Rotor-side PI controller design of DFIG wind turbines based on direct power flow modeling

In this paper, an appropriate nonlinear modeling approach of a DFIG wind system is introduced that permits the design of suitable PI controllers for direct regulation of the active and reactive power produced. Using this model and adopting grid voltage reference frame orientation, it is immediately shown that the stator power components, namely the reactive and active power, can be controlled separately through the d- and q-axis rotor voltage inputs, respectively. This provides the possibility to design simple PI controllers for each voltage input, thus overcoming the conventional cascaded controller designs used so far. Therefore, the proposed methodology does not require either current inner-loops or estimation of any flux components. Furthermore, the proposed modeling and control scheme provides a closed-loop system in a form suitable for applying advanced, Lyapunov-based, stability analysis methods. As shown in the paper, by using these methods, stability and convergence to the equilibrium can be guaranteed. Eventually, to verify the analysis and evaluate the performance of the closed-loop DFIG wind system, extensive simulations under wind speed changes and reactive power reference variations are conducted.