C I R E D Session Number 5 LV DC DISTRIBUTION NETWORK WITH DISTRIBUTED ENERGY RESOURCES: ANALYSIS OF POSSIBLE STRUCTURES A. Agustoni†, E. Borioli†, M. Brenna*, G. Simioli†, E. Tironi*, G. Ubezioa0 *Politecnico di Milano, † CESI, a1 SIEL SpA Italy agustoni@cesi.it, borioli@cesi.it, morris.brenna@polimi.it, simioli@cesi.it, enrico.tironi@polimi.it, gubezio@sielups.it The attention of the end users to the electric power quality problem, the widespread use of the electronic power converters and the need to integrate the new distributed generation and storage systems have increased the interest in considering a public distribution system in direct current. The main idea is to extend the dc section, nowadays present in many electric devices, distributed generation systems and uninterruptible power supplies, at the level of LV public distribution. In this way it would be possible to create an high quality electric island that achieves also the advantages of the number and the complexity reduction of the power converters connected to the distribution network and a greater energy transport capacity with the same conductors. The dc distribution would offer also other advantages: from the point of view of the distributor, it can reduce the frequent power quality problems present in ac network due to the disturbances injected by the rectifier stages of the electronic equipments. Vice versa, the presence of a single ac/dc converter as interface between the dc distribution system and the public network allows to keep under control the harmonic distortion on the network. On the other hand, the end user would have a greater quality of the electric power, intended as continuity of the electric service, and a network in which the integration of the distributed generators is simpler. Moreover, it is possible to stabilize the dc voltage also in presence of perturbations incoming from the ac side. Finally, the manufacturer of the electric appliances can simplify their construction by modifying the input stage, especially for the single phase devices, with a significant cost reduction. Starting from the considerations carried out in the previous paper [1], in which the reasons to move towards a dc distribution have been exposed, in the present job some circuital configurations are analyzed both for technical and economical points of view. In particular, among the solutions proposed in [2], it has been analyzed the scheme constituted by an IGBT interface converter, paying attention to conductor numbers of the distribution line: 2 wires (positive and negative pole) or 3 wires (positive and negative pole, and a neutral conductor). In both cases, a value considered suitable for voltage in the dc section is 800 V, that means +400 V pole-to-ground for one pole and –400 V pole-to-ground for the other one. The first solution appears more suitable in industrial contexts, in which the various loads (e.g. electric motor drives) could be directly fed with 800 V, while the second one can be used in residential or tertiary ambient, in which a lower voltage can be more useful. The various solution (ac, dc 2 wires and dc 3 wires distribution) have been examined from the maximum transmissible power and the economic points of view. Figure 1 shows a comparison of the maximum current in a quadripolar copper cable among the three solutions, in the respect of the thermal and maximum voltage drop limits. It is possible to see that for distances that produce the reduction of the exploitation of the cable due to voltage drop limit (descending part of the curve) the dc solution allows the transmission of a power 2.2÷3.9 times greater than in ac, depending either on the configuration of the conductors or on the dc distribution system. The economic analysis has shown that the dc distribution is more convenient regarding the ac one in the case of powers greater than 100 kW and for distances greater than 400 m, since by increasing the transported power or the length of the line, the better exploitation of the conductors compensates the greater cost due to the interface and balance converters. Finally, a model of the entire system has been implemented in ATP, and some numerical simulations have been carried out. For example Figure 2 shows the stabilization of the dc voltage by storage systems during an interruption on the ac side. [1] A. Agustoni, M. Brenna, E. Tironi, G. Ubezio, "Proposal for a high quality dc network with distributed generation", 17th International Conference on Electricity Distribution, CIRED, 12-15 May 2003, Barcelona, Spain [2] M. Brenna, E. Tironi, G. Ubezio, "Proposal of a local DC distribution network with distributed energy resources", 11th International Conference on Harmonics and Quality of Power, ICHQP, 12-15 September 2004, Lake Placid, New York (f ile casob.pl4; x-v ar t) v : DC_POS-NEU+v:NEU -DC_NEG 0.0 5 0.1 0 0.1 5 0.2 0 0.2 5 0.3 0[s ] 77 2 77 7 78 2 78 7 79 2 79 7 80 2 [V ] Figure 2: dc voltage stabilization during an interruption on the ac side. Figure 1: Comparison between maximum current in a quadripolar copper cable for ac and dc distribution. PUBBLICATO A5031380 (PAD – 671737)PUBBLICATO A5051440 (PAD – 723044)