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Large-scale electric aircraft is a disruptive technology to address the environmental impact of air travel. Fault current limitation is crucial to realise the safety and reliability of the electric aircraft, in particular for large-scale electric aircraft using DC distribution network. This paper investigates the behavior of the resistive SFCL under different cryogenic cooling systems including liquid nitrogen and helium gas circulation cooling systems for the electric aircraft DC distribution network. Electromagnetic and thermoelectric models were used to predict the characteristics of resistive SFCL. The simulation results demonstrate that it is promising to design resistive SFCL using liquid nitrogen cooling as well as helium gas circulation cooling systems, which offer flexibility for SFCL cryogenic cooling system for electric aircraft applications.

The HTSConductor on Round Core (CORC) Cables have been widely recognized as the proper candidate for superconducting applications such as the high-field magnets and power transmission cables. This article presents the modeling of HTS CORC cables using the FEM H -formulation model merged into the package COMSOL Multi-physics. The 2D and 3D H -formulation models of HTS CORC cables were compared and analyzed. There were two important parameters in our CORC design: (1) the gap angle between the HTS tapes in a single layer, and, (2) the degree of twist for HTS tapes (normally identified as the pitch). The 2D CORC model has much faster solving speed over the 3D model and sometimes is able to model the AC loss in a reasonable range, while, the 3D CORC model has the advantages over 2D CORC model with respect to more information in each sector with the different conditions of two parameters above, and more realistic loss calculation.

Flux diverters (FDs) are used in High Temperature Superconducting (HTS) transformers for AC loss reduction and flux optimization. In this paper, a 2D axial symmetric superconducting winding model is proposed and two designs of flux diverters are applied to the windings. A homogenization approach is used to analyze the windings with large turn numbers. The key parameters including the number, width, height and the spatial positions of the FDs are adjusted for AC loss and magnetic flux analysis. The means of obtaining optimum designs of the FDs is provided and can be used to develop new winding designs with FDs, which contributes to better electromagnetic performance and higher efficiency of HTS transformers.

Impulse breakdown voltage is one of the most important factors for the designation of the R-SFCLs. However, no research about the polarity effect on standard lightning impulse in LN2/insulation barrier composite systems has been reported. The objective of this paper is to study the polarity effect on the standard lightning impulse in the LN2/insulation barrier composite systems. The 50% impulse breakdown voltage was measured with a pair of the needle to plane electrodes with the 0.05 mm thick PTFE film. The gap length of the two electrodes was 10 mm. The distance between the needle electrode and the insulation barrier was varied. The time delays during the breakdown progress were recorded by the oscilloscope. The breakdown path was determined by the arc traces on the PTFE films. The results showed that the 50% impulse breakdown voltage of the LN2/insulation barrier composite increased with the insulation barrier get closer to the needle electrode. The results of the time delays reflect that in negative polarity, all the breakdowns occurred in the tail of the waveform. In positive polarity, most of the breakdowns occurred in the head of the waveform. With the barrier get closer to the insulation barrier, the longer time delay T delay was in both polarities. And the probability of wave tail breakdown increased in positive polarities. The arc traces on the PTFE showed that all the breakdowns were occurred through the PTFE film.

Resistive type fault current limiters (SFCLs) have attracted lots of attention from research interests and engineering applications, due to their self-triggering, fast and effective fault current limitation ability. Fast recovery of SFCL after quench is highly attractive, yet still remains challenge when recover with load compared to recovering without load. We investigated the dependence of recovery time of a helical SFCL coil on the ratio of prospective current over critical current I pros/ I c. A multilayer simulation model was built in MATLAB/Simulink to investigate the electro-thermal behavior of SFCL coil during a fault and recovery period, and the model was validated by experimental results. Quench simulations were carried out under different I pros/ I c, ranging from 15 to 119 and in different recovery conditions including recovery with load condition and recovery without load condition, while keeping the fault duration of 100 ms. We observed that the SFCL coil reached 85 K and 195 K at the time when the fault was cleared, in the quench tests with I pros/ I c = 15 and 119, respectively. SFCL coil recovered without load in 0.5 s to 10 s, depending on the ratio of I pros/ I c.

Utilizing renewable energy sources (RESs) to their full potential provides an opportunity for lowering carbon emissions and reaching a state of carbon neutrality. DC transmission lines have considerable potential for the integration of RESs. However, faults in DC transmission lines are challenging due to the lack of zero-crossing, large fault current magnitudes and a short rise time. This research proposes using a superconducting fault current limiter (SFCL) for effective current limitation in PV-based DC systems. To properly design an SFCL, the present work investigates the effect of copper stabilizer thickness on SFCL performance by using an accurate multilayer thermoelectric model. In the MATLAB/Simulink platform, the SFCL has been modeled and tested using different copper stabilizer thicknesses to demonstrate the effectiveness of the SFCL model in limiting the fault current and the impact of the copper stabilizer thickness on the SFCL’s performance. In total, four different thicknesses of the copper stabilizer were considered, ranging from 10 μm to 80 μm. The current limitation and voltage profile for each thickness were evaluated and compared with that without an SFCL. The developed resistance and temperature profiles were obtained for various thicknesses to clarify the mechanisms behind the stabilizer-thickness impact. An SFCL with an 80 µm copper stabilizer can reduce the fault current to 5.48 kA, representing 71.16% of the prospective current. In contrast, the fault current was reduced to 27.4% of the prospective current (2.11 kA) when using a 10 µm copper stabilizer.

Previous studies on magnetization AC loss of high temperature superconducting (HTS) coated conductors (CCs) taken into account the ideally sinusoidal external magnetic field, or non-sinusoidal by considering only the harmonics in phase with the fundamental component. However, realistically magnetic field often contains harmonics with different phase angles and amplitudes. In this paper, magnetization AC loss in HTS CC was studied, considering the phase angle $\varphi $ of each harmonic; assuming superconductor is subjected to external magnetic field which is distorted by the $3^{\mathrm {rd}}$ and the $5^{\mathrm {th}}$ harmonics. Phase angle of each harmonic was considered in the range of 0 to $\pi $ and amplitude of fundamental magnetic field, was assumed as 10, 20, 50, and 100 mT. Different distortion level of magnetic field was considered too. It is concluded that phase angle of field harmonic has a significant effect on magnetization loss of HTS CCs.

HVDC circuit breakers are of increasing importance, as multi-terminal high voltage DC (HVDC) transmission becomes a commercial reality. Multiple HVDC breaker technologies have been developed, and are starting to appear as proof-of-concept installations on real networks. Information about them is however distributed widely in the literature. This article describes the underlying challenges, the leading candidate solutions, and discusses the requirements and construction of the component sub-assemblies. Its goal is to provide a comprehensive overview of the field.

An asymmetric dependence of the critical current on the direction of an applied magnetic field in HTS coated conductors has a non-trivial influence on the AC loss of coil windings. We report the modelled influence of real conductor critical current asymmetry on the AC loss characteristics of a 1 MVA HTS transformer design previously demonstrated by the Robinson Research Institute as well as a stand-alone coil having the same geometrical and electrical parameters as the low voltage (high current) winding of the transformer. We compare two commercial HTS conductors with distinctive differences in their critical current asymmetry and show a maximum variation of 15% and 29% in the calculated AC loss of the transformer and the stand-alone coil winding, respectively, when the conductor orientation is varied in the top and bottom halves of the windings. AC loss simulation giving consideration to asymmetric conductor critical current before winding the transformer could lead to substantial AC loss reduction even using the same amount of conductor and the same transformer design.

High-density power conversion and energy storage solutions were and are being explored for use in Electric Aircraft (EA). A superconducting magnetic energy storage (SMES) system is a promising candidate due to its fast response and ability to satisfy large pulse loads as is expected from EA. For the SMES, Dual Active Bridge (DAB) converters can offer high-density power conversion and provide galvanic isolation. This paper proposes a design methodology for a bidirectional DC/DC converter using DAB converter and a MOSFET-based chopper to interface SMES. This paper includes analysis, performance prediction, and needs for implementation in EA. The DAB/SMES system has been successfully designed, analyzed, and is being optimized for application in EA.

Axial-flux permanent magnet (AFPM) motors have been attracting great interest due to their key advantages of high-torque density and compact structure. Concentrated windings are commonly used for AFPM motors since they significantly reduce the radial length of the end windings. This paper proposes a novel double-sided stator single-rotor motor topology where one stator is offset by π radians. This arrangement can cancel significant space harmonics produced by the concentrated winding and reduce the core and permanent magnet losses. Analytical analysis and finite element analysis (FEA) are used to verify the principle and validate the topology. The simulation results demonstrate that this proposed double-sided offset stator motor can reduce the core loss and permanent magnet loss significantly at base speed compared with the conventional double-sided stator single-rotor motor. In addition, the magnetic core saturation and induced voltage for the double-sided offset stator motor are significantly reduced.

A nearly all-superconducting power system is considered as a promising path to realize Turboelectric distributed propulsion (TeDP) Aircraft. The options of power switches are rather limited and only cryogenic power electronics are into the scope so far. A mechanical switch using LN2 as an arc extinguishing medium (LN2 switch) can be a promising alternative that has lower conduction loss. However, basic research on current interruption in LN2 is still lacking, especially the effect of magnetic fields. The objective of this paper is to understand the effect of the magnetic field on DC current interruption by LN2. Interruptions under different magnetic fields are tested. A two-stage arcing process is observed under the magnetic field. Stage 1 is similar to that in the air while stage 2 is distinguished as interactions between vaporized LN2 and arc. 600 V DC/ 1 kA is interrupted by LN2 under 62.5 mT successfully. The interruption current is 8 times higher than that of without magnetic field applied. The magnetic field strengthens arc cooling by enhancing arc motion in LN2. Adding no burden to refrigeration, an LN2 switch can be a promising application in HTS electrical system.

High temperature superconductor (HTS) techniques can be used to design high efficiency power or traction transformers with considerable AC loss and volume reduction compared with conventional transformers. Further, HTS transformers are environmental friendly, with no risk of fire hazard or pollution. In this paper, superconductor windings of a transformer are modeled and flux diverters (FDs) are applied to the ends of windings for magnetic field and AC loss analysis. A homogenization approach is used to save computation cost of the model. The distribution of AC loss is quantized for analysis. Observations suggest that the flux diverters optimize flux distribution and are conducive to AC loss reduction.

A new switchgear named Liquid Nitrogen Insulated metal-enclosed Switchgear (LNIS) is proposed, which uses LN2 as an insulation medium to substitute SF 6 in Gas Insulated metal-enclosed Switchgears (GIS). Power loss of the LNIS is only about 1/8 of the same level GIS, making the novel switchgear a fascinating technology for future power systems. To make the switchgear more compact and efficient, LN2 is also used as an arc extinguishing medium of disconnecting switches. Thus, closed-loop current that can up to thousands of amps should be interrupted by LN2. However, AC current interruption ability of LN2 is still unknown. The objective of this paper is to obtain the AC current interruption properties of LN2. Experiments are conducted with currents ranging from 700 A to 4 kA. AC current of 2950 A (Peak value) is interrupted by LN2 successfully. The maximum arc energy during the interruption is about 437 J, far less than the Joul heat generated by the rated current flow. Arc energy during interruption doesn't burn extra burden on the cooling system. Thus, AC current interruption ability of LN 2 could meet requirements for LNIS.

Superconducting fault current limiters (SFCLs) are foreseen to be critical and promising candidates in power system applications, due to their automatic fault current limiting performance, low loss, size compactness and light weight. A helical bifilar coil which is composed of one inner winding and one outer winding was developed, and these two windings were wound on the same coil bobbin with opposite directions and connected in parallel. Experiments were carried out to characterize the fault current limitation behavior of the helical bifilar coil in flux flow mode, when the prospective fault current falls in over-critical current regime yet still within 2 times of the coil's critical current. The current limiting performance of the helical bifilar coil in flux flow mode was demonstrated and discussed.

A combination of resistive type superconducting fault current limiters (R-SFCLs) and mechanical-type DC circuit breaker is a reliable way to protect the medium voltage and high voltage DC systems. Using the insulation barrier plays an important role in enhancing the insulation strength of R-SFCL. It promotes the development of R-SFCLs to a higher voltage level. However, the bubbles effect on the DC flashover characteristics of the insulation barrier is unknown. The objective of this paper is to obtain the DC flashover characteristics of insulation film with and without bubbles in liquid nitrogen (LN2). In the experiments, a triangle-plane electrode was put horizontal and immersed in LN2. A 14 W heater was placed right below the gap of two electrodes. Negative flashover characteristics of PTFE and Kapton were studied with different gap distances. The results show that the negative flashover voltage of PTFE film increases with the increase of gap distance both in conditions with and without bubbles. The shorter the gap distance between the triangle and plane electrodes is, the stronger the influence of bubbles on the negative flashover voltage is. The flashover voltage of PTFE is 1.15 times of Kapton both in the conditions with and without bubbles. The results could be used to design the insulation structure of high voltage R-SFCLs.

The mitigation of peak-valley difference and transient power fluctuation are both of great significance to the economy and stability of the power grid. This paper proposes a novel vehicle-to-grid behavior management method that can provide peak-shaving and fast power balancing service to the grid simultaneously. Firstly, a multi-time scale vehicle-to-grid behavior management framework is designed to enable large-scale optimization and real-time control at the same time in vehicle-to-grid scheduling. Then, the grid peak-shaving requirement is modeled as an optimization problem in a centralized V2G state coordinator, where the charging behavior of all grid-connected electric vehicles can be synergistically scheduled. The optimization variable is designed as a group of vehicle-to-grid state control signals that can respond to grid peak-shaving requirements. Further, a V2G power controller is designed to manage the vehicle charging power in real time based on the sampled grid frequency state and discrete state control signals. In the developed scheduling method, the charging power of grid-connected electric vehicles is scheduled by the cooperation between the V2G state coordinator and the power controller. The effectiveness of the proposed methodologies is verified on a microgrid system, and results indicate that the V2G scheduling can achieve multi-time scale grid power balancing. This work can bring dual benefits, enabling system operators to use cheap solutions to manage energy networks and allowing vehicle owners to gain profits from providing V2G services to the grid.

Medium voltage direct current (MVDC) systems have been proposed for a variety of applications including offshore wind farm collector system and transport electrification. An efficient and reliable hybrid DC circuit breaker is critical for MVDC system protection. The operating speed of the hybrid DC circuit breaker heavily relies on the speed of the mechanical circuit breaker. Two types of actuators: moving magnet actuator and moving coil actuator using electromagnetic interaction are compared in this paper for the operation of a medium voltage vacuum interrupter. The Finite Element software package COMSOL is used to simulate the static and dynamic performance of each actuator topology. The paper focuses on comparing the advantages and disadvantages of these two actuator topologies particularly for vacuum interrupter application. The moving coil actuator is found to have better dynamic performance and is more suitable for medium voltage vacuum interrupter application.

A resistive superconducting fault-current limiter (SFCL) has been developed using round magnesium diboride (MgB2) wire. The SFCL coil was wound using an interleaved coil arrangement to minimize the total coil inductance. The SFCL coil demonstrated reliable and repeatable current-limiting properties during testing. However, the wire temperature of the SFCL coil increases quickly during quench tests, and several minutes are required for temperature recovery after the fault is cleared. The SFCL coil therefore was fully integrated with a vacuum interrupter to quickly remove the SFCL coil from the circuit once a fault occurred. This allowed the SFCL coil to recover quickly while a bypass resistor acted as the current limiting resistance. A fast-acting actuator and its control circuit were designed and built to provide automatic control for the operation of the vacuum interrupter. The SFCL with the prototype vacuum interrupter was successfully tested. The energy dissipated in the SFCL coil was significantly reduced by integrating the vacuum interrupter. The fault tests with different potential fault currents also proved that the operation of the vacuum interrupter is independent of the fault current level. This prototype demonstrated the potential of a cost-effective and compact integrated SFCL and vacuum interrupter for power system applications.

HVDC transmission systems using voltage source converters (VSC-HVDC) have been identified as an attractive solution for bulk power delivery over long distance. However if DC grids are to be built using this technology, fast DC circuit breakers are needed. This work proposes a new hybrid DC circuit breaker utilizing a series connected coupled inductor connected, which enables automatic current commutation from the mechanical switch to the semiconductor switch during a fault. The key advantage of the proposed DC circuit breaker is that the current in the mechanical switch automatically reduces to zero when a fault happens. This paper focuses on comparison of different mechanisms to commutate the current from the mechanical switch to the semiconductor switch and create a zero current for the mechanical switch. Three types of hybrid DC circuit breaker topologies including proactive hybrid DC circuit breaker, hybrid DC circuit breaker with commutation booster, and hybrid DC circuit breaker with coupled inductor for a 500 kV HVDC system are simulated and compared using Matlab/Simulink. A prototype hybrid DC circuit breaker with coupled inductor is designed and built. The prototype is capable of interrupting 200 A direct current within 4 ms. The automatic current commutation compensates for the fault detection and location time in the selective protection scheme, which provides faster fault current interruption.

Aircraft technology moves towards electrification in order to achieve environmentally friendly goals. However, one of the main challenges facing the electrification of aircraft technology is the weight of the electric devices necessary for operating a fully electric aircraft. The National Aeronautics and Space Administration (NASA) proposed the use of the superconductive technology in electric aircraft (EA) to overcome this challenge in addition to its other benefits. The proposed EA is called N3-X and it has an on-board DC superconducting network including a superconducting fault current limiter (SFCL). The SFCL is a self-mechanism device that very effectively limits the current within a few milliseconds, thus improving the stability of the system. As the grounding of this network is different from traditional networks, analysing the behaviour of different fault types and how to manage them becomes critical. In this paper, one set of the EA’s systems (one generator, AC/DC rectifier and four motors with their DC/AC variable frequency drives) has been modelled in MATLAB/Simulink environment to carry out the fault analysis and to demonstrate the effect of the SFCL on this network under different types of faults.

Resistive superconducting fault current limiter (SFCL) plays a key role to realise the stability and safety of power grid and the future electric aircraft. This paper presents the design and test of a braid type non-inductive coil for SFCL. One inner coil and one outer coil were wound around a G10 tube using 4 mm-wide SuperPower wires. Two configurations of the braid type non-inductive coil were achieved by connecting the inner coil and outer coil in series and in parallel, respectively. AC loss results of these two non-inductive coils were measured and compared to investigate the AC loss dependency on coil configuration and current distribution. It has been concluded that braid type non-inductive coil has low AC loss due to magnetic field cancellation. Serial connection of the inner and outer coils leads to a good degree of field cancellation, which results in higher critical current and lower AC loss of the non-inductive coil, compared to parallel connection.

This paper proposes a novel control scheme for a hybrid energy storage system (HESS) for microgrid applications. The proposed two-stage control method is used to control the HESS to stabilize a microgrid’s voltage level and extend battery service lifetime during the coupling/decoupling of a microgrid from the main power grid. The conventional HESS control method (the filtration method) is not suitable to compensate for a microgrid’s power demand when it is decoupled from the main grid. This research focuses on using a superconducting magnetic energy storage (SMES) and battery HESS to assist with the microgrid coupling/decoupling process. To compensate for the instantaneous high power demand during decoupling, the battery will need to rapidly discharge. Moreover, batteries have difficulty supporting high discharging rates, which results in ineffective compensation of the power demand. In this paper, the high power density of the SMES system combined with the high energy density of a battery shows good performance on stabilizing microgrid bus voltage during the decoupling process. A novel energy management method for the HESS is proposed to improve the battery performance when the microgird coupled/decoupled from main grid. The sizing design is simplified based on the control methodology. Moreover, a SMES and battery HESS experimental platform is built to validate the proposed control methodology and its reliability.

Resistive superconducting fault current limiters (SFCLs) offer the advantages of low weight and compact structure. Magnesium diboride (MgB 2 ) in simple round wire form has been previously tested and shown to be suitable as a low-cost resistive SFCL. The primary objective of this work was to design a resistive SFCL for an 11-kV substation using multiple MgB 2 wire strands. This paper will look into the options for the coil design. Two types of low-inductance solenoidal coils, namely, the series-connected coil and the parallel-connected coil, were theoretically examined and compared. This paper also reports the experimental results of two multistrand MgB 2 prototype coils used as a resistive SFCL. This paper demonstrates the potential of SFCL coils using multistrand MgB 2 wire for distribution network levels.

Medium voltage direct current (MVDC) distribution networks have been considered for various applications, such as offshore wind farm collector systems, all-electric naval vessels, and aircraft. MVDC circuit breakers are a critical technology to directly manage faults in multi-terminal DC (MTDC) networks. However, DC current breaking is much more challenging than in AC systems because there is no natural zero-crossing of the current waveform to aid fault isolation. This paper reviews existing MVDC circuit breaker technologies and also discusses their advantages and disadvantages. This paper also introduces new topologies that can be applied in MVDC applications. The operation of several hybrid DC circuit breaker topologies with aided commutation is included. The paper illustrates that a hybrid DC circuit breaker with aided commutation can clear a fault within 2–5 msecs with low losses, this shows great potential for future MVDC applications. The implications for the practical design of commercial MVDC circuit breakers are also discussed.

Superconducting machines offer the significant advantage of smaller volume, lighter weight, and increased operating efficiencies compared with traditional electrical machines. To date, superconducting machines have utilized a superconducting dc field winding on the rotor of a synchronous machine. This increases the system complexity because it requires cryogenic cooling on the rotating part of the machine. The stator in these machines is generally composed of a set of conventional ac copper wire coils. Round magnesium diboride (MgB2) wire however has the potential to form low-cost ac stator coils for a superconducting machine. This could enable a stationary superconducting ac stator winding to be fabricated, reducing the complexity associated with the cryogenic cooling. This paper presents initial test results on a prototype ac solenoidal coil to represent a typical ac stator coil for a superconducting machine using MgB2 wire. The diameter of the wire with insulation was 1 mm and the coil was wound in a double-layer solenoidal arrangement. The magnetic flux density distribution, quench current level, long duration operating current level, and the ac losses of the coil were measured and discussed. This paper demonstrates the potential of MgB2 wire to develop a superconducting ac stator winding for a superconducting machine.

A prototype resistive superconducting fault current limiter (SFCL) was developed using single-strand round magnesium diboride (MgB2) wire. The MgB2 wire was wound with an interleaved arrangement to minimize coil inductance and provide adequate inter-turn voltage withstand capability. The temperature profile from 30 to 40 K and frequency profile from 10 to 100 Hz at 25 K were tested and reported. The quench properties of the prototype coil were tested using a high current test circuit. The fault current was limited by the prototype coil within the first quarter-cycle. The prototype coil demonstrated reliable and repeatable current limiting properties and was able to withstand a potential peak current of 372 A for one second without any degradation of performance. A three-strand SFCL coil was investigated and demonstrated scaled-up current capacity. An analytical model to predict the behaviour of the prototype single-strand SFCL coil was developed using an adiabatic boundary condition on the outer surface of the wire. The predicted fault current using the analytical model showed very good correlation with the experimental test results. The analytical model and a finite element thermal model were used to predict the temperature rise of the wire during a fault.

For HVDC links, the substation is a major cost factor. 3D visualization using virtual reality has the potential to play a significant part in cost reduction. This paper outlines the problem and gives two examples: the use of visualization for planning internal drone inspection; and the application of visualization to the design of a superconducting fault-current limiter breaker.

Multi-terminal Voltage-Sourced Converter (VSC) High Voltage Direct Current (HVDC) transmission is expected to play a key role in future power transmission grids. Compared with AC transmission, DC transmission is more vulnerable to faults due to the lower DC side impedances and sensitive power electronics used in the converters. DC protection issues must be solved before any multi-terminal VSC-HVDC grid can be established. In terms of DC protection, apart from using a DC circuit breaker to interrupt and isolate a DC fault, a Fault Current Limiter (FCL) may be employed to limit the fault current. FCLs could potentially form an important part of any future DC protection system. This paper investigates such a FCL. The main contribution of this paper is that, the resistive type superconducting FCL (SCFCL) is modelled in detail to perform DC protection with the ABB hybrid circuit breaker, in a high fidelity four-terminal VSC-HVDC system model. The simulation results show that the SCFCL has great potential to be employed for DC protection.