The linear induction machine (LIM) drive system can get direct thrust and linear motions without transmission, which enjoys strong climbing capability, high acceleration or deceleration ratio, and small mechanical losses. The LIM drive systems have been developed and commercialized in over 20 linear metro lines worldwide. However, due to the large air gap, end effects, and high-power, low-switching frequency drive, the LIM drive system in urban rail transit needs better efficiency. Although the existing efficiency optimization control strategies have improved machine efficiency, the parameter robustness and system efficiency still need to be addressed. This paper proposes a robust efficiency optimization strategy for three-level inverter-fed LIM systems under low switching frequency.

Firstly, the primary flux-based LIM loss model considering end effects is built, where the loss is expressed as a convex function of primary flux. Its parameter sensitivity and limitation are analyzed. Furthermore, combined with the gradient descent method, a hybrid optimal primary flux search method is proposed to eliminate the influence of parameter changes on optimal flux selection. Then, the cost function containing multiple objectives, such as primary flux control, switching frequency constraint, and neutral point voltage balance, is derived. A model-free predictive flux control based on the nonlinear-extended state observer is proposed to manipulate optimal flux flexibly under low switching frequency.

Finally, experimental comparisons with the existing methods on a 3 kW LIM confirm that efficiency and parameter robustness can be improved for the drive system under low switching frequency. The system efficiency with the proposed method can be improved by 1.22% and 0.64% compared with the mature control strategy and the existing efficiency optimization strategy under the working conditions of 8 m/s and 200 N.

The following conclusions can be drawn. (1) The proposed method takes the minimum DC-link current as the search objective, which considers the harmonic loss and inverter loss, thus improving the system’s efficiency. (2) Considering multiple objectives, such as the switching frequency constraint and neutral point voltage balance, a model-free predictive flux control with adaptive switching frequency regulation is developed. (3) By combining the hybrid optimal primary flux search method with model-free predictive flux control, the proposed method effectively avoids the influence of parameter changes and modeling errors on optimal flux selection and manipulation. In this way, the parameter robustness of the efficiency optimization control strategy is significantly enhanced.

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 maximum torque per ampere (MTPA) control strategy can fully use the reluctance torque to output the maximum torque per unit stator current and improve the operating efficiency of interior permanent magnet synchronous motors (IPMSMs). Still, the traditional formula method or the high-frequency signal injection method requires the installation of at least two phase-current sensors to obtain the current information of the motor. Once the current sensor fails, the closed-loop control of the system will fail and cause unpredictable damage. The paper proposes an MTPA control strategy for IPMSM without current sensors. The strategy can accurately realize the MTPA control of the permanent magnet synchronous motor by calculating the optimal voltage control instruction only from the rotational speed information and the mathematical model of the system.

Firstly, the relationship between the control voltage command and the rotational speed is calculated using the formula method according to the mathematical model of the IPMSM and the conditions of the MTPA control. Secondly, inverter nonlinearity can cause the inverter output voltage to deviate from the commanded voltage, resulting in the motor deviating from the MTPA operating point. Therefore, the mean value compensation method is proposed to compensate the command voltage for the nonlinearity. Thirdly, the effect of current estimation error on the calculated voltage compensation value is analyzed. The analysis shows that even if the current estimation error exists, the average error of the voltage compensation value based on the estimated current is still 0. Finally, the effect of parameter deviation on the control strategy is analyzed, and the influence of parameter deviation on the optimal voltage command amplitude and the response current is given.

The experimental results for steady-state conditions show that the A-phase current fundamental wave amplitude of the motor with the proposed strategy is smaller than that with the traditional strategy. The effectiveness of the proposed method is verified. The experimental results at the same load torque under different speeds show that the current vector amplitude error of the MTPA control with the proposed method is small. Its maximum error does not exceeding 0.5%, while the traditional method is 14%~30%. The experimental results at the same speed with different load torques show that the current vector magnitude error of MTPA control with the proposed method is small, with the maximum error not exceeding 1%. In contrast, the traditional method’s maximum error is in the range of 2%~37%. The experimental results of dynamic operation and loaded starting conditions show that the proposed control strategy is robust to parameter deviations and has good dynamic performance.

The following conclusions can be drawn. (1) The proposed method can realize MTPA control of IPMSM without current sensors, which is of great significance to the fault-tolerant control capability of current sensor failures in the IPMSM drive system. (2) The proposed method has good dynamic performance and is robust in the variation of motor parameters. (3) The proposed MTPA control strategy considers the effect of VSI nonlinearity, and the control voltages are compensated, effectively improving the running accuracy in MTPA.

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.

The electromagnetic rail launch process exists in high current, ultra-high speed, high temperature-rise, strong friction, and extreme impact conditions. The high heat generated causes the surface of the aluminum armature to melt, resulting in a transition at the pivot-rail interface from solid-solid electrical contact to a solid-liquid-solid melt process. Eventually, molten aluminum solidifies on the rail surface, forming a complex deposition layer. This deposition layer has implications for the performance of the pivot rail system during subsequent launches. The operational environment characterized by ultra-high-speed friction during repeated launches results in a low melting point in the armature. A portion of molten material forms a liquid transferred onto the rail, enhancing the interface and diminishing the electromagnetic rail's longevity. Consequently, it is imperative to investigate the impact of the aluminum deposition layer on the sliding electrical contact at the pivot-rail interface.

This study conducted small-diameter electromagnetic launching tests with varying launching times to examine the carrier friction wear behavior of the friction sub-material of the pivot rail. The results revealed that a significant amount of molten aluminum was transferred to the rail surface after multiple launches, increasing the roughness of the pivot-rail interface due to the residual deposit layer. As a result, the pivot-rail friction sub-contact deteriorated, characterized by organizational features such as gouges and cracks on the rail surface. The wear intensity escalated with an increase in the number of launches. However, after a certain number of launches, the aluminum alloy oxide layer on the rail surface reached a critical thickness, reducing the wear on the rail body. Nonetheless, mechanical and electrical wear simultaneously intensified the environmental conditions at the pivot-rail contact surface.

Finally, a liquid film fusion deposition model at the pivot-rail interface was developed, and the deposited layer’s impacts on the operational dynamics of the liquid film and the electrical contact condition of the pivot-rail interface were studied. The study involved the calculation of the thickness of the deposited layer and the deposition efficiency for varying launch times. During high-speed launches, the aluminum liquid layer experienced significant viscous forces, and pronounced velocity variations of the liquefied layer at the armature tail exit increased viscous dissipation forces. With multiple launches, heightened interfacial friction can counteract the viscous forces within the aluminum liquid layer, destabilizing the interfacial liquid film. Thickening the aluminum deposition layer on the rail surface can exert extrusion effects on the liquid film, introducing destabilizing factors to the flow of the liquefied layer. Consequently, the aluminum liquid layer, which serves as a lubricant between the armature and the rail, may be extruded from the interface. Therefore, the armature’s normal operation is compromised, and the rail's longevity is diminished.

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.

Developing high-voltage silicon carbide (SiC) devices has enabled breakthroughs in voltage levels, power density, and efficiency in power electronic systems. Research institutions and manufacturers have recently created high-voltage SiC devices with ratings over 10 kV and 15 kV. These devices can increase the voltage level of large-capacity converters to 10 kV or higher and achieve megawatt power levels using only two- or three-level topologies. However, as voltage levels rise, the isolated power supplies for the SiC device drive circuits face greater challenges in voltage-withstand capability. These isolated power supplies draw power from the low-voltage side to supply the high-potential drive circuits. While they only need a few watts, they must withstand isolation voltages from several kilovolts to tens of kilovolts because they connect to the main circuit of the converter.

The high-frequency current transformer (HCT) is a promising isolated power supply structure known for its strong resistance to dv/dt. This advantage comes from the high integration of ultrahigh-frequency electromagnetic coupling and the low coupling capacitance of single-turn coils on the primary side. However, current research on HCT-isolated power supply mainly targets optimizing transmission efficiency, power, and coupling capacitance. There has been little systematic study of its unique insulation characteristics. As a result, the optimization design methods are unclear, and assessing insulation voltage capacity is challenging.

This paper investigates the insulation characteristics of the HCT-isolated power supply for high-voltage SiC devices. It examines six key structural factors: the inner diameter, height, and thickness of the magnetic core, as well as the winding method and wire diameter for both primary and secondary windings. This paper proposes an electric field optimization design method under compact size constraints. Additionally, a high voltage experimental platform was established to clarify the relationship between key structural parameters and the initiation voltage and discharge magnitude of partial discharges. The voltage withstand characteristics of the HCT isolated power supply were also verified. Simulation and experimental results indicate that using concentrated winding for the secondary winding results in a more uniform electric field within the structure. The inner diameter and height of the magnetic core, as well as the turns and diameter of the secondary winding, have significant effects on the electric field and partial discharge. However, the thickness of the magnetic core has a relatively weak influence on insulation capability. This study provides a theoretical basis for the design and optimization of the HCT-isolated power supply and experimentally verifies the specific effects of key structural parameters on insulation performance.

Due to its shared structure, the dual Buck/Boost-CLLC three-port converter has a simple structure and few power devices. The integrated interleaved parallel Buck/Boost unit significantly reduces input current ripple, while the integration of CLLC units endows the converter with excellent buck-boost conversion capability and soft-switching capability. However, the large number and volume of magnetic components in the shared structure are the main factors limiting the size of the power converter. Increasing the switching frequency or using magnetic integration can increase the power density of the power converter. However, in some studies, some schemes integrate two energy storage inductors and the resonant inductor in the converter to enhance coupled inductor current sharing and converter power density. Nonetheless, these schemes can only integrate full inverse coupling at a fixed duty cycle and cannot control the inverse coupling coefficient. Integration schemes with controllable coupling coefficients have been proposed, but two magnetic components remain after integration.

This paper proposes a fully integrated magnetic structure based on a dual Buck/Boost-CLLC three-port converter. By unevenly distributing the windings and establishing low reluctance paths, all magnetic components are integrated into a single magnetic element under variable duty cycle and coupling coefficient conditions. The proposed fully integrated magnetic component achieves inverse coupled inductor current sharing and ripple reduction, thereby enhancing system stability. Additionally, by integrating all magnetic components into a single magnetic element, the increased magnetic flux cancellation within the core further reduces core losses. Fig.A1 shows the proposed fully integrated magnetic structure, which consists of a cover magnetic core and a base magnetic core.

Fig.A1 Structure of the topology and fully integrated magnetic component structure

Firstly, based on the partially integrated structures and the proposed fully integrated structure, magnetic circuit models were established for both partially integrated and fully integrated magnetic components. The magnetic flux distribution and cancellation with different integration methods were compared. It is shown that the proposed fully integrated structure exhibits more magnetic flux cancellation and has lower losses. Next, the

performance-influencing parameters were analyzed, and a loss model was developed. Low losses for the fully integrated magnetic component were achieved through finite element parameterization scanning. Finally, a 500W prototype platform was built, and comparative experiments of non-integrated, partially integrated, and fully integrated magnets were conducted. Steady-state and dynamic experiments verified the feasibility of the integrated magnetic design. Efficiency and temperature comparison experiments validated the effectiveness of the integrated magnetic design.

The results show that the proposed fully integrated magnetic component maintains the same volume and footprint and exhibits more magnetic flux cancellation and uniform temperature distribution. The fully integrated magnetic component achieves an efficiency of 94.6% under full load, demonstrating higher power density and efficiency compared to non-integrated and partially integrated structures.

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.

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.