Interconnection of bulk power systems needs a reliable Security & Stability Control System (SSCS). Preventive maintenance may help to reduce the unavailability of the SSCS. In this paper, the fault tree of the SSCS is derived from lower level components to obtain the failure probability of the SSCS. This includes hidden failure and preventive maintenance. The state space model of the SSCS is derived. With the object of the reliability coefficient and the economic cost reliability coefficient, the equality constraints of the state probabilities, and the inequality constraints of the feasible range of the maintenance period and the acceptable annual economic loss, an optimization model is proposed to find the optimal maintenance period. Based on Lagrange multipliers, the sensitivities of the objective function to the constraints of the maintenance period are derived. Compared with the continually modifying parameters, the proposed optimization and sensitivity models are more efficient, and can handle multiple parameters. The numerical results show the feasibility and the application value of the proposed models. This work is supported by the Science and Technology Project of State Grid Corporation of China “Research on Intrinsic Safety Improvement Technology of Security and Stability Control System for AC/DC Power Grid” (No. 5100-201940006A-0-0-00).