Jinming Xu

The interface converters based on virtual synchronous generator (VSG) help enhance grid stability but face severe overcurrent risks under grid voltage sags. Relying solely on finite control set model predictive control (FCS‐MPC) cost‐function hard constraints for rapid current limiting inevitably truncates optimal voltage vectors, causing severe output voltage distortion during the fault‐ride‐through. To address the inherent conflict between instantaneous current protection and voltage quality during grid faults, this paper proposes a synergistic adaptive virtual resistance current‐limiting strategy for MPC‐VSG interface converters. During a voltage sag, the inner‐loop MPC cost function acts as a safeguard against the initial inrush current. Concurrently, an autonomous outer‐loop mechanism increases the virtual resistance stepwise. As the equivalent resistance rapidly accumulates, the proposed strategy naturally suppresses the prospective current, thereby smoothly deactivating the inner MPC hard constraints. Consequently, the voltage distortion associated with rigid MPC truncation is substantially reduced. Experimental results demonstrate that this strategy rigorously clamps fault currents throughout the entire sag duration, significantly accelerates dynamic recovery and yields improved output voltage quality during the fault‐ride‐through compared to conventional methods.

The strict absolute stability analysis of the overall grid-following inverter (GFLI) system can be achieved by adopting the orthogonal-normalization-based grid synchronizer (ONGS), but the system robustness is limited by the weak grid, injected power and port voltage level. To improve the absolute stability, this paper proposes a novel grid impedance detection and current adaptive compensation-based ONGS (IDCAC-ONGS), which introduces the grid current feedback signal into the synchronizer input. Based on the absolute stability criteria of the GFLI system composed of synchronous nonlinear criterion and traditional impedance criterion, the reason why the proposed current-compensation-based ONGS (CC-ONGS) has better robustness than the conventional ONGS is explained. Compared with CC-ONGS, a harmonic-injection-based grid impedance detection unit is embedded in proposed IDCAC-ONGS, which can select proper compensation coefficient to further improve the absolute stability. Finally, theoretical analysis and experimental results verify the effectiveness of proposed control strategies.

For grid-following inverters, grid voltage sag may induce the loss of synchronization under weak grids. Numerous efforts have been devoted to the transient instability dominated by phase-locked loops, while treating the dc input of inverter as an ideal constant voltage source. However, the dc input is usually connected to the photovoltaic array. The active current reference generated by the dc-link voltage control is not constant, and it may reach its software upper limit under grid fault. The impact of dc-link voltage control and its output limit on the transient stability has been ignored in most existing studies. Therefore, a nonlinear model is established considering the coupling of dc-link voltage control with its output limit and phase-locked loop. It is found that there is still a risk of transient instability after triggering the limit, while using the traditional reduced-order model may give incorrect stability analysis results. Then, the influence of different parameters with current limit is discussed, followed by the stability-oriented controller parameter design. A damping control method is further proposed for the transient stability enhancement. Finally, the correctness of the above analysis is verified through experiments.

Energy storage system based on grid-forming converter (GFMC) is regarded as the key equipment in photovoltaic (PV) system for energy consumption and inertia improvement. However, the design of GFMC aiming at stability improvement of PV & energy storage system (PVESS) is still open to public. Hence, this study takes the PVESS composed of photovoltaic grid-following converter (GFLC) and energy-storage GFMC as the research object, explores the impact of GFMC on the PVESS stability under different operating conditions, and proposes a stability-oriented parameter design method for GFMC. Firstly, the impact of operating conditions on the output impedance of GFLC and GFMC in PVESS is analyzed. Secondly, by treating the GFMC and the grid as an equivalent grid (i.e., an equivalent subsystem) and taking into account the operating status of PVESS, the system stability under different operating conditions is discussed in detail. Results show that the PVESS stability is the worst when the PV output is maximum, and, because the output impedance of GFMC in low-to-mid frequency range is dominated by its filter inductor, the strength of the equivalent grid is seriously affected by the GFMC filter inductor. Hence, as the key factor affecting the PVESS stability, the GFMC filter inductor deserves a careful selection. Subsequently, a comprehensive design for GFMC filter and control parameters is proposed, which can effectively improve the PVESS stability under the weak grid. Hardware-in-the-loop (HIL) experiments are conducted to verify the effectiveness of the proposed method.

For grid-following inverters, grid voltage sag can induce the loss of synchronization under weak grids. Numerous studies have discussed the synchronization instability dominated by phase-locked loops (PLLs). However, since the inverter is a high-order nonlinear system with the coupling of multiple control loops, its transient process is hard to be accurately described by merely considering PLL dynamics. Hence, this study establishes a nonlinear model, which is suitable for not only the small-signal stability analysis at equilibrium points, but also transient stability evaluation under various large-disturbance scenarios. Then, the synchronization stability considering the multiple-loop coupling is assessed with the aid of phase portrait. Unlike existing studies by ignoring inner control loops or applying model reductions, it is proved that inner control loop parameters are expected to be neither too large nor too small. Parameters of multiple control loops which jointly affect the synchronization stability deserve a careful design. Consequently, approaches for acquiring stability boundaries are depicted and a graphical stability-oriented design of multiple-loop parameters is proposed, which can provide an accurate guidance for engineers and researchers to achieve the small-signal and transient synchronization stabilities under large grid impedance and grid voltage sag. Finally, experiments verify the correctness of the analysis.

In the grid-connected inverter, both the phase-locked loop (PLL) and dc-voltage loop (DVL) can lead to the frequency coupling in the weak grid. Instabilities caused by PLL frequency coupling have been widely discussed. For DVL, past studies have analyzed the effect of DVL parameters on the degree of frequency coupling, but it is quite unclear how DVL parameters affect the inverter stability and how DVL can be designed under the weak grid. Hence, by fully considering the abovementioned frequency coupling, an accurate admittance model of photovoltaic grid-connected inverter is established in this article. By deriving the modified admittance matrix, the effects of grid impedance and DVL parameters on the frequency coupling are clearly studied. In addition, by modeling the equivalent inverter output impedance, the impact of DVL on the inverter stability under the weak grid is revealed. As proved, the increase of DVL bandwidth ${f}_{vbw}$ and grid impedance will aggravate the frequency coupling, but the current distortion does not necessarily increase. Meanwhile, the increase of ${f}_{vbw}$ helps to improve the inverter stability under the weak grid. Accordingly, a novel design for DVL in the weak grid is proposed. Experiments verify the correctness of the analysis and design.

The stability problem of grid-following inverters has been widely investigated. Nevertheless, existing literature has not yet proposed a strict stability analysis method for the overall grid-following inverter system with nonlinear nature. By focusing on single-phase systems and using frequency-domain nonlinear theory, this article proposes a universal absolute stability criterion for the single-phase grid-following inverters (SGFIs) equipped with orthogonal-normalization-based grid synchronizer. It will be shown that a SGFI will be absolutely stable under small- and large-disturbances if both the traditional impedance criterion and the synchronous nonlinear criterion are satisfied. The usefulness and applicability of the proposed new criterion are demonstrated by a case study of the conventional LCL -type SGFI system. Finally, the theoretical analysis is fully supported by a series of steady-state and transient experimental tests.

Analyzing the influence of controller parameters and physical system uncertainty is the premise of system stability design. This letter presents an intuitive analysis method for single-parameter variations in linear systems. Different from the root locus analysis, in the proposed method, the effect of single-parameter full-range variations on the system stability can be clearly visualized by simply plotting one Nyquist curve of reconstructed function at an arbitrary parameter point. Finally, the validity of the proposed method is demonstrated by a case study for the single-phase grid-following inverter.

This article is concerned with the strict stability problem for grid-following inverters subject to synchronous nonlinearity and large transient disturbances. First, considering that the complex nonlinear structure of the conventional phase-locked-loop-based grid synchronization scheme may be difficult to match the existing intuitive nonlinear analysis methods, a simplified open-loop grid synchronizer is developed for the grid-following control. Subsequently, the sector-bounded condition of the system nonlinear factor is derived by setting the grid voltage and power control variation intervals, and the frequency-domain conditions for absolutely stable are further obtained with the help of the circle criterion. The derived conditions illustrate that the stability assessment is intuitive because it is only necessary to observe the graphical position of the disk $d(-\psi,-\phi)$ with respect to the Nyquist curve of $-\Omega _{\alpha }(s)$. Finally, the effects of a weak grid and power control variations on system stability are further analyzed, and the effectiveness of the proposed theories is fully supported by transient experimental results.

For single-phase grid-connected inverters, the second-order generalized integrator-based phase-locked loop (SOGI-PLL) is widely used for good harmonics attenuation and frequency adaptability. However, existing studies usually neglect the frequency feedback loop in the modeling and design process, and thus, the potential instability caused by the frequency feedback loop is not well explained. Besides, the improper design of SOGI-PLL can lead to the inverter instability both under ideal and weak grid conditions. Therefore, by fully considering the coupling of the frequency feedback loop, this article establishes an accurate model for SOGI-PLL. Furthermore, effects of different PLL parameters on behaviors of both the SOGI-PLL itself and the inverter are studied, based on which a graphical method for PLL parameters design is proposed. As proved, the frequency feedback loop can deteriorate the stability and performance of both SOGI-PLL itself and inverter system, especially if rapid transient is expected. The modeling method is also extended to the low-pass filter-SOGI-PLL and the improved parameters design is discussed. Experimental results verify the correctness of the proposed PLL model and the abovementioned analysis results. The proposed model is helpful to deeply reveal the instability mechanism and provides a reference for the PLL design under nonideal grid conditions.