High capability applications of Long Gas Insulated Lines in Structures

The paper deals with the possibility of installing Gas Insulated transmission Lines (with acronym GIL) in dedicated or shared structures. The applications involve railway galleries (both in the proper galleries and in service or pilot tunnels), highway networks, gas lines. The high GIL power capability together with very low power losses configures these applications as high transmission efficiency corridors. Some issues of this application will be shown in this paper (steady-state regime of the line, voltage drops, power losses, voltage unbalance factor, ampacity, electro-magnetic field impact and limit lengths). Moreover, the paper discusses the necessary studies to be carried out and the problems that have to be faced in order to develop this kind of application towards a feasible technical solution. ThE authors, when facing the question of land constraints for electric infrastructures, have often begun with this outstanding passage taken from an EPRI book [1] that postulates the necessity to develop common corridors in order to satisfy the ever-growing exchanges between areas of big industrialised systems: COMMON CORRIDORS For both ecological and economic reasons future expansion of transmission systems especially near urban areas will have to consider sharing their rights-of-way with other land users. Independent use of land by single utilities can lead to inefficient use of natural resources. Development of new rights-of-way in future years will have to consider the location of suitable highway networks, gas lines, sewage systems, nearby utility transmission lines, telephone lines, underground transmission systems, and recreational areas. Such development will require a coordinated effort by the government and industry. A current study in Reference [2] outlines the approach that the electric utilities may follow to encourage full use of their transmission line corridors. This EPRI idea regarding USA, even if with wide territory resources, is very well applicable to other places. This topic is so felt and argued in Europe that the International Council on Large Electric Systems (CIGRÉ) has established a Joint Working Group entitled "Application of long high capacity Gas Insulated Lines in structures" to study the feasibility and the characteristics of this technical solution. This JWG will last three years and will deliver a final Brochure. A dismantled Joint Working Group 23/21/33.15 "Gas Insulated Transmission Lines (GIL)" has delivered a brochure in February 2003 [3] but this is completely devoted to short length and directly buried GIL so that a prolongation of it in the direction of long length and installed in shared, dedicated structures, in tunnels or in galleries is highly needed. The question can be naturally applied to other underground technologies most of all to XLPE cables. In fact there is another CIGRÉ Working Group B1.08 entitled "Cable systems in multipurpose or shared structures" which is facing this different solution. The rationalisation of fundamental services in the land, such as electricity and transport both via rail and via highway, represents a higher degree of consciousness in conciliating environmental issues and technology. Of course it is also a big technical challenge because it is not trivial that there is a full compatibility between possible different infrastructures. Moreover there are a lot of projects of new galleries between neighbouring countries (separated by mountain chains) and these possibilities represent absolutely unique opportunities for integrating (railway or highway) transport and energy transmission within the same corridor or within the same structure. This could strengthen the scarce transnational network between States whose electrical networks are responsible of giving high hindrances to an effective international electric market. Sharing different nature structures and power transmission line within the same corridor represents an optimization of service rights-of-way: conciliating technological society needs with respect towards nature and territory seems to be the challenge of 21st century.