generation Massimo Gallanti*, Maurizio Delfant**, Marco Merlo**, Mauro Pozzi**, Valeria Olivieri** CIRED 2009 20th International Conference and Exhibition on ELECTRICITY DISTRIBUTION Praga, 8-11 Giugno 2009 * CESI RICERCA * POLITECNICO DI MILANO Nowadays, and in the next future, the increase of the Dispersed Generation (DG) will alter significantly the operation of distribution networks. The attention is focused here on the HV/MV interface, where an inversion of power flow on the HV/MV transformer feeding the MV network, can take place as a consequence of the DG increase. This particular condition has not been considered so far in the design of the electric protection devices of the HV/MV substations and can lead to critical situations. For the purposes of this paper, the time percentage when the MV network injects power into the HV (transmission or subtransmission) network is assumed as a possible indicator of the “active” behavior of MV networks. The proposed research focuses on the analysis of the degree of DG penetration capable to lead to such an inverse power flow: the aim is to point out how much DG can be connected to the MV electric distribution networks assuming to accept a predefined threshold, in terms of yearly hours, of Inverse Power Flow (IPF).