This paper proposes a multi-objective expansion planning tool in order to support and facilitate the characterization of a transmission or generation investment as beneficial or not. The tool is able to provide information for decision analysis based on economic, environmental and societal criteria, in terms of security of supply, using trade-off curves and weighted internalized external costs. The results describe the impact of policy strategy and transmission network utilization on the social welfare and the social benefits of externalities internalization. Furthermore, the change in system prices and system losses has been studied as well. It has been shown that the level of internalization is a key issue in order to compensate the negative environmental impact of high utilization of transmission lines. In case that green investments are promoted, it may happen that the best solution regarding social benefits doesn't reflect always to the highest system price or to the highest system losses.