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To cope with the DC fault isolation issue of a hybrid high-voltage direct-current (HVDC) system, a promising option is to coordinate superconducting fault current limiter (SFCL) and DC circuit breaker (DCCB). In this paper, a multi-objective Pareto optimization method is proposed for the combinatorial use of a resistive-type SFCL and a DCCB in a hybrid HVDC. Firstly, theoretical modeling and coordinated action process of the SFCL and DCCB are expounded. Then, a multi-objective function that collaboratively minimizes the SFCL resistance and the breaking capability of the DCCB is established, and a non-dominated sorting genetic algorithm-II (NSGA-II) is utilized to acquire the satisfying Pareto optimal solution set. The optimizing calculation is done in a 320 kV hybrid HVDC for validating the approach efficacy. Besides, transient simulations are implemented by PSCAD/EMTDC, and comprehensive performance analyses are executed to evaluate the rationality of the combinatorial optimal schemes. The findings indicate that the DC fault current is reduced and interrupted with the minimal rating of the resistive-type SFCL and the DCCB, and also, the HVDC system's safe and stable operation will be ensured. Therefore, the proposed approach is adequately verified for its availability and feasibility.

For a voltage-source-converter (VSC) based high voltage direct current (HVDC) system using modular multilevel converter (MMC) topology, it is vital to address the emergency DC fault current problem, and superconducting fault current limiter (SFCL) is a very serviceable and competitive solution. This paper proposes a performance evaluation approach for applying SFCL in MMC-HVDC transmission system. Firstly, according to different action stages of SFCL and DC circuit breaker (DCCB), the fault development process of the MMC-HVDC is elaborated. The fault current peak, dissipated energy of DCCB and fault clearance time are probed, and the critical limiting impedance is theoretically deduced. Then, three aspects of electrical stress, thermal stress and fault handling speed are specially considered, and a performance evaluation system with multiple indicators is built to assess SFCL functions. Using MATLAB, the performance assessment of a resistive SFCL and an inductive SFCL in a 320 kV/1.6 kA MMC-HVDC system is done. From comparative analyses, it is concluded that the resistive SFCL overall outperforms the inductive SFCL, and while the critical current-limiting resistance and inductance are gained. The suitability and feasibility of the proposed performance evaluation approach are validated.

This paper proposes an improved flux-coupling-type superconducting fault current limiter (FC-SFCL) with multiple working modes, and investigates the efficacy of the improved FC-SFCL in guarding a medium-voltage wind turbine generation system (WTGS). The improved FC-SFCL is mainly consisting of a coupling inductance with superconducting and conventional coils, two high-speed switches, a shunt resistance and an additional limiting resistance. Under normal circumstances, the improved FC-SFCL shows little impedance to not disturb the system. In terms of fault severities, the two high-speed switches are flexibly and separately controlled to change the current paths, and the improved FC-SFCL can appear two-level limiting impedances to affect the system's transient response. The performance validation of the improved FC-SFCL in a 35 kV class WTGS is done. From the theoretical derivation and simulation assessment, the results well testify the effectiveness of the improved FC-SFCL. The fault currents in the WTGS are timely suppressed to reasonable levels, and the fluctuations in the power and torque of the WTGS are visibly reduced. A beneficial voltage quality improvement is to maintain the grid connection of the WTGS and make it ride through the serious short circuit fault successfully.

The fault current limiter (FCL) based on high coupled split reactor (HCSR), namely HCSR-FCL, is a new technology developed in recent years to realize short-circuit current suppression and interruption. This paper investigates the impact of the internal ground fault on the HCSR-FCL, based on the analysis of its basic principle and internal structure, and proposes two improved schemes for the wiring mode and the current-transformer (CT) configuration. Through the theoretical and simulation analysis, the performance of the original scheme and the two improved schemes under various internal ground fault cases are compared. The results show that when the HCSR-FCL of the original configuration scheme is in the current-sharing state and its switch-arm (SA) has an internal ground fault, the current-limiting arm (CA) will lose its shunting function, and the fast switch has to cut off the short current more than twice of the normal fault and bear the excessive voltage, threatening the safety of the FCL and the stability of the system. Both of the two improved schemes can solve this problem, and ensure the reliability and the effectiveness of the FCL, to provide a reference for the development and engineering application of the HCSR-FCL.

Virtual synchronous generator (VSG) techniques are friendly applied in inverter interfaced renewable energy sources (IIRESs), and that enables to recover the rotor inertia and provide the damping support. However, the VSG generally owns weak overcurrent ability, and its transient angle stability (TAS) under faults may be destroyed. For solving the issue, This paper puts forward a coordination method of a resistive superconducting fault current limiter (SFCL) and an additional power controller (APC). The relevant mathematical model is built, and the TAS is probed based on the extended equal area criterion (EEAC). In the coordination philosophy, the SFCL represses the fault current and prevents the disconnection of the VSG, and thereupon the designed APC is to actively adjust the VSG's power reference and positively reduce the acceleration of the virtual rotor. It is aimed to fully realize the transient power balance, and the change of the virtual rotor angle can be mitigated. Furthermore, the MATLAB platform is adopted to create a simulation model. Multiple fault scenarios are simulated, and the findings certify the efficacy and superiority of the proposed approach. The VSG successfully rides through the serious and moderate faults, and meanwhile, its transient angle stability is visibly increased.

Building multiple microgrid (multi-MG) clusters is conducive to accessing high penetration of renewable generations, consuming electric energies locally, and reducing operating losses. Nevertheless, it causes a challenge that the interconnectivity of multi-MG clusters may be seriously affected by short-circuit faults. To figure out the issue, this paper puts forward a solution of using flux-coupling-type superconducting fault current limiters (SFCLs), which are positioned at the points of common coupling (PCCs) among the multi-MG clusters and the distribution system. Considering the power exchange and balance under faults, the interconnectivity characteristics of the multi-MG clusters are theoretically analyzed, and the potential actions of the SFCLs to boost the interconnectivity are expounded. Using MATLAB, a modified IEEE 13-node distribution system containing multi-MG clusters and flux-coupling-type SFCLs is modeled, and different fault scenarios are simulated to assess the efficacy and suitability of the proposed solution. From the findings, the SFCLs can very usefully relieve the power fluctuation and mitigate the PCC voltage drop for the multi-MG clusters. Not only the operation of tripping off the multi-MG clusters is efficaciously avoided, but also a stronger power support capability withstanding the fault rush is appreciatively obtained.

Power electronic transformers (PETs) can be used to interconnect different voltages and forms of electrical systems, and enable to offer higher operation efficiency and flexible power flow controllability. This paper focuses on the fault ride-through (FRT) problem of a PET interconnecting medium voltage (MV) and low voltage (LV) power systems, and the solution of a resistive superconducting fault current limiter (SFCL) is proposed. Firstly, theoretical modeling of the PET and the SFCL is addressed, and a suitable control strategy is applied to realize reliable power exchange between the LV and MV systems. Then, in regards to a fault at the PET's output-terminal, the fault developing process is probed, and considering the SFCL's quench behavior, the PET's current variations are clarified. Further, a 3 MW PET with a SFCL is modeled in MATLAB/Simulink, and through changing the fault severity and the SFCL parameter, different simulations are implemented. The results reveal that the SFCL stabilizes the PET from multiple aspects. A visible current-limiting function is achieved, and the voltage dynamic fluctuations at the AC and DC sides of the PET are effectively alleviated. Meanwhile, the power delivery from the LV to the MV system is smoothed, and a successful FRT fulfillment is gained.

DC distribution system (DCDS) has many preferable advantages to integrate renewable energy sources and native-DC loads than AC distribution system (ACDS), and the benefits of higher power density, greater energy conversion and wider supply radius can be obtained. In this paper, a methodology of sizing the techno-economic of a medium-low voltage DCDS is proposed. Firstly, the system framework and its equipment composition are stated. Then, considering the distribution capacity, supply ability, operation loss and reliability, the technical indicators of the demonstrated DCDS are analyzed and quantified. In regards to the DCDS's distribution-level and consumer-level, the calculation models of the annual costs are presented. Further, different scenarios are imitated to verify the efficacy of the proposed sizing approach, and the techno-economic comparison is implemented in the DCDS and ACDS with similar structure configurations. The calculation results reveal that the DCDS shows a visible economic performance at the consumer-level, and for the scenario of accessing high proportion DC loads, it is more cost-effective than the ACDS. Finally, the correctness of the cost models is proven, and this study can offer a reliable reference for building the medium-low voltage DCDS.

Sensitive magnetometers that can operate in high magnetic fields are essential for detecting magnetic resonance signals originating from small ensembles of quantum spins. Such devices have potential applications in quantum technologies, in particular quantum computing. We present a novel experimental setup implementing a differential flux measurement using two DC-SQUID magnetometers. The differential measurement allows for cancellation of background flux signals while enhancing sample signal. The developed protocol uses pulsed readout which minimizes on-chip heating since sub-Kelvin temperatures are needed to preserve quantum spin coherence. Results of a proof of concept experiment are shown as well., Comment: 5 pages, 6 figures, 1 table

In practical applications, the critical current of high temperature superconducting (HTS) tapes is usually influenced by different magnetic field magnitudes and different relative magnetic field orientations. In order to design HTS power devices accurately, it is vital to study the critical current behavior of HTS tapes under different magnetic field. In this work, a series of experiments have been performed to study the critical current characteristics of seven types of commercial HTS tapes from Sumitomo, AMSC, SuperPower and SuNAM at 77 K, low-strength and 0–90° magnetic field. The experimental results and analysis have been introduced in details, including the field angle dependence of critical current of the HTS tapes. The normalized critical current ranking and anisotropy index of seven types of commercial HTS tapes are given. Furthermore, a method for estimating critical current at some magnetic field angles based on the maximum and minimum critical current values of the HTS tapes is proposed.

Directing at the robustness increase of a high voltage direct current (HVDC) system with line commutated converter (LCC) and voltage source converter (VSC), this paper proposes to coordinate resistive superconducting fault current limiters (SFCLs) and hybrid-type DC circuit breakers (HDCBs) for dealing with the DC line fault. The theoretical modeling of the SFCLs is presented, and the DC fault current characteristics are analyzed. Considering that the fault interruption involves multi-stages, the coordination of the SFCL and the HDCB in the LCC/VSC station is elaborated. Using PSCAD/EMTDC, the simulation model of a 320 kV HVDC system is built. Changing the SFCL size and the HDCB operation delay is simulated, and the performance indexes such as the fault current level, breaking time and dissipated energy are compared. The proposed coordination is verified to be very effective in handling the fault. Especially for the VSC station, it enables to visibly limit the fault current rising, shorten the transient voltage duration, and facilitate a significant decrease in the dissipated energy of the HDCB. For the LCC station, the proposed coordination can serve as a competitive backup to ensure a timely fault breaking. Consequently, the DC line fault is removed more rapidly and reliably.

Yttrium Barium Copper Oxide (YBCO) tape has a superconducting layer with a thickness of 1 μm. In the actual operation of YBCO superconducting devices, they may be subjected to mechanical strains produced by electromagnetic and thermal forces. In some cases, these forces may repeat tens of thousands times during their lifetime. We designed and built a cyclic strain test apparatus for HTS tapes, and tested the critical current degradation characteristics of YBCO tapes with two different stabilizing layers. Most of the test samples show extremely small critical current degradation even after 60000 cyclic mechanical strains. But the tapes with stainless steel layer are more likely to suffer from thermal stability problems. Increased current density was observed in the lateral direction when magnetizing the degraded samples.

To solve the low-voltage ride-through (LVRT) issue of a virtual synchronous generator (VSG) under severe grid fault, our research group proposes to use a modified flux-coupling-type superconducting fault current limiter (SFCL). In this study, the configuration structure of the VSG with the SFCL at the point of common coupling (PCC) is designed. The fault analytical circuit of the VSG is presented, and the functions of the SFCL are stated from the aspects of current restraint, voltage compensation, and energy dissipation. Using MATLAB/Simulink, symmetrical and asymmetrical faults are both simulated to assess the transient behaviors of the VSG, and the SFCL parameters are changed to clarify the quantitative effects. From the comparison results, the SFCL parameters are properly determined to provide favorable performance. The SFCL can visibly decrease the fault current of each phase within the permissible level, and facilitate the VSG to have a preferable voltage profile. Furthermore, the SFCL can positively stabilize the VSG by enhancing the power delivery, mitigating the frequency fluctuation and consuming the excess energy. A satisfactory LVRT operation is gained, and the efficacy of the SFCL on reinforcing the VSG robustness is validated.

Besides its well-known magnetic sensitivity, the nano superconducting quantum interference device (SQUID) has an almost instantaneous response time. However, transferring an ultrafast magnetic signal to a nano-SQUID is a technical challenge. We describe a very efficient direct-coupling structure between the SQUID and the coplanar waveguide (CPW). The center conductor of the CPW and its grounding planes are connected by a short line. We fabricated devices having direct and indirect coupling between the short line and nano-SQUIDs. The coupling efficiencies of both schemes are compared quantitatively. By injecting microwaves of frequencies 0.1-18 GHz through the CPW, we found that microwave power of 100 nW and 158 μW is required to suppress the critical current of nano-SQUIDs for the direct and indirect-coupling devices, respectively. By applying a dc current to the short line, we were able to measure the flux-modulation curves of SQUIDs. To achieve a flux quanta in SQUIDs requires currents of 0.46 and 11 mA for the direct- and indirect-coupling devices, respectively. Therefore, the direct-coupling scheme improves the efficiency by a factor of 39 from 0.1 to 18 GHz and 24 at dc, respectively. Furthermore, while sending a pulse-modulated signal of 1 μW power, a SQUID incorporating the direct-coupling scheme produced a working flux modulation.

Normal zone propagation velocity (NZPV) and minimum quench energy (MQE) are the important parameters for quench protection of superconducting magnets. In this research, a series of experiments were conducted on single-length conductor, small coils, and parallel double conductors made of YBCO and BSCCO high-temperature superconducting (HTS) tapes at the liquid nitrogen temperature, and their normal zone propagation velocities and minimum quench energies were tested and analyzed. The experimental results show that the NZPVs of HTS single-length tape, parallel double tapes, and coils are closely related to the critical current ratio i of HTS tapes. When i is in a certain range, the NZPV is approximately proportional to i, and similarly, MQE is inversely proportional to i.

Regarding a dc microgrid integrated with wind, photovoltaic generators, and energy storage, this paper suggests a resistive-type superconducting fault current limiter (SFCL) to protect it from short-circuit fault. The modeling and control of the dc microgrid are first stated, and then, the resistive SFCL's impacts on the dc microgrid's fault current and bus voltage sag are analyzed in brief. Using MATLAB, simulations on different SFCL parameters are done, and the results confirm the SFCL's contributions for the dc microgrid include the following. First, the dc fault current is limited quickly and effectively. Second, the bus voltage drop is compensated enough to ensure that the renewable sources achieve the fault ride-through operation. Third, the power transfer efficiency of the dc microgrid under the fault is appropriately enhanced. Finally, the resistive SFCL's conceptual design is done. Not only is the magnetic field/current density distribution of the HTS tape given, but the Ampere Force and ac loss are also estimated. In summary, the feasibility of the resistive SFCL for the dc microgrid is verified.

To strengthen the fault ride-through support for a doubly-fed induction generator (DFIG)-based wind turbine, this paper studies the combined use of a resistive superconducting fault current limiter (SFCL) and a superconducting magnetic energy storage (SMES). The installation sites of the SFCL and SMES are the DFIG's stator and dc-link, respectively. The SFCL offers the expectations of current-limitation and terminal-voltage compensation, and the SMES brings the functions of controlling dc-link voltage and absorbing unbalanced power. The theoretical modeling, control method and influence mechanism are studied, and the simulations of symmetrical and asymmetrical faults are done in MATLAB. From the results, the contributions by the combined SFCL and SMES include: First, the overcurrents in the stator and rotor of the DFIG are quickly suppressed. Second, the variations in the DFIG terminal voltage and dc-link voltage are well reduced. Third, the unbalanced power and electromagnetic torque fluctuations of the DFIG are availably mitigated. Finally, a preliminary capacity optimization considering multiple energy features is discussed to further promote the proposed approach.

A 150 kJ/100 kW conduction-cooled HTS superconducting magnetic energy storage magnet, which consists of 17 double pancake coils and operates at 20 K, was developed and manufactured in the Huazhong University of Science and Technology. To evaluate the thermal characteristic of the magnet, this paper conducts a series of experiments and carries out the loss analyses. When the maintaining time and peak value of the flat top current in the magnet are constant, the maximum temperature rise will first decrease and then increase with the continuous augment of the current ramp rate, and it means that there is a valley value for the maximum temperature rise during an operational cycle time including charging, standby, and discharging stages. Similarly, the total losses, which include the ac heat loss and the resistive loss caused by the joints between every two double pancakes and extra resistance of the high temperature superconducting (HTS) coils, will fall with the increase of the ramp rate to its valley value, and then it will increase with the ramp rate. Experimental results show that the temperature variation of the magnet is in keeping with the heat loss variation.

As superconducting magnetic energy storage (SMES) and battery are complementary in their technical properties of power capacity, energy density, response speed, etc., this paper proposes an SMES-battery energy storage system to stabilize a photovoltaic-based microgrid under different faults. The related theoretical modeling is stated, and the control and coordination methods of the SMES-battery are put forward. Using MATLAB, a comparison of with the SMES-battery and only with the battery is carried out. From the main specifications of the SMES magnet, the ac-loss calculation is also performed. The results show that i) the SMES-battery is better than the battery to timely deal with the transient faults of the microgrid; ii) the SMES-battery enables to ensure a seamless mode-transition for the microgrid under the external fault, and reduce the fault current in the point of common coupling to avoid an unnecessary off-grid under the internal fault; and iii) the SMES magnet's ac-loss power has a maximum value of 380 W, and it is acceptable for the future design of conduction-cooled structure and cryogenic system.

It is widely known that the power supply would be interrupted during mode switching between grid-connected and islanded operation in a microgrid, which might lead to voltage and frequency fluctuations of the microgrid. As a power-type energy storage device, superconducting magnetic energy storage (SMES) is capable of providing rapid power response for either charge or discharge within a few milliseconds. In order to study the feasibility of applying SMES to microgrid, a microgrid model with SMES was built with Matlab/Simulink platform, and the compensation performance of SMES on the power fluctuation of microgrid was simulated and analyzed. The compensation power and current of the SMES were obtained. In this regard, a conceptual design of the SMES using high-temperature superconducting (HTS) double-pancake coils composed of YBCO tape was proposed. The loss properties were evaluated based on a finite element model. The results demonstrate that HTS SMES could realize the seamless switching and provide uninterrupted power supply for the microgrid. Moreover, the energy storage system can also keep safe and stable operation during power compensation.

In this paper, a flux-coupling-type superconducting fault current limiter (SFCL) is suggested to protect a 15-MW class doubly fed induction generator (DFIG)-based wind farm. Detailed conceptual design and performance evaluation of the SFCL are conducted. From the technical requirements and operational parameters of the DFIG wind farm, a 35-kV/500-A flux-coupling-type SFCL is designed, taking into consideration: 1) the winding type, core selection, and inductance values of the coupling transformer (CT); and 2) the structural style, tape length, and quenching resistance of the superconducting coil (SC). By use of different simulation tools, the electrical and electromagnetic characteristics of the SFCL are evaluated. The results show that the maximum magnetic field of the CT is 2.4 T, and the ac loss of the SC is 0.51 W. Not only the electromagnetic properties and loss of the SFCL are acceptable, but also using the SFCL in the wind farm enables to suppress the fault current, compensate the voltage sag, and mitigate the wind power fluctuation. Thus, the robustness of the DFIG-based wind farm against the fault is enhanced, and the effectiveness of the proposed SFCL design is verified.

This paper suggests a small-scale superconducting magnetic energy storage (SMES) to enhance the transient behaviors of a 100 kW grid-connected photovoltaic (PV) system, and conducts the conceptual design and performance evaluation. Considering the PV system requirements, the stored energy of the SMES is 90 kJ, and the YBCO tapes are adopted to make a solenoid SMES magnet. Based on the genetic algorithm, the magnet parameters including critical current, tape length, parallel and perpendicular magnetic fields are optimized. Using simulation tools, the effects of the SMES on the PV system are assessed, and the electromagnetic properties, stress, and loss of the SMES magnet are analyzed. The results prove that 1) the SMES improves the fault-ride-through capability and realizes an effective protection for the PV system. 2) A relatively large perpendicular magnetic field is found in both ends of the SMES magnet, and the maximum field is induced in the middle of the SMES magnet. 3) The maximum stress of the SMES magnet is acceptable, and the mechanical strength is satisfied. 4) The ac loss is crucial for voltage maintenance performance of the SMES at the fault. Under a short clearance time, the heat accumulation may have a moderate controllability.

To model the nonlinear characteristic of the flux linkage for the switched reluctance motor, a modified flux linkage analytical model and a fast model are proposed. The models are based on an original polynomial model which fits the ψ - θ curve at different phase currents by analyzing the error percentage curve of the original model in this paper. The coefficient of the polynomial is discussed for a fast model in order to solve the modeling problems quickly. Then, the electromagnetic torque of switched reluctance motor is calculated by the modified model and the fast model, which agree well with the finite element results. Based on the modified model, the simulation model is built in MATLAB/Simulink. Also, an experimental platform is established to validate the proposed models.

Controlled reactors can adjust the output capacity to stabilize system voltage, control reactive power, and improve system stability according to operating conditions. The design of reactance and control system determines whether a controlled reactor can meet the requirement of stable operation. In this paper, the operating principle of a high temperature superconducting leakage flux-controlled reactor (HTS-LFCR) is introduced. The design of the reactance and the optimal initial turn-on angle of the control system are discussed. An HTS-LFCR is fabricated and tested. The experimental results show that the reactance design values are basically consistent with the experimental tests; when the initial turn-on angle of the control system is 90°, the reactor can achieve a smooth transition from one operating mode to another.

For a high penetration microgrid, improving its low-voltage ride-through (LVRT) capability under some minor or temporary faults can contribute to reinforcing power support and reducing network instability. In this paper, the comparison of a superconducting fault current limiter (SFCL) and a dynamic voltage restorer (DVR) for LVRT capability enhancement of a 10-kV microgrid is conducted. Concerning the microgrid which includes distributed photovoltaic generation, energy storage and loads, the effects of the SFCL and the DVR are compared in detail, and related theoretical analysis, simulation study, and economical evaluation are carried out. From the demonstrated results, the suggested two devices can both assist the microgrid to achieve the LVRT operation, but the economics of the DVR are weaker than the SFCL. Moreover, compared to the DVR, the SFCL enables the energy storage unit to offer better control effects in power stabilization. From technical and economic perspectives, the SFCL is a more preferable choice than the DVR.

Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current–voltage characteristics in the absence of any shunting resistance.

With the development of modern power systems, the occurrence of fault current and system sensitivity to fault current have increased. Along with extensive damage to network hardware, considerable consumer losses can result from fault current events. However, with the extensive application of voltage source converter based power systems, large dc fault current becomes an important issue due to the bottleneck of the capacity of the dc breaker. To provide effective dc power system fault protection, a novel dc superconducting current fault limiter (dc SFCL) was designed. Here, the working principles and the mathematical model of the designed SFCL are presented. The simulation test based on SIMULINK and the demonstrator experiment were performed to confirm the correctness of the theoretical analysis. The test results show that the dc SFCL exhibits a rapid response time and good current limiting performance in dc systems.

Concerning that a short-circuit fault happens inside a microgrid system, parts of distributed generation (DG) units may not meet the fault ride-through (FRT) requirements and will be enforced to disconnect. This paper suggests a modified flux-coupling-type superconducting fault current limiter (SFCL) to improve the DG's FRT capability, and investigates the relay protection coordination in the microgrid. The SFCL's structural principle is introduced, and according to the fault characteristic of the microgrid under its grid-connected and islanded statuses, the modified SFCL's application fields are conducted. Furthermore, the directional overcurrent protection and differential protection are proposed for the microgrid's two statuses, respectively. Based on the transient simulations in the MATLAB, the SFCL's positive effects on enhancing the DG's FRT capability can be confirmed, and also it will not affect the relay protection coordination. A comparison of the conventional distance relay protection and the proposed protection scheme is performed, and the latter has a better applicability than the former to match the SFCL. Finally, regarding the impacts of fault type and fault resistance on the FRT operation and the proposed protection, technical discussions are performed preliminarily, and the application value of the SFCL integrated with the proposed protection is clarified further.

For that wind plants are connected to an electric power grid through voltage source converter-based high-voltage direct-current (VSC-HVDC) transmission, the improvement of the VSC-HVDC system's robustness against a dc short-circuit fault is critical. This paper suggests the inductive and resistive superconducting fault current limiter (SFCL) to alleviate the fault current and enhance the system's transient behaviors. Each of the two SFCLs is installed in series with the dc transmission line, and once the fault is detected, the current-limiting inductance or resistance will be activated to affect the system characteristics. Herein, wind plants are based on doubly-fed induction generators, and related theoretical derivation and transient simulation analysis are done. From the results, both of the two SFCLs can limit the dc fault current, improve the dc-voltage sags and suppress the power fluctuations. By comparing the specific performance indexes including current-limiting ratio and restraining capability to voltage/power fluctuations, the resistive SFCL is more preferable than the inductive SFCL. At the end, the SFCL optimization technique considering current-limitation, voltage stability, and device cost are discussed, so as to further promote the SFCL's application in the VSC-HVDC system.

In this paper, a resistive-type superconducting fault current limiter (SFCL) is suggested to improve the transient performance of a microgrid system during a fault. The microgrid is connected to the main network at the point of common coupling, where the resistive-type SFCL is applied. When a short-circuit fault happens at the connecting line, the SFCL can mitigate the fault current, and its action signal will be sent to the master distributed generation (DG) included in the microgrid. Accordingly, the switching between the master DG's two control patterns can be flexibly performed; furthermore the microgrid system is expected to achieve a smooth transition between its grid-connected and islanded modes. Theoretical analysis and a technical discussion are conducted, and the simulation model of a typical microgrid with the SFCL is built in MATLAB. From the demonstrated results, employing the resistive-type SFCL can effectively limit the transient fault current to a lower level, help guarantee the microgrid system's power balance, and enhance its voltage and frequency stability.

We fabricate $\hbox{MgB}_{2}$ ultrathin films via the hybrid physical–chemical vapor deposition (HPCVD) technique. With the proper background pressure and $\hbox{B}_{2}\hbox{H}_{6}$ flow rate, we fabricate a series of ultrathin films ranging from 10 to 40 nm. These films grow on the MgO (111) substrate and are all $c$ -axis epitaxial. The ultrathin films show good connectivity, very high $T_{c}(0) \approx 35{-}38\ \hbox{K}$ , and very low residual resistivity $\rho (42\ \hbox{K}) \approx 1. 8{-}20. 3\ \mu \Omega \cdot \hbox{cm}$ . With the increase in the film thickness, the critical transition temperature increases, and the residual resistivity decreases. In addition, the 10-, 20-, and 40-nm films show extremely high critical current density, i.e., $J_{c} \approx 2. 0 \times 10^{7}$ , $2. 3 \times 10^{7}$ , and $2. 8\times 10^{7}\ \hbox{A/cm}^{2}$ at 5 K, respectively, which indicate that the films fabricated by HPCVD are well qualified for device applications.

For a power distribution system with distributed generation (DG) units, its fault current and induced overvoltage under abnormal conditions should be taken into account seriously. In consideration that applying superconducting fault current limiter (SFCL) may be a feasible solution, in this paper, the effects of a voltage compensation type active SFCL on them are studied through theoretical derivation and simulation. The active SFCL is composed of an air-core superconducting transformer and a PWM converter. The magnetic field in the air-core can be controlled by adjusting the converters output current, and then the active SFCLs equivalent impedance can be regulated for current-limitation and possible overvoltage suppression. During the study process, in view of the changes in the locations of the DG units connected to the system, the DG units injection capacities and the fault positions, the active SFCLs current-limiting and overvoltage-suppressing characteristics are both simulated in MATLAB. The simulation results show that the active SFCL can play an obvious role in restraining the fault current and overvoltage, and it can contribute to avoiding damage on the relevant distribution equipment and improve the systems safety and reliability.

The current distribution and stability of low temperature superconductor (LTS) and high temperature superconductor (HTS) hybrid superconductor, made of NbTi wire and YBCO coated-conductor, are described. The current distribution, quench propagation velocity (QPV) and the minimum quench energy (MQE) are numerically analysed by finite element method (FEM) based on the different power-law models since the value in LTS is much larger than that in HTS. It is shown that the transport current in LTS is larger than in HTS if both of them have the same critical currents, and the ratio of transport current in LTS to that in HTS decreases with increase of the total transport current near their critical currents. The stability of the hybrid NbTi/YBCO composite conductor is also simulated, which indicates that MQE in the hybrid conductor is much higher than in NbTi wire but smaller than in YBCO conductor. However, the quench propagation velocity is in the range of low-temperature superconductor through the high-temperature superconductor. With advantages of high engineering current density and improvement of stability, the hybrid superconductor is expected to have potential application in large scale magnet and conducting-cooled magnet particularly.

Superconducting fault current limiters (SFCLs) offer a solution to control and suppress fault current levels on utility distribution and transmission networks, and are believed to be one of the most exciting applications of superconductors in power systems. Consequently, the various types of SFCLs have been developed in many countries. But, there is a very long way for SFCLs from prototypes or paper concepts to the practical applications and commercialization because of many unsolved problems. In this paper, a novel flux-coupling type SFCL is presented. This SFCL is composed of two superconducting windings and a controllable switch. In order to validate its fault current limiting performance, a simulation for the operation of this kind of SFCL in a modeling power system has been done. The simulation results show that it is feasible technically. Based on that, the economic feasibility by introducing a flux-coupling SFCL into the high voltage grid is estimated. Finally, the optimal locations for SFCLs are proposed.

A novel flux-coupling type superconducting fault current limiter (SFCL) is proposed in this paper. This type SFCL is composed of two superconducting coupling coils, a controlled switch and two metal oxide arresters (MOAs). The primary coil and the secondary coil, which are wound in the reverse direction and are respectively in parallel with the two MOAs, are connected in parallel through the controlled switch and a main circuit breaker. Under normal condition, due to the non-inductive coupling characteristics between the two superconducting coils, the impedance of the proposed SFCL will be too small to affect the main circuit. After the fault happens, the controlled switch will be opened from its initial closed state, and meanwhile the MOAs will prevent the two coils from being damaged by the overvoltage caused by the switching operation. Since the flux between the coils cannot cancel out each other, the non-inductive coupling will disappear, and then the limiting impedance will be triggered for current-limitation. In order to evaluate the current-limiting performance of the novel flux-coupling type SFCL, a laboratory scale prototype was made. The two superconducting coils were fabricated with Bi2223/Ag, and an anti-parallel IGBT pair was chosen as the controlled switch. According to the test results, it was found that, the fault current could be limited quickly and effectively, and there was no overvoltage during the operation process of the controlled switch.

This paper presents a superconducting magnetic energy storage (SMES) based dynamic voltage restorer (DVR) to protect consumers from the grid voltage fluctuations. Due to the characteristic of high energy density and quick response, a superconducting magnet is selected as the energy storage unit to improve the compensation capability of DVR. This paper analyses the operation principle of the SMES based DVR, and designs the DVR output voltage control method. The control system mainly consists of two parts, the PWM converter controller and the DC/DC chopper controller. The PWM converter controller adopts double-loop control strategy, with an inner current regulator and an outer voltage controller. Combining the coordinated control of DC/DC chopper, the DVR can regulate output voltage accurately and quickly to compensate the system voltage fluctuations. Using MATLAB SIMULINK, the models of the SMES based DVR is established, and the simulation tests are performed to evaluate the system performance.

The three-phase voltage compensation type active superconducting fault current limiter (SFCL) is composed of three air-core superconducting transformers and a three-phase four-wire PWM converter. The primary winding of the air-core superconducting transformer is in series with AC main circuit, and the second winding is connected with the PWM converter. The electromagnetic condition in the transformer can be changed by the compensating current offered by the PWM converter. In this paper, the electromagnetic characteristics of the single phase air-core transformer are calculated by the circuit-field coupled time-stepping finite-element method. The magnitude of the magnetic field and current, phase difference between magnetic fields and current and loss characteristic under different compensating current are studied. The analysis results are very important for the establishment of the control strategy and the transformer's design in the engineering realization.

In this paper, a high-temperature superconducting (HTS) loss-free delay network for high-power pulses is performed. Based on the voltage and current pulse experiments carried out at 300 and 77 K, experimental results demonstrate that, under the matched resistive load condition, compared with a normal conducting lossy delay network, the HTS loss-free delay network can effectively reduce the amplitude attenuation of voltage pulses and the front-edge distortion of current pulses.

This paper presents a new topology of the active superconducting DC fault current limiter (DC-SFCL) applied in the hybrid AC/DC power supply systems. The proposed DC-SFCL is composed of superconducting transformer, DC chopper and voltage source converter (VSC). The primary winding of the transformer is connected in series with DC transmission line, and the second winding is connected with AC power system through the VSC and the DC chopper. The active DC-SFCL can suppress the fault current of DC system through power exchange between the DC and the AC system. Moreover, the DC-SFCL can act as an active power filter for both the DC system and the AC system to solve the problem of power quality. Using MATLAB SIMULINK, the simulation model of the hybrid AC/DC power supply systems with the proposed DC-SFCL is created. Simulation results validate the dynamics of system, and the performance of the active DC-SFCL is confirmed.