[Objective] With the increasing penetration of power electronic devices in the new power systems， the system is gradually evolving from a “physical synchronization” paradigm dominated by synchronous generators to a “control-based synchronization” paradigm dominated by grid-connected inverters. Due to the strong nonlinearity， limited overcurrent capability， and multi-inverter coupling characteristics of grid-connected inverters， synchronization stability issues exhibit multi-level features， extending from local dynamics to system-wide interactions.[Methods] To address this， this paper presents a comprehensive review from two perspectives， single-machine infinite-bus systems and hybrid systems. For the single-machine infinite-bus systems， virtual power angle dynamic models describing the synchronization behavior of grid-following （GFL） inverters， virtual synchronous generator-based， and virtual oscillator-based grid-forming （GFM） inverters with the grid are introduced. Based on the virtual power angle curves， the key factors affecting the synchronization stability of different inverter types are analyzed， and representative stabilization control strategies are summarized. For hybrid systems， considering the interactive coupling among control loops of GFL and GFM inverters， equivalent power angle dynamic models under both islanded and grid-connected modes are established， and the mechanisms of synchronization instability caused by multi-machine interactions， together with corresponding stabilization control strategies， are systematically reviewed.[Conclusions] Based on an analysis of existing research progress， future research directions on the synchronization stability of grid-connected inverters are discussed from the perspectives of multi-time-scale modeling， multi-scenario stability assessment， and coordinated stabilization control.

[Objective] When uncoordinated control strategies are employed during the parallel-connected operation of multiple energy storage converters in new-type power systems, they cause unbalanced state of charge (SOC) distribution among energy storage systems, as well as frequency overshoot and low-frequency oscillations. To address these problems, this paper proposes an improved virtual synchronous generator (VSG) control strategy that combines SOC-based power balancing and adaptive fundamental damping design. [Methods] First, a grid-forming parallel-connected energy storage system model is established using voltage-source-type VSG control as the grid-forming outer loop. Second, a set of active power balancing coefficients is derived based on the real-time SOC of energy storage systems. Third, a control strategy combining fundamental damping with adaptive damping is designed on the basis of these balancing coefficients. Finally, the performance and effectiveness of the proposed control strategy are verified and analyzed using the MATLAB/Simulink digital simulation environment. [Results] Using an aluminum battery pack as the energy storage system, after introducing the proposed improved VSG control strategy, a power distribution difference of 5% to 7% is observed between the high-SOC and low-SOC systems. Under typical disturbance conditions, the frequency drop amplitude is reduced by approximately 20% and the overshoot amplitude is reduced by approximately 30%; under complex operating conditions, the frequency deviation is reduced by 10% to 15%. [Conclusions] The proposed control strategy maintains SOC balance among parallel-connected units by setting balancing coefficients, thereby reasonably optimizing power distribution. Furthermore, by incorporating an adaptive damping strategy based on the obtained balancing coefficients to correct frequency deviations, the proposed approach effectively enhances the dynamic response performance, frequency stability and multi-machine coordinated operation capability of energy storage systems. Therefore, the proposed control strategy holds certain theoretical significance and engineering application value for the construction of new-type power systems with high penetration of renewable energy.

[Objective] Addressing the challenge that grid-forming energy storage converters，operating in a single mode，struggle to adapt to variations in grid short-circuit ratio and complex fault disturbances，this paper proposes a dual-mode switching strategy based on amplitude and phase synchronization（APS），and uses the improved particle swarm optimization（IPSO）to identify its key parameters. [Methods] First，the limitations of conventional grid-forming/grid-following switching strategies are analyzed. The mechanism by which the cumulative voltage phase error in the power loop induces reactive power/voltage deviations，thereby amplifying transient impacts during mode switching，is revealed. Based on this，an APS compensation mechanism is proposed to simultaneously correct the voltage amplitude and phase signal during the mode switching process，ensuring smooth changes in the inner-loop current reference signal. Second，to overcome the difficulty in tuning the parameters of the conventional strategy's tracking loops，an IPSO algorithm based on nonlinear inertia weights and learning factors is used to adaptively identify the parameters of the four sets of tracking loops. This enhances the tracking performance and disturbance suppression effect of the energy storage converter on the operation points of the grid-following and grid-forming modes. [Conclusions] Validation was conducted via an electromagnetic transient model of a MW-level grid-forming energy storage system built in MATLAB/Simulink. The results showed that the proposed control strategy could successfully achieve a transient power impact of less than 0.02 p.u.，and could operate stably in the scenarios of continuous switching and operation point fluctuation. [Conclusions] Compared with the conventional switching strategy，the APS-IPSO-based strategy enables energy storage converters to achieve low-impact switching and high stability during grid-following to grid-forming transitions，providing a theoretical basis for the subsequent deployment of energy storage or new energy units with mode switching in new energy stations.

[Objective] To enhance frequency stability in systems with a high penetration of converters，this paper investigates a dynamic evaluation and quantification method for determining the frequency support requirements of grid-forming（GFM）converters. [Methods] First，a multi-machine system frequency response model is developed by incorporating the power-frequency characteristics of coupled GFM converters. Second，considering both the rate of change of frequency（RoCoF）and the maximum transient frequency deviation under anticipated disturbances，a comprehensive assessment method is constructed to evaluate the frequency support requirements of GFM converters. Finally，based on the Brent iterative algorithm，the critical thresholds of virtual inertia and primary frequency regulation gain for GFM converters are dynamically assessed under various operating conditions. Based on the modified IEEE-39 bus system，multiple scenario-based case studies were conducted. [Conclusions] The results show that the introduction of GFM converters can raise the minimum system frequency to 49.5 Hz，significantly enhancing system stability and highlighting the necessity of deploying GFM converters in highly power-electronic systems. Furthermore，a performance comparison of three algorithms demonstrates that the proposed iterative method requires only 11 iterations on average，reducing the iteration count by over 20% compared to traditional approaches，thereby validating the effectiveness of the proposed assessment method and iterative algorithm. [Conclusions] The proposed assessment method clarifies，from a system-level perspective，the frequency support requirements imposed by the grid on GFM converters，providing a theoretical foundation for their optimal allocation and parameter tuning.

[Objective] To address the issues of power angle instability and output current overload in grid-forming（GFM）inverter during symmetrical grid faults，this paper proposes a fault ride-through strategy based on power command constraints. [Methods] First，a transient model of a droop-controlled GFM inverter is established to analyze the transient characteristics of the system under grid voltage sag conditions，revealing the impact of power commands on transient stability. Second，based on the circuit relationship between the inverter and the grid，the characteristics of fault currents and their primary influencing factors are identified. Finally，a fault ride-through method based on active power command constraints is proposed，which only requires calculating and setting the active power command value to restore power angle stability and limit fault currents. [Conclusions] Simulations performed in MATLAB/Simulink demonstrate that the proposed strategy effectively enhances power angle stability，achieving fault ride-through. [Conclusions] The proposed fault ride-through strategy based on power command constraints effectively addresses power angle instability and overcurrent issues in GFM inverters during voltage sag by constraining active power commands，providing a feasible solution for enhancing the fault ride-through capability of renewable energy grid-connected systems.

[Objective] The large-scale integration of inverter-based renewable generation introduces new challenges for power-system stability. Although virtual synchronous generator（VSG）control imparts virtual inertia and damping，grid short-circuit faults can induce excessive currents that threaten synchronization and may cause inverter disconnection. This paper proposes a VSG fault ride-through strategy based on dynamic virtual velocity regulation. [Methods] The transient behavior of the VSG under fault conditions is analyzed to identify the source of overcurrent，and dynamic virtual angular velocity regulation is introduced to maintain power-angle stability. The reactive-power control loop is reinforced to improve reactive-power compensation，and a voltage-current dual-loop structure based on the balanced-current method is employed to ensure balanced three-phase output currents. [Conclusions] Matlab/Simulink simulations show that，during grid short-circuit faults，the virtual angular velocity and power angle stay close to their rated values； short-circuit current is limited to within 1.5 times the VSG’s rated current； the VSG provides reactive power in accordance with national grid connection codes； output currents remain three-phase balanced； and fault impact currents are effectively suppressed. [Conclusions] Compared with existing methods，the proposed strategy more effectively stabilizes virtual angular velocity and power angle，limits short-circuit and fault impact currents，accelerates transient response，maintains balanced three-phase outputs，and ensures secure，stable VSG operation under grid short-circuit conditions.