An alternative modeling and controller design guaranteeing power stability for DFIG wind systems

Doubly-Fed Induction Generators (DFIG) are widely used in wind power systems due to their inherent capability of controlling the produced active and reactive power at desired levels, in a large range of wind speeds. In this paper, a nonconventional modeling approach of a DFIG wind system is introduced that permits to control directly the active and reactive power produced. To this end, first, the complete nonlinear dynamic model that contains as states the stator active and reactive power is extracted in the synchronously rotating dq reference frame. Assuming operation under grid voltage reference frame orientation, it is easily shown that the stator power components can be controlled separately through the d- and q-axis rotor voltage inputs. Hence, unlike the complex conventional cascaded controller designs for DFIGs, in this paper, a simple design of proportional controllers for the stator power components is adopted. For this scheme an advanced, Lyapunov-based, stability analysis is conducted that guarantees stable operation and convergence to the equilibrium. This closed-loop scheme is further completed by an outer PI controller design that tracks the rotor speed to the optimum, providing the active power reference for the maximum power point operation. Finally, the analysis and the performance of the closed-loop DFIG wind system are verified through simulation results.