Comments on "Control and performance of a doubly-fed induction machine intended for a flywheel energy storage system"

The paper [1] proposes a very simple and attractive algorithm for controlling both active and reactive powers of a DFIM with a unique feedback control loop. In this technical note, it is proved that the presented algorithm in [1] yields a marginally stable closed-loop system (for the rotor currents). This fact could imply undesired oscillations in the rotor variables, which, eventually, can be large for certain disturbances. The control law in [1] (which has been cited in more than 100 journal and conference papers) has probably been tested and implemented several times in a real application. However, as the rotor current oscillations are with the same frequency than the power grid (or twice in the three-phase framework), they may have been attributed to the presence of grid-coupling noise or small unbalanced effects instead of the inherent marginal stability of the closed-loop dynamics. The problem detected could be overcome using two nested loops, as many references propose (see a general example in [3] or a recent application in [4]), or adopting amore advanced control design technique [5].