AC Losses In Coils Research Articles

Experimental AC loss study on REBCO coil assemblies coupled with an iron cylinder

In high-temperature superconducting (HTS) power devices, the presence of iron cores changes the magnetic field profile around the HTS coil windings, potentially affecting their AC loss characteristics. AC loss measurements for HTS coil windings coupled with an iron core using the electrical method can lead to a significant error, owing to the indirect estimation of the iron core loss through using a copper test coil. To investigate the cause of the experimental error and the influence of an iron core on coil AC losses, transport AC losses of REBCO double pancake coil (DPC) assemblies coupled with an iron cylinder were measured. A 40-turn 1DPC and an 80-turn 2DPC assembly wound with 4 mm SuperPower wire were employed in the measurements. To ensure the same iron core loss using the HTS coil assembly and the copper coil, 2D finite element method simulations were conducted iteratively to design the iron core and the copper coil to get the same local magnetic field distributions in the designed iron core for the two cases. The main cause of the error is due to the difference in local magnetic flux densities in the iron core generated by the HTS coil assembly and the copper coil even when the ampere-turns of the coils are identical. We showed that the simulation-guided measurement method can assure accurate AC loss measurement in the HTS coil assemblies coupled with iron cores. Compared with the AC losses in the 1DPC and 2DPC coil assemblies without the iron cylinder, the presence of the iron cylinder significantly increases the coil losses. Frequency dependence is observed in the coil AC losses of the 1DPC and 2DPC assemblies when coupled with the iron cylinder. This is due to the eddy current induced in the iron cylinder generating a magnetic field, which influences the coil AC loss.

Experimental study on AC loss of 1G and 2G HTS coils with an identical structure, geometry and number of turns

Devices made of High Temperature Superconductor (HTS) will suffer AC loss during AC operations, which has unnegligible influence on energy consumption, overall efficiency, cooling cost, and operational stability of the devices. In this study, comparative measurements on 1G and 2G HTS coil samples with an identical structure, geometry, and number of turns were systematically conducted with a calorimetric method at different current values (I rmss) and frequencies (fs). The AC loss characteristics of the HTS samples were analyzed and compared based on the experimental results. The results show that eddy current and coupling play a more dominant role than magnetic hysteresis in the total AC loss of coils made of HTS tapes. The AC loss of both samples increases with I rms and f, but the AC loss is more sensitive to I rms than to f. Under the same conditions, the 1G coil sample has a slightly larger AC loss than the 2G one.

Design and test of a setup for calorimetric measurements of AC transport losses in HTS racetrack coils

The estimation and measurement of AC losses in coils based on high-temperature superconductors (HTS) are relevant aspects of HTS applications. They influence the cooling power requirement, operating temperature and efficiency, which can be decisive in the construction and implementation of superconducting equipment, such as superconducting electrical machines. The measurement of losses due to AC transport current (without external magnetic field) is not an easy task. Several efforts have been made to measure these AC transport losses in superconducting coils with a calorimetric approach by trying to minimize the influence of the environment, improve the accuracy and ensure the reproducibility of the results. This work presents the design and construction of a setup to measure transport AC losses in high-temperature superconducting coils based on a calorimetric approach (boil-off method). The evaporated cryogen (nitrogen) related to the dissipation of energy is collected by using a 3D printed bubble collector that guides the gas into a flow sensor. A box-inside-a-box approach is used to surround the measurement chamber with a cryogenic environment. This approach allows re-directing the heat transfer from the surroundings into an intermediate zone (space between external and internal box). Since this intermediate zone operates under cryogenic temperatures, the noise and the heat transfer in the internal part of the setup are reduced. A statistical analysis of the results based on a standard load cycle, average value, and standard deviation calculations allows assessing the variability in the measurements and expressing the results as average value and uncertainty range. The calibration and reproducibility of the measurements are verified with a set of resistors under different conditions and during different weeks. Finally, the AC transport losses in a racetrack coil for an electrical machine application are measured and compared with 3D simulation results based on the homogenization of the T-A formulation.

The relationship between AC loss of YBCO superconducting coil and current waveform, amplitude, frequency and temperature

With the development of superconducting technology, superconducting devices have begun to be used more and more in the AC field, and AC loss is an important factor affecting the application of superconducting devices in the AC field. In this paper, the AC loss measurement experiment of the YBCO coil in the temperature of 20K and 77K is carried out. By applying sinusoidal current and triangular wave current at the frequency of 50Hz to 500Hz, the coil's AC loss is obtained. This paper compared the AC loss of the coil at different temperatures, analyzed the relationship between the AC loss and the current waveform, and gave the correlation formula between the AC loss and the current amplitude and frequency.

Preliminary Design of CFETR TF Prototype Coil

China fusion engineering test reactor (CFETR), based on ITER technology and bridged between ITER and DEMO, has been supported by China government to start technologies R&D and engineering design. The field of CFETR at plasma core is 6.5 T, maximum field of TF coil is about 14.8 T. TF coil is wound by Nb3Sn and NbTi CIC conductors. Coil weight is about 650 tons with height about 21.7 m and width about 12.3 m. Prototype TF coil manufacture design consists of mechanical & electro-magnetic design and analysis, conductor design and analysis, coil AC loss analysis, thermal–hydraulic analysis and coil cooling, quench detection and coil protection, coil winding, case manufacture, and coil assembling. The TF prototype coil is one sub-task of the CRAFT project, which will last for 5 year and 8 months and is constructed by ASIPP and SWIP cooperatively. The preliminary design of the TF prototype coil has been finished at the present stage. In this paper, the basic design requirements of TF magnet are introduced firstly. Next, the preliminary design and analysis of the TF coil is described, which includes the winding package (WP) design, the coil case (CC), the conductor design and analysis, the electromagnetic, mechanical and AC losses analysis. Then the process and tooling design for the WP and CC manufacturing and assembling is presented. Finally, a summary and a plan are given.

AC loss calculation in REBCO coils or stacks by solving the equation of motion for current using an integration approach

The flux motion and its collective behaviour known as AC loss is the basic characteristic of high temperature superconducting (HTS) materials and controls the economic efficiency, stability and magnetic field uniformity of superconducting equipment. In so far, we have no method to calculate the AC loss of HTS devices in an efficient way. Normally, it will take days to simulate AC loss of HTS coils even with moderate number of turns. In this paper, we surprisingly found that an integral method is remarkably efficient in calculating AC loss of the coils in large scale. By comparing with the fastest method at present, the integral method is proved to be 4.4 times faster than the present one. We give the calculation details and discuss why this method has an advantage in calculating AC loss of the coil with a large number of turns.

AC Loss Characterization of HTS Pancake and Solenoid Coils Carrying Nonsinusoidal Currents

Application of high-temperature superconducting devices become promising in power networks, and transportation, including ship, train, and electric aircraft propulsion systems, with the advantages of light weight, compact size, and high efficiency, compared to conventional devices. In reality, electric networks-either in grid or transportation propulsion system-are polluted with harmonics due to the widespread use of power electronic devices and nonlinear loads. It is essential to explore the dependency of harmonic ac losses of different coil configurations carrying nonsinusoidal current. We modeled and compared harmonic ac loss behaviors in three coil configurations, single pancake coil (SPC), double pancake coil (DPC), and solenoid coil (SNC), where SPC and SNC are wound by identical wire length and DPC has twice conductor number compared to SPC. The research work has been carried out by means of H-formulation finite element method in a 2-D axisymmetric modeling environment of COMSOL Multiphysics. We explored and reported ac losses in these three coil structures carrying nonsinusoidal current with the third and the fifth harmonic orders, respectively, under different total harmonic distortion (THD) and fundamental current levels. It has been concluded that ac loss in these coils first decreases with the increase of the third harmonic content, when THD of the third harmonic <; 0.2; and increase with the increase of the third harmonic when THD >0.2. AC loss in coils monotonically increases with the increase of the fifth harmonic, drastically. We found that ac loss in SPC carrying the third harmonic and the fifth harmonic at different THD are more than 3.8 times of that in DPC; ac loss in SPC carrying either third or fifth harmonics at different THD are around 4.5 times of that in SNC.

Numerical estimation of AC losses in Bi2212/Ag wire and coil

The multi-filamentary Bi2Sr2CaCu2O8-x (Bi2212) round wire is the candidate for high field magnets, which is macroscopically isotropic and has large critical current in high field. AC losses generated in coils by ac transport current and magnetic field will lead to the rise of local temperature and consumption of liquid helium. However, the simulations of AC losses in coils are limited by the complicated geometry. This paper presents a homogenization model for computing the current density distributions, field distributions and AC losses in coils wound by multifilament Bi2212/Ag round wire. Based on two-dimensional axisymmetric H-formulation (H is the magnetic filed), the field dependent critical current density and the nonlinear relation between the electric filed E and the current density J are used to describe the macroscopic behavior of the superconductor. To validate the homogenization model, the AC loss is compared with the result of origin model which has the actual filament configuration of round wire. The homogenization model is powerful to estimate the AC losses of cables and coils made by Bi2212/Ag round wire.

Multiscale Model for Simulation of Large-Scale YBCO Solenoid Coils With &lt;italic&gt;J&lt;/italic&gt; Infinite-Turn

Obtaining distributions of current density, magnetic field, and ac loss in coils is essential for practical applications. However, simulating large-scale 2-generation high-temperature-superconducting, YBCO coils is a challenging task. In this paper, we employ a multiscale model to successfully implement electromagnetic modeling of large-scale YBCO solenoid coils in two-dimensional axisymmetric geometry with the approximation of infinite turns (J infinite-turn). Based on the approximation, current penetration, magnetic field, and ac loss of coils can be obtained. The calculation results of multiscale models with J infinite-turn and uniform current density (J uniform) are compared with those from reference models which consider superconducting characteristics on each turn of coils with actual size based on H-formulation. Compared with J uniform, J infinite-turn improves the quality of current density and background magnetic field significantly and demonstrates good accuracy with errors less than 10% in terms of ac loss, whilst speeding up calculation by two orders of magnitudes than reference model. Also, the approximation shows more reliability if the coil is larger with more turns in pancakes.

The Influence of Flux Diverter Structures on the AC Loss of HTS Transformer Windings

Compared to conventional transformers, high temperature superconducting (HTS) transformers have higher efficiency and fewer hazards, whose main loss comes from the ac loss of high temperature superconducting windings carrying ac current. Their radial leakage magnetic fields are perpendicular of HTS winding tapes, which is the main parameter affecting the ac loss. In the study of superconducting magnets, the flux diverter has been used to reduce the radial leakage magnetic field. The effect of the flux diverter installed at the top of the transformer windings on the coil ac loss was explored in this paper. The influence of different structure, slotting, and the number of slots on the transformer was also discussed, which laid a theoretical foundation for the optimization of flux diverter on the 6.6 MVA HTS transformers. Besides, in this paper, we choose the homogenization treatment method to reduce the computing time and memory when modeling.

Numerical study on AC loss reduction of stacked HTS tapes by optimal design of flux diverter

High temperature superconducting (HTS) coils are key parts of many AC applications, such as generators, superconducting magnetic energy storage and transformers. AC loss reduction in HTS coils is essential for the commercialization of these HTS devices. Magnetic material is generally used as the flux diverter in an effort to reduce the AC loss in HTS coils. To achieve the greatest reduction in the AC loss of the coils, the flux diverter should be made of a material with low loss and high saturated magnetic density, and the optimization of the geometric size and location of the flux diverter is required. In this paper, we chose Ni-alloy as the flux diverter, which can be processed into a specific shape and size. The influence of the shape and location of the flux diverter on the AC loss characteristics of stacked (RE)BCO tapes is investigated by use of a finite element method. Taking both the AC loss of the (RE)BCO coils and the ferromagnetic loss of the flux diverter into account, the optimal geometry of the flux diverter is obtained. It is found that when the applied current is at half the value of the critical current, the total loss of the HTS stack with the optimal flux diverter is only 18% of the original loss of the HTS stack without the flux diverter. Besides, the effect of the flux diverter on the critical current of the (RE)BCO stack is investigated.

Design of a Cooling System for a REBCO Insert Coil in a Cryogen-Free 25 T Superconducting Magnet

A cryogen-free 20 T superconducting magnet with a 52 mm room-temperature bore was developed and installed in Tohoku University in 2013. This magnet consists of a Bi2223 insert coil, which generates 4.5 T, and outer low-temperature superconducting (LTS) coils. Both coils were cooled by a GM/JT cryocooler with 4.2 W-class cooling capacity at 4.3 K. To generate a higher magnetic field, a new cryogen-free 25 T superconducting magnet using a REBCO insert coil, which generates 11.5 T, and new outer LTS coils is now under development. The magnetic field contribution of this REBCO insert coil is considerably higher than that of the previous Bi2223 insert coil, and the ac-loss of the insert coil during field ramping rises to approximately 9.7 W. The LTS coils have to operate at about 4 K, but the REBCO coil can operate at various temperatures above 4 K. In addition, the cooling capacity of a GM cryocooler is greater than that of a GM/JT cryocooler around 10 K. Thus, the REBCO insert coil is cooled to about 10 K by using two GM cryocoolers, independently of the LTS coils, which are cooled by two GM/JT cryocoolers. To protect the cryocoolers from the leakage field of the magnet, the two GM cryocoolers cool circulating helium gas through heat exchangers, and the gas is transported over a long distance to another heat exchanger provided for the REBCO insert coil. The maximum temperature of the REBCO insert coil was calculated under the most severe condition where an insert coil ac-loss of 9.7 W was generated continuously. And it was confirmed to be less than the target maximum temperature of 12 K.

Modeling of AC Loss in Coils Made of Thin Tapes Under DC Bias Current

Many applications, such as magnets and SMES, are usually charged and discharged under a bias DC current, which may increase the AC loss. For their design, it is necessary to understand and predict the AC loss. This article analises the AC loss in magnet-like coils under DC bias contribution super-imposed to the AC current. The analysis is based on a numerical model that takes the interaction between magnetization currents in all turns into account. The studied example is a stack of 32 pancake coils with 200 turns each made of thin tape, such as $Re$BCO coated conductor. We present the current density, the instantaneous power loss, and loss per cycle. We have found that the loss increases with the DC bias current. The instantaneous power loss is the largest in the initial rise of the the AC current. In following cycles, the power loss is higher in the current increase than in the decrease. The loss per cycle is the largest at the end pancakes. In conclusion, the highest cooling power should be supplied to the top and bottom pancakes and during current rise, specially the initial one. The presented model has a high potential to predict the AC loss in magnet-size coils, useful for their design.

Calculation of AC loss in coated conductor coils with a large number of turns

For superconducting coils with many turns, it is widely believed that the magnetic field from magnetization currents is negligible compared with the one generated by the whole coil. This paper shows that this is not the case, from a few turns up to a considerable number of turns (thousands). We introduce a method to accurately calculate the AC loss in coils with virtually any number of turns that takes the magnetization currents into account. The current density and AC loss are analyzed for two windings made of a coated conductor consisting of a stack of 32 pancake coils totaling 768 and 6400 turns, respectively.

Transport AC losses of YBCO pancake coils wound from parallel connected tapes

The transport AC losses (Qtr) of YBCO pancake coils wound from n = 1, 2, 4 non-transposed parallel connected (n-tpc) tapes have been investigated experimentally and theoretically. It was found that the Qtr AC losses of the coils with several n-tpc tapes consist of hysteresis AC losses (Qh) and coupling AC losses (Qcc). AC losses Qh of these coils are nearly the same as the AC losses of single-tape coils (n = 1). In contrast, Qcc are unique for coils with n > 1 n-tpc tapes. We found that the distribution of the coupling currents in the coil turns is defined by the magnetic flux penetrating between n-tpc tapes. This flux is related to the self-field component of the coil parallel to the tape surface, which is antisymmetrically distributed (an odd function) with respect to the winding center. This antisymmetrical distribution leads to a compensation effect of the coupling currents if n-tpc tapes are insulated along the turn length, in spite of low resistance between n-tpc tapes in the soldered coil ends. Therefore, AC losses Qcc of such coils are negligibly small compared to AC losses Qh. On the other hand, AC losses Qcc per current cycle are frequency dependent and have a maximum defined by the time constant τeff if tapes are non-insulated along the turn length. In this paper we estimated τeff for several special cases. In particular, we found that Qtr of YBCO pancake coils wound from several n-tpc tapes are comparable to Qtr of single-tape coils if the operating frequency f is far from the characteristic frequency 1/τeff.

5T級DI-BSCCO小型コイルの特性評価 (1) -コイルの交流損失-

5T級DI-BSCCO小型コイルの特性評価 (1) -コイルの交流損失-

AC loss measurements in pancake coils wound with 2G tapes and Roebel cable: dependence on spacing between turns/strands

AC loss measurements in five single pancake coils wound with 4 mm wide commercial high temperature superconductor wires were carried out to investigate the dependence of coil AC loss on separation between the superconductor layers in the neighbouring coil turns (g) and coil turn number (N) for a given number of ampere-turns NI. The highest frequency was set at approximately 1 kHz. The AC losses measured at different frequencies agreed well with each other. AC loss in the coils with the same N increases with decreasing g. When g is increased to 1.5 times the tape width (6 mm), the loss level is similar to that in an isolated wire. Transport AC loss per unit length in the pancake coils increases with increasing turn number. However, when g is increased to 1.5 times the tape width, the loss level in the coils with different turn number is almost the same. This indicates that, at the same NI, a coil with greater N is advantageous, even considering the conductor length difference. Therefore to achieve a given level of NI for a coil and minimize AC loss, we should favour coils with more turns. Coil AC loss can be measured using voltage taps attached to copper blocks outside a coil with reasonable accuracy. This is important for measuring AC loss in a coil with a complicated structure where voltage taps are not able to be attached inside the coil. AC loss in a nine-turn pancake coil wound with a 5 m long 9/2 Roebel cable (i.e. with 9 mm × 2 mm width strands) was measured using voltage loops arranged in each Roebel strand in the central turn of the coil. The AC loss in the coil was compared with two straight 9/2 Roebel cables with and without spacing between the strands. The combination of inter-strand spacing and turn spacing is an effective way to reduce AC loss in a single pancake coil wound with a Roebel cable.

Transport AC Loss Measurements in Superconducting Coils

Transport AC loss measurements were performed on several Second Generation (2G) High Temperature Superconducting (HTS) pancake coils. A good agreement was observed between the AC loss values of the coils obtained using two separate techniques. Self-field critical current density of the tape did not have direct effect on the AC losses in coils. Relative orientation of the tapes in 2-tape stack affects AC losses in coils using the stacked conductor. Losses scale with maximum perpendicular field seen by the conductor in the coil suggesting that the magnetization loss is the predominant component of coil losses. Absence of frequency dependence of the losses indicates that the eddy current losses are not significant in the coils up to 400 Hz.

Modeling and Electrical Measurement of Transport AC Loss in HTS-Based Superconducting Coils for Electric Machines

AC loss can be a significant problem for any applications that utilize or produce an AC current or magnetic field, such as an electric machine. The authors are currently investigating the electromagnetic properties of high temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. In this paper, a 2D finite element model based on the H formulation is introduced. The model is then used to calculate the trans port AC loss using both a bulk approximation and modeling the individual turns in a racetrack-shaped coil. The coil model is based on the superconducting stator coils used in the University of Cam bridge EPEC Superconductivity Group's superconducting permanent magnet synchronous motor design. The transport AC loss of a stator coil is measured using an electrical method based on inductive compensation using a variable mutual inductance. The simulated results are compared with the experimental results, verifying the validity of the model, and ways to improve the accuracy of the model are discussed.

An improved FEM model for computing transport AC loss in coils made of RABiTSYBCO coated conductors for electric machines

AC loss can be a significant problem for any applications that utilize or produce an ACcurrent or magnetic field, such as an electric machine. The authors investigate theelectromagnetic properties of high temperature superconductors with a particular focus onthe AC loss in superconducting coils made from YBCO coated conductors for use in anall-superconducting electric machine. This paper presents an improved 2D finite elementmodel for the cross-section of such coils, based on the H formulation. The model is used tocalculate the transport AC loss of a racetrack-shaped coil using constant and magneticfield-dependent critical current densities, and the inclusion and exclusion of a magneticsubstrate, as found in RABiTS (rolling-assisted biaxially textured substrate) YBCO coatedconductors. The coil model is based on the superconducting stator coils used in theUniversity of Cambridge EPEC Superconductivity Group’s all-superconductingpermanent magnet synchronous motor design. To validate the modeling results, thetransport AC loss of a stator coil is measured using an electrical method basedon inductive compensation by means of a variable mutual inductance. Finally,the implications of the findings on the performance of the motor are discussed.