As modern power system toward high renewable energy integration with wind, solar, and storage sources, the increasing share of inverter-based resources leads to stability challenges dominated by multi-loop control with wide-band frequency characteristics. In systems with high penetration of renewable energy, converter-based systems have presented new features, such as large-scale integration and long-distance transmission, causing system faults to exhibit large-signal transient characteristics. The hybrid connection of grid-following (GFL) and grid-forming (GFM) converters has emerged as a potential solution to enhance the stability and efficiency of new energy transmission. However, the high order and strong nonlinearity of these hybrid systems pose challenges to the assessment of their transient stability. Therefore, this study is dedicated to designing an effective method for evaluating the transient stability of GFL/GFM converter hybrid systems.

The research methodology starts with the construction of a detailed fourth-order nonlinear model of the hybrid system, integrating phase-locked loops and virtual synchronous generators, which serves as the basis for the proposed transient stability solution method based on alternating calculation. Further, by calculating the mutation portion at the failure moment, the method derives the computed initial values for each system of the transient process. The essence of the rotation calculations lies in performing energy calculations and resolving the angular velocities in the power angle domain, subsequently mapping them back to the time domain. In the method implementation, energy calculations are first performed for a certain converter system, the dynamics of this system is used to further estimate the motion of the other system in this step, and the order of calculations for the two systems is exchanged to perform the alternating calculations. In this process, the correspondence between the power angles of GFL and GFM control is established, which enables the complex interactive motion patterns of the hybrid system under severe disturbances to be evaluated. During the alternating computation process, for the GFL/GFM system, the equivalent kinetic energy change over the step is computed by integrating the relevant equations that take into account the damped power and kinematic properties, avoiding the uncertainty associated with neglecting damping. During the continuous iterative computation process, the computed values are exchanged and updated between the two systems to ensure accurate transient behavior of the system. Eventually, the computation is stopped after the judgment condition of stability is satisfied.

The experimental and simulation results confirm the feasibility and effectiveness of the proposed method. It accurately depicts the variations in power angles, angular velocities, and GFM converter voltages during the transient processes of the hybrid system. The computational time of this method is significantly reduced compared to existing numerical methods, with at least an order of magnitude improvement. Additionally, the method is applicable to calculating the critical clearing time (CCT), achieving a resolution within 5 ms in the presented examples. It can also accurately characterize the out-of-sync operation of GFL and GFM converters during the fault recovery process.

In conclusion, this study provides a practical solution for evaluating the transient stability of hybrid converter systems. The developed method based on alternating calculation in the discrete domain exhibits clear physical mechanisms and relatively low computational requirements. It has the potential to be incorporated as a subsystem in large-scale simulations to accelerate the simulation speed.

For bundled conductors, the shadowing effect of upwind sub-conductor will affect the airflow and droplets distributions of downwind one, resulting in the difference in icing characteristics. Traditional icing calculation process generally ignored these differences and, hence, only giving an identical icing mass result of each sub-conductor. This affects the study of the aerodynamic characteristics and deicing methods of icing bundled conductor. Although some scholars have pointed out that the shadowing effect between sub-conductors will influence the icing process, there is no quantitative study. Therefore, this paper further explores the shadowing effect and relevant influencing factors of bundled conductors through numerical simulation and test research. Furthermore, based on the analysis of the shadowing effect and the superposition principle, a rapid calculation method of ice mass accreted on the bundled conductor is proposed.

Firstly, the distributions of airflow and droplets around bundled conductor are solved by Eulerian-Eulerian two-phase flow model. Secondly, combined with the mass and thermodynamic balance equations, the icing mass and shape accreted on bundled conductor under various icing environments are obtained. Then a new parameter called shadowing coefficient is defined to investigate the shadowing effect and influencing factors as well. The results show that: Shadowing effect is weakened with increasing absolute value of shadowing angle and bundled-spacing, but intensified with the increase of median volume diameter (M_VD) of droplets; Meanwhile, the shadowing effect experiences a growth and then drops down along with the increase of wind speed, and reach to the max at 15 m/s range 5~20 m/s.

Based on the superposition principle and shadowing effect analysis, a rapid calculating method for ice mass on bundled conductor is proposed. Where iced bundled conductor is regarded as a linear combination of non-shadowed sub-conductor (single conductor) icing intensity and shadowing coefficient, so the icing intensity of various types of bundled conductor can be obtained only requiring the icing intensity of single conductor and the shadowing coefficient in the corresponding environment. Then the rapid icing calculation formulars of 3,4,6,8-bundled conductor under various shadowing angle is given by geometry analysis, respectively, which simplifies the calculation of the icing mass on bundled conductor.

Finaly, a 4-bundled conductor nature icing test was carried out at the Xuefeng Mountain Energy Equipment Safety National Observation and Research Station to validate the accuracy of the numerical simulation and rapid calculation method. Results show that under the environment parameters of ambit temperature T_f = -2℃, M_VD = 25.4 μm, liquid water content L_wc = 0.61 g/m³, wind speed V = 10 m/s and shadowing angle θ = 2°, the difference in icing intensity between rapid calculation and test results was within -4.01% to -19.77%, the icing thickness differences of sub-conductors were between 1.66% to -6.36% and the differences in shadowing coefficient were between 4.05% to 5.33%, which well verifies the accuracy of the rapid calculation method proposed in this paper.

Grid-forming energy storage technology serves as a critical solution for enhancing power system stability. Transformerless energy storage systems, characterized by high efficiency, modularity, and direct medium/high-voltage grid integration, have emerged as the preferred choice for large-scale grid-connected energy storage. However, the reduced electrical distance between transformerless systems and the grid results in significantly lower grid impedance, posing severe challenges to the stability of grid-forming control. The underlying mechanism lies in the voltage-source operation of grid-forming converters: under low grid impedance conditions, minor voltage deviations between the converter and grid can trigger substantial current surges, ultimately leading to instability. To address these challenges, this study establishes a full order small signal model to analyze the impact of low grid impedance on stability and proposes impedance enhancement strategies.

The research begins by developing a dynamic model that integrates virtual synchronous generator (VSG) control, voltage-loop regulation, and grid interactions. Pole trajectory analysis reveals two critical instability mechanisms: 1) Excessively low grid inductance shifts system poles to the right-half plane, inducing instability; 2) Insufficient grid resistance reduces damping ratios, exacerbating oscillatory behavior. These combined effects diminish system stability margins and may provoke subsynchronous oscillations. To mitigate these issues, a dual-layer impedance enhancement strategy is proposed: (1) Physical impedance reconstruction: The equivalent internal voltage control strategy repurposes filter inductance as coupling impedance by relocating the controlled voltage from the point of common coupling (PCC) to the converter side. This hardware-free modification enhances physical coupling impedance without requiring additional components. (2) Adaptive virtual impedance: A composite virtual impedance module combines static impedance for damping optimization and a dynamic current-limiting component. The static virtual impedance elevates damping ratios near to 0.707, while the current-limiting module dynamically adjusts impedance parameters based on real-time overcurrent thresholds, ensuring fault current suppression.

In the analysis of impedance enhancement effect, it is shown that equivalent internal voltage control causes the dominant pole of the system under strong power grid to shift to the left into the stable region, while the introduction of adaptive virtual impedance further enhances damping characteristics and improves dynamic response performance. The proposed impedance enhancement strategy enhances the system stability by introducing filtering impedance at the physical level and superimposing virtual impedance at the control level, thereby increasing the equivalent coupling impedance of the system from a single grid impedance to the combined effect of the three.

Experimental validation on a cascaded H-bridge transformerless energy storage platform under zero grid impedance conditions confirms the strategy's effectiveness. The proposed method eliminates oscillatory instability observed in conventional approaches, achieving smooth active power step responses without overshoot. During grid frequency fluctuations (±0.5 Hz), the system provides 0.67(pu) active power support, demonstrating effective grid-forming capabilities. Under symmetrical voltage sags (0.5(pu)), it delivers 0.5(pu) reactive power while constraining currents within 1.2(pu) safety thresholds, validating robust fault ride-through performance. Experimental and theoretical analyses confirm: (1) The proposed impedance enhancement architecture synergizes physical-layer reconstruction with control-layer virtual compensation, demonstrating superior stability improvement over conventional methods through coordinated impedance augmentation. (2) A pole trajectory analysis-based parameter optimization framework achieves concurrent enhancement of stability and dynamic performance, with virtual impedance implementation optimizing damping ratios to eliminate oscillatory instabilities. This work validates the effectiveness of the proposed strategy in extreme low-impedance scenarios, providing technical support for grid-forming transformerless energy storage applications in power grids.

After the high proportion of wind power is connected, it brings a series of problems to the stability of the frequency and voltage, and the grid needs wind power to assume the main responsibility for ensuring power supply. Existing studies have shown that the virtual synchronous grid-forming equipment, including grid-forming wind turbines, has good frequency/voltage active temporary and steady-state support capabilities, and has better temporary and steady-state adaptability in weak grid scenarios. Based on these advantages, grid-forming wind turbines are expected to play a greater role in the future grid supported by low inertia and weak voltage. However, the maximum power point tracking (MPPT) operation mode of the wind turbine and its own limited rotational energy lead to the restriction of the transient support capacity of the grid-forming permanent magnet synchronous motor(PMSG), while the grid-forming PMSG based wind-storage generator has an additional energy source due to its access to energy storage, and its transient support performance has been greatly improved, which is an effective solution. At present, the access mode of energy storage is mostly parallel energy storage on the DC side or AC side, in which the energy storage driven by grid-following control adopts the passive mode of responding to the frequency acquisition signal to support the grid frequency, and most of the energy storage driven by virtual synchronous grid-forming control is connected to the AC side of the wind turbine, and the energy storage cannot be incorporated into the virtual synchronous control system. Therefore, this study is dedicated to proposing a transient support capacity improvement strategy for grid-forming PMSG based on wind-storage integration.

Firstly, the power energy storage represented by the supercapacitor was selected to form a grid-forming PMSG based Wind-storage generator with grid-forming PMSG, and the dynamic model of the grid-forming PMSG and grid-forming PMSG based wind-storage generator were established, and the constraints of the wind turbine dynamics on the frequency support capacity and transient stability of the grid-forming PMSG were summarized by analyzing the transient response of the grid-forming PMSG under frequency and voltage drops.

Then, combined with the energy flow characteristics of the grid-forming PMSG based wind-storage generator in the transient support process, the transient response power of the Grid-forming control is decomposed into inertia response power signal and damping response power signal to drive energy storage, and a transient support capacity improvement strategy for grid-forming PMSG based on wind-storage integration is formed, which realizes the flexible allocation and invocation of rotor kinetic energy and energy storage, and incorporates the rotor kinetic energy and energy storage into the active support system of virtual synchronous control to improve the frequency support capacity and transient stability of the grid-forming PMSG based wind-storage generator.

Finally, after simulation verification under various conditions, the strategy can improve the transient support capacity of the grid-forming PMSG based wind-storage generator, including: (1) The frequency support capacity has been improved, which reduces the constraints of MPPT on the frequency support capacity of the wind turbine. (2) The fault ride-through capability is improved, the transient fluctuation of rotor speed and DC bus voltage is effectively suppressed, the redundant energy generated by fault ride-through is effectively absorbed. (3) Combined with the fault ride-through power angle stability control strategy, the redundant energy during the fault period is converted into energy storage energy, so as to avoid the reduction of wind turbine power generation efficiency and reduce energy waste.

Under the impetus of "dual carbon" targets, new energy sources are increasingly integrated into the power grid through power electronic converters, leading to a gradual decline in the proportion of synchronous machines. To enhance the stability of "highly renewable and highly flexible" systems, the flexible controllability of converters can be leveraged by employing grid-forming control to provide reliable voltage and frequency support to the system. Virtual synchronous generator (VSG) control emulates the operating characteristics of synchronous generators to achieve voltage and frequency regulation, providing active frequency and voltage support capabilities while effectively increasing the inertia level of new energy units. VSG control has garnered significant attention due to its active support features; however, the factors influencing its voltage support capability are not yet fully understood, necessitating further research on VSG control strategies that balance voltage support with short-circuit current limitations.

To address these issues, this paper first analyzes the equivalent impedance of each control stage of grid-forming converters based on VSG control during steady-state operation and establishes an equivalent circuit model of the system. Secondly, based on the system's equivalent circuit, the expression for terminal voltage is derived, quantifying the relationship between terminal voltage, internal electromotive force, and system impedance, and analyzing the factors affecting the voltage support capability of VSG. Subsequently, improvements to VSG control are made considering both current limitation requirements and voltage support capability, proposing adaptive control strategies for virtual impedance and voltage compensation coefficients. Finally, the accuracy of the theoretical analysis and the effectiveness of the proposed strategy are verified using the Matlab/Simulink electromagnetic simulation platform.

The analysis results show that reducing virtual impedance, reactive power voltage droop coefficient, or increasing the voltage compensation coefficient can enhance the voltage support capability of VSG. However, decreasing virtual impedance and reactive power voltage droop coefficient reduces the system's equivalent impedance, while increasing the voltage compensation coefficient increases the system's internal electromotive force, thus imposing higher demands on the system's current-limiting capacity. By adopting the proposed adaptive control strategy for virtual impedance and voltage compensation coefficients, virtual impedance can be self-adaptively configured according to the system state, ensuring voltage support capability under the premise of meeting current-limiting requirements.

Through theoretical analysis and simulation experiments, the following conclusions can be drawn: (1) When the grid-forming converter system based on VSG control enters a steady state, its various control stages can be represented by equivalent impedance, which characterizes the relationship between terminal voltage, internal electromotive force, and system impedance. (2) The voltage support capability of VSG is related to virtual impedance, reactive power voltage droop coefficient, and voltage compensation coefficient. Reducing the reactive power voltage droop coefficient, decreasing virtual impedance, and adding voltage compensation control to the reactive power loop can all improve voltage support capability. (3) Voltage support capability and short-circuit current limitation of VSG interact. Through adaptive control of virtual impedance and voltage compensation coefficients, short-circuit currents can be fully utilized, maximizing the voltage support capability of VSG without exceeding the short-circuit current limit.

The dynamic characteristic of grid-forming inverter (GFM) is mainly affected by the control strategy, and the interaction with the power grid may cause instability such as oscillation. At the same time, the interactive coupling between different time-scale controllers in GFM makes the stability analysis more complicated. Modal analysis based on the state-space model (MASS) uses the participation factor (PF) to quantify the contribution of each state variable to a particular pattern. However, the number of electrical components in new power systems is increasing explosively, and the difficulty of state-space modeling of the whole system is increasing rapidly. In addition, state-space modeling requires detailed system structure topology and complete control parameters of each electrical component, and inverters usually only have impedance models that describe the characteristics of voltage and current ports, with gray box or black box characteristics.

In order to explore the interaction characteristics among all electrical components of the system, the dynamic model of the whole system is constructed by the closed-loop feedback formula of the whole system dynamic matrix. Based on this foundation, the modal analysis based on impedance model (MAI) can evaluate the contribution of each power device to oscillation modes at the device level. However, MAI treats inverters as single, holistic components, which limits its ability to identify dominant system dynamics at the control loop or state variable level. Decomposing different control loops into equivalent circuit components enables the stability analysis of internal inverter dynamics. However, the decomposition of synchronization control loops remains to be explored. This paper proposes an extended modal analysis based on impedance model (EMAI) method to address the current challenges faced by MAI.

First, a decomposition method for the GFM impedance model based on the matrix inversion lemma was proposed, dividing GFM dynamics into synchronous dynamics (SD), dominated by the power frequency synchronization loop (PFL), and electromagnetic dynamics (ED), governed by the voltage control loop (VCL). The detailed categorization of dynamics facilitates an in-depth exploration of the complex coupling mechanisms among controllers operating on different time scales. Subsequently, overall impedance participation factors and participation ratios (PR) were introduced to characterize different internal dynamics of GFM, enabling the evaluation of SD and ED contributions at the control loop level. These metrics help identify the dominant system dynamics and trace the root causes of system instability. Finally, an explicit parameter PF was introduced to precisely locate the critical control parameters of identified loops, serving as a metric for optimizing control parameters and enhancing system damping.

The analysis yields the following conclusion: as the frequency of oscillation modes decreases, the dominant dynamics within each GFM gradually shift from ED to SD. MAI can provide an overall assessment of GFM participation but fails to capture the dominant dynamics of individual GFMs. EMAI not only identifies interactions between various GFMs and the grid but also evaluates the contributions of ED and SD within GFM through overall impedance participation factors, thereby pinpointing the primary causes affecting system dynamics to specific control loops. Moreover, the results of EMAI and MASS in assessing the participation levels of different GFM dynamics are highly consistent, validating the effectiveness of the EMAI method. Furthermore, the explicit parameter PF provides effective recommendations for improving system damping and enhancing stability. EMAI offers nuanced insights into system stability analysis, enabling the rapid identification of the root causes of system instability.

In China, the current renewable energy resources mainly use grid-following converters as grid-connected interfaces, which cannot provide inertia and damping support for power systems. In order to enhance the support capacity of renewable energy resources, grid-forming inverters are emerging as a promising solution as they can emulate the dynamic property of synchronous generator and provide support. However, the grid-forming inverter faces significant risks of transient synchronous instability. Current research primarily focuses on single grid-forming inverter systems, which cannot be applied to multi-machine systems due to complex interactions between converters. Quantitative transient analysis and the method of stability region estimation for multiple paralleled grid-forming inverter systems are absent.

To fill this gap, taking transient interaction and power coupling into consideration, the large-signal equivalent model of multiple grid-forming inverters system is established. Based on this model, a set of Lyapunov functions is constructed, which accounts for damping dissipation, reactive power loop dynamics, and transient interactions, enabling intuitively and accurately plotting the stability region for multi-machine system. Then, by comparing the sizes of the stability regions, the impact of control parameters and grid parameters on the stability boundaries of grid-forming multi-machine systems is quantified. Furthermore, the influence of damping dissipation, reactive power loop dynamics, and transient interactions on the transient stability margin is explored. Finally, hardware-in-the-loop experiments validate the accuracy of the estimated maximum stability region.

The following conclusions can be drawn from the analysis in this paper: (1) Due to the complex interaction, the equivalence model and transient characteristics of multi-machine system are more complex than those of single-machine system. (2) The Lyapunov function set, which takes into account voltage dynamics, damping dissipation and transient interaction, can accurately estimate the maximum stability region of multiple grid-forming inverter systems, and predict the transient synchronization stability via the location of the fault clearing point. (3) By comparing the size of the stability region, the increase of reference power, fault depth, and line impedance will reduce the stability region, and the increase of damping coefficient, inertia, and reactive droop coefficient will enlarge the stability region. The voltage dynamics and damping dissipation can increase the stability margin of the system, and the transient interaction between units can reduce the stability margin.

Electric vehicles (EV) have the characteristics of both traffic and mobile load, and their charging behavior will have an interactive impact on the power grid. With the rapid increase in the number of electric vehicle and the continuous improvement of their penetration rate, charging guidance for large-scale EVs has become an important measure to alleviate the contradiction between local limited charging resources and strong charging demand. Therefore, considering the influence of future traffic information changes on navigation strategy, this paper proposes a fast guidance strategy for electric vehicle charging based on dynamic traffic inference.

First of all, a dynamic traffic information prediction model based on spatio-temporal self-supervised learning (ST-SSL) is established. A self-supervised learning (SSL) module for spatial and temporal heterogeneity of traffic data is designed to achieve accurate prediction of multi-period traffic flow information. Secondly, a multi-time dynamic impedance modeling method for urban road network considering future traffic information changes is designed, a charging navigation strategy considering multi-demand scenarios and multi-navigation objectives of users is established, and a solution method based on dynamic Dijkstra algorithm is proposed to realize the selection of the optimal charging station and the planning of the optimal navigation path. Based on the global charging navigation results, the service range of urban charging stations is dynamically evaluated, to achieve rapid charging guidance for electric vehicles. Finally, taking the actual road network of a certain area in Los Angeles as an example, the accuracy of the prediction model and the effectiveness of the guidance strategy are proved, which can effectively perceive the dynamic traffic information and quickly realize the service range division of urban charging stations and the charging guidance for electric vehicles.

In this paper, a fast guidance strategy for electric vehicle charging based on dynamic traffic inference is proposed, based on the case simulation results, the main conclusions can be obtained as follows. (1) The model based on ST-SSL can make full use of the spatial and temporal heterogeneity of traffic data, improve the prediction effect of traffic flow information, and provide an effective data basis for the construction of dynamic traffic impedance. (2) The proposed multi-scenario and multi-objective charging navigation strategy based on dynamic impedance can effectively perceive traffic information and take into account the diversified needs of users, effectively reduce the cost of charging navigation for different users, and reasonably guide the load distribution of electric vehicles. (3) The proposed dynamic Dijkstra algorithm can recommend the optimal path according to the future traffic information, which can be used as a navigation algorithm to plan the driving path, and can also recommend the customized optimal charging station according to the needs of users. (4) The division of charging station service range based on the global charging navigation results can effectively evaluate the service range of charging station, and provide an important reference for the construction planning of charging station. Based on the evaluation results, the charging navigation strategy is quickly assigned to each node, which effectively reduces the computing resource consumption of charging navigation.

The increasing distance of offshore wind farms from coastal areas has created an urgent need for the development of long-term extra high voltage direct current (EHVDC) cables. Factory joints are commonly used to connect sections of submarine cables, forming extensive cable systems. Therefore, studying factory joint is crucial for advancing long-length cable lines. This study investigates the physicochemical and dielectric insulation characteristics of XLPE samples under various vulcanization pressures, highlighting the effects of these pressure changes on the properties of 500 kV EHVDC cross-linked polyethylene (XLPE) cable joints.

Commercially available 500 kV EHVDC XLPE pellets were used to prepare the XLPE samples via hot-press method. Initially, a specified quantity of XLPE pellets was distributed between two iron plates. The pellets were preheated at 120℃ for 5 minutes and then heated at 180℃. Cross-linking was subsequently performed under different vulcanization pressures of 1.3 MPa, 1.6 MPa, 1.9 MPa and 2.5 MPa respectively. The fabricated XLPE specimens underwent physical characterization through Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and gel content analysis. While electrical measurements included current density analysis, pulsed electro-acoustic (PEA) analysis, and DC breakdown test.

The physiochemical results indicate that increasing vulcanization pressure enhances the crosslinking degree of XLPE samples, transforming the material from a linear molecular structure to a 3D network structure and breaking macromolecules into smaller, mobile molecules. The increased mobility of these small molecules leads to improved crystallinity, resulting in a higher crystallinity structure. Additionally, the recrystallized macromolecular chains have higher melting temperatures, raising the overall melting temperature of the samples. However, higher vulcanization pressure also produces crosslinking by-products that are difficult to decompose and volatilize. The combination of high temperatures and pressures causes thermal expansion forces perpendicular to the lamellae, increasing lamella spacing, creating more amorphous regions, and effecting the insulation performance of the samples.

Regarding electric insulation performance, the DC breakdown strength and space charge injection threshold strength of the fabricated XLPE samples initially increase and then decrease with the increase in vulcanization pressure. Conversely, conductivity current and average space charge density first decrease and then increase. An optimal vulcanization pressure of 1.9 MPa was identified, at which the XLPE samples exhibited improved electrical insulation properties. Below this pressure, the increased trap energy levels inhibit carrier transport, thereby reducing the number of free carrier paths and hindering the formation of conductive channels, ultimately increasing the breakdown strength of the XLPE samples. However, at vulcanization pressures above 1.9 MPa, the increased crosslinking byproducts create more shallow traps, which lower space charge injection and accumulation, ultimately distorting the sample's internal electric field. Additionally, the increased lamella spacing creates more amorphous regions, reducing the carrier transport barrier and further decrease the breakdown strength of the prepared XLPE samples.

Based on the results, it can be concluded that appropriately increasing the vulcanization pressure of factory joints improves the physicochemical and electrical properties of XLPE. However, excessively high vulcanization pressure can have a detrimental impact on the electrical insulation properties of cable factory joints.