Application Of Multi-Criteria Methodology For Planning An Electric Transmission Line

The multi-criteria procedure has already been presented in a previous CIGRÉ paper [1] and successfully applied to several existing and planned transmission lines [14]. In this paper it has been particularized to take into account a transmission link whose part of the route has a constrained passage inside a tunnel. The tunnel whose primary purpose could be railway or highway actually becomes a multipurpose system in which railway or road transport and electricity share the same structure. Multipurpose structures have deserved a very careful consideration by CIGRÉ Community that has started two Working Groups aiming at studying this technically challenging option. The harmonious growth of the electric grid requires tools for the evaluation of the social performance of new transmission investments, in terms of economic efficiency, environmental impact and technical capabilities. In particular conditions, like urban areas and possible presence of tunnels, the optimization in planning new Extra High Voltage (EHV) lines has required a methodology aimed at comparing the different technological options in the frame of the reference of different "exogenous" conditions. In this paper the multi-criteria procedure allows investigating on the most convenient (from an environmental and economic standpoint) transmission line option for an EHV link. To this aim, the following technical options have been taken into consideration: an AC mixed line composed of a cascade of overhead and cable lines, bipolar HVDC link with metallic return and an entirely AC XLPE cable line. A strong constraint of the considered lines is that the link must pass inside a multifunctional infrastructural tunnel or penetrate urban areas. For this reason an entirely overhead line has to be discarded. Various performance indicators have been defined. The technical ones are related to power losses (losses in the lines and in the conversion stations for HVDC) and effects of shunt reactive compensation for XLPE cables. The economic indicators take into consideration the installation costs, the costs for operation and maintenance (including the monetary value of active power losses) and those for dismantling at the end of the line life. With regard to the environmental impact, the indicators are referred to the magnetic field, land occupation and right of way requirements, the presence of compensation equipments, constraints on the use of land, acoustic and electromagnetic pollution, visual impact, disamenities and impacts during construction. The paper will yield two outputs: the best transmission technology from an economical standpoint and that from an environmental one so that each involved subject can clearly weigh the importance of two different results and the respective synergies.