The paper focuses on the identification and numerical ranking of critical elements in a power transmission network. This problem is closely linked to the identification of critical infrastructure in a power grid. A quantitative ranking of the power system elements is necessary for the prioritization of actions enhancing system resilience, as the most critical elements require higher attention ensuring protection, functionality, redundancy and reliability. The paper presents a methodological approach based on alternating current multi-period optimal power flow problem formulation and application on a reference network replicating a country scale system. The multi-period optimal power flow method was tested and applied to the reference system and a number of critical elements were identified. The paper further discusses possible protection and mitigation measures.