By developing the National Transmission Grid (NTG), the Italian transmission system operatorTERNA pursues the aims of security, reliability, efficiency, continuity of electrical energy supply andcost reduction of transmission and of supplies. This aim is pursued through suitable planning of theNTG development, aimed at reaching an appropriate level of quality of the transmission service andreduction of possible grid congestion, while complying with environmental and landscape lawrestrictions.The planning process begins with the collection, selection and analysis of essential information about:1. forecasts of increases in electricity demand and its geographical distribution;2. location and power capacity of new power generation plants;3. forecasts of electrical energy exchanges with other countries;4. development programmes of other grids with third party access interoperating with the NTG.TERNA verifies the operating conditions of the forecasting grid in static security, using the so-called"N – 1 security criterion". This criterion enables an initial sizing of the forecasting transmission grid,providing for an adequate redundancy of the elements, so that the following conditions are satisfied:5. in typical load situations for the forecasting grid, with related foreseeable productionprogrammes and with all elements of the system in service (fully operational grid), supplies at allpoints of withdrawal are guaranteed without violating the normal operating limits (current andvoltage) of the grid elements in continuous operating conditions;6. starting from the operating states in the above mentioned typical situations, a contingent (oranyhow non-deferrable) out of service of any element of the NTG does not simultaneously cause:7. exceeding of the permitted operating limits for the grid (current and voltage) in continuousoperating conditions;8. load interruptions, except for interruptions that concern only the out-of-service element(connection in antenna).TERNA already owner of about 92% of the transmission grid, in 2006 acquired part of 220 kV gridfunctional to the supply of the major Italian metropolitan area (i.e. Milan, Turin and Rome); the classiccriteria for planning and developing the transmission has immediately shown itself not suitable forsuch metropolitan grid.A new approach has been used as a pilot project in the Milan Area, and it was adopted successively forTurin and Naples areas.In the urban context, the transmission grid planning rises new kinds of problems.At first, the problem are technologies: the new infrastructures planning couldn’t consider theenvironmental context, in which they will be collocated; so that the using of solutions like cable and/or
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