This paper presents the modelling of the evolution of plasma equilibrium in the presence of external poloidal field circuits and passive structures. The optimization of plasma scenarios is formulated as an optimal control problem where the equations for the evolution of the plasma equilibrium are the constraints. The procedure determines the voltages applied to the external circuits that minimize a certain cost-function representing the distance to a desired plasma augmented by an energetic cost of the electrical system. A sequential quadratic programming method is used to solve the minimization of the cost-function and an application to the optimization of a discharge for ITER is shown.