Gas Insulated System or GIS is the modern form of an electrical substation. The power system voltage is rising day by day. Along with this, the system current also rises. Hence, power switchings need a proper safe and reliable operation.
We used to use air in the first generation power switching devices. The blast of air can quench arcs during interruptions.
After that, the voltage demand of the system again raised. Then oil became a suitable medium for quenching the arc. First generation oil circuit breakers were of bulk oil type. A bulk oil breaker contains oil emerged mechanism. So, the equipment consists of a big oil tank.
Then the minimum oil circuit breaker came in the market. Here, interruption of current only occurred in a smaller oil chamber. The manufactures used solid insulation to insulate that chamber.
After oil, SF6 gas came into the picture. SF6 gas has both insulating and arc quenching property like oil.
The figure below is showing a photo of Gas Insulated System.
Advantages of Using SF6
For the insulating purpose, we could use many inert gases. One of such gases is nitrogen. Although, those gases do not have excellent arc quenching capability. The SF6 gas is not flammable. But oil is flammable. So, SF6 gas reduces the chance of fire hazard to a very minimum or zero.
The dielectric strength of sf6 gas is three times higher than that of air. Again if we increase the pressure of the sf6 gas, it enhances the dielectric strength of the gas further. So, we can construct a significantly smaller switching system with SF6.
Brief History of Gas Insulated System
After the year of 1960, we started using SF6 gas for high voltage system. The German constructed the world’s first gas insulated system. It was in Berlin, in the year of 1968. The new SF6 systems came with continuous research.
In the first generation of gas insulated systems, we used to use steel for enclosures. At that time, aluminum modules were rarely available on those days. The manufacturers had to make different curved parts of the steel enclosure from steel sheets. The steel sheets required hammering and handwork to give the required shape. The manufacturers used to join numbers of such curved shaped parts together to make a complete enclosure. They did it with o-rings and nut bolts. Although, steel enclosures are quite strong and stable. Many initially installed still enclosed GISs are still functioning today.
Aluminum module enclosures came into use after 1980. Aluminum has reduced the size of modules to much smaller. Also, the aluminum casting module gives much aesthetic look.
Operating Lifespan of Gas Insulated System
The lifespan of gas insulated systems is quite long. It is more than 50 years. Its reliability is much more than the lifespan of AIS. AIS means Air Insulated System. The maintenance cycle of 1st-generation GIS was of 5 years. The 2nd-generation and 3rd-generation GIS have ten years maintenance cycle. The 4th and 5th generation SF6 systems have more than 25 years of the maintenance cycle. In modern gas insulated systems, we do not require any significant inspection before 25 years of its installation. The outer surface of steel enclosures requires a color coating to prevent corrosion.
On the other hand, aluminum module enclosures are entirely free from corrosion. Hence practically it does not require any color coating. But to give an aesthetic look, we may paint the surface of aluminum modules.
Environmental Impact of Gas Insulated System
The ecological impact of GIS is much lower than that of AIS. We usually install a Gas Insulated System under the roof. We usually do not put it in the open air.
Basic Features of Gas Insulated System
The system reduces the probability of electrical faults and failures. It is because SF6 gas physically isolates the power circuit from the atmosphere. Again, the internal supporting insulators have high dielectric strength. Because of these, the probability of internal system fault becomes almost zero in the Gas Insulated Systems. So, we can install an SF6 system in an aggressive environment, too. The system gives the desired performance even in coastal, desert and highly polluted zone.