The present paper proposes an optimal tuning of Power System Stabilizer (PSS) parameters for synchronous generators. Many approaches have been proposed in the relevant literature which gave significant contribution for the most convenient choice for the parameters of the aforementioned control systems. It is difficult to recognize the superiority of a certain approach above the other ones, but some deficiencies can be observed especially when dealing with very large multi-machine systems. In this case the inadequacy of the methodology developed with reference to the machine connected to the infinite power network emerges. In this paper by using the modified Heffron-Phillips model, an optimal control strategy is adopted by locating the eigenvalues in a convenient circle. The linearized model allows to properly handle the variations of the network parameters, which are described as disturbances. The optimal control strategy is combined with the structural constraints of actual power system stabilizers. The effectiveness of the proposed approach is tested by verifying the damping action in the face of variations in both the amplitude and the phase of the secondary bus voltage of the step-up transformer.