A gas insulated substation (GIS) is a high voltage substation in which the major conducting structures are contained within a sealed environment with a dielectric gas known as SF6, or sulfur hexafluoride gas as the insulating medium. In comparison, a conventional (AIS) or Air-Insulated Substation, uses atmospheric air as the dielectric gas medium, as these types of substations primarily consist of outdoor facilities.
Topics covered in this course: Overview of a GIS System Hybrid GIS Substations Advantages and Disadvantages of GIS Systems SF6 gas, leak detection, and gas monitoring systems Physical arrangements used in GIS International Standards on GIS Economics of GIS vs AIS Contaminants and Oxidation in the Gas Enclosure Modules for a GIS System Conductors in a GIS System Circuit Breakers and Transformers used in GIS Systems Switching and Arrester Devices used in GIS Systems Connections in a GIS Systems The Control System of a GIS Testing and Installation Operational Procedures for a GIS
What is a Gas Insulated Substation A gas insulated substation (GIS) is a high voltage substation in which the major conducting structures are contained within a sealed environment with a dielectric gas known as SF6, or sulfur hexafluoride gas as the insulating medium. In comparison, a conventional (AIS) or Air-Insulated Substation, atmospheric air is the dielectric gas medium, as these types of substations are almost always located at outdoor locations.
Origin of GIS technology GIS technology originally began in Japan in the 60's, where there was a critical need to develop substations with a greatly reduced footprint. GIS was slowly adopted by various other countries over the following years. After about 5 years of evaluation and testing, GIS system construction increased to about 20% of new substations in countries with severe space limitations. For countries with space readily available, the higher cost of GIS relative to AIS has limited its adoption to special needs cases. In the US, only about 2 to 5% of new substations are constructed as GIS.
Clearances for GIS, roughly a tenth of that for AIS With GIS technology, the clearance needed for phase to phase or phase to ground for all equipment is much less than that of an AIS or air insulated substation . The total space required for a GIS is roughly a tenth of that needed for a conventional AIS facility. While the conventional, AIS requires several feet of air insulation to isolate a conductor, SF6 gas insulation only needs inches, allowing a GIS facility to fit into areas far smaller than that of a AIS facility. A GIS is mainly constructed where real estate space is expensive or scarce.
Environmentally Protected System GIS technology is a good choice to use in artic or desert locations, as it can be enclosed in a building which is environmentally protected from extreme conditions. In addition to protecting the system components from extreme heat and cold, GIS technology encloses the electrical components within a Faraday cage which shields the system from potential lightning strikes.
Gas Tight Modular Design Each compartment housing the live sections of the GIS system is gas tight, with respect to one another. This ensures that the gas is not allowed to pass to an adjacent module, as well as ensures that gas monitoring is independent. 'O' rings - are installed at the equipment and enclosure flanges to insure the gas tight integrity of the modules. Gas monitoring system - this includes an integrated alarm system, as well as automated tripping and lockout features in case of low pressure conditions due to leakage. This is critical, as when the pressure of the dielectric gases drop, so does the insulating capability of the SF6 gas.
High Voltage Installations Gas insulated technology becomes more desirable as the voltage requirements increase. As an example, the footprint of a 765kV conventional substation is considerably large. A station footprint of this sized station would be significantly less were it to employ GIS.
Urban Installations GIS technology can be used for installations in areas where the cost of real estate or the aesthetic appeal and safety is a significant consideration. When acquiring land for a conventional AIS station in some densely populated areas, it can be cost prohibitive, politically sensitive (when ent domain is required to seize the land), or downright impossible to locate. Imagine attempting to build a high voltage AIS in urban areas such as downtown New York city.
Expansion of Existing AIS - Mixed Technology Switchgear One approach to the expansion of existing AIS, is mixed technology switchgear (or hybrid GIS.) This approach is especially suitable for the expansion of an existing substation, without requiring large areas of additional real estate, which is often unavailable. This design uses GIS breakers, switches, and voltage/current transformers with interconnections between the breaker positions and connections to other equipment using air-insulated conductors .
The GIS System's Foundation GIS systems are usually installed on a monolithic concrete pad or directly on the floor of a housing facility. The system is attached by bolting or welding the GIS support frames to embedded steel plates of beams, or by the use of chal drill anchors. Expansion Joints Expansion drill anchors - These are not recommended, as the dynamic loading from circuit breaker operations may loosen the expansion anchors over . Bus expansion joints – This type of expansion joint may be recommended in the large GIS installations. The expansion joints are installed between the various sections of the GIS to adjust to the "fitup" in the field. They can also be installed to address thermal expansion of the GIS system.
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