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The contribution of distribution networks to the improvement of security of national electric systems: present systems and future innovations

pubblicazioni - Articolo

The contribution of distribution networks to the improvement of security of national electric systems: present systems and future innovations

C I R E D Session Number: 3 The Contribution of Distribution Networks to the Improvement of Security of National Electric Systems: Present Strategies and Future Innovations C. D’Adamo *; M. Sforna †; G. Mauri +; A. Pievatolo  *ENEL Distribuzione; †GRTN; + CESI;  IMATI * † +  Italy *christian.d’adamo@enel.it; †sforna.marino@grtn.it; +gmauri@cesi.it; antonio.pievatolo@mi.imati.cnr.it Recent blackouts happened in many countries worldwide have brought to the attention of the public opinion the importance of the security and reliability of national complex electric systems. In a liberalised market, with many different operators involved, ancillary services are provided by several actors. Electric utilities take part to the reduction of the vulnerability of the electric system according to Independent System Operators guidelines and rules . In this paper, "security of the electric system" is assumed to be the capacity of a complex electric system to keep in service in case of wide scale or severe events. Improving the security of an electric system means to reduce the possibility that severe network events can evolve to general blackouts. To prevent these blackouts, load shedding strategies are implemented in both transmission and distribution networks. They generally consist in load reduction strategies, both automatic and manual. Obviously, the strategies implemented are redundant in order to assure the reliable service of the electric system. In Italy, most of emergency plans are implemented on HV/MV substations and on MV grids. In particular, load reduction strategies in sub-frequency transients are actuated through frequency sensitive relays. If the emergency could be previewed with advance, rotation power cuts (PESSE) can be actuated to reduce the load in a selective way on the whole national territory. Recent blackouts demonstrated how emergency plans could be improved in a medium term and in a long term perspective. By the collaboration among the Italian ISO and Distribution Companies, new devices and strategies are being experimented in distribution networks. In particular, simulations demonstrated that new minimum voltage relays can be used to suddenly reduce the load, in order to avoid the separation of national grid from the European interconnected transmission system. These new relays can improve in a significant way the security of a complex interconnected system anticipating the separation of a national sub- system from the major interconnected one. An experimental installation of these new devices is beginning in Italy. Settings of such devices should be experimented on field in order to avoid inappropriate triggers. Another important contribution to the improvement of the security of Italian electric system is the contribution of dispersed generation (DG) in HV and MV grids to the under-frequency transients. Actually, Italian connection rules for dispersed generation do not specify the participation of generators connected to distribution networks in case of under- voltage transients due to electric system’s collapse. The Italian ISO and the major Distribution company agreed that, in future, DG will participate to ancillary services, and defined the network’s requirements. By the participation of DG to under-frequency transients , it will be possible to enlarge generation margins, improving the security of the whole system. Finally, in long term perspective, interest can be in the use of Electronic Meters to manage the peak of demand, using both tariff leverage and/or focused load reductions. Even if Demand Side Management (DSM) cannot be properly considered an emergency plan for the security of electric system, it can contribute to stabilize the whole system, assuring a sufficient generation margin, through a better use of energy. Schemes and layouts of local automation systems are presented. Limits are considered. In conclusion, never than before, distribution network is playing an important role to the security of national electric system. To the aim, in medium term, from demand side, new and more efficient load shedding devices are being experimented. At the same time, from generation side, dispersed generation can contribute to the improvement of reserve margin even in sub-frequency transients and in emergency conditions. In future, new perspectives can be opened by Demand Side Management even if security plans should be however implemented in complex electric systems. PUBBLICATO A5031083 (PAD – 670817)PUBBLICATO A5051961 (PAD – 724609)

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