With the rapid development of high proportions of renewable energy and power electronic equipment, these new features have posed severe challenges to the safe and stable operation of modern power systems. To enhance the connection performance between systems with a high proportion of power electronic equipment and the grid, this paper proposes a frequency coordination control method based on margin factors. Firstly, this paper discusses the power distribution problem for the primary frequency regulation among multiple wind farms. After comprehensively considering factors such as the operating conditions of the wind farms (such as rotor speed and converter capacity), the equal proportion margin factor allocation method is used to send the power command to each wind farm. Secondly, for the setting of the virtual inertia and virtual damping coefficients of each wind farm, a method based on releasing kinetic energy to adjust the frequency regulation parameters is studied. According to the trend of the frequency response, the states of the wind farms are classified, and on this basis, the frequency regulation parameters are set by combining the available frequency regulation kinetic energy. Finally, simulations are conducted to verify the accuracy and effectiveness of the proposed method. The results of these simulations indicate that this method can reasonably utilize the frequency regulation resources of each wind farm, effectively improve the frequency stability of power systems characterized by a large number of power electronic devices, and ensure the frequency safety of modern power systems.

The high inertia energy storage synchronous condenser (HIESSC), which combines the advantages of doubly fed machines and flywheel energy storage, has recently attracted attention as a solution to provide both grid frequency and voltage. However, under weak grid conditions, the HIESSC may experience power limitations due to power flow and interactions between converter control and grid dynamics. This article provides a comprehensive analysis of the HIESSC, focusing on improving its bidirectional active power transfer capability under weak grid conditions. A steady-state model of the HIESSC is first developed, incorporating the effects of reactive power and power angle characteristics. Based on this, the static power limit (SPL) of the HIESSC under common reactive power control loops is analyzed and compared. A unified full-frequency small signal impedance model is then introduced to explore the dynamic power limit (DPL) and key destabilizing factors. Furthermore, a voltage feedforward compensation method for both the rotor-side converter and grid-side converter is proposed to mitigate destabilizing factors and extend the DPL. The theoretical analysis and proposed method are validated through hardware-in-the-loop (HIL) experimental results.

Grid-following grid-connected converters (GCC-GFLs) in renewable energy systems are prone to PLL-dominated oscillations under weak-grid conditions. The integration of a high-inertia energy storage synchronous condenser (HIESSC) can enhance voltage and frequency support, but the coupled electromechanical and converter-control dynamics may still leave the GCC-GFL exposed to weakly damped interaction modes. Based on the identified feedback-path mechanism of the GCC-HIESSC system, this short communication proposes a damping power control method to suppress the dominant oscillatory mode. The key idea is to reinforce the active-power-related negative-feedback path and offset the local positive-feedback effect. An auxiliary damping branch is introduced into the DC-voltage control loop, and the damping power is generated from the PLL-related dynamic signal without changing the steady-state operating point. A practical gain-selection guideline is further provided by considering the equivalent loop-gain margin. Comparative analysis with representative damping methods and hardware-in-the-loop experiments under different operating conditions verify that the proposed method enhances the system stability margin.

Unlike the single-machine grid-connected system of virtual synchronous generator (VSG), the multi-machine parallel grid-connected system of VSG has more complex interaction relationships among devices. Therefore, it is difficult to accurately analyze the dynamic performance of the system only by using traditional analysis methods. In order to study the frequency characteristics between VSG and VSG as well as between VSG and the power grid, this paper first derives the general small-signal model of the multi-machine parallel grid-connected system of VSG. Secondly, by combining the motion trajectory of the participation factor of the dominant state variable of the system characteristic root, the influence relationship of factors such as the active power control loop parameters on the low-frequency characteristic root of the system is analyzed in detail, and the influence law of the active power control loop parameters on the frequency between VSG and VSG and between VSG and the power grid is obtained. Finally, the correctness of the frequency stability influence law of the system obtained in this paper is verified by simulation.

The deep coupling between wind power, photovoltaics, converters, and other power electronic equipment (PEE), as well as between PEE and the power grid, leads to a new voltage-dominated cascading failure (VDCF) mode in power electronic-based power systems. Disconnection and blocking of PEE can be triggered by VDCF, posing a serious threat to power-system stability. Driving of VDCF is attributed to local reactive power imbalance. A propagation mechanism fundamentally different from active power redistribution in traditional faults is exhibited by VDCF, making weak line identification difficult. Therefore, by analyzing the voltage response characteristics of PEE, the action state domain of voltage response events has been established, revealing the evolutionary characteristics and internal mechanisms of VDCF. For variable scenarios involving fault lines, locations, and impedances, a voltage estimation knowledge graph (VEKG) for power systems based on white-box dendritic network (WBDN) has been developed to improve voltage estimation accuracy. A VDCF weak line search method based on the VEKG is proposed considering the coupled effects of voltage and duration of voltage response events. Case study shows that the proposed method can accurately estimate the voltage at PEE grid connection points and identify VDCF weak lines accurately.

The transient behavior of DC-link voltage (DCV) significantly affects the low-voltage ride-through for phase-locked loop (PLL)-based grid-connected doubly-fed induction generator (DFIG) systems. This study investigates the DCV transient behavior of a PLL-based DFIG system under asymmetrical grid faults. First, by considering the coupling characteristics of positive and negative sequence (PNS) components, a nonlinear large-signal model of DCV is developed. Furthermore, the transient characteristics of DCV under varying parameters are analyzed using phase trajectory diagrams. In addition, the transient stability (TS) mechanism of DCV during asymmetrical faults is examined through an energy function approach. The analysis indicates that the transient instability of DCV is primarily associated with the control characteristics of PNS PLLs, while the TS level of DCV is mainly determined by the power coordination control between the rotor side converter and grid side converter. Moreover, a coordinated control strategy is proposed to enhance the TS of DCV under asymmetrical grid faults. Finally, both simulation and experimental results are presented to validate the theoretical analysis and the effectiveness of the proposed strategy.

The high inertia energy storage synchronous condenser (HIESSC), which integrates the doubly-fed machine technology with flywheel energy storage, has recently emerged as a promising means to support both frequency and voltage regulation in renewable energy sources (RESs). Despite its novelty, the impact of the HIESSC on system oscillations and the interactions among the control loops of RES stations remains unclear. This paper examines a grid-connected system comprising the HIESSC and a grid-connected converter with grid-following control (GCC-GFL). Firstly, a single-input-single-output interaction model is developed through equivalent block diagram transformations. By incorporating the grid-side coupling impedance and transfer functions of interacting subsystems, the proposed model reveals how the HIESSC modifies the dynamic characteristics of GCC-GFL through a feedback control perspective. Subsequently, the impact of HIESSC operating modes and operating points on the small-signal stability is analyzed. Finally, the effectiveness of the proposed methods and analyses is validated by the experiments.

The virtual synchronous generator (VSG) control is increasingly adopted in multi-paralleled photovoltaic generation systems (MP-PGSs) due to its enhanced grid-support capability. While the transient stability of VSG-controlled photovoltaic generation systems (PGSs) under symmetrical grid faults is well-studied, instability mechanisms under asymmetrical grid faults (AGFs) remain underexplored. This paper establishes a multiple coupling analysis model for transient stability analysis of VSG-controlled MP-PGSs under AGFs. Based on this model, the impacts of coupling dynamics including sequence coupling and mutual coupling on the transient stability of VSG-controlled MP-PGSs are investigated. Furthermore, the influence laws of key parameters on the transient stability are analyzed. To improve the low voltage ride-through capability of MP-PGSs under AGFs, a multi-objective stabilization control method is proposed, which satisfies both the grid codes and current limitation requirements. Finally, simulation results validate the correctness of the theoretical analysis and the effectiveness of the proposed control method.

During asymmetric grid faults, significant transient interactions occur between the grid and wind turbines (WTs) based on doubly fed induction generators (DFIGs), as well as among different DFIG-based WTs, increasing the risk of transient instability in DFIG-based multi-WT parallel systems (DFIG-MWPS). To quantitatively assess the transient stability (TS) of DFIG-MWPS during asymmetric grid faults, this letter proposes an assessment algorithm operating on the DC-link voltage (DCV) control timescale. The proposed method defines the TS boundary from the perspective of energy conversion. Compared to qualitative approaches, it quantitatively evaluates system stability based on the positive and negative sequence (PNS) active and reactive currents from each WT during grid faults, and determines the TS level of the DCV. Finally, hardware-in-the-loop experimental results validate the accuracy of the proposed assessment algorithm.

The interaction between rotor-side and grid-side converters (RS-GSCs) has a significant impact on the low-voltage ride-through (LVRT) performance of grid-connected wind turbine based on doubly fed induction generator (WT-DFIG) during severe symmetrical grid faults. Hence, to improve the LVRT performance of the WT-DFIG during symmetrical grid faults, this letter proposed an LVRT control strategy, which is implemented by coordinating the current references of RS-GSCs. Compared to existing LVRT strategies, by coordinating the active and reactive currents of the RS-GSCs, the proposed strategy could enhance not only the transient stability level of DC-link voltage, but also the LVRT performance of the WT-DFIG. Finally, the validation was carried out using an experimental system.

Similar to synchronous generators (SGs), symmetrical short-circuit faults can reduce the stability margin of grid-forming renewable power generation (GFM-RPG), thereby heightening the risk of transient instability. While existing studies primarily examine single-machine infinite-bus systems, this work explores transient stability challenges inherent in paralleled GFM-RPG systems. First, through rigorous mathematical derivation, it establishes that the transient characteristics of paralleled systems can still be effectively characterized by a second-order motion equation. Subsequently, by applying the extended equal area criterion (EEAC) and numerical solutions to differential equations, the study uncovers the governing principles behind the variations in the critical clearing angle (CCA) and critical clearing time (CCT) for the paralleled GFM-RPG system under various operating conditions. Finally, to mitigate potential instability risks, two corrective strategies, namely adaptive damping enhancement and power switching control, are proposed to improve the transient stability of the paralleled system during symmetrical faults. Simulation results confirm the accuracy of the theoretical analysis and demonstrates the effectiveness of the proposed strategy.

Wind power generation systems based on doubly-fed induction generator (DFIG) are subject to the shafting oscillation under small disturbances such as uncertainty in wind speed and grid short-circuit faults, which will cause oscillations in the output power. In this paper, according to the damping characteristics of shafting oscillation in DFIG system, the mechanism of DFIG electromagnetic torque on shafting damping in common control modes is deduced. On this basis, the impact of wind speed and current inner loop control proportional coefficients on shafting oscillation is analyzed in detail. The shafting oscillation mechanism of DFIG system has been profoundly revealed from the physical level, and the theoretical found for the analyzing and suppressing of low-frequency oscillation in DFIG wind turbine (WT) system is laid. Finally, the DIgSILENT/PowerFactory simulation platform was used to model and verify the relevant simulations.

In an offshore wind farm connected with a high-voltage direct current (HVDC) transmission system based on modular multilevel converter (MMC), a symmetric fault on the outgoing line at the sending end (SFOLSE) exhibits complex controlled characteristics in the fault current, which can undermine the reliability of relay protection. Detailed analysis of the control interaction between the wind farm and the MMC sending station (MMCSS) is conducted to ascertain the fault current characteristics. Considering the constraints imposed by the existence of a stable operating point (SOP) during SFOLSE, the phase angle difference distribution law for short-circuit currents sourced from both the wind farm and MMCSS is analyzed. Furthermore, the influence of control interaction on the reliability of distance protection is discussed. The results show that the additional impedance exhibits specific distribution characteristics under the influence of control interaction. In addition, the setting ratio of the dq-axis current reference for wind farm distance protection is analyzed, and the impact of wind farm control on the adaptability of distance protection under the constraints of the grid-connected guideline is evaluated. The main risk scenarios of misoperation are clarified, and the correctness of the analytical results is validated through PSCAD time-domain simulations.

The transient synchronization characteristics and instability mechanism of the permanent magnet synchronous generator(PMSG)-based grid-forming wind energy conversion system(GFM-WECS) under symmetrical grid fault have received little attention to date. In this paper, considering the dynamics of DC-link voltage, the transient stability and an improved control strategy of PMSG-based GFM-WECS are studied in detail. Firstly, considering the dynamic interactions between the machine-side converter and the grid-side converter, the large-signal equivalent model of GFM-WECS is established. Furthermore, a novel Lyapunov function is derived to evaluate the transient stability margin and instability boundary of GFMWECS during grid voltage sag. Additionally, the impacts of current-limitation control on the transient stability of GFM-WECS are revealed. Then, a stability evaluation index is proposed to evaluate the transient stability margin of GFM-WECS. Moreover, an improved control strategy is proposed to enhance the transient response characteristics and low voltage ride-through(LVRT) capability of GFM-WECS under symmetrical grid fault. Finally, simulations and experimental results are conducted to verify the effectiveness of the proposed control strategy.

In this article, from the perspective of DC-link voltage (DCV) control, the transient interaction mechanism of multi-paralleled doubly fed induction generator (DFIG)-based wind turbines (WTs) is investigated during asymmetrical grid faults. Firstly, considering the coupling characteristics of positive and negative sequence (PNS) components and the interaction characteristics between the rotor side converter (RSC) and grid side converter (GSC), a large-signal nonlinear model of multiple-parallel DFIG-based WTs in DC-link voltage control time-scale is obtained. Furthermore, by using the energy function method, the dynamic interaction mechanism of multiple-parallel DFIG-based WTs is analyzed. The influence of different parameters on the transient characteristics of DC-link voltage is analyzed by using phase trajectory diagram. The dominant factors affecting the transient stability of the WTs and stability level of DC-link voltage are obtained. In addition, considering the interaction among WTs, the dynamic interaction between RSC and GSC, as well as the requirement of grid codes, a transient stability optimization strategy during asymmetrical grid faults is proposed to improve the transient stability level of the DC-link voltage and the transient stability of multiple-parallel DFIG-based WTs. Finally, simulation and experimental results validate the correctness of theoretical analysis and the effectiveness of the proposed strategy.

During grid symmetrical faults, grid-connected voltage-source converters (VSCs) face transient synchronization instability risks during low-voltage ride-through (LVRT). Research on the synchronization stability mechanism and control strategies of grid-connection VSC systems is crucial for maintaining stable operation after severe disturbances. This paper proposes a control strategy to enhance transient synchronization stability based on the equivalent rotor swing equation of VSC. The proposed approach improves both the existence of equilibrium points and the transient synchronization process in grid-connected VSC systems, thereby significantly enhancing their transient synchronization stability during operation. Finally, time-domain simulations confirm the effectiveness of the proposed strategy.

The high-voltage direct current (HVDC) transmission system based on line-commutated converter-modular multilevel converter (LCC-MMC) has become one of the crucial solutions for delivering offshore wind farm power over long distances. How to achieve black-start capability for wind farms via LCC-MMC transmission system after power outages has emerged as a critical issue requiring urgent resolution. The active power recovery and reactive power balance in offshore wind farms connected via LCC-MMC transmission system is analyzed. Considering the configuration of static and dynamic reactive power compensation devices at LCC converter station, along with factors such as wind energy utilization rate, startup efficiency, and power quality, a coordinated black-start scheme involving grid-forming wind turbines, the HVDC transmission system, and reactive power compensation devices is proposed in this paper. The effectiveness of the proposed black-start scheme is verified on the MATLAB/Simulink platform.

The dynamic coupling between grid-following (GFL) and grid-forming (GFM) converters, as well as their interaction with synchronous generators (SGs), may have a significant influence on the control characteristics of renewable energy converters and the operating modes of SGs during grid faults. This coupling complicates the transient synchronous stabilization of hybrid power systems. This study focuses on the hybrid system integrating GFL with GFM converters. Initially, an equivalent mathematical model is developed based on the synchronous mechanisms of different power sources. Subsequently, using the phase plane and space vector diagram methods, the interactions among various power sources and their effects on the transient stabilization of the system are analyzed. Finally, a collaborative strategy for improving transient synchronous stabilization is proposed. This approach involves adjusting the current and the power reference values of the GFL and GFM converters, respectively, to maintain the equilibrium points of each power source approximately constant before and after grid faults. As a result, it minimizes the dynamic interaction between power sources and enhances the overall transient stabilization of the hybrid system under the voltage dip scenarios.

In this article, the equivalent power angle transient response of the grid forming (GFM) during fault are derived in detail. Based on the control system of GFM, the power angle characteristic equation is proposed, which investigates the effects of the GFM system’s control parameters and the operation status on the general output characteristics of the system. Then the transient synchronization characteristics of the GFM system are derived from the equation. The relation between the actual value and the instruction value of the output power is used as the criterion to judge transient stability of the GFM system. Considering the detection delay, the transient synchronization mechanism of the GFL grid-connected system is studied by analyzing operating trajectory of equivalent power angle (EPA) and using the equal area criterion (EAC). In addition, the sequential switching control schemes from normal stage to fault stage are introduced. Moreover, the stability assessment method based on the EAC is proposed, and the transient synchronization processes of different time sequence stages are studied. Finally, the theoretical analysis is verified by the time domain simulations.

The high voltage direct current transmission technology, widely implemented in offshore power grid-connected projects, has recently attracted significant attention due to oscillation-related challenges. This paper analyzes the influence of zero-sequence current control strategy in modular multilevel converters on the dynamic stability of offshore wind farms integrated with MMC-HVDC grid-connected system. Firstly, the MMC impedance model incorporating zero-sequence circulation controller dynamics is established using harmonic state space method. Subsequently, the influence of zero-sequence circulation controller implementation on system stability is systematically analyzed. The analytical results demonstrate that zero-sequence circulation controller integration effectively increases the bridge arm resistance, enhances the system damping, and suppresses the impedance resonance peak in MMC islanded control mode. Finally, a parameter optimization scheme for zero-sequence circulation controller is proposed from the perspective of damping enhancement, with simulation results validating the theoretical analysis and demonstrating the effectiveness of the proposed approach.