AC Loss Reduction Research Articles

Reduction of AC Loss in HTS Coils of Superferric Magnets for Rapid-Cycling Synchrotrons by Changing Cross-Section of Coils and Iron Yoke Geometry

We have been studying the potential of energy consumption reduction in rapid-cycling synchrotrons (RCSs) by using superferric magnets with coils wound with high temperature superconductor (HTS). By conducting three-dimensional electromagnetic field analyses on superferric magnets with different coil cross-sections and iron yoke geometries, ac loss of each magnet design was estimated. Rough estimations of the iron loss were also conducted to obtain overall energy consumption of each magnet design. By the comparison, influence of coil cross-section and iron yoke geometry on electricity consumption of the magnet were evaluated.

Coupling Time Constant Measurements of Spirally-Twisted Striated Coated Conductors With Finite Transverse Conductance Between Filaments

Striation is effective for AC loss reduction of coated conductors, but it must be associated with twisting. Instead of direct twisting like in low Tc metallic superconductors, we spirally-twisted “striated and copper-plated coated conductor”, in which filaments are electrically connected. The thickness of the plated-copper was varied to examine its influence on filament decoupling. The magnetization losses of these conductors were measured along a transverse magnetic field with a small amplitude, where the coupling loss was dominant. From the frequency dependence of the measured magnetization loss, the coupling time constants of samples with different copper thicknesses, different spiral pitches, and different core diameters were determined. This study demonstrates filament decoupling by spiral twisting the striated and copper-plated coated conductors.

Magnetization Loss Estimation of HTS Solenoid Coils Wound With CORC

CORC is one of the candidates of the HTS cable for high field magnets. We have been interested in applying CORC to HTS power transformers that require not only large currents but also very low AC losses. Striation and twisting are the solutions for reducing AC losses. We suggested an estimation method for a rough calculation of magnetization losses in a striated CORC coil. First, we measured and calculated the magnetization losses of short samples of straight CC tapes and CORC. Before considering the striation effect, we analyzed the characteristics of slit HTS tapes. In order to emulate the slitting of an HTS tape, we prepared three kinds of HTS tapes with different widths: 2, 4, and 6 mm. Slitting will certainly create the effect of reducing the magnetization loss, but the effect is not directly dependent on the number of the slits nor the applied field magnitude. We measured and compared the magnetization losses of a wide HTS tape to those of narrow HTS tapes. And the narrow tapes were arranged in parallel and measured, so the net width was same as the width of the wide tape. Finally, we applied the measured data of the magnetization loss of several samples to the calculation of the losses of solenoid coils wound with CORC in order to take into account the loss reduction effect of slit HTS tapes.

Numerical Study on AC Loss of an HTS Coil Placed on Laminated Silicon Steel Sheets With Distorted AC Transport Currents

AC loss is an intractable and inevitable issue on high temperature superconducting (HTS) coils. In some HTS devices with the iron core, AC loss in HTS coils have been investigated. However, practically, partial distorted currents can flow in HTS coils, and the AC loss in this situation has rarely been conducted. Here, a normal and distorted current were respectively applied into a double-layer racetrack coil (DRC) to induce AC losses. The numerical and experimental results were obtained with and without laminated silicon steel sheets (LSSS), which showed that the AC loss of the DRC significantly increased when put on the LSSS. Furthermore, compared with the loss generated by the fundamental waveform, currents with 3rd harmonic in phase and 5th out of phase, both with 10% total harmonic distortion (THD) could increase AC loss by 9.3% and 7.8%, respectively. To reduce AC loss of the DRC on the LSSS, two methods are proposed, and it is found that increasing the 5 mm distance vertically and horizontally between the DRC and LSSS could significantly decrease the AC loss by 20.7% and 18.0%, respectively. This study could offer an indispensable reference for designing and operating HTS coils with the iron core for AC applications.

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.

AC Loss Calculation on a 6.5 MVA/25 kV HTS Traction Transformer With Hybrid Winding Structure

HTS wire cost is a critical factor for successful commercialization of HTS traction transformer technology. Wire cost might be minimized without significantly increasing AC loss by introducing a hybrid winding structure: the end-part of the windings is wound with high-cost high- I c wires; the central-part of the windings is wound with low-cost low -I c wires. We report AC loss simulation results on HTS windings with both HV and LV windings wound with REBCO wires. The 2D axisymmetric FEM simulation was carried out using H -formulation. The HV windings are wound with 4 mm-wide wires and LV windings are wound with 8/5 (eight 5 mm – wide strands) Roebel cables. Both HV and LV windings have a hybrid structure in order to reduce the wire cost. Flux diverters are placed at the end of the windings to reduce AC loss. Significant HTS wire cost reduction could be achieved without compromising AC loss by using hybrid windings. This may help commercialize HTS traction transformer technology.

3D Finite Element Electromagnetic Analysis of a 14-Strand HTS Roebel Cable

Roebel cables with HTS strands exhibit reduced AC losses and possess large current carrying capacity. Even though works on several 2D and 3D numerical modelling and simulations are available, little efforts are made on computer-aided design (CAD) modelling, finite element analysis (FEA) and simulation of Roebel strands and cables. In this paper, finite element electromagnetic analysis and simulations of a 14-strand HTS Roebel cable is performed. The results obtained are then compared with another work which uses an H-formulation technique to model the problem. Although several numerical simulations have been carried out to optimise the HTS Roebel cable, no FEA work has yet been conducted on the complete 3D structure of Roebel cable. This work can therefore be considered as an initiator of the 3D finite element electromagnetic analysis of HTS Roebel cables.

Asymmetric Bar Winding for High-Speed Traction Electric Machines

A new asymmetric bar winding concept along with the analysis and benefits for high-speed traction electric machines is presented. The objective is to reduce the ac losses, especially at high speeds, utilizing optimized and asymmetric conductor heights within a slot for bar-wound stators. Detailed winding diagram, height optimization, ac loss analysis, and thermal performance are presented for both symmetric, i.e., conventional, and asymmetric bar windings. The proposed idea is validated using the closed-form analytical equation and 2-D time-stepped finite-element analysis (FEA). A substantial reduction of ac losses and improvement in continuous power over the wide operation range is achieved as demonstrated for a 12-pole, 100-kW high-speed (15 000 r/min) PM traction machine. Thermal performance analysis using forced liquid cooling is also included. The design and analysis methodology is presented to support high-speed traction electric machine designers meet the ever-increasing demand on efficiency and performance with bar-type windings.

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.

Interior Winding Synchronous Motor for Reducing AC Loss in Ultra High Speed Rotation Range

Interior Winding Synchronous Motor for Reducing AC Loss in Ultra High Speed Rotation Range

Reduction of Winding AC Losses by Accurate Conductor Placement in High Frequency Electrical Machines

In the design of electrical machines, the increase in power density has become one of the main research themes. With most of the power loss in high power density electrical machines often being generated in the stator winding assembly, it is necessary to reduce these losses. The effects of strands and bundle positions in the slot on the ac losses in the winding are often overlooked. Taking as a case-study an existing high-frequency machine, this article analyzes and provides an in-depth insight into such effects. Exploiting the rapid advancements in precision three-dimensional printing, it is found and experimentally verified that by controlling the position of the conductors within the top of the slot the ac losses can be markedly reduced.

Exploiting asymmetric wire critical current for the reduction of AC loss in HTS coil windings

Critical current and AC loss in coil windings are two important factors for various HTS applications. Many coated conductors exhibit asymmetry in the variation of the critical current with magnetic field angle. This asymmetry results in different coil critical current values depending on the orientation of the conductors in the coil windings. We report critical current and AC loss results at 77 K and 65 K for three hybrid coil assemblies which have a common central winding and different arrangements of the end windings. We found a difference greater than 13% in both the critical current and the AC loss results for the different arrangements. The results imply that if we wind coil assemblies smartly even using the same materials and the same design, we can not only improve critical current but also reduce AC loss significantly.

Parallel magnetic field dependence of AC losses in periodically arranged superconductors with ferromagnetic substrates

Unlike superconductors on nonmagnetic substrates (SC/NM tapes) in a parallel magnetic field, the ferromagnetic substrates can cause obvious change of current density and magnetic field distributions in superconductor coated conductor (CC) tapes. In this case AC losses of CC tapes exposed to a parallel magnetic field cannot be neglected. In this paper, we investigate the electromagnetic response and AC losses of CC tapes using H-formulation model. The ferromagnetic loss of the substrate in a CC tape and the effect of magnetic field dependent permeability on AC loss are also considered. It can be found that the AC loss of a CC tape is much larger than that of a SC/NM tape, especially in weak applied magnetic fields. It is an effective way to significantly reduce AC loss of a CC tape by appropriately increasing the width of the ferromagnetic substrate. We also consider the CC tape carrying DC transport current and simultaneously exposed to a parallel AC magnetic field. The influence of the DC transport current on the AC loss is investigated. Moreover, AC losses of CC tape-array systems mainly depend on the arrangement and the vertical interval between CC tapes. We demonstrate that AC loss can be reduced effectively by decreasing the vertical interval between CC tapes.

Study on AC Loss Characteristics of 3-Phase HTS Coils With Iron Core and Its Reduction

Study on ac loss characteristics of HTS coils with an iron core to simulate an armature winding of ac rotational machine and a method to reduce ac losses in HTS coils combined with the iron core in 3-phase condition are reported in this paper. In this study, the 3-phase test model which combines 6 HTS coils wound by YBCO tapes and the iron core used for an armature of conventional induction motor is fabricated for the ac loss measurement. It is shown that the measured ac losses in the 3-phase condition coincides with the sum of the ac loss of each phase measured in the 1-phase condition. The effectiveness to suppress the perpendicular magnetic field applied to the tape in the coil by use of magnetic materials around the coil winding on the ac loss reduction in the 3-phase condition is investigated. The measured ac loss with placing the magnetic material around the coil winding in the slot of the iron core remarkably decreases. The results of the numerical magnetic field analysis confirmes that the perpendicular magnetic field significantly decreases by arranging the magnetic material and supportes the validity of the measurement results. The results obtained in this study demonstrates that the effectiveness to modify the magnetic field distribution by use of the magnetic material around the coil winding on the ac loss reduction in 3-phase HTS coil combined with the iron core.

AC Loss Measurement in HTS Coil Windings Coupled With Iron Core

Utilizing high temperature superconducting (HTS) magnets coupled with iron core on the rapid cycling synchrotrons is considered as an effective way to reduce their construction and running costs. However, the heat generation by ac loss in HTS wires is a critical obstacle for the application, and prediction and reduction of ac loss in HTS magnets coupled with iron core are essential. In this work, we built one HTS double pancake coil (DPC), a stack of two DPCs, and an iron core comprising laminated steel to couple with the coil windings. The HTS-coated conductors are 200 A-class 4-mm–wide Shanghai Conductors. We report the ac loss measurement results on the coil windings with/without the iron core at three different frequencies. The ac loss in the HTS windings was significantly increased due to the existence of the iron core. Increasing the number of stack of the DPCs may help reduce the difference in ac loss values in the coil windings with/without iron cores.

AC Loss Modeling in Superconducting Coils and Motors With Parallel Tapes as Conductor

Many superconducting power applications contain REBCO coils. Superconducting motors are very interesting for mobile applications like aviation, thanks to their high power per unit weight. In order to reduce the inductance, multi-tape conductors are often used in windings but parallel tapes present coupling AC loss. In this article, we analyze the AC loss of single coils and a full stator of a motor made of windings of parallel tapes, taking several coupling situations into account. Our computer modeling method combines self-programmed and commercial finite-element methods to calculate the current density and AC loss. We found that isolating the tapes along the length provides almost the same AC loss reduction as that obtained by full transposition. This reveals that there are still many unexploited strategies to reduce the AC loss in multi-tape conductors.

Design Study of 10 MW REBCO Fully Superconducting Synchronous Generator for Electric Aircraft

Future aircraft are expected to have low noise pollution, low emission, and low fuel consumption. A fully turbo-electric propulsion system was proposed by NASA with these aims. The advantages of fully superconducting rotating machines are their weight and efficiency, owing to a high current density resulting in no iron core and no joule heat. In this paper, 10-MW fully superconducting generators with REBa 2 Cu 3 Oy (RE: Rare Earth, Y, Gd, Eu, and others, REBCO) superconducting tapes were studied in relation to their output power density and efficiency. The target output power density is over 20 kW/kg. Fully superconducting generators of 10 MW were designed, then a numerical simulation of the rated operation was conducted. The ac loss of the superconducting windings, iron loss of the yoke, total efficiency, dry weight, and output power density were evaluated, and their dependence on the operating temperature and the synchronous rotating speed were investigated. Here laser-scribing of the wires into a multifilamentary structure and transposed parallel conductors were introduced for ac loss reduction and current capacity enhancement. Consequently, the efficiency reached 99.2% in the case where the REBCO superconducting tapes were scribed into a 10-filament structure. The best model achieved a power density of 26.7 kW/kg. The iron yoke accounts for approximately half the total weight of all the models. The result suggests that the development of superconducting shields brings significant improvement to the output power density.

Experimental Test and Analysis of AC Losses in Multifilamentary MgB2 Wire

AC losses in superconductors are essential for the design of cooling system for large scale power applications. Magnesium diboride (MgB2) superconducting wires have been investigated and manufactured over the last decade due to cheap raw materials and flexibility for coil design. In addition, multifilamentary MgB2 wires have been manufactured to reduce ac losses. In this paper, self-field ac losses of multifilamentary MgB2 wires with magnetic barrier are investigated using both experimental and numerical methods. A short straight wire sample and a coil sample are tested under various temperatures and frequencies between 16- and 128-Hz. The test results show that the transportation loss is independent of the operating temperature. On basis of both theoretical and numerical study, it is found that hysteresis loss in superconductor accounts only for a small fraction of the transportation losses, ferromagnetic hysteresis loss in the magnetic barrier dominates when the transport current is low, whereas eddy current loss dominates when the transport current is close to the critical current.

Numerical AC Loss Analysis in HTS Stack Carrying Nonsinusoidal Transport Current

Bifilar coil structure is promising for reducing ac loss in resistive-type superconducting fault current limiters (FCLs). However, the electric power networks could be contaminated with harmonic components due to nonlinear loads and switch converters. We built a numerical model to calculate transport ac loss in a bifilar stack composed of two coated conductors, carrying distorted current, by means of H-formulation. We compared the calculated losses in the bifilar stack exposed to nonsinusoidal current with the third and fifth harmonics plotted as a function of total harmonic distortion (THD), where THD is defined as the ratio of current magnitudes for the harmonic frequency and fundamental frequency, I hf,peak /I ff,peak . AC loss results in the bifilar stack carrying current with the third harmonic in each conductor were compared with those in an inductive stack and single tape carrying the same current as the conductors in the bifilar stack. Results show that harmonic components have substantial influence on ac loss in the bifilar stack. AC loss values in the stack with fifth harmonic increased monotonically with increasing THD, while ac loss value in the stack with the third harmonic had minimum value when THD is approximately 0.2.

Reducing AC losses in high-temperature superconducting coated-conductor wires towards more efficient superconducting electric power applications

Since the discovery of high-temperature superconductivity, over 30 years ago, there has been great interest in utilising these materials to develop next-generation electric power applications that would replace conventional transformers, cables and motors and generators. There also exist novel technologies enabled by superconductivity, such as superconducting fault current limiters (SFCLs) and superconducting magnetic energy storage (SMES). The high current carrying capacity of high-temperature superconducting (HTS) coated-conductor wires, with zero (DC), or low (AC) losses, enables more efficient devices with smaller footprints and reduced weight. Continual advances in manufacturing techniques have resulted in the commercial availability of long lengths (km+) of high-quality wire from several manufacturers worldwide. This has improved the technological readiness level and commercial attractiveness of such devices.