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.

The small-signal stability of interconnected converter systems can be tackled by the impedance-based analysis method, where the Bode plots of the individual impedances derived from subsystems are utilized for a design-oriented analysis. However, those methods only apply to the interconnected systems without having open-loop right half-plane (RHP) poles. This paper thus proposes a general stability analysis method based on Bode plots of individual impedances, which allows considering the open-loop RHP poles and shaping the impedances for stabilizing the system. Experimental case studies on the current control interactions of two paralleled grid-tied inverters validate the effectiveness of the method.

The utilization of a large number of power electronic converters in the high-speed railway system results in the low-frequency instability for the single-phase vehicle-grid system. In order to probe into the problem, the impedance-based approach is adopted. This paper focuses on the dq-frame impedance modeling and verification of the single-phase converters in the electric multiple units. The closed-loop input impedance modeling methods for both single converter and interleaved converters considering the second-order generalized integrator-based phase-locked loop (PLL) are put forward. Then, the dq-frame impedance measurement method for single-phase systems based on the Hilbert transform is proposed, and the modeling approach is verified by the frequency-domain simulations on the switching models. The good agreement of the model and simulation illustrates that the modeling method is suitable and accurate enough until half of the switching frequency. It is elucidated that the single-phase rectifier's impedance has the off-diagonal characteristic and the negative-impedance characteristic on the dd channel at low frequencies. Moreover, both the current and voltage controls have influences on the negative impedance shaping, while PLL does not.

This article presents a systematic evaluation on the impedance passivity of voltage-controlled voltage-source converters. The commonly used single- and dual-loop control structures with different linear controllers are compared extensively, considering the effect of the time delay involved in the control loop. A virtual impedance control, co-designed with different voltage control schemes, is then proposed to eliminate the negative output resistance till half of the sampling frequency, which improves the system stability for grid-forming converters in grid-connected applications. Both frequency-domain analysis and experimental results validate the theoretical findings.

Nowadays, the power electronics-based systems sometimes present the low-frequency instability owing to the interaction between the power converters and the grid, such as the low-frequency oscillation (LFO) happening in the single-phase vehicle-grid (electric multiple units (EMUs) - traction power grid) systems of high-speed railway. To address this problem, the impedance-based stability analysis method is adopted. Given the grid impedance and the dq-frame converter impedance in the EMUs, several popular impedance-based stability analysis methods are compared, and their features and applicability are summarized. The generalized Nyquist stability criterion is then applied to more cases to validate the proposed impedance model for the single-phase converters, and excels in good estimation of the stability boundary and oscillation frequency. The analysis results are validated by the simulation and dSPACE tests. Through the comprehensive analysis in this paper, the mechanism of LFO in high-speed railway system is further elucidated, and it is suggested that the negative impedance of the converters will contribute to the LFO.

The voltage low frequency oscillation (LFO), between traction network and electric locomotives or electric multiple units in high speed railway, is a typical phenomenon happening in cascade systems with power electronic converters accessed, which is regarded as the stability problem of a vehicle-grid (V2G) system in general. To address the problem, the mathematical model of the V2G system is deduced in detail first, and a generalized dq -decoupled small-signal model of a single-phase pulse width modulation rectifier is established in this process. Then, a novel stability criterion based on forbidden region for dq-decoupled multi-input multi-output cascade systems is proposed to analyze the stability of the V2G system. The proposed criterion has less conservatism than singular-value criteria and norm criteria, and can be applied for systems with either high or low power factor. The comparison with other criteria and simulation results verifies that the proposed forbidden region-based criterion performs well in the analysis of the critical condition of LFO. Moreover, the impact factors of LFO, both grid parameters and vehicle parameters, are researched based on the proposed stability analysis criterion and the simulation verification, which provides design guidance for the V2G system to restrain LFO.