One of the major preoccupations in the design of control systems in power systems is the evaluation of its stability robustness. Classical and modern control techniques use linear models for the controller synthesis. This introduces many uncertainties into the design and the robustness of the control and, for this reason, many control systems under certain conditions can not be operated properly. This paper presents the application of the structured singular value (/spl mu/) analysis in order to model power system uncertainties and to evaluate the stability robustness. Moreover, this paper presents the calculation of a boundary function of stability depending on the control system. Robustness evaluation for different types of SVC controllers located in the interconnection of two power systems by a weak tie-line has been developed.