Small-signal modeling and analysis of type-III wind turbines considering coupling through DC bus and over frequency

Abstract

This work presents a new modeling methodology based on separate ac and dc perturbations to obtain refined small-signal models of the wind turbine. It is shown that certain resonances close to the fundamental can only be explained through the use of the refined models because the dc dynamics and additional coupling cause a dip in the impedance. This dip has both capacitive phase and negative damping which causes an unstable resonance with the inductive grid. This method is then extended to type-III wind turbines to obtain its refined small-signal models and study the turbine-grid system. These refined models can predict the newly observed resonance close to the fundamental. It was identified that the resonance might occur when the input admittance and the coupling response have similar magnitudes, and this only happens close to the fundamental. The refined impedance models are then used to propose an active damping mitigation method based on impedance shaping, this damper reduces the overall resistance on the rotor side of the induction machine in a narrow frequency band to avoid negative damping in the frequencies where the grid and turbine impedances intersect. The proposed damper is compared against similar alternatives found in the literature and found to have equal or better performance in mitigating the resonance.

Type-III wind turbines have been known to enter unstable resonance when connected to a series compensated grid, this resonance is severe and can damage the wind farm within hundreds of milliseconds. Recently, a new resonance has been observed between a type-III wind farm and an overhead transmission line without series compensation, this new resonance appears as two frequencies mirrored around, and close to, the fundamental. Impedance based analysis is a proven tool that has been used to study and solve a wide range of resonance issues, but existing models cannot predict the origin of this resonance or the appearance of the mirrored components.