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 2^nd 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. 

GAS INSULATED SUBSTATIONS FOR INDIAN GRID: ADOPTION OF NEW TECHNOLOGIES

Last up-dated 16 January 2017

" The Gas Insulated Substation Technology was introduced for the first time in 1968 and is widely used today because of its significant advantages."

Today's power system insists for a more efficient and reliable solution to handle the new challenges. Power Transmission and Distribution in densely populated urban area is a great challenge because of land availability, and low noise and electromagnetic limitations. With the development in technology and enough improvement in substation and switch-gear design, Gas Insulated Lines (GIL) and Gas Insulated Substations (GIS) are finding increasing use under these conditions. These sub-stations are more safe and reliable, requires less maintenance, and have a longer life. They are highly adaptable to individual requirements.  

How they are different from Conventional Sub-stations?

In a GIS, major equipment such as circuit breakers, CTs, PTs, isolators, bus-bars etc are enclosed in a sealed environment of Sulphur Hexafluoride (SF₆) gas. Due to the excellent dielectric properties of SF₆ gas, the clearance required between phase to phase and phase to earth is much smaller and hence the design is compact. The enclosure is made up of Aluminium which is light in weight and is non-corrosive. Reduced weight of enclosure leads to reduced load on foundations/ floors and thereby reduces the cost also.

Relative merits of Gas insulated substations:

Gas insulated substations have an estimated lifespan of more than 50 years. They have high mechanical stability and resistance to arcing, and hence offers a high degree of personal safety. As these sub-stations are very compact, even larger units of relatively higher voltages are easy to handle during transportation and installation. They can be shipped as completely assembled units, pre-filled with SF₆ gas. The control interfaces of these sub-stations enable the connection to any analog or digital control, protection and monitoring system.  

Scope of Gas insulated substations in India

The scope and future of Gas insulated substations in India is quite promising considering the rapid growth in power sector. Bulk power is transmitted at 400/765 kV and still higher voltage to come, the scope and opportunity for GIS is quite substantial. 

GIS, which are available in voltage range of 72.5 kV to 765 kV, is preferred for hydro-power plants and coastal area substations because of its compact and enclosed design and low maintenance requirements. In these substations, additional protection against thunderstorm, lightning, static discharge etc is not required. The footprint of a typical GIS is about 10 times less than the conventional air insulated sub-station. Because of the ever increasing land cost, GIS has now become a commercially viable solution in India. 

Prominent Gas insulated substations Projects in the Country

Recently Alstom T&D India (globally known as Alstom Grid) has been awarded to supply two 400/220/66 kV Gas insulated substations at Wangtoo and Gumma in Himachal Pradesh. The scope of the project covers the design, manufacturing, testing and commissioning of the GIS. The same company has been awarded a contract by Power Grid Corporation of India to design, manufacture, install and commission two Gas insulated sub-stations at Betul in M.P. and Navsari in Gujarat. These 400/220 kV substations will help to evacuate the power generated at Mouda STPP and Kakrapar Atomic PP. Alstom T&D India is manufacturing Gas insulated substations in India since 2009.

Similarly another Energy Sector industry L&T construction, in collaboration with Korean company, has successfully commissioned India's first 765 kV gas insulated sub-station in Pune, Maharashtra in 2015. This particular sub-station will enhance the grid connectivity and power transmission in Western India.