Every component of an electric power system is susceptible to failure. The power transmission system connects generating units to local distribution systems, and its central operational role means that the scheduling of preventive maintenance of transmission lines must be carefully planned. This planning aims to ensure uninterrupted power supply by reducing equipment failures and accidents, increasing the quality of the energy supplied, and generally maintaining network reliability. The transmission maintenance scheduling problem is concerned with selecting the optimal periods to remove specified transmission lines from operation to carry out preventive maintenance. We propose a mixed-integer linear optimization formulation of this problem for a planning period of one year. This formulation schedules preventive maintenance while ensuring that the transmission system stays connected even in case of an unexpected line failure. The resulting large-scale optimization problem is solved using a new decomposition algorithm that divides the large problem into two smaller optimization problems. One of these problems is solved with CPLEX through Benders decomposition, and the second validates the solution found. We report computational results with the IEEE 24-bus system that demonstrate that our algorithm achieves the required accuracy and solves the problem more efficiently than solving the complete formulation without decomposition.