The aim of the proposed paper is to extend the well known complex torque coefficient method for synchronous machine rotor oscillations, to auxiliary devices (voltage regulators, power system stabilizers (PSS), ...) in electrical networks. To illustrate the effectiveness of the proposed extension, the study is performed on a model power system. Results obtained by this method are compared with those given, in the one hand, by an eigenvalue analysis, and in the other hand by numerical simultations.