. The equipment selection is based on the application requirements of customers’ power systems and operational preferences. Application requirements include service voltage, continuous current, and interrupting capabilities. In general, voltage, continuous current, and interrupting ratings are similar between GIS and metal-clad Switchgear. Continuous current and interrupting ratings for metal-clad Switchgear can reach 4000 A and 63 kA, respectively, whereas GIS is typically limited to 3000 A and 40 kA.

Operational flexibility:

Metal-clad Switchgear provides more operational flexibility compared to GIS. If application needs changed, modifications to primary circuits and future expansion could be easily made with metal-clad equipment. Primary circuit modifications/future expansion may not be easily made or possible with GIS equipment. Therefore, future application needs must be taken into consideration when selecting metal-clad and GIS.

Equipment space, size, and location considerations:

Due to differences in insulating mediums used in metal-clad Switchgear and GIS, there are significant differences between the metal-clad and GIS equipment sizes. At 27 and 38 kV, GIS offers a much smaller footprint than metal-clad Switchgear. Utilizing GIS at those voltages can reduce the footprint by up to 60 per cent, saving building costs and freeing up space for other use. At 5 and 15 kV, lineups of metal-clad Switchgear and GIS have similar footprints. The footprint savings at 27 and 38 kV are attractive and are one of the main reasons most GIS is applied for 27 and 38 kV applications.

Maintenance and environmental considerations:

Metal-clad equipment with withdrawable circuit breakers and auxiliary components, such as VTs and CPTs, are easy to install, operate, test, and maintain. However, the performance and dielectric integrity of metal-clad equipment is negatively affected by environmental contaminants and humidity. Therefore, more elaborate and frequent maintenance may be required depending on the application location and surrounding environment. In the case of GIS, primary circuits and components are contained within a sealed tank or covered with solid encapsulated insulation. They, therefore, are not subject to destructive effects of the surrounding environment, thus requiring minimal or no maintenance for long periods.

Safety:

By its defining characteristics and design requirements, metal-clad equipment uses more total parts than equivalent GIS equipment. Therefore, in general, GIS is considered more reliable compared to metal clad. In addition, GIS is considered inherently arc-resistant because it is impervious to environmental contaminants and foreign intrusion.

Total cost considerations:

As the voltage decreases, metal-clad Switchgear is more cost-effective, as the cost of metal-clad Switchgear is less at lower voltages, while the cost for GIS remains similar across voltage ratings. For equipment at 5/15 kV voltage classes, the cost of GIS can be upward of two or even two-and-a-half times as much as metal-clad equipment.

Conclusion

Equipment, AIS or GIS, must be selected based on the equipment’s ratings and capabilities to satisfy actual application requirements. GIS equipment offers a smaller footprint and space savings but may have a higher initial cost. However, GIS requires minimal or no maintenance over long periods compared to regular or more frequent maintenance for metal-clad, hence in the long run, GIS may offer a cost advantage over metal-clad. Also, GIS is not impacted by environmental influences and therefore, it is expected to provide more reliable performance and safety over metal clad. Where future expansion/changes are expected, metal-clad should be considered as it provides more operational and expansion/modification flexibility. Presently, most switchgear applications at 5 and 15 kV uses metal clad as it is less costly than GIS and offers a choice of ratings to suit most applications. Experience with installation, operation, and maintenance of metal-clad Switchgear can make it more attractive than GIS.

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