Supervisor:
Xiongfei Wang

Co-Supervisor:

Assessment Committee:
Yajuan Guan (Chair)
Yaow-Ming Chen, National Taiwan University, Taiwan
Professor George Weiss, Tel Aviv University, Israel

Moderator:
Fangzhou Zhao

Abstract:

Grid-forming (GFM) control has become an attractive solution for voltage-source converters (VSCs) used in power electronic-based power systems. The dc-link voltage regulation needs to be implemented with GFM-VSCs in scenarios that lack a stiff dc voltage source and necessitate the operation of both inverter and rectifier modes. However, their control dynamics and interactions may result in instability issues. To systematically address those challenges, this Ph.D. thesis focuses on the analytical modeling, dynamics analysis, and enhanced control strategies for GFM-VSCs with dc-link voltage regulation under various operating conditions.

The dynamics of dc-link voltage can be utilized to synchronize GFM-VSCs with ac networks, thereby leading to the dc-link voltage-synchronization control (DVSC). The small-signal synchronization stability of GFM-VSCs using various DVSC approaches is comparatively analyzed. When employing the single-loop DVSC approach, GFM-VSCs connected to dc-link constant-power sources and loads encounter low-frequency oscillations. To address this issue, the active power and the q-axis voltage are fed forward to the frequency deviation, which are configured in parallel with the DVSC to achieve synchronization with ac networks. The proposed control approach enhances the stability of GFM-VSCs under a wide range of grid strengths and at both inverter and rectifier modes.

The inner control loops are another critical concern of GFM-VSCs. An analytical approach is proposed to reveal the impact of inner control loops on the system damping. First, an impedance model is employed to characterize the behaviors of different inner-loop schemes. On this basis, the complex torque coefficient method is used to characterize the influence of inner-loop dynamics on the outer-loop dynamics and the associated instability issues within synchronous and sub-synchronous frequencies. Finally, the proposed approach is exemplified with virtual admittance and vector-current control as inner control loops.

In addition to the outer-loop and inner-loop control strategies, the external interactions of GFM-VSCs with ac grids and dc-link sources and loads are investigated using impedance-based analytical methods. The dc-link impedance reveals the interactions between GFM-VSCs and various loads connected through the dc link, including constant-power, constant-resistance, and constant-current loads. Furthermore, ac impedance-based analyses evaluate the impact of dc-link sources and loads on ac-side impedance characteristics and the resulting interactions with ac grids. It is revealed that the constant-power load poses the highest risk of instability in the rectifier mode, while the constant-current source presents the highest risk of instability in the inverter mode.