In distributed power supply and distributed energy storage technologies, a bidirectional AC-DC converter is an important energy conversion device connecting AC-DC hybrid microgrids, and its performance index directly affects the overall performance and effect of hybrid microgrids. Compared with the two-stage topology, the single-stage dual active bridge (DAB) AC-DC converter removes the intermediate DC bus capacitance with a large capacitance value, reduces the conversion link, and has apparent power density and cost advantages. The traditional DAB AC-DC converter mainly adopts the modulation strategy of phase shift, which has the problems of high current stress and narrow soft-switching range. In addition, only adopting the phase shift control leads to the nonlinear relationship between the system input current and the shift ratio, increasing the control complexity. Therefore, this paper proposes a linearization-based minimum current stress control strategy for the converter to address the problems of modulation nonlinearity and high current stress in a single-stage dual active bridge AC-DC converter. This control strategy reduces the converter’s control complexity and current stress, ensuring a wide zero voltage switch (ZVS) range of the switching tubes.

Firstly, the switching characteristics of the extended phase-shift (EPS) modulation strategy are analyzed. For the nonlinearity between the input current and the shift ratio, the input current i_ac and the shift ratio D₁ are linearly related by introducing the phase shift index k and the maximum switching frequency f_smax. The expressions of the shift ratio D₁ and the switching frequency f_s are obtained combined with power factor correction. The switching characteristics of the EPS modulation strategy are analyzed. The trajectory of the phase-shift index k under the minimum current stress is obtained by the differential polarity method. Then, the expression of the shift ratio D₂ is obtained. Finally, the soft-switching ranges are analyzed for switch tubes S₂, S₅, and S₈. Except for the DC-side switch tube S₅, which is difficult to realize soft-switching in the small range under extreme light-load conditions, the other two switch tubes can realize ZVS in the wide range in other cases.

This paper verifies the proposed control strategy by combining simulation and experiment. Firstly, regarding simulations, the proposed control strategy can achieve the linearization between the input current and the shift ratio, effectively reducing the converter current stress. An experimental prototype is constructed with an AC 50 V input, DC 12 V output, and 100 W output power. The current stress is compared before and after optimization under different input voltages and the soft-switching realization under different load conditions. The control strategy effectively reduces the current stress of the converter while ensuring that the switching tubes have a wide ZVS turn-on range.

Recently, the bus voltage of data center power architectures has been gradually increased from the traditional 12 V to 48 V to reduce the current in the distribution lines, thus reducing distribution losses. In 48 V bus-powered architectures, the uninterruptible power supply (UPS) system is connected in parallel with the 48 V bus, which causes the bus voltage to fluctuate over a wide range (40 V to 60 V). To better manage the bus voltage and energy flow, a bidirectional DC-DC converter must be inserted between the UPS and power-using systems. The four-switch Buck-Boost converter is attractive because of its high efficiency and wide voltage regulation capability. In order to reduce the energy consumption in data centers, it becomes crucial to improve the efficiency of FSBB.

This paper analyzes the voltage gain of the FSBB converter. Then, a graphical approach compares the control strategies of the FSBB converter. The unimodal control strategy has large ripples. The bimodal control strategy system is unstable. Tri-modal solves the problems of duty cycle limitation and system stability, but the duty cycle varies greatly when the transition mode is switched. Four-mode control can add a control mode in the transition section to realize smooth conversion between different modes, and the ripple of inductor current is small. It is a control strategy with excellent performance.

Then, the minimum ripple condition of the inductor current is analyzed based on four-mode control. The inductor current ripple of the FSBB converter is minimized when the phase shift time between the Buck and Boost bridge arms is controlled to zero. The average value of the inductor current is related to the output voltage, input voltage, maximum duty cycle, and output current, and it is almost the same for all four-mode control strategies. Therefore, the control method to minimize the inductor current can be obtained by simultaneously controlling the phase shift time to zero.

Next, an accurate loss model of the FSBB converter is developed. When the voltage gain is constant, the loss of the converter increases as the load current increases. When the load is fixed, the lower the switching frequency, the lower the loss. Under the same load conditions, if the voltage gain is greater than 1, the loss decreases with the gradual increase of the gain at the same switching frequency until the loss is minimized when the voltage gain equals 1. On the contrary, if the voltage gain is less than 1, the loss gradually decreases as the gain gradually increases. Thus, this paper proposes a frequency reduction control strategy to reduce the converter loss in the transition mode and improve the conversion efficiency by reducing the switching.

Finally, an experimental platform is established to test the inverter control strategy for the FSBB converter in steady states. The minimum ripple control strategy is then validated. The results show that the transition mode’s inductor current ripple with the proposed control strategy is much smaller than the conventional four-mode control strategy. Among them, the inductor current ripple of the proposed minimum ripple control strategy is 35.2% of the conventional control strategy under the operating conditions of 51 V input voltage and 48 V output voltage. After reducing the switching frequency, the inductor current ripple of the minimum ripple control strategy is still smaller than that of the conventional control strategy. The efficiency of the proposed low ripple inverter control strategy is improved over the whole load variation range compared with the traditional fixed frequency control. Among them, the peak efficiency of the proposed inverter control strategy reaches 98.52% and full-load efficiency 98.4%, which is improved by 2.46% and 2.35% compared to the conventional scheme, respectively.

The high peak-to-average ratio of low-frequency pulse power loads seriously affects the safe and stable operation of airborne power supply systems. The conventional approach requires stacking numerous energy storage capacitors due to the DC bus voltage ripple limitation, which substantially increases the system’s volume. Although current active pulse power suppression method can reduce the required capacitance by increasing voltage fluctuations, the considerable power ratings and additional power processing stages of active suppression circuits impact system efficiency significantly.

This paper presents a low-frequency pulse power active suppression method based on voltage compensation. The active suppression circuit is inserted between the DC bus and the energy storage capacitor C_d. By compensating for the voltage difference between C_d and DC bus with the output voltage v_s of the active suppression circuit, the voltage range of C_d is not constrained by the DC bus, allowing for a reduction in C_d. Since the active suppression circuit only compensates for capacitor voltage fluctuations, its power rating and losses are much smaller than the average power of pulse loads, which greatly reduces the volume, weight, and losses. The active suppression circuit takes power from the DC bus. Considering that its input and output terminals are non-common ground and have a wide output voltage range, the LLC-DC transformer (DCX) cascaded Buck converter is chosen for the active suppression circuit. The LLC-DCX functions operate in an open loop as a high- frequency DC transformer, and a dual-loop control strategy is implemented for the Buck converter. The outer voltage loop adjusts the voltage fluctuation range of C_d, while the inner current loop suppresses the input current ripple.

The design guidelines for key parameters are also presented, with size and efficiency as the main considerations. The size of the power supply is influenced by C_d, while the power rating of the active suppression circuit affects system efficiency. Therefore, a detailed study of both aspects is conducted. The results reveal that once the load is determined, C_d decreases as the voltage fluctuation Δv_d increases, and the decreasing rate gradually slows. Additionally, the power rating of the active suppression circuit increases linearly with the average voltage V_dav and Δv_d. The lower limit of V_dav is also affected by Δv_d to ensure that the output voltage of the Buck converter remains positive. Therefore, a balance must be achieved between V_dav and Δv_d to optimize capacitance and power rating.

An experimental prototype is constructed. The experimental results are consistent with the theoretical analysis, and the active suppression circuit effectively regulates the voltage fluctuation range of C_d and suppresses the input current. Efficiency tests reveal that the active suppression scheme maintains an efficiency above 98.1% throughout the entire range, with a peak efficiency reaching 99.1%. This scheme is compared with existing active suppression schemes, showing clear advantages. In addition, results from various literature are normalized and compared.

With the remarkable growth of renewables, distributed power generation systems (DPGSs) are starting to take over the dominant role of synchronous machines. As an essential interface between renewables and power grids, the grid-connected inverter plays an important role in the safe and stable operation of DPGSs. Among two types of grid-connected inverters, i.e., grid-following (GFL) and grid-forming (GFM) ones, attention has gradually turned to the GFM inverter in recent decades, owing to its synchronous-machine-like characteristics and capability of operating in weak grid or even forming a stand-alone grid. However, similar to the GFL inverter, the GFM inverter may exhibit non-passive characteristics in the mid/high-frequency bands, leading to mid/high-frequency resonance risk.

The existing research mainly focuses on sub-synchronous oscillation, but the mid/high-frequency resonance issue still needs to be explored. In order to mitigate the mid/high-frequency resonance and harvest the desired performance, this paper provides the optimal design procedure for controller parameters from the perspective of internal stability and the impedance reshaping method via the grid current feedforward from the perspective of external stability.

Firstly, a mathematical model of the voltage-current double-loop controlled GFM inverter is established. The control block diagram of the inverter’s control system is depicted, and its equivalent transformation is performed. Accordingly, the impedance model of the GFM inverter is obtained as a controlled source in series with the output impedance.

After that, the stability of the GFM inverter is divided into internal stability and external stability, which characterize the stability of the equivalent voltage source and the interaction stability between the equivalent impedance and the grid, respectively. From these two stability dimensions, the stability mechanism and resonance risk of the GFM inverter are analyzed based on the Nyquist stability criterion and the passivity theory, and the main factors affecting the system stability are revealed.

According to the internal stability constraints, stability margin requirements, and steady-state error, an optimal design procedure for the control parameters is provided, which avoids repeated trials and ensures internal stability and low steady-state error. Additionally, based on the external stability constraints, the impedance shaping scheme with the grid current feedforward is proposed, and the corresponding feedforward function is derived. The proposed scheme is simple to implement and can effectively enhance the inverter's robustness against grid impedance variations.

Finally, experiments are carried out on a 10 kW GFM inverter prototype. The results confirm that under different grid conditions, the inverter can with the designed parameters and the proposed scheme continuously operate stably, and the power quality is high, which verifies the theoretical analyses and the proposed scheme.

The large-scale development of wind power is a major demand for the development and utilization of new energy sources, and the high-performance service of the wind turbine fleet is an important guarantee for realizing the goal of the national carbon peaking and carbon neutrality goals. With the continuous increase of stand-alone capacity and installed capacity, wind conditions, sea conditions, and other complex environments make the synergistic optimization between service performance of large-scale wind turbine fleet- safe operation capacity-power generation benefits complex, and the unit safety and accurate warning and service quality control face serious challenges.

Firstly, the advantages and disadvantages of condition monitoring and fault diagnosis of key components of WTGs and reliability assessment are sorted out and compared. The current status of their service quality regulation is investigated. Secondly, the impacts of WTG’s healthiness, corrosive environment, and thunderstorms on the service quality of WTGs are elaborated, and the impacts of WTG FM strategy on the service quality are summarized. Then, the factors that affect the service quality of the wind turbine fleet are analyzed. The characteristics of voltage control strategy, operation and maintenance, and tail current control are analyzed.

High-quality power generation, operation and maintenance strategies, and tailing effects are analyzed at the level of the wind turbine fleet based on the service quality control methods of key components, wind turbines, and the wind turbine fleet. An outlook of the possible future direction is made to enhance the service performance of the wind turbine fleet and promote the healthy and sustainable development of the wind power industry.

As China advances its dual carbon strategy, integrating new energy sources into power grids has grown significantly, making power system operations more complex and dynamic. For deep learning-based models used in transient stability assessment to be reliable, the training data and the data encountered in real-world applications must be independent and identically distributed. However, because power systems are time-varying and uncertain, models trained offline may not perform well in new operational scenarios. This paper proposes a transient stability assessment-discriminative domain adaptive (TSA-DDA) framework to address variations in operating scenarios.

Firstly, an inter-domain dual distribution adaptation method was proposed. While aligning the marginal probability distributions of the source and target domains, this method also used Bayes' theorem to align the conditional probability distributions, achieving optimal domain adaptation. Secondly, both mean and variance differences between the source and target domains were comprehensively considered in the domain adaptation process. A new transfer regularization term was constructed to measure the inter-domain distribution differences, improving the model's domain adaptation capability. Finally, a discriminant Softmax function with adjustable parameters was developed to make intra-class sample features more compact while keeping inter-class sample features away by adjusting the parameters. This improvement can enhance the applicability of the assessment model to power grids.

In the case studies, the TSA-DDA framework's ability to address variations in operational scenarios was first validated on the New England 10-machine 39-bus system. Subsequently, four alternative TSA-DDA frameworks, each with specific modules removed, were established to evaluate the effectiveness of individual components. The prediction accuracy of the target and source domain test sets was compared using a fine-tuning algorithm and the TSA-DDA. The TSA-DDA’s capacity for continual learning is confirmed. The TSA-DDA was then benchmarked against mainstream transferred learning approaches to verify its effectiveness in scenarios with limited new data. Finally, to assess the generalization capability of the proposed scheme, experiments were conducted on a larger and more complex provincial power grid in Southwest China. The experimental simulations utilized the PSD Power Tools and Dynamic Simulation Program to offer high-fidelity power system simulation data for model training and testing.

The conclusions of this paper are given as follows. (1) The inter-domain dual distribution adaptation method comprehensively measures differences in marginal and conditional probability distributions between domains from both mean and variance perspectives. It constantly forces the feature extractor to narrow these differences, ensuring effective feature alignment across domains and enhancing the model’s adaptability. (2) The discriminant Softmax function improves the model’s learning of discriminative features by compacting intra-class features and separating inter-class features, which enhances the performance of the domain adaptation framework in transient stability assessment tasks. (3) Using voltage trajectory clusters with clustering and convergence properties as model inputs, the proposed framework ensures effective transferability across systems with varying structures and scales.

The dynamic transformer rating (DTR) and thermal life loss of oil-immersed transformers under on-site variable load operation conditions are closely related to the transient temperature rise of the equipment. However, as an implicit solution method, the traditional finite element analysis and finite volume method need to be iteratively solved in each sub-step of transient thermal analysis, which has many computational resources and is time consuming. It is challenging to meet the requirements of fast calculation. The rapid and accurate solution of the temperature field (especially the hot spot temperature rise) of the oil-immersed transformer in field operation is the premise to realize the digital operation and maintenance of the transformer and the DTR evaluation. Therefore, this paper proposes a lattice Boltzmann (LBM) physical in the loop simulation model for coupled electrical networks. The real time evaluation of DTR under electrical network constraints is realized through the rapid solution of the transformer temperature field.

Firstly, the D2Q9 model is used to solve the fluid flow and thermal lattice Boltzmann equations (LBEs) to capture the transient oil flow and temperature rise process inside the transformer. In the Simulink environment, the equivalent current source model is used to construct the electrical network constraints of multi-level load scenarios, and the established transformer LBM model is used as a component for numerical encapsulation to complete the construction of the physical-in-the-loop simulation model. Secondly, to verify the effectiveness of the proposed method, the finite volume method (FVM) is used to simulate the same oil-immersed transformer model. The grid independence test determines the optimal number of grids. The number of grids is 1 250×420 for LBM modeling and simulation, and the number of units is 43 654 for FVM meshing. Compared with the constructed LBM-Simulink model with the FVM model, LBM still has the advantages of speed and memory occupation when the number of lattices is higher than the number of FVM units. If commercial software is used, this advantage will be further expanded. Thirdly, the steady state solution results of the hot spot temperature rise of the established LBM model are compared with the FVM solution, and the error is 2.60%. According to the load curve given in the transformer guidelines, it is used as input to solve the transient temperature rise of the established LBM and FVM simulation models. Finally, the results show that the LBM and FVM calculations are better than the transformer guide calculation. The maximum error between the hot spot temperature calculated by LBM and FVM is 6.44%. Moreover, the hot spot temperature rise trend of LBM is consistent with the transformer load guidelines, which verifies the effectiveness of the proposed method.

Based on the constructed LBM model, the load capacity of oil-immersed transformers under constant 25℃ and typical ambient temperature changes in summer and winter are evaluated at 6~18 hours during the day. The results show that under the premise that the relative insulation life loss of oil-immersed transformers is less than 1. The maximum load capacity coefficients are 1.20, 1.10, and 1.60 under the constant ambient temperature of 25℃, typical temperature changes in summer, and typical temperature changes in winter. The simulation model based on the proposed LBM provides an effective method for real-time monitoring of temperature rise, load capacity evaluation, and dynamic capacity increase of oil-immersed transformers.

The low-voltage power supply and distribution system is directly connected to the user at the end of the power system. Its wide distribution, diverse applications, and complex structure make overhauling difficult and lack safety maintenance. Due to its negative resistance characteristics, the series arc can decrease line current, exhibiting high concealment of fault characteristics. It is a loophole in traditional relay protection methods. The series arc fault can produce high temperatures in a short time, which can cause a fire very quickly. The temperature characteristics of AC fault arcs have not been thoroughly studied, the development process and influencing factors of fault arc temperature are not apparent, and the mechanism of arc ignition and disaster needs to be clarified. This paper builds a real experimental platform for arc ignition, constructs a numerical simulation model of AC arc fault based on magnetohydrodynamics, verifies the temperature characteristics of arc fault through simulation and experiment, clarifies the ignition mechanism of arc fault, and puts forward suggestions for the improvement of relevant standards.

Firstly, based on the IEC 62606 standard, combined with a temperature acquisition device, an experimental platform for arc fault ignition risk is built to simulate arc faults. The current, voltage, temperature, and thermal imaging images are collected. Secondly, the physical characteristics of AC fault arc and related test standards are analyzed, and a complete set of fault arc simulation schemes is designed. Thirdly, the control equation, calculation domain, and boundary conditions of the arc fault magnetohydrodynamic simulation model are defined, the material parameters are designed, and the division of the simulation grid is refined. Finally, by analyzing the simulation model's calculation results, the fault arc's temperature characteristics are obtained, and experiments verify the simulation results.

The simulation results show that the temperature of the AC fault arc increases periodically, and the maximum temperature of the arc appears near the instantaneous peak value of the current. At this time, the influence range of arc temperature also increases significantly. The arc temperature is a cumulative process but develops rapidly in half an AC cycle. The arc current level and arc gap distance are the main factors influencing the maximum temperature of the arc, and the current level plays a decisive role in directly affecting the severity of the arc fire risk. The maximum temperature of the arc increases linearly with the current level below the 32 A current level, and the maximum temperature growth rate slows down after the 32 A current level.

The existing arc fault product standards can effectively limit the maximum temperature of arc fault and the influence range of arc temperature. However, even in the time specified in the standard, the arc center temperature can still reach more than one thousand degrees. Therefore, the standard can be improved by limiting the influence range of arc temperature to reduce the fire risk. Low current arc ignition ability cannot be ignored. The current level range covered by the relevant standards should be expanded, and the maximum removal time of 1 A and 2 A current level arc faults is recommended to be 3 s and 1.5 s, respectively. The standard action characteristic requirements should be refined to prevent arc fault hazards and reduce electrical fires comprehensively.

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.