Supervisor:

Xiongfei Wang

Co-Supervisor:
Heng Wu

Dr. Jos, van der Burgt (DNV, Netherlands).

Assessment Committee:
Yajuan Guan (Chair)
Fei Gao, School of Energy and Computer Science, University of Technology of Belfort-Montbeliard, France
Oriol Gomis-Bellmunt, Electrical Engineering Department, Technical University of Catalonia, Spain

Moderator:

Fangzhou Zhao

Abstract:

Operating the Type-IV wind turbine (WT) with grid-forming (GFM) control, henceforth referred to as GFM-WT, emerges as an attractive solution for integrating large-scale wind energies into power systems. However, the application of GFM control may introduce different electromagnetic and electromechanical dynamics from those with traditional grid-following control, thereby imposing new challenges on the small-signal stability of wind turbine systems. To tackle this challenge, this PhD project develops comprehensive small-signal models for GFM-WTs various control structures, based on which, stability analysis is carried out to provide insights into the parametric impact on the electromagnetic and electromechanical dynamics of GFM-WT. Finally, recommendations for controller tunning are given to stabilize GFM-WTs.

 

The dc-link voltage control (DVC) of GFM-WTs can be implemented either in the Grid-Side Converter (GSC), referred to as GFM-GWT, or in the Machine-Side Converter (MSC), referred to as GFM-MWT. This PhD thesis starts with the investigation of the electromagnetic dynamics of GFM-MWT excluding the mechanical dynamics of the WT, which is referred to as GFM-MPMSG hereafter. By comparing the dc output impedance of the MSC and the dc-link capacitor, it is found that the dynamics of MSC have a negligible impact on the ac dynamics of GFM-MPMSG, and thus, can be ignored when analyzing the stability of GFM-MPMSG when connected to the ac grid.

 

Following the electromagnetic dynamics analysis, the PhD thesis delves into the analysis of electromechanical dynamics of GFM-MWT by incorporating the small-signal models of the mechanical parts of the WT. The reduced-order small-signal model of GFM-MWT is developed and analyzed to characterize the impact of GFM control on torsional dynamics during grid-phase change events. Based on the reduced-order model, the complex torque coefficients method is further adopted to analytically derive the simplified expressions of natural frequencies, damped frequencies, and damping ratios of torsional modes. It is revealed that a significant negative impact is introduced by the GFM-MWT on the torsional dynamics mode, thereby posing a higher risk of torsional vibrations.

 

The above analysis is further extended to GFM-GWT and GFM-MWT with varying DVC structures. It is demonstrated that the GFM-GWT configuration contributes to positive damping, whereas the GFM-MWT configuration exhibits a negative damping effect, depending on the specific DVC architecture implemented within the MSC. Additionally, a sensitivity analysis employing the partial derivative algorithm, based on the feedforward neural network, is performed to reveal the parametric impacts of electrical and mechanical system constants and controller gains, thereby offering insights for controller tuning to dampen the torsional vibrations.