In this paper the authors apply methodological strategies for the structural vulnerability assessment in high voltage power networks based upon the combination of power flow models and scale-free graph statistic indexes. Thus, it is possible to study risk scenarios based on events that may trigger cascading failures within a power system. The usefulness of graph theory techniques has been validated on previous works of the authors, and may be applied in analysis of the vulnerability of different power electric networks. A case study for vulnerability analysis is carried out through methodologies that allow comparison on random error and deliberate attack tolerance evaluation in transmission electric systems from countries like Colombia and Spain. Such vulnerability assessment methodology takes into account the current conditions of the power networks (base case), as well as the impact of expansion plans into infrastructures as defined by their governments. Consequently, the authors show the advantage on the use of graph theory based techniques for vulnerability analysis of electrical power systems.