A gas-insulated switchgear is a composite device encapsulated in a solid metal frame that houses different electrical devices such as circuit breakers, bus bars, transformers, earth switches, surge arrestors, etc. All these devices are immersed in sulfur hexafluoride gas (SF6) inside shielded compartments that are bordered by barrier devices.
The main function of a GIS substation is to switch, separate, transform, measure and distribute electrical energy in power systems.
The main feature of a GIS device is the use of SF6, an inert gas with exceptional insulation properties, and chemical and thermal stability.
By comparison, a GIS unit only requires centimeters for effective insulation, while an air-insulated switchgear unit would need meters to perform the same function. All these features make GIS a much more reliable device with less space and maintenance requirements than AIS.
These devices have been used with great success since the late 1960s and early 1970s.
Switchgear using SF6 protects electrical power supply and distribution by interrupting the flow of current in an electrical circuit when necessary.
Under normal conditions, the breaker contacts are closed, but when a failure occurs in the electrical system, these contacts separate and an arc occurs between them.
The displacement of the mobile contacts is synchronized with that of a valve that allows the entry of high-pressure SF6 into the chamber where the arc is occurring.
The properties of SF6 allow the absorption of free electrons in the path of the arc, forming ions that do not carry electrical charge — because they become too heavy. The dielectric strength of the gas increases notably, so the arc is extinguished.
Due to its ability to rapidly recombine, as the arc is extinguished, the pressure of the SF6 also decreases, so it is stored in a low-pressure tank and then in a high-pressure tank ready for reuse.
Thanks to these properties, SF6 insulating is much more effective than air insulating — about three times — which makes these substations ideal for medium and high-voltage power systems.
