Supervisor:
Xiongfei Wang

Co-Supervisor:
Heng Wu

Assessment Committee:
Amin Hajizadeh (Chair)

Prof. Kai Strunz, Technical University of Berlin

Prof. Garcia Fernandez, University of Oviedo, Gijon Spain

Moderator:
Fangzhou Zhao

Abstract:

Abstract: Voltage-Source Converters (VSCs) have gained widespread usage in the integration of renewables into modern power systems. This thesis addresses the transient stability of VSCs during grid faults and the transient overvoltage during fault recovery, with a particular focus on the impact of widely used grid-following (GFL) control. The research work in this thesis is of importance for ensuring the secure and reliable operation of power systems, particularly considering the growing integration of VSC-based resources.

The first focus of this thesis is to assess the transient stability of GFL-VSCs during the fault ride-through (FRT) operation, taking into account the dynamics of voltage-dependent current injection (VDCI). To gain a deeper understanding of the effects of dynamic interactions between VDCI and the phase-locked loop (PLL) on the transient stability of GFL-VSCs connected to weak AC grids, a comprehensive analysis is performed. By incorporating the influence of VDCI, a static model is employed to investigate the existence of equilibrium points (EPs) during grid faults. Subsequently, a dynamic model is developed to assess the convergence capability of GFL-VSCs towards EPs during grid faults. The study reveals the impact of the K-factor that is resulted from the VDCI characteristic on the transient stability of the system.

Second, this thesis examines the effects of various reactive current injection modes (absolute and relative) on the transient stability of GFL-VSCs during FRT operations. To that end, the dynamic interaction between the current injection profile and the PLL is captured by the developed nonlinear model. The influences of these two current injection modes on the existences of EPs and the transient dynamics is then characterized based on the model. Additionally, to ensure the transient stability of the system, this study provides recommendations for selecting the reactive current injection modes.

Third, this thesis analyzes the transient overvoltage (TOV) phenomenon of GFL-VSCs during fault recovery. The mechanism of TOV is analyzed considering the slow and fast current reference update modes. The necessity of analyzing the influence of modulation saturation (MS) mode on TOV is also highlighted. For the fast current reference update mode, the extended TOV duration is identified, which is resulted from the current reference surge introduced by mode switching from full reactive current injection to VDCI. Then, the TOV mitigation method is developed.