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Tuesday, 29 December 2015

GAS INSULATED TRANSMISSION LINE: A BACKBONE FUTURE TRANSMISSION SYSTEM?

The power transmission system of today will see basic changes in the near future and Gas Insulated Transmission Line (GIL) is one of them. When overhead (OH) lines cannot be built, then the GIL offers an alternative solution by going underground with the same quantum of power as the OH line.

The gas insulation technology was introduced to sub-stations in the late 1960s and is widely used today because of its significant advantages. Application of GIL was started way back in the 1975, when the Siemens co. installed a GIL inside a tunnel for one of the pumped storage power stations in Germany. 

These lines can be laid above the ground, under the ground directly in soil or in underground tunnels depending upon the requirements. The introduction of 2nd generation GIL in 2001 using Nitrogen (N2) and Sulphur Hexa-fluoride (SF6) mixture and pipeline laying techniques to reduce the cost makes the GIL a long distance, bulk power transmission system with greater reliability and availability.

Construction of Gas Insulated Transmission Lines:


Gas insulated transmission line consists of a number of modular components which are assembled to make the complete GIL. They consists of an Aluminium conductor inside a tubular enclosure. The conductor is rested on cast resin insulators which keeps the conductor right in the centre of the outer enclosure. The outer enclosure is a strong aluminium alloy tube which provides the needed mechanical protection. The spacing between the conductor and the enclosure is filled with a mixture of Nitrogen and SF6 gas to provide the required electrical insulation.

Usually the SF6 percentage is small (about 20%). When these lines are buried directly into the ground, the outer ‘Al’ alloy enclosure is coated with a polyethylene layer throughout its entire length. Ultrasonic inspection technique is used to check the leakage of gas mixture. Fig 1 shows the construction of a GIL.

Fig.1: Construction of a GIL

Advantages of Gas Insulated Transmission Lines:


The advantages of Gas Insulated Transmission Lines (GIL), as compared to other transmission systems, are:

  • higher power transfer capability (3000 MVA/system for a rated voltage of 550 kV), 
  • superior Electro-Magnetic Compatibility (EMC), 
  • low losses (less than 150 W/m for a loading of 1800 MVA) and 
  • no fire or explosion hazard and hence higher safety. 

The insulating system is not subjected to aging and hence reduces the risk of internal failure. Since these lines are fully enclosed, hence are entirely protected from the environmental impacts. These lines have a very less maintenance requirement, only external inspection is needed. Because of these features the GIL system has an expected life of more than 50 years. The housing i.e. the outer enclosure is solidly grounded and therefore makes the GIL a safe system.  

The capacitance of GIL is very low (55nF/km) as compared to XLPE cables, and hence the compensation required in the form of reactors is usually not needed for lines up to 70 km. These lines are suitable for direct connection to sub-stations and no modification in the protection techniques is required.

Gas insulated transmission lines are gas tight and are sealed for lifetime and because of which they have superior operation throughout their comparatively higher lifetime. GIL have a very low magnetic flux density, 15 to 20 times less than conventional power transmission lines and hence are more suitable for power transfer through populated areas, EMC sensitive areas, and along with telecommunication systems. These lines are un-affected by high ambient temperature and severe atmospheric pollution.

Fig.2 shows the comparative analysis of magnetic flux density of GIL with other transmission systems.

Fig.2: Comparative analysis of magnetic flux density (in microTesla) of GIL with other transmission systems.

The GIL installation process consists of assembly of pre-fabricated modules at the installation site. The key elements are light in weight and can be easily transferred to the site location. During the installation gas tightness is to be ensured at all cost and which very much depends on the welding process.

Application of Gas Insulated Transmission Line

Gas insulated transmission line can be used in the voltage range of 245 to 550 kV with the current capacity up to 4.5 kA. Gas Insulated Transmission Lines in underground tunnels can be very viable option in future, as the land above the tunnel can be fully restored for agricultural use. Gas insulated transmission line can be installed vertically also, and hence can be used with underground power plants.  The tunnels used for GIL can also be used for ventilation purpose in the case of underground power plants which reduces the overall cost of the system. 

GILs have all the qualities to become the backbone of future transmission systems. They can also be deployed to transfer bulk power in the GW range from large off-shore wind farms through undersea tunnels with greater reliability.