Abstract:In order to address the synchronization stability of grid-forming converters considering current limiting constraints，a switching dynamic model of the system of a single converter connected to the grid based on direct current voltage synchronization control （DVSC） strategy considering current limiting is established in this paper. Grid-forming converters without current limiting constraints will only operate in the constant voltage control mode after fault clearance，while those with current limiting constraints may stabilize in different modes like constant voltage control mode or constant current control mode. Therefore，the transient stability of studied model is analyzed using the segmental equal area criteria （SEAC） in this paper and the analytical expression of the critical clearing angle （CCA） is derived，revealing the mechanism by which the saturation current angle affects the stability of the system. In addition，a setting method of current saturation angle that can enhance the transient stability margin is proposed in this paper，which not only prevents the system from being locked into the current limiting mode after fault clearance but also achieves the maximum critical clearing angle and deceleration area. Finally，simulations based on MATLAB/Simulink are carried out to verify the correctness of the expression of critical clearing angle and setting method of current saturation angle.

Abstract:The grid-forming converter has voltage source characteristics，providing damping and frequency support to high-impedance weak grids，and demonstrates good stability. However，instability may occur under low impedance grids. This paper focuses on the small-signal modeling and stability analysis of multi-voltage source converters （VSCs） for grid-forming converters under low-impedance condition. Firstly，a small-signal model of multi-VSCs for grid-forming converters connected to the grid under low impedance is established，clarifying the system’s dynamic characteristics and coupling mechanism. Secondly，through eigenvalue trajectory and participation factor analysis，the dominant oscillation modes and their impact on system stability under different grid strengths and different control parameters are discussed. Finally，MATLAB time-domain simulations are conducted to verify the accuracy of the modeling and stability analysis results. The research results show that the increase in grid strength，reduction in active power，decrease in the droop coefficient，and reduction in interconnection impedance all weaken the stability of multi-VSCs for grid-forming converters，with the power loop and voltage loop being the dominant control loops affecting system stability.

Abstract:A high proportion of wind power generation is integrated into the grid through power electronic devices，which lack the inertial response of traditional synchronous generators. This limitation makes it challenging to maintain system frequency stability under external disturbances. Grid-forming direct-drive wind turbines operate with virtual synchronous control，enabling frequency support without interference from the phase-locked loop. To mitigate significant frequency and power fluctuations in grid-forming direct-drive wind turbines under virtual synchronous control，an active frequency support control strategy with adaptive inertia and damping is proposed. Firstly，mathematical model and small-signal model of the wind turbine system are established. Key parameters in the grid-side control loop are analyzed using characteristic root locus analysis to evaluate the impact on system frequency response. Based on this analysis，a parameter-adaptive frequency support control strategy is formulated. The validity of the small-signal model and the effectiveness of the proposed control strategy are verified through simulations on the MATLAB/Simulink platform. The results indicate that the proposed strategy effectively mitigates the frequency and power fluctuations induced by disturbances in the system.

Abstract:The internal storage system of a wind turbine usually needs to be connected to the DC bus using a DC/DC converter. However，due to the poor dynamic performance of the energy storage DC/DC converter using traditional PI control，it is easy to cause a large drop in bus voltage or even the risk of undervoltage shutdown during the grid inertia/frequency support process. This paper proposes adopting auto-disturbance rejection control to improve the anti-disturbance performance and dynamic performance of LLC type energy storage DC/DC converter，and employing the improved gray wolf algorithm to perform offline self-optimization on the six core parameters of the auto-disturbance rejection controller. The algorithm introduces dynamic neighborhood search into the position update strategy of the traditional gray wolf algorithm，which effectively improves the convergence speed of the self-optimization algorithm. The proposed improved gray wolf optimized auto-disturbance rejection control method can effectively shorten the bus voltage recovery time，quickly coordinate the energy exchange between the storage wind turbine and the grid，and effectively improve the bus voltage stability and inertia/frequency support capability of the storage wind turbine. The MATLAB/Simulink simulation results verify the feasibility and effectiveness of the control method proposed in this paper.

Abstract:Grid-forming converters controlled by virtual synchronous generator （VSG） provide a certain amount of inertia and damping support for new energy sources to be connected to the grid. However，the power coupling generated by the VSG control link causes system frequency oscillation，which can seriously threaten the system safety. To solve the unclear mechanism of power coupling interaction on frequency oscillation when multi-VSGs are connected to the grid，a method of impedance modeling of multi-VSG grid-connected system considering power coupling is proposed. The frequency oscillation characteristics are analyzed by combining with parameters changes. Firstly，the power coupling mechanism of single VSG is analyzed. Then the impedance model of multi-VSG grid-connected system is established which reveals that output angular frequency under multi-VSGs is mainly affected by three parts：its own power，interacting power and grid angular frequency. In addition，a method based on the dynamic relative gain array is proposed to quantitatively analyze the laws of system parameter variations on the interaction effects among VSGs. The study reveals the impact trends of parameter variations in multi-VSG systems on system coupling interactions across high-frequency and low-frequency bands. Finally，the accuracy of the theoretical analysis is verified by time-domain simulation of a three-VSG grid-connected system model constructed in PSCAD.

Abstract:As a large amount of renewable energy is integrated into the grid via power electronic converters，the power system is faced with challenges，such as reduced inertia，diminished disturbance resistance，and declining transient support capabilities. The stability of the renewable energy interconnected with the grid and the capability of supporting the grid are enhanced by the autonomous synchronization and active support capacity of grid-forming converters. The grid-connected control strategy of grid-forming converters is closely related to the stability of the device and system under transient voltage disturbances. This paper focuses on disturbances caused by grid voltage sags and reviews the transient control strategies of grid-forming converters. Firstly，current synchronization control strategies of grid-forming converters are summarized. Next，the mechanism of overcurrent generation in grid-forming converters is revealed，and the overcurrent behavior and support characteristics of converters during transient periods are analyzed. Then，transient control strategies are classified into two major types，and the principles，research status，advantages and disadvantages of these two types of control strategies are comparatively discussed. Finally，future research directions for grid-connected issues of grid-forming converters in high-penetration renewable energy power systems are prospected.

Abstract:With the large-scale distributed generation equipment gradually replacing the traditional synchronous generator，grid-connected systems are faced with a series of challenges such as loss of inertia，voltage and frequency stability decline. The introduction of grid control technology gives the necessary inertia and frequency support capability to distributed generation equipment. In the field of grid control technology，virtual synchronous generator technology has been widely used，but the traditional proportional integral （PI） control strategy it relies on has some problems，such as weak anti-interference ability and insufficient transient stability. In view of these shortcomings，an improved sliding mode active disturbance rejection control strategy for grid type photovoltaic energy storage system is proposed，which applies to the outer voltage loop of the system to improve the system performance. Among them，the fraction order extended state observer is designed to enhance the observation ability of the internal state of the system，and the fixed-time convergence sliding mode controller is designed to effectively suppress the chattering phenomenon in the traditional sliding mode control by means of its characteristic of converging to the equilibrium point in a fixed time，thus significantly enhancing the robustness and anti-interference ability of the system. Finally，through the construction of simulation model in MATLAB environment and experimental verification，the simulation and experimental results show that the control strategy proposed in this paper effectively reduces the transient voltage fluctuations of the system and speeds up the system response speed，which plays a positive role in improving the system performance.

Abstract:With the access of high proportions of new energy，synchronous generators are gradually phased out. On one hand，system inertia is reduced. On the other hand，during short-circuit faults，the low-voltage ride-through characteristics of new energy may result in short-term power disturbance，leading to rapid drops in grid frequency. Frequency emergency control，as a critical measure to safeguard frequency stability after grid faults，may encounter issues such as delayed actions or improper control actions resulting in high or low frequency events. To address these challenges，a frequency response model incorporating frequency emergency control is established. The boundaries of emergency control effectiveness under typical conditions are studied. Furthermore，a frequency response model that considers both frequency emergency control and grid-forming new energy is developed. Based on this model，the influence of various frequency control parameters，such as power reserves of new energy and droop control coefficients，on the effectiveness of frequency emergency control is analyzed comprehensively. Requirements for frequency regulation performance of grid-forming new energy to ensure effective emergency control are provided. Finally，the frequency model and the proposed effectiveness boundaries of emergency control are verified using simulations on the IEEE 10-machine 39-bus system. The results show that the participation of grid-forming new energy in frequency regulation can effectively extend the operating boundaries of emergency control，and its power reserve has a significant impact on the upper and lower boundaries of frequency emergency control effectiveness.

Abstract:For grid-forming energy storage integrated into power systems dominated by new energy，voltage drops in the grid are caused by the instability of new energy generation. Leading to transient power angle instability and overcurrent issues in grid-forming energy storage. A transient power angle stability control strategy based on power angle deviation feedback is proposed in the article. Firstly，grid-forming energy storage adopts virtual synchronous generator （VSG） control，and a corresponding model is established based on VSG control. Secondly，the relationship between active power，power angle stability, and output current is analyzed based on VSG power angle characteristics, and the causes of power angle instability are further analyzed using phase portrait theory. Based on this，an adaptive adjustment strategy for the power angle deviation feedback coefficient is designed，considering the power angle stability range and active power deviation. The active power deviation is controlled through power angle deviation feedback, thereby suppressing power angle amplification，maintaining power angle stability，and effectively mitigating overcurrent. Finally, time-domain simulations verify the correctness of the theoretical analysis and the effectiveness of the proposed control method.

Abstract:In order to take advantage of the flexible adjustment ability with new energy in unit combination and make up for the lack of system inertia caused by new energy access，a unit combination model is constructed in this paper，which takes the inertia support and primary frequency regulation capabilities of grid-forming new energy. The model incorporates dynamic frequency constraints and grid-forming control parameters，utilizing the shedding capacity stored through new energy curtailment. Then，the nonlinear mixed integer programming model is transformed into a multi-objective two-layer optimization problem for iterative solution. Thus，the optimal start-up mode of synchronous generators，the optimal reduction of new energy and the corresponding control parameters are obtained which meet the system frequency constraints. A high proportion of wind power transmission grid in Gansu province is taken as an example to verify the method. The results show that compared with the existing scheme that only considers frequency constraints and the unit combination scheme with constant grid-forming control parameters，the unit combination scheme with variable grid-forming control parameters proposed in this paper can further improve the system operation economy and new energy utilization rate under the frequency security constraints.

Abstract:The grid-forming converter （GFM） is prone to transient instability，when a fault occurs in the power grid. At present，the research on voltage ride through control of GFM is comprehensive. However，the influence of power loop difference and interaction on transient stability is rarely analyzed. Therefore，the expression and power-loop model of GFM are derived for transient conditions，based on the typical power-synchronous loops. The expression establishes a bridge to analyze the influence of control loop differentiation. Secondly，the influence of power loop difference on the value of control parameters is analyzed. The difference of active loop is the scaling of control parameters，and the difference of reactive loop is the change of control structure and parameters. Then，the interaction regularity of power loop difference is described. The inertia promotes the coupling of active loop to reactive loop，and voltage correction coefficient alleviates the deterioration of reactive loop to active loop. Finally，the following conclusions are concluded. Increasing damping and decreasing inertia is beneficial to improve power angle and frequency stability，while smaller proportional，integral parameters and larger voltage correction parameters are beneficial to improve voltage stability.

Abstract:In order to solve the problem of high and low voltage fault ride-through of modular multilevel converter （MMC） under weak grid，a multi-link control strategy including capacitance voltage balancing control of integral，phase-to-phase and upper and lower bridge arm，improved phase-locked loop （PLL） and multi-resonance control is proposed. Firstly，the pre-filtering stage of the double second-order generalized integrator （DSOGI） is improved，and nonlinear proportional integral （PI） control is adopted in the PLL. Secondly，a capacitance voltage equalization control method based on the power redistribution for MMC under weak grid is proposed，and the active power redistribution of the submodule is realized by controlling the DC component of the bridge arm current. Then，considering the inconsistency of the upper and lower arm losses or asymmetrical parameters，the traditional voltage equalization control of the upper and lower bridge arm is improved by adopting the method of the active power redistribution of the upper and lower bridge arm. The simulation results show that the proposed control strategy realizes the uninterrupted operation of the system under the fault ride-through，solving the problem of inaccurate phase and frequency detection of PLL during high and low voltage ride-through and the current imbalance caused by three-phase voltage imbalance.

Abstract:The large-scale connection of distributed resources requires the flexible control ability of distribution network to be enhanced continuously. How to make full use of multi-level flexible resources to assist system operation has become an urgent problem. Therefore，a hierarchical autonomous collaboration strategy to support multiple types of resource access to distribution network is provided in this paper. Firstly，the characteristics of flexibility resources under multi-level are analyzed，and a probabilistic model for distributed resource output to reduce the influence of its uncertainty factors is adopted. Secondly，a hierarchical zonal optimization and dispatch model is constructed for the main substation-feeder-station area. The station area layer carries out internal autonomy and passes the equivalent results to the feeder layer. The feeder layer divides the area based on the network architecture and the operating characteristics of the resources，so as to realize the main-distribution cooperative optimization taking into account the security and economy of the system，and the spectral penalty parameter based adaptive alternating direction method of multipliers （SPPA-ADMM） is used for the solving. Finally，the improved IEEE 33-node example is selected for simulation. The simulation results show that the parallel control method adopted in this paper can effectively improve the efficiency of optimization solution，which verifies that the proposed strategy has guiding significance for the operation regulation of multiple distributed resources.

Abstract:With the continuous advancement of new power system construction，the impact of uncertainty is becoming increasingly prominent. Consequently，it is urgent to enhance the flexibility of new power systems to respond to fluctuations in source load power and ensure the safe and stable operation of the system. To this end，this paper reviews methods for improving the flexibility of new power systems considering distributed resources. Firstly，based on the inherent characteristics of the new power system，the challenges and technical bottlenecks in improving the flexibility of the new power system are analyzed. Secondly，following the historical-current-future timeline，this paper summarizes the flexibility improvement paths of the new power system from three perspectives：flexibility quantification evaluation，flexibility optimization scheduling，and flexibility configuration planning. Finally，the key challenges in enhancing the flexibility of new power systems are summarized from the three aforementioned aspects.

Abstract:The large-scale integration of wind farms and power electronic devices reduces the total inertia and frequency regulation capability of power grid. The new energy systems，including wind power generation system，are required to provide sufficient inertia support to ensure the stability and safety of frequency. The wind power generation system can provide virtual inertia to the power grid by applying the kinetic energy of wind turbine rotor or the electrostatic energy of DC capacitor. However，how to coordinate the two sources to achieve optimal utilization of resources and efficient inertia support is the current research difficulty. On the other hand，the participation of DC capacitor of wind power generation system in virtual inertia provision may lead to sustaining deviation of DC voltage from the rated value. It is difficult for the wind power generation system to cope with subsequent possible disturbances，and the continuous frequency regulation capability of wind power generation system is constrained. Therefore，an improved piecewise coordinated frequency control method for wind power generation system considering voltage restoration is proposed. Firstly，a piecewise coordinated frequency control of wind power generation system considering virtual inertia provision from wind turbine rotor and DC capacitor is constructed. Then，a novel virtual inertia control of DC capacitor considering DC voltage restoration is proposed，based on which an improved piecewise coordinated frequency control of wind power generation system considering voltage restoration is developed. Finally，simulation analysis is carried out in PSCAD/EMTDC. The simulation results show that the proposed control method fulfills the participation of DC capacitor in virtual inertia provision，and it can restore the DC voltage rapidly without affecting the inertia support performance of wind power generation system. The proposed method optimizes the use of frequency regulation resources and improves the grid support capability of wind power generation system under cascading frequency disturbances.

Abstract:A two-stage robust optimization model of power system considering source-load uncertainty is proposed，to address the serious lack of system scheduling flexibility caused by the source-load uncertainty in new energy power systems. According to the characteristics of source-load uncertainty，the K-means method and robust optimization theory are combined to quantify the flexibility demand of the power system at multiple time scales. Firstly，the robust dispatch model is established，and the flexible regulation potentials of thermal power units，pumped storage and other resources are fully exploited.The flexible transformation of thermal power units and pumped storage pumping status are included in the first stage of the model，and the output of the flexible resources is taken as the second stage of the decision variables. The optimization objective of the model is to minimize the cost of retrofitting，carbon emission and operating costs. The two-stage robust model is transformed into relatively independent main problems and sub-problems，and the column constraint generation （C&CG） algorithm and strong dyadic theory are adopted to iterate repeatedly to approximate the optimal solution. Finally，the proposed optimal scheduling strategy is verified through examples，so that the proposed optimal scheduling strategy can integrate all kinds of resources based on meeting the demand for flexibility，which achieve the balance of economy，environmental protection，and flexibility in the system，and improve the ability to resist the risk of uncertainty in the source load.

Abstract:Based on travelling wave features, a diagnostic method is proposed to address the complexity of manual threshold setting process and the difficulty of detecting high-resistance faults in the fault diagnosis of multi-terminal direct current grid based on modular multilevel converter （MMC-MTDC）. Firstly, the blocking effect of boundary elements on high-frequency signals is identified by analyzing the fault characteristics of the system. Secondly, empirical mode decomposition （EMD） is employed to decompose power signals into intrinsic mode function （IMF）, and the energy values of the IMF is utilized as fault features to train the CNN-BiGRU network composed of convolutional neural network （CNN） and bidirectional gated recurrent unit （BiGRU）. On this basis, the Kepler optimization algorithm （KOA） and attention mechanism （AM） are employed to enhance the CNN-BiGRU network to realize the fault diagnosis of the MMC-MTDC. Finally, the simulation model is built in PSCAD/EMTDC. The results show that the method can not only realize the detection of bus faults and line faults but also solve the problem of easy refusal of protection under the high resistance state while meeting the requirements of protection reliability and speed.

Abstract:Load aggregators should fully consider the impact of fixed and inverter air conditioning groups' characteristics and the interaction willingness of users in different scenarios on the adjustable potential，when integrating and managing air-conditioning load resources. Firstly，two air conditioner monomer models and aggregation models are constructed for engineering applications，based on the detailed analysis of the differentiated working state of fixed and inverter air conditioners. Secondly，quantitative analysis is carried out to analyze the interaction willingness of users in different scenarios，days types，and time-of-use electricity prices. An air conditioning adjustable potential calculation model is proposed considering interaction willingness. Then，the multi-scenario adjustable temperature interval is obtained based on the interaction willingness of the users and used as a constraint to construct an optimization model for control strategies. The osprey-Cauchy-sparrow search algorithm （OCSSA） is applied to solve and obtain multi-scenario control instructions for fixed and inverter air conditioners. Finally，the high precision temperature control command is accurately calculated through the proposed control method，and the requirements of preset load reduction command is successfully met in the final control results. By considering different user interaction willingness，the ability to adaptively and accurately control fixed and inverter air conditioning loads is demonstrated in various scenarios.

Abstract:With the accelerating popularization of photovoltaics （PVs） and electric vehicles （EVs），distribution networks are facing issues such as voltage violations and voltage fluctuations. On the one hand，conventional voltage regulation resources are featured by slow response and limited lifespan，making them unable to rapidly respond to the temporary voltage issues caused by PVs and EVs. On the other hand，EVs and PVs interact with the power grid through charging stations and inverters，and their fully controllable power converters are capable of adjusting their operating settings in real time，making EVs and PVs ideal resources for reactive power-based voltage support. To fully leverage the reactive power support capabilities of EVs and PVs，a two-stage control scheme for the voltage regulation in distribution networks is proposed，including a day-ahead control stage and an intraday control stage. The day-ahead control stage provides a day-ahead operational scheduling of on-load tap changers and capacitor banks through global optimization algorithms to avoid potential voltage violation issues. The intraday control stage dynamically adjusts the reactive power output of EVs and PVs based on their real-time operating states，minimizing voltage deviations and fluctuations in the distribution network through reactive power compensation. Finally，the effectiveness of the proposed two-stage control method for voltage regulation in distribution networks is verified via case studies on a modified IEEE 123-bus test feeder. The simulation results show that the reactive power compensation capabilities of EVs and PVs can improve the voltage distribution in the distribution network to some extent.

Abstract:Accurate and effective monitoring of SF₆ decomposition components inside the gas-insulated equipment （GIE） is crucial for the equipment fault diagnosis and condition assessment. However，the existing external detection methods are limited by sampling points and the diffusion rate of these components，making it difficult to accurately determine the concentration of the components. Therefore，in this paper，a method for detecting the SF₆ decomposition component SO₂ based on surface-coated micro/nano fibers （MNF） is proposed，which has the potential to be applied in built-in fiber optic sensing for GIE online monitoring. The optical gas-sensitive response performance of the graphene is analyzed based on the density functional theory. A fiber-loop ring-down （FLRD） gas detection system is constructed，and typical SF₆ decomposition component detection experiments are carried out. The simulation results show that the optical properties of graphene change significantly after the adsorption of SO₂ gas molecules，indicating its excellent optical gas-sensitive response performance to SO₂. The experimental results also show that the gas detection system can detect the trace SO₂ gas at room temperature. It shows a good linear relationship within the concentration range of 0~200 μL/L. The maximum error of SO₂ detection is 4.89%，and the sensitivity is 0.81 ns/（μL·L^–1）. The detection performance is improved by adding an optical fiber amplifier，and the sensitivity of the system reaches 1.24 ns/（μL·L^–1）. The method proposed in this paper provides a new approach for online monitoring of SF₆ decomposition components using GIE built-in optical fiber sensors.

Abstract:To address the issue of decreased reliability in power transmission caused by the incapability of existing parallel protection gaps to actively extinguish arcs，a method utilizing the power-flow current flowing through coil to generate a magnetic field，and employing electromagnetic force to drive a piston to compress air and jet high-speed airflow for arc extinction is proposed. Consequently，a double-ended electromagnetic air-blast segmented arc-extinguishing device is designed. Based on the theory of magnetohydrodynamic，the theoretical model is built by using finite element simulation software，and the arc-extinction process of the device is simulated and analyzed. The results show that the arc-extinguishing airflow maximum speed is up to 330 m/s. Arc-extinguishing airflow rapidly act on the arc from the upper and lower ends，under the action of high-speed airflow，the arc gradually thinned out to break the dynamic equilibrium. The arc column temperature decreases to 2 000 K within 3 ms，and the arc channel pinch off，so as to realize the ″no arc channel″. The arc-extinguishing time is only 9% of the time before the installation of the device. The device can greatly shorten the arc-extinguishing time，and improve the reliability of the power supply system.

Abstract:Partial discharge poses a serious threat to the insulation performance of electrical equipment，with streamer discharge being the main form of gas discharge in high-voltage fields and a key issue in the study of gas gap discharge. Due to the influence of external factors such as humidity and air pressure during the operation of electrical equipment，studying the effects of different external conditions on insulation defect discharge is of great significance for the design and operation of transmission lines and substations in high-altitude （low-pressure） and high-humidity areas. The current research status of the impact of external conditions on the discharge process at home and abroad is analyzed in this paper. Based on the fluid dynamics model，a simulation model of streamer discharge at sharp defects is established in the COMSOL environment. Using this model，the influence of external conditions on the streamer discharge process of sharp defects with a gap distance of 3 mm is studied. The research results indicate that a decrease in air pressure accelerates the propagation speed of the streamer，slightly reduces the maximum electric field intensity at the streamer head，and exacerbates the ionization degree of the streamer discharge. An increase in humidity accelerates the propagation speed of the streamer，with little impact on the maximum electric field intensity at the streamer head but exacerbates the ionization degree of the streamer discharge. Taking the example of a streamer head distance of 1.5 mm from the cathode，a decrease in air pressure from 101.325 kPa to 81.060 kPa results in an acceleration of streamer propagation speed by 0.56$ \text{×}{\text{10}}^{\text{6}} $ m/s，a decrease in head electric field intensity by 0.633$ \text{×}{\text{10}}^{\text{6}} $ V/m，and an increase in head luminosity by 1.675$ \text{×}{\text{10}}^{\text{28}}\;{\text{m}}^{{-3}}/\text{s} $. An increase in humidity from 3.04 g$ \text{/}{\text{m}}^{\text{3}} $ to 13.04 g$ \text{/}{\text{m}}^{\text{3}} $ results in an acceleration of streamer propagation speed by 0.08$ \text{×}{\text{10}}^{\text{6}\text{}} $ m/s，a decrease in head electric field intensity by 0.015$ \text{×}{\text{10}}^{\text{6}} $ V/m，and an increase in head luminosity by 0.865$ \text{×}{\text{10}}^{\text{28}}\;{\text{m}}^{{-3}}/\text{s} $.