Rotor-side cascaded PI controller design and gain tuning for DFIG wind turbines

A novel and efficient solution to the problem of designing and tuning the gains of cascaded PI controllers used in DFIG wind systems is addressed in the present paper. The overall design takes into account the nonlinear dynamic model of the system while the analysis of the inner-loop controllers leads to some new concepts, namely that of the effective time constant, which in turn leads to a gain tuning rather independent from the system parameters. The controller structure is completed by exploiting the time-scale separation assumption in the design of the outer-loop controller. This approach provides the possibility to obtain a simple systematic method for tuning the outer-loop controller gains, based on the well-analyzed in the literature second order transfer function. Hence a more robust control scheme with some essentially enhanced stability and transient properties results, as it is clearly verified by extensive simulation tests.