The progresses in research of complex network theory inspire scientists to make the analysis from a new structural perspective in terms of nodes and their connection patterns. However， a pure topological approach， as proposed in the current literature， fails in capturing the specific features of power systems. Two different aspects， namely operation state and network structure， are proposed for solving power grid security problems and their relationships are discussed. Therefore it is pointed out that power security metrics should keep the dominance of complex network approach in structure analysis and capture special features of power system engineering. Furthermore， on the basis of net-ability and entropic degree， a concept of transmission betweenness is proposed. With the drop of net-ability， the critical transmission lines is identified， and entropic degree and transmission betweenness are used to quantitatively measure the connectivity and responsibility of buses respectively. Finally， the new metrics are applied to the Italian power transmission network for analysis and comparison to verify the effectiveness of the proposed method.