With the increase in wind power penetration, the stable operation of wind turbines under the new power system is facing severe challenges. The grid-forming wind power technology operates in a self-synchronous mode, which can provide voltage and frequency support for the system without being affected by the phase-locked loop, and is also suitable for operation under weak power grids. However, the current research for the grid-forming (GFM) permanent magnet synchronous generator (PMSG) ignores the DC-link dynamics generated by the wind turbine, which makes the sub-synchronous oscillation (SSO) phenomenon under different grid conditions and lacks a physical explanation. In this paper, the SSO problem in the grid-forming PMSG is studied, and the study reveals that the reduction in the DC-link voltage control bandwidth of the machine-side converter (MSC) is the main cause. To this end, an improved damping method is proposed, which introduces a low-pass filter branch in the reactive power control loop and takes the DC-link voltage tracking error as a compensation term. The small-signal analysis and simulation results show that the proposed method has significant effectiveness.

Grid-forming energy storage converters (GFM-ESC) are essential for stabilizing power electronic-based grids owing to their voltage and frequency support capabilities. However, current research primarily focuses on the discharging mode of GFM-ESC, while grid-supporting characteristics in the charging mode remain underexplored. This article first identifies a phenomenon that the GFM-ESC is stable in the discharging mode but exhibits low-frequency resonances in the charging mode. To investigate the instability mechanism of this oscillation, a reduced-order small-signal impedance model for GFM-ESC is developed, revealing that the current matrix, resulting from the dynamics of the reactive power loop, is the primary factor causing instability. Therefore, this article proposes an asymmetric matrix control strategy based on q-axis voltage feedforward, which optimizes the reactive power loop to reshape the current matrix, thereby counteracting its negative impact on stability and ensuring stable operation across different modes. Finally, simulation and experimental results validate the correctness and effectiveness of the proposed control strategy.

The phenomenon of frequency coupling is widely observed in grid-forming (GFM) converters due to the presence of asymmetrical controls and nonlinear blocks. However, the factors influencing frequency coupling have not been thoroughly explored. This paper introduces a systematic small-signal impedance model of GFM converters that intuitively reflects the factors affecting frequency coupling and provides a detailed analysis of how coupling impedance affects stability. It is demonstrated that the influencing factors of coupling impedance are an active power loop and reactive power loop. Specifically, the active power loop influences coupling impedance characteristics near the fundamental frequency, while the reactive power loop impacts the entire frequency range. This paper first reveals that the reactive power loop has a more pronounced effect on frequency coupling than the active power loop. Additionally, the variation in the steady-state operating points also affects the degree of frequency coupling of the GFM converters, primarily affecting the coupling impedance characteristics beyond the fundamental frequency, and the low operating points tend to affect system stability adversely. Finally, simulation results validate the accuracy of the mathematical model and theoretical analysis.

With the large-scale integration of renewable energy sources and power electronic converters, grid-forming energy storage converter (GFM-ESC) with voltage and frequency support capabilities is emerging as an important component in power-electronic-dominated systems. Although GFM-ESCs are generally stable under weak grids, this paper reveals that under ultra-weak grids, limitations in power transfer capability, specifically the static power transfer limit (SPTL) and dynamic power transfer limit (DPTL), can still cause stability issues and under-utilization of converter capacity. The SPTL is determined by grid impedance, while the DPTL further considers the control dynamics. It is first demonstrated that the SPTL exhibits asymmetry between discharging and charging modes, primarily attributed to grid resistance, which limits the power transfer in charging mode. A third-order nonlinear dynamic model is then developed, with phase trajectory analysis used to evaluate the impact of various control parameter configurations on the DPTL. To address the instability induced by DPTL under ultra-weak grids, an adaptive voltage compensation strategy based on reactive power deviation is proposed, which improves both power transfer capability and capacity utilization. Finally, experimental results validate the theoretical analysis and the effectiveness of the proposed strategy.