CONTRIBUTION OF DISTRIBUTION NETWORK TO THE SECURITY OF THE ITALIAN POWER SYSTEM: PRESENT STRATEGIES AND FUTURE INNOVATIONS

摘要

Recent black-outs happened in many countries worldwide brought to the attention of the public opinion the importance of the security and reliability of national electric systems. The evolution of electric systems from national grids, initially born to satisfy the load growing demand, to large transnational interconnected systems (e.g. UCTE system in Europe) introduced new criteria for coordination and cooperation. Electricity market liberalization, by other side, established different operators involved in generation, transmission, distribution and trade of energy, and new rules for access, planning, operation and management of electric systems. In this context, ancillary services, including system's defence systems, are provided by several actors. Electric utilities take part to the improvement of security of the electric system according to Independent System Operators guidelines and rules. Generally, emergency systems implemented in transmission and distribution network consist in load shedding actions, both automatic and manual. In Italy, most of emergency defence systems are implemented on HV/MV distribution sub-stations and on MV grids. In particular, load shedding strategies during sub-frequency transients are actuated through frequency sensitive relays. If emergency could be previewed with a certain advance, rotation power cuts could be actuated in order to reduce the load in a selective way, on the whole national territory or a part of it. Recent events demonstrated how defence systems could be improved in a medium term and in a long term perspective. By the collaboration among the Italian ISO and Distribution Companies, new strategies are being experimented in distribution network and will be presented in the paper. In long term perspective, interest can be in the use of Electronic Meters to manage the peak of demand, using both tariff leverage and focused load reductions. Even if Demand Side Management (DSM) cannot be properly considered an emergency plan for the security of electric system, it could contribute to stabilize it, assuring a sufficient generation margin, through a better use of energy. Schemes and layouts of local automation systems are presented. Limits are considered.