This paper presents an overview of the synchronization stability of converter-based resources under a wide range of grid conditions. The general grid-synchronization principles for grid-following and grid-forming modes are reviewed first. Then, the small-signal and transient stability of these two operating modes are discussed, and the design-oriented analyses are performed to illustrate the control impact. Lastly, perspectives on the prospects and challenges are shared.

This paper addresses robust design of the active-power and dc-link control loops of power-synchronization control. Robustness is obtained by analytic gain selections which give large enough stability margins. The proposed design allows robust stability irrespective of the grid strength and of the operating point, the latter with one exception. The proposed design is compared to design based on the principle virtual synchronous machine. Experiments show that the time-domain results correlate well with the frequency-domain results.

The interconnection stability of a grid-connected voltage-source converter can be assessed via the dissipative properties of its input admittance. In this paper, the modeling of the current control loop is revisited with the aim to improve the accuracy of the input-admittance model above the Nyquist frequency of the converter control system. Particular care is taken in the modeling of the pulsewidth-modulation and sampling processes. It is shown that, in order to eliminate the risk for destabilization of poorly damped grid resonances, dissipativeness of the input admittance is desirable not only for frequencies below, but also above, the Nyquist frequency. Validation is made by simulations and experiments.

Power-synchronization control (PSC) and vector current control (VCC), the latter including cascaded outer loops, are, respectively, considered to be grid-forming and grid-following schemes. They have significant structural differences. Despite that, it is here shown that the two schemes can be unified by making a series of minor modifications to PSC. This results in a universal controller, allowing various combinations of the two schemes to be explored. Fundamentally, it allows PSC to be used as guideline for a robust VCC design, permitting stable and well-performing operation irrespective of the grid strength.

With the increasing deployment of offshore wind power plants (WPPs), the grid-forming (GFM) battery energy storage system (BESS) has recently emerged as an attractive solution to improve the dynamic performances of WPPs. However, the control interactions of the GFM-BESS and offshore WPP, under different grid strengths, tend to complicate the controller-parameter tuning. This paper presents a modeling method for analyzing control interactions of offshore WPP and GFM-BESS, which sheds clear insights into the critical controller parameters to the system dynamics. Differing from conventional methods, a frequency-domain model of GFM-BESS, obtained by taking the Laplace transform of the corresponding state-space model, is developed first. This allows the impedance model of offshore WPP, including a black-box model of long transmission cable, to be flexibly integrated. Based on the model, both closed-loop transfer-function and pole-based dynamic analyses are then performed. Electromagnetic transient simulations corroborate the effectiveness of the model and analysis.

This paper revisits the design of the current controller for grid-connected voltage-source converters (VSCs), considering the dynamic impacts of the phase-locked loop (PLL), weak grids, and of voltage feedforward (VFF) control. First, a single-input single-output transfer-function-based model is proposed to characterize the interactions of control loops. It is analytically found that the proportional gain of the current controller essentially aggravates the instability effect of PLL in weak grids, while the cutoff frequency of the low-pass filter used with the VFF loop has a non-monotonic relationship with the PLL-induced instability. Then, based on these findings, a guideline for re-designing the current controller of PLL-synchronized VSCs is developed, which enables a co-design of the current controller and VFF controller. Lastly, simulation and experimental results confirm the validity of theoretical analyses.