AC loss study on a 3-phase HTS 1 MVA transformer coupled with a three-limb iron core

High-temperature superconducting(HTS)technology provides an alternative approach to achieve compact transformers.Addressing AC loss in the HTS winding is crucial for HTS transformer applications.Most numerical AC loss studies on HTS transformers have neglected the influence of iron cores.This work carries out an AC loss study to explore the impact of an iron core on the HTS windings in a 3-phase HTS 1 MVA transformer coupled with it.AC loss simulations for the transformer winding both with and without the iron core are conducted by adopting the three-dimensional(3D)T-A homogenization method.When the iron core is incorporated,the saturation magnetic fields of iron materials,flux diverters(FDs)with different geometries,and variations in turn spacings in the LV winding composed of Roebel cables are considered to investigate their influence on the AC loss of the transformer winding.The inclusion of the iron core leads to a 1.2%increase in AC loss for the transformer winding while simulating at the rated current.We attribute this slight difference to the non-inductive winding structure of the transformer winding,where a strong magnetic field generated in the space between the LV and HV windings effectively shields the influence of the iron core.

Numerical simulations on the AC loss of REBCO stacks under rotating magnetic fields

AC loss presents a significant challenge for high-temperature superconducting (HTS) rotating machines. To date, the behaviour of total AC loss (Qtol) (with current) and magnetization loss (Qm) (without current) in a single HTS tape under rotating magnetic fields (RF) have been explored. However, a research gap remains in understanding how these findings translate to the more complex HTS windings of rotating machines. Further exploration is needed to understand the loss behaviour of more complex HTS structures, such as HTS stacks. In this work, Qtol and Qm, in the HTS stacks under RF and a perpendicular AC standing wave magnetic field are numerically investigated. Two different RF models are considered: one is the Uni-RF model, characterized by a uniform field with equal field amplitudes and phases at each position, and the other is a non-uniform field created by a rotating Halbach array, referred to as the Hal-RF model. The dependence of AC loss on parameters such as the number of tapes in the stacks, tape width (2a), and the inclination angle (α) of tapes, which refers to the angle between the normal direction of the stack and the vertical direction, have been explored. The number of tapes in the stacks ranges from 1 to 16, α ranges from 0° to 90°, and the tape width includes 4 mm and 40 mm. Additionally, different rotating field directions are also considered. Interestingly, the analytical values from Brandt and Indenbom equation for Q_(m) of a superconducting strip (BI-strip) are close to Q_(m) results of the stacks under the standing wave at high fields, while they are over twice as high as those in the Hal-RF model at 1 T. This suggests the BI-strip equation is not reliable for predicting Q_(m) under RF at high fields. We also show in the Hal-RF model that different rotation directions of the field lead to varying Q_(m) and Qtol when asymmetric Jc (B, θ) data are applied. Moreover, it has been observed that the inclination angle has no impact on Q_(m) under uniform RF while significantly impacts both Q_(m) and Qtol in the Hal-RF model.

A full-wave HTS flux pump using a feedback control system

Transformer‐rectifier flux pumps are DC superconducting power supplies capable of charging superconducting magnets to high currents and stored magnetic energies.Here,we demonstrate a full‐wave superconducting flux pump assembled from high‐temperature superconducting(HTS)wire that utilizes superconducting switches controlled by applied magnetic field.A negative DC offset occurs in the superconducting secondary of the circuit during operation which is related to the output load current.A feedback control system is proposed and demonstrated to account for the negative DC offset.Increasing the primary current proportional to the load current during operation allowed for the maximum output of the flux pump to be increased from 35 A to more than 275 A.These results are reproduced using a coupled electrical‐and magnetic–circuit model formulated in the MATLAB Simulink®package.

I_(c) measurement of twisted multifilamentary MgB_(2) wires with non-magnetic sheath over a wide range of temperatures and fields

All‐superconducting rotating machines have the potential for meeting the high power density and high efficiency required for electrical aircraft applications.However,very high AC loss encountered in superconducting armature windings could hinder their development.Multifilamentary MgB_(2) wires are one of the promising candidates for the stator windings,due to their potentially low AC loss properties with small filament size and twist pitches.As the first step,the dependence of critical current and n‐value on magnetic fields and temperatures I_(c)(B,T)and n(B,T),which are basic input parameters for AC loss simulation,needs to be measured.In this work,we present transport I_(c)measurements in three non‐magnetic multifilamentary MgB_(2) wires(MgB_(2)/Nb/CuNi/CuZn):one large wire with a 0.70 mm diameter and 25 mm twist pitch,and two small wires with a 0.48 mm diameter each and a 10 mm and 30 mm twist pitch respectively.A four‐probe direct current method is used to measure I_(c) of the MgB_(2) wires with variations in temperature(15-35 K)and magnetic field(0-5.5 T).Full I_(c) data for the small wire with 10 mm twist pitch was obtained,and the n‐values were mostly less than 20.While the I_(c) data for the large wire at low fields was more limited due to heating,the n‐values were higher and could be up to around 100.The difference is attributed to the different filament sizes.Experiments also found that there is no significant hysteresis in the transport critical current measured by decreasing or increasing magnetic fields due to the non‐magnetic sheaths.This non‐hysteretic characteristic is critical for lowering AC loss because the additional losses from magnetic sheaths can be eliminated.From the magnetic‐field dependence of critical current density,an empirical expression has been developed that provides suitable extrapolations to lower fields for the large wire.

Optimizing coil configurations for AC loss reduction in REBCO HTS fast-ramping magnets at cryogenic temperatures

AC loss is one of the critical issues for designing REBCO fast‐ramping magnets operating at cryogenic temperatures.There are many ways to reduce AC loss for coil windings.However,it is not clear which method is the most effective way to minimize AC loss in the coil windings for a given Ampere‐turns.In this work,we numerically studied coil configurations of several small superconducting magnets constructed from 12 mm SuperPower REBCO coated conductors,for fast‐ramping application with the same Ampere‐turns to identify the lowest AC loss among them.The HTS magnets have a total turn number of 50 and inner diameter of 30 cm,carrying AC current operating in the temperature range of 20–40 K at 25 Hz.We incorporated several existing loss reduction strategies including spacing between the turns for single pancake coils,grading Ic values for the solenoid configuration,and applying flux diverters to shape the magnetic field around the coil windings.The simulation was implemented using a homogenized H‐formulation.Across all studied loss reduction methods,the use of flux diverters has the largest impact in AC loss reduction.The AC loss values in the solenoid winding comprising a stack of five single pancake coils with 0.1 mm turn‐to‐turn gap with the flux diverters agree well with those in the single pancake coil for 2 mm turn‐to‐turn gap with the flux diverters.Solenoid type coil configurations with flux diverters generate much smaller AC loss than the single pancake type with flux diverters when they generate the same center magnetic field.

The impact of magnetic field periodicity on the hysteresis loss in superconducting magnetic bearings

Since the discovery of high‐temperature superconductors(HTS),superconducting magnetic bearings(SMB)have attracted much attention for practical applications such as flywheel energy storage systems,electrical machines,gyroscopes,etc.,because of their ability to provide passive stable levitation under high‐load conditions.Despite providing contactless linear and rotational motion,SMBs gradually decelerate by AC losses mainly generated by magnetic field inhomogeneity.The main component of AC losses at low rotational speeds is hysteresis loss,which is said to be independent of rotational speed,intrinsic to HTS,and proportional to the cube of magnetic field inhomogeneity.Although the state‐of‐the‐art analytical expression of hysteresis loss in SMBs captures the general physics of the loss mechanism,it ignores the periodicity of the magnetic field in one complete rotation of the bearing.In this paper,the analytical expression of hysteresis loss is modified,taking into account the impact of magnetic field periodicity and the distribution of loss over the bearing surface.The new expression is tested by performing spin‐down experiments with magnets of different levels of inhomogeneity in an actual SMB environment.The impact of magnetic field inhomogeneity on the dynamic behaviour of the bearing is also investigated.The results show consistency between modified analytical calculations and experimental data.