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A new flexible mixed-integer formulation is presented in this paper for solving the single-stage ac-transmission expansion planning problem. This new formulation handles exactly the discrete nature of equipment additions without resorting to linearized incremental approaches, but still uses integer linear programming. The interconnection of a disconnected system is handled directly without resorting to an initial connected system. The optimal configuration at the end of the planning step under consideration minimizes an annually amortized cost function including the investment cost for equipment additions and the operating cost in terms of real-power transmission losses. Using a new efficient linear load-flow model, this new formulation is characterized by a complete decoupling between real and reactive equations. Examples have been incorporated to illustrate the potentials of this new optimum planning formulation.