A Direct Stator Current Controller for a Doubly-Fed Induction Machine

In this paper, a new control scheme is presented for the doubly-fed induction machine with specific applications to renewable energy (wind farms in particular). The proposed control algorithm offers the advantages of proven stability and remarkable simplicity. In contrast with the classical vector control method, where the doubly-fed induction machine is represented in a statorflux oriented frame, a model with orientation of the stator voltage is adopted. This allows for a decomposition of the active and reactive powers on the stator side and their regulation on the rotor side. A main contribution of the paper is the proof that a linear PI control with a skew-symmetric matrix regulating the stator currents ensures global stability for a feedback linearized doubly-fed induction machine, the specific condition that the PI gains must satisfy is derived as a simple inequality. The proof is based on a Hurwitz test for polynomials with complex coefficients that does not seem to have had prior application in control theory. The feedback linearization stage only uses the direct measurement of the rotor and stator currents and is thus easily implementable. Furthermore, it is also shown that a simpler PI control is also stable for a large range of control gains. Finally, the control system is tested in simulations.