This paper presents a transmission network clustering method based on spectral graph theory, which is applied to Transmission Network Expansion Planning (TEP). This novel method allows to identify sub-systems in an efficient way by applying both topological and system criteria. The identified subsystems are used to solve the TEP problem in a distributed way: by combining a Reduced Linear Disjunctive Model with an active power injection mechanism based on the minium effort network criterium. Results show a significant improvement in execution times without sacrificing the quality of the expansion plan.