Simulation of large power systems require the reduction of the complexity of the network models. Different works propose methodologies to aggregate buses and to model the connections between those clusters, that are computationally expensive and developed to work around a specific operating point, presenting significant errors when that set-point changes. This paper proposes a methodology to define a reduced network model consisting in aggregating network buses, using clustering algorithms, and modelling the connections between them as an optimization problem as well as a performance assessment index to evaluate its accuracy. It differentiates from state of the art by the use of multiple operation scenarios, that help to improve its robustness to the system operation set-point, and through its application to a realistic case-study of the European transmission network.