Refined Small-Signal Sequence Impedance Models of Type-III Wind Turbines

Refined sequence impedance models are presented for type-III wind turbines that use doubly-fed induction generators (DFIG). Previous small-signal sequence impedance models of DFIG did not consider the dynamics of the dc bus voltage connecting the stator- and rotor-side converters, and ignored small-signal responses at non-perturbation frequency. This work models the turbine in a new way by first modeling each converter independently and fully characterizing their ac/dc dynamics, and then combining these transfer functions to obtain the small-signal models of the whole turbine. These improved small-signal models consider dc bus dynamics as well as the current response at non-perturbation frequency that occurs due to the nonlinear nature of the converter and control. Turbine-grid system stability analysis that accounts for the coupled current responses is also presented. The transfer functions and small-signal models are validated against detailed simulation. The developed models are also compared against previous models to demonstrate the improved accuracy.