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This paper investigates control methods for the unified power flow controller in order to improve the stability of a power system hence providing security under increased power flow conditions. These include a direct PI controller with decoupling (PI-D), MIMO internal model control, multivariable GPC, multivariable GPC with decoupling algorithm (GPC-D) and adaptive fuzzy logic controls. The performances of the controllers are evaluated under different operating conditions of the power system. The results demonstrate that IMC, GPC, multivariable GPC-D and FLC are very effective in improving the transient power system stability and very robust against variable transmission line parameters.