Transport Current Losses Research Articles

Modeling and Analysis of HTS Linear Motors in High-Dynamic Applications

In this paper, high-temperature superconducting (HTS) linear motors are modeled and analyzed for a high-dynamic application requiring a periodic, to-and-fro motion with substantial acceleration. Two topologies of a coreless double-sided HTS linear motor (HLM) are investigated: moving magnet motor (MMM) – a set of three-phase AC commutated HTS coils in the stator with permanent magnets in the mover, and moving coil motor (MCM) – a set of DC operated HTS coils in the stator with three-phase AC commutated normal coils in the mover. A two-step design process is adopted where transport current losses are used to determine feasible HTS coils to operate between 20 and 60 K, subject to requirements on input power for the cryogenic cooling system. Using the homogenized T-A formulation, a computationally fast 2D finite element method (FEM) model of the motor is developed to compute these losses. Additionally, a semi-analytical tool calculates the force produced by the motor. The MCMs with feasible HTS coils are then optimized to minimum volume. An MMM with the same force density is designed from the optimum MCM. The optimized motors are further analyzed using FEM for losses during high-dynamic motion. Results show that an MCM topology can reduce the motor volume by at least a factor of 10 when compared to conventional permanent magnet linear motors. The MCM delivers the required force in the same volume with significantly lower losses than an MMM.

Magnetization loss and transport current loss in ReBCO racetrack coils carrying stationary current in time-varying magnetic field at 4.2 K

ReBCO racetrack coils may be used in high-dynamic superconducting linear motor systems, typically replacing either permanent- or electromagnets in the DC stator. Even so, in order to achieve a significant increase in force density, the superconductor needs to carry a high transport current while simultaneously experiencing the time-varying magnetic field from the copper mover coils. To aid with the design of such devices, a 2D numerical model has been developed that predicts the AC loss under motor-relevant conditions, i.e. under the combined influence of a stationary transport current and an alternating external magnetic field. The main aim of the experiments described in this paper is to validate this model with dedicated AC loss measurements. To this end, we constructed a set-up that simultaneously measures magnetization-, transport current- and overall AC loss. Two identical insulated sub-scale ReBCO racetrack coils were tested at 4.2 K while carrying a stationary transport current of up to 700 A in a sinusoidal, alternating magnetic field up to 1.5 T, applied perpendicular to the broad face of the windings. Just like with metallic superconductors, the transport current significantly increases the AC loss level and lowers the penetration field. The inductive, electric and calorimetric data were found to be consistent with each other, validating the experimental calibration methods involved. Furthermore, the numerical model accurately predicted all AC loss components in the coils without any fitting to the data and can thus reliably be used in the design of superconducting machines.

A Comprehensive Investigation on the Accuracy of Electrical Measurement of Transport Current AC Losses in HTS Tapes

The measurement of transport current AC losses in HTS tapes with the electric method has been extensively discussed in the literature. It is well known that the configuration of the circuit used to acquire the voltage signal can affect the measured losses, potentially leading to inaccurate results. To reduce this undesired effect, the rectangular circuit arrangement which is widely adopted involves twisting the pair of voltage taps at a given distance from the tape middle axis. However, the explanations reported in the literature to identify the correct distance are not fully exhaustive, and an alternative interpretation is here described. Moreover, given that the conventional voltage taps arrangement creates a significant circuit area to which electromagnetic noise can be linked, an alternative configuration is proposed, aiming to reduce the linked flux. This work presents a theoretical analysis to quantify the impact of the measurement circuit configuration on the losses. To ensure clarity, the model equations involved are derived step-by-step. The numerical results are applied to a thorough investigation on the accuracy of the AC losses measured on a sample tape, at different frequencies and current amplitudes. Both the conventional and the alternative arrangements are studied, varying their main geometrical parameters, at the same operating conditions. The correction terms are then applied to the measurement results, highlighting the advantages and drawbacks of the different configurations.

Characterization of HTS Coils for Superconducting Rotating Electric Machine Applications: Challenges, Material Selection, Winding Process, and Testing

The second-generation high-temperature superconducting tapes (2G-HTS) such as the REBCO coated conductors are known for their strong mechanical properties and high current carrying capacity with low ac losses compared with first-generation HTS tapes as well as low-temperature superconductors. Therefore, they seem to be good candidates to assemble coils for superconducting rotating electric machines (SuREMs). In this article, to investigate the influence of a soft magnetic composite (SMC) material on the dc and ac HTS coil performances at cryogenic temperatures, GdBCO- and YBCO-based commercial tapes from different manufactures were used to assemble ten HTS coils in double-pancake configuration. These tapes were wound using two different core structures, i.e., fixed and movable cores, having the same circular shape but different diameters as well as materials, i.e., fiberglass and SMC. Critical currents and ac transport current losses of these coils were measured using the electrical method. To evaluate the parallel and perpendicular components of the profile of magnetic flux density for each assembled HTS coil, a finite element model has been developed using Maxwell 2D. Eventually, the performance of different coils for SuREM application was discussed.

Influence of field-dependent critical current on harmonic AC loss analysis in HTS coils for superconducting transformers supplying non-linear loads

There are two main obstacles in front of the development of high temperature superconducting (HTS) technology for electric power network applications; tape price and cooling cost. In order to reduce cooling cost, it is vital to evaluate AC transport current loss of the tapes precisely and then reduce it by some design innovative approaches. In addition, AC transport current loss in HTS material is a critical design variable for large-scale power network applications such as HTS transformers, superconducting fault current limiters, and power cables, since they are continuously carrying the network/load current during their operating life. In existing power networks, harmonic production sources are commonly used and thus, currents are distorted. Therefore, the effect of nonsinusoidal current on the critical apparatus in the network such as transformer must be studied. In this paper, AC transport current loss of a single-turn 2G YBCO HTS coil was modeled and numerically calculated under nonsinusoidal transport current using finite element method. Furthermore, influence of dependency of critical current density to magnetic field on the AC transport current loss of HTS coil when carries distorted currents was considered. It was observed that nonsinusoidal current causes excessive losses in HTS coil. On the other hand, a case study on an HTS transformer supplying non-linear load was considered to study the loss increment as well as heat load change. It was observed that current harmonics increases the AC loss, and heat load of transformer and decreases the efficiency, consequently.

Loss characteristics of superconducting pancake, solenoid and spiral coils for wireless power transfer

Wireless power transfer (WPT) is an emerging technology with widespread applications, such as wireless charging for electric vehicles (EVs), which has become a major point of interest. Conventionally, it is used for stationary charging, but also dynamic systems emerge. Key drawbacks of standard WPT systems are the limited transfer distance between the copper coils and the transfer efficiency. By employing high-temperature superconductors (HTS) as coil material these limitations can be alleviated. However, HTS coils have highly nonlinear ac loss characteristics, which will be studied. This paper investigates the transport current loss and the magnetisation loss of HTS coils individually and when combined in the high frequency range relevant to WPT for EVs. A multilayer 2D axisymmetric coil model based on H-formulation is proposed and validated by experimental results as the HTS film layer is inapplicable at such frequencies. Three of the most commonly employed coil configurations, namely: double pancake, solenoid and circular spiral are examined. While spiral coils experience the highest transport current loss, solenoid coils are subject to the highest magnetisation loss due to the overall distribution of the turns. Furthermore, a transition frequency is defined for each coil when losses in the copper layer exceed the HTS losses. It is much lower for coils due to the interactions between the different turns compared to single HTS tapes. At higher frequencies, the range of magnetic field densities, causing a shift where the highest losses occur, decreases until losses in the copper stabilisers always dominate. In addition, case studies investigating the suitability of HTS-WPT are proposed.

A Comparative study of transport current loss in HTS coils for superconducting wireless power transfer

This paper focuses on the transport current loss modelling for HTS coated conductor coils under high-frequency AC current, which is essential when applying superconducting technology in wireless power transfer system (WPT). The pancake coil, spiral coil and solenoid coil designs were proposed, whose study were carried out based on the Finite Element Method in 2D using H-formulation. The transport current loss in each layer for each coil when applying high-frequency AC current were simulated in the FEM platform. The variation of the transport current loss distribution from different layers of the HTS coated conductor according to changing current input was simulated and compared for different operating frequencies. Finally, the frequency and current dependence of the transport current loss in different layers was shown and compared. The simulation demonstrates that the transport current loss in copper layer could be much higher than the transport current loss in the HTS layer at high current and frequency level due to the skin effect. The results also present that the pancake coil design and spiral coil design could be applied to relatively lower frequency WPT application while solenoid coil design can achieve lowest total loss in higher frequency application. These results are essential when designing HTS coils for WPT system.

Loss analysis of superconducting wireless charging for electric vehicles

Wireless power transfer (WPT) is an emerging technology with widespread applications. It can be used for charging in multiple applications, such as wireless charging for electric vehicles (EVs), which has become a major point of interest. Conventionally, it is used for stationary charging, but also dynamic systems emerge. Key drawbacks of standard WPT systems are the limited transfer distance between the copper coils and the transfer efficiency. By employing high-temperature superconductors (HTS) as coil material these limitations can be alleviated. However, HTS coils have an interesting ac loss characteristic which will be studied. This study investigates the transport current and magnetisation loss of HTS coils in the frequency relevant to WPT for EVs. In addition, a comparison between the full anisotropic dependency of the critical current and only considering perpendicular field components is conducted. A homogenous 2D axisymmetric coil model is used to examine three of the most employed coil configurations and their loss characteristics depending on turn numbers.

AC Losses Analysis in stack of 2G HTS tapes in a coil

The model coil with the racetrack geometry based on second generation (2G) High Temperature Superconducting (HTS) tapes has been developed for an electrical machine where a winding pack is a stack of 2G HTS tapes. It is important to evaluate transport current AC losses and possible methods to reduce them in a winding. In a device made of 2G HTS tapes, the main the AC losses are the hysteresis ones. Only numerical simulation permits to predict them in full. The FEM model for calculation of the hysteresis losses developed before for 2G HTS power cables was modified for a stack of the 2G HTS tapes in a coil. In this paper the methods to increase a computational speed are presented. Possible ways to reduce AC losses are discussed also. The details of the model and comparisons of calculations with measurements of the AC loss are presented.

Modelling of electromagnetic loss in HTS coated conductors over a wide frequency band

Electromagnetic loss is a crucial factor to evaluate the performance of high temperature superconducting (HTS) coated conductors (CCs). Although the loss characteristics around power frequency have been well studied, it is still unclear how loss varies towards high frequencies above kHz level, which is important to a wide range of applications, such as wireless power transfer. This paper is to investigate the frequency dependence of both magnetization and transport current loss in HTS CCs and therefore, to provide comprehensive analyses through a detailed multilayer model reflecting their actual structures. In this paper, a T-formulation based multilayer numerical model for HTS CCs, consisting of HTS layer, substrate, silver overlayer, and copper stabilizers, has been developed. Both magnetization loss and transport current loss over a wide frequency range, from 50 Hz to 1 MHz, have been simulated and discussed. The results obtained by the existing thin film model based on T-formulation, multilayer model and homogenization model by H-formulation are also presented and compared. The proposed multilayer model has been validated by experimental measurements, which has proven the widely adopted HTS thin film model to be inapplicable for frequencies above 100 Hz. In addition, most magnetization losses occur in the copper stabilizers above 1.2 kHz due to skin effect. Results can be used to study the performance of HTS devices towards high frequencies at kHz or even MHz level.

Анализ потерь в обмотках и стопках из ВТСП лент второго поколения

The model coil with the racetrack geometry based on second generation High Temperature Superconducting tapes has been developed for an electrical machine where a winding pack is a stack of 2G HTS tapes. It is important to evaluate transport current AC losses and possible methods to reduce them in a winding. In a device made of 2G HTS tapes, the main the AC losses are the hysteresis ones. Only numerical simulation permits to predict them in full. The FEM model for calculation of the hysteresis losses developed before for 2G HTS power cables was modified for a stack of the 2G HTS tapes in a coil. In this paper the methods to increase a computational speed are presented. Possible ways to reduce AC losses are discussed also. The details of the model and comparisons of calculations with measurements of the AC loss are presented.

AC Loss Characteristics in kHz frequency band of a High Temperature Superconductor Coil for a Wireless Power Transmission System

Improvement of transmission efficiency is an important issue in a large-capacity wireless power transmission (WPT) system using copper coils for automobiles and electric railways. We have investigated the application of a high-temperature superconducting (HTS) coil to a high-power WPT system operating in the frequency region of 10 kHz. We measured the transport current losses of GdBCO and Bi2223 tapes and the total ac losses of GdBCO and Bi2223 coils in the kilohertz frequency band. We analyzed the current distribution within the tape using a finite element method (FEM) program. When the current load factor was low and the frequency was above 1 kHz, the ac losses per cycle increased with the frequency. This is because the eddy current loss in stabilized layers was larger than the hysteresis loss at the frequency of above 1 kHz. One the other hand, the ac losses per cycle were constant regardless of the frequency at the high-current load factor. This is because the hysteresis loss was larger than the eddy current loss regardless of the frequency. We investigated the maximum transmission efficiency at a resonance frequency in an HTS WPT system and a Cu WPT system. Although the quality factor of the HTS coil gradually decreased with the frequency and the operating current, the maximum transmission efficiency of the HTS WPT system was much higher than that of the Cu WPT system. We believe that the HTS coil is suitable for a large-capacity and high-efficiency WPT system in the frequency region below 10 kHz.

A Comparative Study of AC Transport and Eddy Current Losses for Coil Made of HTS Tapes Coated with Copper Stabilizer

In this work, the evaluation of AC loss of the pancake coils wound by HTS-coated conductors, by employing the finite element method in 2D, is presented. The transport current loss of the superconducting tape and the eddy current loss of the copper stabilizer as a function of the amplitude for four frequencies of the applied current are examined numerically by utilizing a newly developed calculation method based on the A − V formulation embedded in COMSOL Multiphysics software with an AC/DC module. The number of the coil turn is 10, and the radius is about 60 mm. The superconducting layer width and height in the simulations are 12 and 1 mm, respectively. The width and height of the copper layers are 12 and 80 mm, respectively. The critical current density of tapes is taken as 300 A.

Experimental Investigation of AC Loss Characteristics of Stacked HTS Tapes in an Iron Core

We report the ac loss characteristics of stacked HTS tapes in an iron core. To estimate the loss distribution, we measured each stacked HTS tape using a calorimetric method. To clarify the influence of an iron core, we measured the ac losses of the stacked HTS tapes without the iron core. The ac transport current losses of the stacked HTS tapes positioned in the iron core were greater than those without the iron core. The distribution of the magnetization losses by the leakage magnetic field was that the loss at the outside of the HTS tape was highest because shielding current prevented the magnetic field from penetrating to the inside of the stacked HTS tapes.

REBCO Coils System for Axial Flux Electrical Machines Application: Manufacturing and Testing

A REBCO coil system for axial flux electrical machines applications is presented. Three couples of series-connected coils have been placed on a fiberglass disc and wired in star configuration to simulate a three-phase, ironless, and superconducting armature concentrated winding. REBCO-based commercial tape has been used to wind each of the six coils in double-pancake configuration around a fiberglass trapezoidal-shaped core. DC current-voltage curve and ac transport current losses plots of each coil have been obtained at liquid nitrogen temperature. Six copper coils having the same shape and geometric parameters of REBCO coils have been also manufactured to assemble a conventional armature concentrated winding. Laboratory testing on different armature windings has been conducted at different temperatures (77 K and 300 K) to measure the impedance (amplitude and phase) value, the back electromotive forces waveform, etc. The experimental results of both configurations (conventional and superconducting) are compared to study the feasibility of a superconducting armature winding prototype of an axial flux electric machine.

Magnetic field direction dependence of AC losses in Bi2223/Ag tape

In this paper, we applied a magnetic field in multiple directions to a Bi2223/Ag tape and measured the transport current and magnetization losses in each position using the four terminal and pickup coil methods. We rotated the tape face and the tape axis to the ac magnetic field by 15° segments. We report the magnetic field direction dependence of ac losses. When the magnetic field direction is parallel to the tape face and the tape axis is rotated in the magnetic field, each magnetization loss shows a crossing point. When the magnetic field is not parallel, the magnetization losses are increased as the angle of the tape axis to the magnetic field increases. We compared the total losses with additions of the transport current loss without the magnetic field and the magnetization loss without transport current in multiple magnetic field directions, and these results did not correspond.

Temperature dependence of critical current and transport current losses of 4 mm YBCO coated conductors manufactured using nonmagnetic substrate

The temperature dependence of the critical current and AC losses were measured on American Superconductor Corporation's (AMSC) second generation high temperature superconducting (2G HTS) wire produced by Rolling Assisted Biaxially Textured Substrate (RABiTS) and Metal Organic Deposition (MOD) process. Wires manufactured with two types of substrates were characterized. The magnetic substrate with composition Ni5a%W exhibits a magnetic signature and has non-negligible AC losses in AC power applications. A new nonmagnetic substrate with an alloy composition Ni9a%W has been developed by AMSC to address the AC losses in 2G HTS. The data presented show that the performance of the new conductor is identical to the conductor with magnetic substrate in terms of critical current density. The data on AC losses demonstrate the absence of ferromagnetic loss component in the new conductor and significantly reduced AC losses at low to moderate values of I/Ic. The reduced losses will translate into reduced capital costs and lower operating costs of superconducting electrical devices for AC applications.

Influence of Geometrical Configurations of HTS Roebel Cables on Their AC Losses

We calculated ac losses in the Roebel cables with various configurations considering their 3-D structures in order to clarify the effect of their geometrical configurations on their ac loss characteristics: the number of strands, the transposition length, and the width of the gap between the two rows of stacks of coated conductors were varied. We also calculated the ac losses in the coated-conductor stacks of which cross-sections were identical to the cross-sections of the straight section of the Roebel cables. Magnetization losses without transport current and transport current losses without externally applied magnetic field were calculated for each case. The difference in the ac loss characteristics of the Roebel cables and those of the coated-conductor stacks is significant for magnetization losses. We discussed this difference via a comparison of the longitudinal ac loss distribution.

Utilizing Triangular Mesh With MMEV to Study Hysteresis Losses of Round Superconductors Obeying Critical State Model

Nature's minimum energy principle formulated in minimum magnetic energy variation (MMEV) and coupled with the Bean's critical state model (CSM) has resulted in feasible tools to model hysteresis losses in superconductors. These tools have been applied for single wires as well as for multi-turn coils in two-dimensional modelling domains. However, so far the discretization of the modelling domain has always relied on regular rectangular meshes. Therefore, the mesh representation of round filaments suffers from large discretization error if the mesh is not refined considerably more than triangular meshing would need. In this paper, we study the utilisation of triangular mesh in such a hysteresis loss modelling tool. We present the required extension to the already available knowledge that is needed to implement such a modelling tool. With our home-brewed tool, we study the convergence of the simulated transport current losses in the cross-section of a round wire represented with triangular and rectangular meshes of different types and of different densities. According to the results, triangular meshing is considerably more efficient than rectangular meshing for simulating transport current losses in the investigated situation.

AC Loss Measurements of a Trapezoidal Shaped HTS Coil Using an Electrical Method

High temperature superconducting (HTS) coils of different shapes (typically circular or trapezoidal) wound on iron or ironless core are fundamental components of many superconducting electrical power devices. A 150-turn (75 turns/pancake) trapezoidal coil in double pancake configuration has been designed and realized in our laboratory of ENEA Frascati. Various epoxy resins and YBCO tapes have been tested in the temperature range room to liquid nitrogen, leading us to the choice of AmSC (American Superconductor) tape for the winding and araldite resin for the impregnation process. The trapezoidal shape has been chosen because of its suitable geometry for practical applications, the results being complementary to what was previously achieved on round shaped HTS coils. The AC transport current losses have been measured using a compensated electrical method and expressed in terms of a linearly frequency dependent resistance. A linear dependence of the losses resistance from frequency was expected and found in agreement with previous results. The current-voltage curve has been measured in zero externally applied field condition, the results being in good agreement with a numerical simulation. The magnetic field distributions, at different air gaps from coil top and zero externally applied filed condition, have been simulated and reported as well.