Strategical planning is one of many research fields in the design of electrical distribution systems. The problem of strategical planning is a multiobjective combinatorial problem and the search space may often be quite large concerning to the options. The aim is to identify a strategy of expansion of a given distribution system in a given timeframe. For this problem, the search space is created beforehand by running a multiobjective optimisation algorithm for the optimal design of distribution networks for different load levels related to different years. The sets of Pareto-optimal solutions obtained for each load level at each year are equivalent in terms of the considered objectives, these being minimum losses, installation costs, and minimum unavailability. The problem of the identification of the optimal expansion strategy through these chronologically intermediate solutions leading to the final target configuration at the last year has been solved herein using an ACS (Ant Colony Search) algorithm. In order to verify the efficiency of the ACS algorithm, a small size application has been carried out and results have been compared to those obtained with enumeration. Then, a Simulated Annealing (SA) approach was used for a larger size test problem and results were compared to those obtained using the ACS. For this problem, the ACS demonstrated to be more robust than SA with higher quality results.

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Abslracl-Distributed Generation will pby an increasing role hi the elfdric power system of the near future. The high penelration of DG in the next ycnrs represents a potential resource for the electric power system On the otber h n d it brings about several problems concerning with distribution system planning, control and mtmgenlent. Thr alm of i h l s paper :is to analyze some operating conflicts due to thr presence of ~nchronous and asynchromm generators directly CoMKted to distribution network.. The workalso proposes a possible solution. based on a new device defined as Pauli Decnupling Device (FDD). The principle of operalion of this device has b e n presented and the iquhility has been proved tiy'nr~ns orcomp&r dniulations Index Tems--Dislributed Genrratiob Fault Decoupling Device, Fault Current, Power Qnalily, Voltage Sags. I. INTRODUCnONHE traditional role of distribution networks is the T. interconnection between generation and transmission systems and costuner's loads These network are, generally, radial with unidirectional power flow from primary substation to loads. As a consequence they were defned as "passive" bccausc they were not desiped io receive any generation plants. The main reasons for ihc radal configuration are the simplicity of operation and the economy of the overcurrent protection system [I].The liberalization of the electricity market, the .need for more flexible electric system, the search for more efficient exploitation of energy sources, uing also renewable ones and the need to protect sensitive loads against network disturbances have given impetus, in the last years, to the development of new technologies for electric power generation and distribution. Distributed Generation (DG), defined a s the generation of electric power by limited size plants (from 10 kW to 15 rVfW).interconnected at substation, distribution feeder or load Icvel, will play an increasing role in the electric power system of the near future. If the high penetration of DG predicted in the next years (up to 20% of all new generation going on line by the year 2010) represents a potential resource for the electric A. Cenpuccia (email: campocci~dieps.unipa.it). G. Cocchiara (timail: cocchian@!diepa.unip.it), M. C . lppolito (email: 'ippalita@diepaunipa.it) and G. M a m a (mam@diqannipa.il~ BIC with the Dcparuncnt of Eleclrical Eogineaing, Palerno University -Viale,&lle Scienze, 901 28 -Palmlo. Italy.0-7S03-7967-5/03/$17.00 82003 IEEE power system (losses reductio& deferme.nt or reduction for new plank investments, renewable sources exploitation, energy savings, power quality and utility system reliability improvement), on the other side it brings about several problems concerning with distribution system planning; control and management [2]-[3].

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