Fast and accurate DC fault-current evaluation is crucial for the design and protection of multi-terminal MMC– HVDC grids, especially under low-impedance metallic pole-to-pole (P2P) short circuits with steep current rise. This paper proposes a semi-analytical EMT-oriented fault calculation method for an n-terminal DC grid in which a metal-oxide varistor (MOV) is connected in parallel with the outlet smoothing reactor to realize source-side energy suppression. In this paper, the term shunt-MOV reactor denotes an outlet smoothing reactor whose terminals are shunted by an MOV. The key challenge is to embed the nonlinear MOV-shunted smoothing reactor into a sparse nodal matrix framework without sacrificing computational efficiency. To this end, an odd-symmetric MOV characteristic is represented by a piecewise-affine PWL companion, where both the conductance term and the affine current-injection term are explicitly stamped into the EMT nodal equation. A one-shot (iteration-free) segment selection rule based on a predictor voltage is further adopted so that each time step requires only one sparse linear solve, with an optional single corrective reselection when needed. The method is specialized to a three-terminal star MMC–HVDC grid with bipolar stacking and a near-end low-impedance metallic P2P fault. EMT validations confirm that the MOV-shunted smoothing reactor effectively limits the fault current and reduces its rise rate, and that the proposed semi-analytical calculation closely matches the EMT benchmark for the limited fault current. The verified source-side limiting capability suggests a reduced energy stress on downstream DC protection devices, such as DC circuit breakers, pending device-level sizing assessment.

Gravity energy storage systems (GESS) have emerged as a promising long-duration energy storage technology capable of supporting large-scale renewable integration and enhancing grid resilience. However, the modeling framework for the hoisting electromechanical subsystem in wire-rope-based GESS remains underdeveloped, thereby limiting the accurate characterization of its transient grid-connected behavior, dynamic operating response, and cross-domain coupling effects. Existing studies commonly simplify wire ropes and related transmission components as rigid bodies or low-dimensional mechanical elements, failing to adequately account for their flexibility and the resulting high-dimensional nonlinear dynamics. Although related studies in mine hoisting and elevator systems have addressed mechanical vibration phenomena, they primarily focus on mechanical-side effects, such as shock loading and guide-structure response, whereas the mechanism by which flexible mechanical vibrations propagate through electromechanical coupling and influence electrical dynamic performance remains inadequately understood. To address this gap, this study establishes a distributed-parameter model for the wire-rope hoisting mechanism based on Hamilton’s principle and solves the corresponding vibration governing equations using the Galerkin method to capture nonlinear multi-modal dynamics. An electromechanical coupling model is then developed to elucidate how rope-vibration-induced tension fluctuations propagate through the drive chain, resulting in torque ripple, electrical interharmonics, and low-frequency grid-side oscillations. A Bessel-function-based analytical representation is further introduced to explain the formation of interharmonic clusters and beat-frequency phenomena under converter modulation. An experimental prototype is constructed to validate the proposed modeling framework. The measured vibration spectra, beat-frequency characteristics, and torque ripple align closely with analytical predictions, confirming the model’s capability to capture key propagation paths from rope vibration to electromechanical oscillation and grid-side dynamic response. The results provide a solid theoretical foundation for vibration mitigation, dynamic analysis, and control design of hoisting electromechanical subsystems in gravity energy storage applications.

Superconducting magnetic energy storage (SMES) is widely recognized for its fast dynamic response, high energy storage efficiency, and flexible active and reactive power regulation capability. To address the challenges associated with large-capacity SMES systems, this paper proposes an efficient power conditioning system (PCS) topology and corresponding control strategy. A cascaded H-bridge inverter combined with a chopper circuit is adopted to realize high voltage and high-power conversion for a 4 MW / 8 MJ SMES system. The cascaded modular structure enables direct connection to a 10 kV grid while improving output waveform quality through phase-shifted PWM. A detailed simulation model of the SMES system and grid interaction is established in MATLAB/Simulink. The operating modes of the SMES system, including charging, persistent current operation, and discharging, are analyzed, and the converter control strategy for active power regulation under unity power factor operation is implemented. The results demonstrate that the proposed topology and control strategy allow the SMES system to respond rapidly to grid load disturbances, provide rapid active power compensation, and maintain stable grid voltage and high power factor.

TMR (Tunneling Magnetoresistance) magnetic sensors offer advantages such as high sensitivity, low power consumption, and compact size, making them promising for applications in fields like biomedical and geological exploration. Currently, most TMR magnetic sensors can only detect magnetic flux density in the horizontal in-plane direction due to the in-plane magnetic anisotropy (IMA) of tunnel junctions. This paper proposes a novel TMR -superconducting composite structure that enables TMR to detect out-of-plane magnetic flux density, and its sensitivity can be adjusted by modifying the shape and size of the superconducting layer. This study provides significant theoretical and experimental support for the design of high sensitivity magnetoresistance sensors and magnetoresistance sensor arrays.

A bulk high-temperature superconductor (HTS) employed in the high-temperature superconducting levitation device is susceptible to cracks or defects during its fabrication and utilization. This may cause changes in the electromagnetic (EM) and vibrational behaviors of the HTS, even affecting the normal operation of the device. To this end, based on the magnetic, thermal and motion field equations for the transversely isotropic superconducting material, we establish a 3D coupled model of a levitation system containing an HTS with a slot defect and permanent magnet (PM), to investigate the vibrational and EM behaviors of the HTS with the defect above the PM, and discuss the effects of the depth of the defect, frequency of the external excitation, and combined defect on the behaviors in the time or frequency domains. The results show that the intrinsic frequencies of the HTS with defects of different depths are in the ranges of 6-8 Hz, 13-16 Hz and 19-20 Hz. With the increase of the defect depth, the intrinsic frequencies and EM losses increase but the vibration amplitude decreases. Under the action of the simple harmonic excitation with the same amplitude, the larger the defect depth, the lower the vibration amplitude and the better the stability of the phase trajectory. With the increase of the excitation frequency, the vibration amplitude decreases and the losses increase. However, when the excitation frequency is close to the intrinsic frequency, the HTS resonates and its internal fluxes undergo the violent motion. When the HTS contains a composite defect consisting of two single defects, its intrinsic frequencies are higher than those for the HTS with either defect included. And its EM losses are greater than the sum of their losses. The modeling method presented here can provide an idea and a reference for the research of EM-vibration behaviors of superconducting levitation systems containing defects.

The methods of suppressing numerical oscillations as caused by the diode or diode bridge electromagnetic transient models are presented and compared in terms of their advantages and disadvantages. These methods include the critical damping adjustment, advanced numerical integration methods, resistance-capacitance damping methods and also interpolation and extrapolation methods. To improve the computational speed and accuracy of real-time simulation as the realities, the interpolation and extrapolation method is at least best one, as employed and implemented in electromagnetic transient simulations. Several cast studies are carried out to verify the effectiveness of the interpolation and extrapolation method, especially for multiple diode or switching conditions.

Tunneling Magnetoresistance (TMR) magnetic sensors offer advantages such as high sensitivity, low power consumption, and compact size, making them promising for applications in fields like biomedical and geological exploration. Currently, most TMR magnetic sensors can only detect magnetic flux density in the horizontal in-plane direction due to the in-plane magnetic anisotropy (IMA) of tunnel junctions. This article proposes a novel TMR-superconducting composite structure that enables TMR to detect out-of-plane magnetic flux density, and its sensitivity can be adjusted by modifying the shape and size of the superconducting layer. This study provides significant theoretical and experimental support for the design of high sensitivity magnetoresistance (MR) sensors and MR sensor arrays.

To address the challenge of providing long-term and stable power supply to wireless sensors embedded inside wind turbine blades, this paper proposes an electromagnetic energy harvester that converts rotational energy into electrical energy based on the principle of electromagnetic induction. The harvester consists of permanent magnets and coils. When the wind turbine blade rotates in the vertical plane, the permanent magnets undergo periodic vibration under the influence of gravity, resulting in a changing magnetic flux through the area enclosed by the coils and thereby inducing an electromotive force (EMF) in the coils. Theoretical analysis and simulation are conducted to investigate the effects of various parameters on the output power of the harvester. Furthermore, an improved configuration featuring three permanent magnets and five coils is proposed. Modeling and simulation demonstrate that the maximum output power of the improved harvester is significantly enhanced—by a factor of 33.9 compared to the original design.

Nitrogen/tetrafluoromethane (N2/CF4) has emerged as an effective refrigerant mixture, which plays a significant role in various high-Tc superconducting (HTS) power devices and energy systems. Several studies have documented the superior performance of such zeotropic binary mixtures in heat transfer and energy conservation compared to pure liquids. However, boiling heat transfer characteristics of liquid mixtures on the molecular scale are not fully understood, especially the mechanisms associated with the role of the additive in regulating thermal properties. Here, we performed the molecular dynamics simulations for the pure N2 and the mixtures containing 20 and 40 mol% of CF4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N2/CF4 mixture shares similar features with the pure N2, but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF4 delays the onset of film boiling, and the operational temperature range could be enlarged by almost 40% compared to pure N2 in terms of the 40 mol% mixture. Moreover, the mixture with CF4 additive maintains a small interfacial resistance even if the substrate temperature exceeds the critical value of film boiling for pure N2, highlighting the potential to use such mixtures for devices that may suffer from high heat flux levels. Finally, analyses regarding the mixture composition and the solid-liquid interactions confirm the essential role of the additive CF4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF4 improving the heat transfer performance of the original N2. These findings provide a better understanding of the advantage of N2/CF4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses.

With the rapid acceleration of urbanization, the scale of urban distribution networks has experienced significant growth. However, the prevalence of issues related to cable equipment quality disparities, inadequate construction process oversight, unfavorable operating conditions, and other factors has led to a pronounced increase in safety concerns for both individuals and the grid resulting from cable failures. Furthermore, the limitations of conventional online monitoring methods have posed considerable challenges in meeting the monitoring demands of widely distributed and large-scale urban distribution networks. To address these multifaceted challenges, this paper introduces the design of a low-power wireless online monitoring system grounded in the Internet of Things (IoT) paradigm. This system is adept at concurrently monitoring multiple status signals within urban distribution networks. The proposed methodology entails the establishment of a monitoring sensing network for distribution networks, built upon IoT principles. It encompasses the optimization of low-power hardware circuitry, power management strategies, data communication protocols, and software control mechanisms, among other facets. Through the aforementioned research endeavors, the successful realization of a low-power design and application of IoT-based distributed monitoring systems for multiple signals within power distribution networks has been achieved. This accomplishment serves as a robust technical foundation for ensuring the dependable operation of urban power distribution networks.

The superconducting fault current limiter (SFCL) improves the power grid safety by restricting the drastically increased current when the circuit fault occurs. However, the SFCL suffers from a long recovery time and can hardly recover to the superconducting state before the breaker attempts to reclose, which hinders its practical application. This paper presents the implementation of the rapid-recovery SFCL with efficient heat dissipation even if encountering a large fault current. This is enabled by thermal performance regulation of the working coolant through adding tetrafluoromethane (CF4) into the liquid nitrogen (LN2). The proposed mixed coolant features a high critical heat flux that suppresses the onset of film boiling. Controlled experiments were performed with the fabricated current-limiting coil immersed in the N-2/CF4 mixed coolants containing 0, 20 and 40 mol% of CF4. Measured results regarding both quenching and recovery processes are reported, demonstrating an 85.4% reduction in recovery time for the 40 mol% composition compared to the conventionally used pure LN2. Analyses of the electrical parameters of the coil verify its effective quenching performance in the proposed immersion cooling system with an overall current limiting rate of over 55%. These findings lay a foundation for designing the rapid-recovery-type SFCL and facilitating its application in power systems.

The asymmetry in hard X-ray (HXR) emission at the footpoints (FPs) of flare loops is a ubiquitous feature closely associated with nonthermal electron transport. In this study, we analyze the asymmetric HXR radiation at two flare ribbons, which is thermal-dominated during a long-duration C4.4 flare that occurred on March 20, 2023, combining multi-view and multi-waveband observations from the Advanced Space-based Solar Observatory (ASO-S), Solar Orbiter (SolO), and Solar Dynamics Observatory (SDO) spacecraft. We find that the H i Lyman-alpha (Ly α ) emission presents similar features to the He ii λ 304 emission, both in the light curve and spatio-temporal evolution of a pair of conjugate flare ribbons. The spectra and imaging analysis of the HXR emission, detected by the Spectrometer Telescope for Imaging X-rays (STIX) in 4-18 keV, reveal that the two-ribbon flare radiation is thermal dominated by over 95 ≈1.0×10^10 cm^-3 , of the low-lying flare loops ( < 32 Mm ), we find that the shorter path from the LT to the northern FP enables more electrons to reach the northern FP more easily after collisions with the surrounding plasma. Therefore, in this thermal-dominated C-class flare, the asymmetric location of the flare LT relative to its two FPs plays a dominant role in the HXR radiation asymmetry, while such asymmetry is also slightly influenced by the magnetic mirror effect, resulting in larger HXR radiation at the FPs with weaker magnetic strength. Our study enriches the understanding of particle transport processes during solar flares.

This paper presents a study on the impact of circuit parameters on the transmission of electrical energy in wireless power transfer systems designed for intelligent sensing devices within the urban electric power Internet of Things (IoT). Relying on the essential principles of resonant mutual inductance models, the paper conducts an analytical investigation into the phenomena of power-frequency splitting characteristics, efficiency-frequency splitting characteristics, and efficacy synchronization characteristics within wireless energy transmission technologies. The investigation includes a detailed analysis of a wireless power transfer system model operating at 100 kHz, delineating how varying circuit parameters influence the system’s efficiency. Via the utilization of graphical software and computational programming for simulation modeling, this research delved into the dynamics between key parameters such as equivalent load and coupling coefficient and their influence on distinct splitting phenomena. This rigorous approach substantiated the validity of the proposed power-frequency and efficiency-frequency splitting characteristics outlined in the study. Based on the analytical results, it is shown that selecting an appropriate equivalent load or utilizing impedance matching networks to adjust the equivalent load to a suitable size is crucial in consideration of the system’s output power, voltage withstand level, and transmission efficiency. The research findings provide a theoretical basis for the design of wireless power supply systems for non-directly buried cable front-end sensing devices.

Nitrogen/tetrafluoromethane (N2/CF4) has attracted wide attention for its reliable dielectric performance and potential to be utilized as a coolant and insulator of power apparatus. Several studies have investigated the basic properties of N2/CF4, but dielectric responses of such a mixture under extreme electric fields are not fully understood, primarily on a molecular level. Here, we modeled the N2/CF4 complex and calculated the molecular properties under different external electric fields through computational quantum chemistry methods. Results suggest that the electric-field-induced polarization can be apparently observed in both positive and negative direction cases; the deformed electronic structure leads to an increased dipole moment and a narrowed energy gap. In addition, the molecular vibrational modes are sensitive to external electric fields, resulting in the splitting and shifting of IR spectra. Moreover, the external electric field also affects the excited-state properties of N2/CF4, and the emerged absorption peaks under extreme electric fields were analyzed in detail. These findings provide a better understanding of the N2/CF4 properties under external electric fields and lay a foundation for designing the electrical insulation systems of the power apparatus.

Liquid nitrogen/tetrafluoromethane (LN _2 /CF _4 ) mixture with a wide liquefied temperature range of 50 to 100 K might be a promising coolant for high- T_c superconducting (HTS) apparatus. However, certain features have not yet been clarified before further application, particularly the insulation properties regarding the composite insulation system consisting of such a cryogenic mixture and polypropylene laminated paper (PPLP). In this paper, AC and DC breakdown experiments and simulations of LN _2 /CF _4 - PPLP composite insulation system are carried out with various molar fractions of each constituent part taken into account using sample cables insulated by the foregoing composite insulation. Results indicate that such a mixture/PPLP composite insulation system possess a superior breakdown strength compared to pure LN _2 /PPLP system; LN _2 /CF _4 can therefore be an advantageous choice for HTS power apparatus.

Superconducting bulk has great advantages in shielding magnetic fields owing to its high critical current density. Since the shielded area is usually larger than the size of a single bulk, superconducting bulks need to be stacked together to form a shielding layer. However, bulks stacked together can cause the magnetic flux to concentrate at the gaps between bulks. In order to increase the shielding effect, multiple shielding layers are usually required. This increases the shielding cost. In this article, a shielding structure that makes an angle between gap and external field nonparallel is explored. This structure can increase the gap reluctance and reduce the magnetic flux leakage, which provides a method to get a uniform shielding area in a relatively economical way. The shielding effects are studied by experiment and finite element calculation. The established simulation model can be used to further explore and optimize new shielding structures.

Currently, copper-oxygen high-temperature superconducting materials have strongly anisotropic electromagnetic properties, which are difficult to describe in the equations. And in the rotating machine containing the permanent magnet (PM) rotor and bulk high-temperature superconductor (HTS) stator, when the HTSs have defects, their electromagnetic-stress behaviours may affect the mechanical stability of the equipment and even cause it to not work properly. In this paper, we proposed an anisotropic electromagnetic equation based on the H -formulation and established a three-dimensional coupled model with the magnetic, thermal and stress fields to study the electromagnetic-stress behaviours of a HTS with a slot defect during the swing of a PM, discussing the effects of the interior angle and inclination angle of the defect on the behaviours. The results show that the interior and inclination angles of the defect have a large influence on the electromagnetic-stress behaviours. For the electromagnetic characteristics, the main influence is on the rotational losses. Especially the losses at an inclination angle of 60° are 16.5 times those without damage. This is related to the ‘thin wall’ structure near the upper surface of the HTS. The stress concentration point appears on the defect boundary. The novelty of this paper is the proposal of the anisotropic electromagnetic equations based on the H -formulation and the study of the electromagnetic-stress behaviours of a superconducting swing system containing a PM and HTS with a slot defect from a three-dimensional perspective. The research results of this paper can be the references for the design and structural protection of superconducting rotating machines.

Nowadays, copper Litz wire is widely used in the electrical applications. This wire is a particular type of multistrand wire, which is designed to reduce the skin effect and proximity effect losses in conductors. As the frequency dependence of losses in YBCO coils and Litz coils is different, how to determine the frequency band in which superconducting coils have an efficiency advantage over copper coils is a problem that needs to be solved. In response to the above concerns, a theory of the applicable frequency band of YBCO coils is proposed. Both experimental and numerical methods are used to study the characteristics of the applicable frequency band. Results show that the applicable frequency band of the YBCO coil decreases as the transport current increases, while its dependence on the radius of the coil is small. The obtained conclusions can be used as a reference when superconducting coils are used to replace copper coils.

Transport ac loss is a crucial reference for evaluating the performance of high-temperature superconducting materials. Although the transport ac loss of $\text{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{{\rm{7 - \delta \ }}}( {\text{YBCO}} )$ single tapes has been well studied, the transport ac loss characteristic of YBCO coils with different substrates in the kilohertz frequency band is still unclear, which is important for a wide range of applications, such as wireless power supply systems and high-frequency superconducting transformers. In response to the aforementioned concerns, both numerical and experimental methods are used to study the frequency dependence of transport ac loss in YBCO coils with magnetic and nonmagnetic substrate over a frequency range, from 55 Hz to 10 kHz. The mechanism of the loss of each turn and the distribution of the transport current of each turn are clarified. It is found that there is a frequency dependence of the distribution of both the loss and the transport current in each layer of each turn, and this distribution is highly dependent on the spatial location and the magnetic properties of the substrate.

NdFeB rare earth permanent magnets are widely used in permanent magnet motors due to their high remanence, high coercivity and high magnetic energy product. However, their relatively poor thermal stability and low senice temperature limit the application of NdFeB magnets. The coercivity can be effectively improved by doping with heavy rare earths to compensate for the flux loss at high temperature. However, the antiferromagnetic coupling between heavy rare earth atoms and Fe atoms will also reduce the remanent magnetization and magnetic energy product of NdFeB magnets. The influence of heavy rare earth doping on the remanence and magnetic energy product of NdFeB magnets can be reduced by process selection and process optimization control while increasing the coercivity as much as possible. The improvement of the performance of heavy rare earth doping can effectively avoid the local demagnetization of magnets, thus expanding its application prospects and potential in permanent magnet motors.