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This thesis is a study of long-term voltage stability in power systems following a severe disturbance. An optimization method for solving the optimal power flow problem is used on power system models. The method tries to optimize the generated active power while fulfilling the requirements from the consumers. Dynamic limitations of the generators are taken into account and an extension of the optimization routine which optimizes over several time steps is constructed. The results from the optimization are then applied to a power system simulator during a scenario where the system is bound to collapse. Whether the stability of the system is improved or not is analysed and discussed.