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In this paper, new energy functions that incorporate the action of controllable series devices (CSDs) are presented. They make it possible for the direct method to acquire the critical clearing time (CCT) of a single machine-infinite bus (SMIB) power system and the certain series flexible AC transmission systems (FACTS) devices directly. Dealing with this matter, optimal control strategies for the controllable series compensation (CSC) and the static synchronous series compensator (SSSC) are first derived. These and later tasks make use of the advantages of mathematical models which represent the controllable parameter of a FACTS device in relation to the transmitted real power through the given line. Using the derived control laws and some assumptions, Lyapunov-like energy functions were carried out. The results of a numerical example attained by a direct method and a digital simulation are shown to be promising. The final consideration suggests where future efforts should be directed.