In the following article, results of the investigation on the effect of feeding subsynchronous currents from the electric grid in the generator are presented. In the end, torsional oscillations of the shaft train are excited, which can reach very high amplitudes in case of resonance. In time with the torsional oscillations, the synchronous generated voltage induced in the stator winding oscillates, which leads to the feed of subsynchronous currents from the generator into the electrical network. The focus is directed to the effect of these currents in the system “generator-grid”. Therefore, different analytical approaches to subsynchronous power excitation are presented and evaluated. In particular, the analytical equations of generator power generated by subsynchronous electric currents are derived. This article shows that, in addition to the damping properties in dealing with torsional oscillations, the interaction between the grid frequency, the natural electrical grid frequencies and the mechanical torsional natural frequencies is also important.