In this paper one introduces the modeling of power flow constraints for transmission grid in the multi-sectoral multi-energy long-term investment planning tool ETEM-SG. This extension allows a better representation of demand response for flexible loads triggered by real time marginal cost pricing. Since the loads and the generation units are geographically distributed these prices should be represented by nodal marginal costs. Now ETEM-SG contains a linearized DC power flow model that represent the transmission grid with the main constraints on the power flowing through the different arcs of the electricity transmission network. Robust optimization is used to take into account the uncertainty on the capacity limits resulting from inter-regional transit. A numerical illustration is carried out for a data set corresponding grossly to the Leman Arc region.