Traffic tunnels could be used to install power transmission systems

Changes in power generation driven by decentralised and dispersed generation locationsthe use of power transmission equipment up to the edge of its ratings and the influences coming from the deregulated power market including electricity trade will change the structure of power transmission systems.

The installations of wind farmsbio gas generationhydro power plantsand solar plants are widely spread in Europe. Many of them are installed at rural locations far away from the load centres. High voltage transmission lines will be needed to connect the regions of generation with the load centres. Existing transmission lines need upgrading and new lines are required to be built.

Substations

The assets in installed substations and on transmission lines are used more to their rated valuesor to the edge. This will bring the transmission system more to their technical limitations. In a consequence it will reduce the reserves in the network for power transmission and may influence the reliability and availability.

Relatively small failures or interruptions in the power network will cause blackouts or regional power supply interruptionswhich may develop to wide area interruptions of electric power supply.

Deregulation

Deregulation in Europe is changing the transmission network in a way that not necessary the generation station in the neighbourhood is delivering the energyin some cases it may come from far away places. With the trade of electricity also the power flow in the transmission network changes and a new structure will be needed.

This underlines the need of new transmission lines to provide sufficient power transmission capacity. New transmission lines built as overhead lines may need much time to get the permission by authorities and the public before they can be built.

One possibility to improve the transmission network today is to use new built traffic tunnels in conjunction with GIL. Multipurpose and multiuse tunnels. GIL is a Gas-Insulated Transmission Line which has the possibility to transmit high amounts of electric power (2000–3000MW) in one system.

It can be added to a street or rail-road tunnels for more effective and efficient use of the expensive investment of a traffic tunnel. The GIL offers solid and reliable transmission of electric power with high availabilitylow power transmission lossesand a long lifetime.

These features of GIL has also be noticed by the European Commission for Energy which statedthat in any case when new traffic tunnels are planned in Europe the possibility of share use of traffic and electric power transmission shall be proven. This paper explains the advantages and possibilities of power transmission in traffic tunnels.

Traffic tunnels

The European Union is proceeding in the goal to bring the citizen of each country more close together.

One very important link between the countries is the traffic of goods and persons. When trade is increasing also traffic is increasing. Large highway and railway projects have been started in the last years and many more are in the planningeg the Brenner Basis Tunnel for high speed train connection between Austria and Italy or new high speed train connection between Lyon in France and Milano in Italy.

The highway tunnel between Dover in the UK and Calais in France has been brought in for discussion. High speed train connection in Germany through the Thuringian Forrest to connect Berlin to the South West of Germanywhich was blocked until the reunification in 1989.

Many more such projects will come up in the next yearsbecause trade and travelling people will increase with each step of creating a single Europe.

Pilot tunnel

Typical traffic tunnels of long lengths and high traffic use two separate tunnel tubesone in each directionand a third tunnelwhich is also called pilot tunnel. Each of the two running tubes could hold a railroad trackthen the diameter would be in the range of 5–7m or it would hold a two-lane highwaythen the diameter would be 9–11m.

The pilot tunnel is used for investigation of the mountain and soil during construction and is not used later on for operating the traffic tunnelthe typical diameter is 4–5m.

hen the traffic on the road is not so highthen a single tunnel with
bi-directional traffic can be chosen. Values of about 5000 vehicles
per hour give some limits for such tunnelsalso restrictions are coming from geographical conditions and length.

The longest road tunnel in the world is the 24.5km long Laerdal Tunnel in Norwayit is laid out for 2000 vehicles per day. The longer the tunnel the lower the traffic. The Gotthard Tunnel in Switzerland is 16.3km long and can take 5000 vehicles per daya medium traffic value (Table 1).

To meet the higher traffic numbers of larger than 5000 vehicles a day and longer distances then the traffic tunnels are built in a two-tube layout with one directional traffic in each tube.

The Channel Tunnel connecting Calais in France with Dover in the UK is such challengewhere the high speed train Eurostar transports load cars and trucks through the tunnel on length of 47km).

A cross section would show two running tubes for the trains and a service tunnel in the centre. In this Channel Tunnel the service tunnel is used for all services during operation and has in distances of kilometres cross passages between the running tubes.

This is part of the tunnel operation concept. It would also be possible to add two systems of GIL in the pilot or service tunnel and reaching the possibility of transmitting 4000MVA of electric energy in two three-phase GIL systems.

Planning

The only requirement for electric power transmission in the service tunnel is to have it planned right from the beginning. To add later onwhen the planning of the traffic tunnel is concludeda GIL is almost impossible.

Today the availability of knowledge and equipment to build large tunnels and the wide basis of experience opens new ways of applications.

Larger sizes and longer lengths will be realised in the future. Wye junction (TV-WSTunnelverzweigung West-Süd) in the western tube of the Gotthard Base Tunnel at Multi-Function Station FaidoCanton TicinoSwitzerland. The right tunnel is the main western bore (EST-WSEinspurtunnel West-Süd)the left tunnel
(VT-SVerbindungstunnel Süd) is a connection into the main eastern bore (EST-OSEinspurtunnel Ost-Süd).

GIL Technology

Gas Insulated Transmission Lines (GIL) is a technology which offers high power transmission rating over long distances.

The Channel Tunnel with its 47km length is a good example where electric power transmission of 2000–3000MW per three-phase high voltage system could have been added. The pilot tunnel offers enough space to install two 3-phase systems.

The GIL has a solid aluminium alloy enclosure and conductor pipe made of electrical aluminium. Two systems can be installed in a tunnel of 3.5m and greater. Two three-phase GIL systems are placed in a tunnel with a small rail track for installation works.

Since more than 30 years the GIL offers solutions for high power transmission in the high voltage field with very high reliability. Modernpipeline laying related technologies to install long lengths of GIL systems have made its application more attractive because of cost reductions.

The GIL offers today the opportunity to use traffic tunnels not only for the road or rail traffic but also for the transmission of electric energy.

This is possible because of the safe and reliable GIL system with no additional dangereven in the case of an internal failure. The solid metallic enclosure protects the surrounding and the solid grounded GIL has very low electrical and magnetic fields even under full electrical load.

Project Planning

Large scale traffic tunnels do have very long planning and project implementation times. Ten years or more are not seldom.

The basic designthe financing partnersthe ownershipthe operation and political and social interests are generally fixed in a very early phase of the planning. As soon as this basic concept and the purpose of the tunnel are fixed a basic change is unrealisticbecause all investigations have to be started again.

The question of ownership and limits of responsibilities in a large scale traffic tunnel is a fundamental one. It will be fixed early and with it the financial responsibilities and risks are fixed.

When this has been fixed it is very difficultalmost impossibleto be changed at a later stage. That means that the combination of traffic tunnel with electric power transmission only can be reached at the beginning of the projectbefore ownership and responsibilities are finally fixed.

Usually electric power transmission companies and traffic tunnel owner/operator companies are different. Also different authorities give the permits for building and operation of traffic tunnels and electric transmission systems.

Thereforeit is strongly recommended to get into contact as early as possible to generate benefits for all entities linked to the project.

Adding a GIL to a traffic tunnel will add one more player/investor to share the invest¬ment and operation cost.

Future Trend

The future trend is higher traffic on road and railsmore electric energy transmissionand the need to use assets more effective. Traffic tunnels are huge assets and there should always be a focus on the possibility to share the tunnel with electrical power transmission systems. This future trend can be seen today in the starting phase.

Several projects are under discussion today and planning has started.

This future trend to combine traffic tunnels with GIL will only be successful if the awareness is there. In a very early stage of planning the possibility of adding a GIL for electric power transmission needs to be provenand that the related entities are getting together to exchange their needs and requirements.

One example can be given here: There is the plan to add a Channel Traffic Tunnel for cars in parallel to the train tunnel of today between Dover and Calais. If this project would be realised it would be a great advantage when traffic and power transmission with GIL are combined. The existing European network would be strongly linkedbeside two HVDC lines already in operation.

Conclusion

The conclusion is that it is needed today to spread the informationthat the combination of high electric power transmission lines with GIL is ready to be added to long traffic tunnels.

To make engineers and authorities aware that such a combination must be brought into the planning and decision path very early.

The benefits is for all participants in such a project: tunnel investorselectrical utilityand operator. It is strongly recommended to prove the need of electricpower transmission lines at locations where traffic tunnels are planned. This would be done in an early stage because it might not be possible later.

Dr-Ing Hermann Koch is with Siemens EnergyErlangenGermany. ww1.energy.siemens.com