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Gas-Insulated Transmiss ion Line - GIL

http://electrical-engineering-portal.com/gas- insulated- transmission- line-gil January 6, 2013

Gas-Insulated Transmission Line GIL

The gas-insulated transmission line (GIL) is asystem for the transmission of electricity at highpower ratings over long distances. In cases

where overhead lines are not possible, the GIL isa viable technical solution to bring the powertransmit ted by an overhead line undergroundwithout a reduction of power transmissioncapacity.

As a gas-insulated system, the GIL has theadvantage of electrical behavior similar to that ofan overhead line, which is important to theoperation of the complete network. Because ofthe large cross section of the conductor, the GIL

has low electrical losses compared with o thertransmission systems (overhead lines andcables).

This reduces the operating and transmission costs, and it cont ributes to reduct ion of globalwarming because less power needs to be generated.

Safety of personnel in the vicinity of a GIL is very high because the solid metallic enclosureprovides reliable protection. Even in the rare case of an internal failure, the metallic enclosure isstrong enough to withstand damage. This allows the use of GILs in street and railway tunnelsand under bridges with public t raff ic. No f lammable materials are used to build a GIL. The use of

GILs in traff ic tunnels makes the tunnels more economical and can solve some environmentalproblems.

If GIL is added to a traff ic tunnel, the cost can be shared between the electric power supplycompany and the ownerof the traf f ic part (train, vehicles).

The environmental advantage is that no addit ional overhead line needs to be built parallel tothe tunnel. Because of the low capacit ive load of the GIL, long lengths of 100 km and more canbe built. Where overhead lines are not suitable due to environmental factors or where theywould spoil a part icular landscape, the GIL is a viable alternat ive because it is invisible and doesnot disturb the landscape.

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GIL Construction

The GIL consists of three single-phase encapsulated aluminum tubes that can be direct ly buriedin the ground or laid in a tunnel. The outer aluminum enclosure is at ground potential.

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Gas-Insulated Transmission Line - Contruct ion

Theinterior,theannularspacebetweenthe

conductorpipe andtheenclosure,is filledwith amixture ofgas, mainlynitrogen(80%) withsome SF6

(20%) toprovideelectricalinsulat ion. A reverse current, more than 99% of the conductor current value, is induced in theenclosure. Because of this reverse current, the outer magnet ic field is very low.

GIL combines reliability with high transmission capacity, low losses, and low emission ofmagnetic fields. Because it is laid in the ground, GIL also satisfies the requirements for powertransmission lines without any visual impact on the environment or the landscape. Of course,the system can also be used to supply power to meet the high energy demands ofconurbations and their surroundings.

The directly buried GIL combines the advantage of underground laying with a transmissioncapacity equivalent t o t hat o f an overhead power line.

SOURCE: Hermann Koch, Siemens