This thesis contains the synthesis, analysis and simulation results of an automatic optimal voltage controller for the Southern Norwegian power grid. Currently the high voltage power grid is controlled manually by operators switching control components. The optimal controller handles the voltage control of the system, as well as keeping the number of control actions to a minimum.The system model is derived from power system analysis. Due to a highy nonlinear system model and integer decission variables in on/off control components, the controller is based on mixed integer nonlinear programming (MINLP). The MINLP uses BONMIN as a solver, and is implemented with the AMPL programming language.It was found that a MINLP controller is good choice for voltage control in transmission systems. The controller handles voltage limits, as well as reducing the number of control actions.The thesis also contains comparison between different solution methods for applying the optimal voltage controller, as well as other approaches to the automatic voltage control problem.