This paper proposes a hybrid algorithm for the ac/dc transmission expansion planning (TEP). The stochastic simulation method would consider random outages of generating units and ac/dc transmission lines as well as load forecast errors. The mixed-integer linear programming problem is decomposed into a master planning problem with integer investment decision variables and subproblems which examine the feasibility of master planning solution and calculate the optimal operation schedule over the planning horizon. The independent system operator would utilize the proposed method to select the optimal set of ac/dc transmission lines for satisfying TEP criteria: supplying load forecasts, minimizing investment costs, and optimizing market operations. The proposed set of dc transmission system may use either current source converters or voltage source converters. Numerical examples illustrate the effectiveness of the proposed TEP model. Index Terms-DC transmission lines, optimal system operation, reliability, transmission expansion planning.

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This paper presents the modeling of high-voltage direct current (dc) transmission systems with voltage-source converters (VSCs) in security-constrained unit commitment (SCUC). The impact of VSC-DC transmission system on the economics and the security of integrated ac/dc transmission systems is discussed. The nonlinear ac/dc equations are linearized and the Newton-Raphson method is utilized to solve the linearized network in the base case and contingencies. The SCUC solution will determine the optimal hourly schedule of controllable VSC-DC transmission systems in electricity markets. Numerical examples show the efficiency of the proposed model. Index Terms-High-voltage direct current transmission lines, security-constrained unit commitment, voltage-source converters.

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