Gas Insulated Lines



Gas Insulated Lines or GIL are perhaps the most promising technology that can deliver reliable, efficient, high capacity underground electricity transmission. For that reason alone, we all need to understand the rudiments of this technology,its benefits and costs.

Patents for GIL have been held since the 1960’s and in Germany, engineering giant Siemens has had a system running beneath the black forest for over 30 years. And in all that time it has run faultlessly, exhibiting no degradation. So GIL is not a new technology.

GIL development is not restricted to Germany though. In Korea, manufacturing company LS Cable & System now manufacture GIL and the above large picture shows a section through their product. Japan and America also have their own manufacturers with Japan having a very early successful implementation of the technology running for years. So GIL is a worldwide technology.

You will see from the large picture above that GIL is an essentially simple technology. A long while ago, it was discovered that the majority of the energy flowing in a high capacity transmission cable actually flows through the outer part of the conductor. So if you don’t need the core of the conductor, why have one? Hence GIL being based on the tubular aluminium design you see above.

GIL is in fact a tube within a tube. The inner one conducts the electricity, the outer contains the insulating gas. The two are kept apart using epoxy (as in Araldite) spacers. Once assembled, the sections of GIL that form a complete transmission system are filled with a pressurised mixture of gasses, 80% nitrogen (which you are surrounded by right now) and 20% sulphur hexafloride. This last gas is a greenhouse gas and so stringent precautions are taken to prevent leakage.

Assembly of GIL (shown top left) involves bringing lengths of the aluminium components to site and flawlessly welding them together using fully automated welding machines that X-ray the weld continuously to test for weld quality.

GIL can be installed in a number of ways which include direct burial and wall mounting in a tunnel (pictured above left). Whilst direct burial has been done in Europe, tunnel mounting brings benefits of serviceability and upgradability. This is because GIL has the enormous benefit of giving out very low heat and electromagnetic emissions compared with cables. In turn, this allows GIL to be racked close together on tunnel walls. If you need to increase capacity, you simply add more GIL lines into the tunnel. And because the emissions are low, engineers can service one GIL while all the others are live. This is not possible with underground cable transmission systems.

We are indebted to Siemens for the images below which illustrate how GIL can be installed. The key point to note is that GIL in tunnels have a very small footprint in the landscape when compared to either underground cables or overhead lines.

GIL are highly efficient due to the fact that the conducting aluminium pipe can be of a large cross sectional area when compared to overhead line or cable. This also reduces the heat output and therefor energy wastage.

Any conductor passing electrical current produces an electromagnetic field. With GIL however, the field produced by the conductor is 180° out of phase with the equal one produced by the outer casing and thus the fields cancel one another.

The last time we obtained a costing for GIL it was appx. £6million/km though given its relative newness, we can confidently expect this cost to drop as the technology becomes more widespread.

In concluding this very simplistic summary of GIL we should point out that though the technology varies slightly, the basic design remains constant. We will end this article with a picture of the Siemens version of the technology to compare with the Korean one that heads this page.

Our final image is intended to show that GIL is not a technology that is alien to either the UK or National Grid. Siemens Gas Insulated Lines as per the image below left are installed in North London and can be seen in this aerial shot of Elstree Substation.

So why is GIL so important in the opinion of Stour Valley Underground?

    GIL in tunnels has a very low environmental impact compared to other transmission methods

    GIL is highly efficient

    GIL can be installed in a 4m wide tunnel, cables need a 60m wide swathe of land to do the same job

    GIL emits little heat or electromagnetic radiation

    GIL is extremely reliable

    GIL can be serviced while all adjacent lines are live cutting down time and maintenance costs

    GIL can be manufactured in the UK

    GIL is affordable


For more technical data on GIL visit Siemens here or download their informative PDF leaflet on GIL here.

Read more on the 30 year trial of GIL under the Black Forest here

For more from LS Cable&Systems on their GIL technology go here

An article on a GIL project in Japan can be found here

A reference for the first American GIL installation in New Jersey in 1972 can be read here

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A lay persons guide to one of the most important transmission technologies of today