Sastry Pamidi

Study of second generation, high-temperature superconducting coils: Determination of critical current

This paper presents the modeling of second generation (2 G) high-temperature superconducting (HTS) pancake coils using finite element method. The axial symmetric model can be used to calculate current and magnetic field distribution inside the coil. The anisotropic characteristics of 2 G tapes are included in the model by direct interpolation. The model is validated by comparing to experimental results. We use the model to study critical currents of 2 G coils and find that 100 μV/m is too high a criterion to determine long-term operating current of the coils, because the innermost turns of a coil will, due to the effect of local magnetic field, reach their critical current much earlier than outer turns. Our modeling shows that an average voltage criterion of 20 μV/m over the coil corresponds to the point at which the innermost turns’ electric field exceeds 100 μV/m. So 20 μV/m is suggested to be the critical current criterion of the HTS coil. The influence of background field on the coil critical current ...

Alternating current loss of second-generation high-temperature superconducting coils with magnetic and non-magnetic substrate

It is widely believed that the second-generation high-temperature superconducting (2G HTS) tapes with magnetic substrates suffer higher transport loss compared to those with non-magnetic substrates. To test this, we prepared two identical coils with magnetic and non-magnetic substrates, respectively. The experimental result was rather surprising that they generated roughly the same amount of transport loss. We used finite element method to understand this result. It is found that, unlike in the single tape where the magnetic field-dependent critical current characteristic can be neglected and the effect of magnetic substrate dominates, the magnetic field-dependent critical current characteristic of 2G tape plays as an equally important role as magnetic substrate in terms of HTS coils.

Frequency-dependent critical current and transport ac loss of superconductor strip and Roebel cable

The frequency-dependent critical current of a superconductor strip and Roebel cable has been studied using a 2D finite element simulation. It is shown that the critical current of the superconductor increases with frequency as f1/n, where n is the exponent of the power law flux creep model. Transport ac loss in a superconductor strip decreases with frequency as f − 2/n when the amplitude of the applied ac current is far less than its critical current. However, when the applied current is large and becomes comparable to the critical current, the transport ac loss decreases with frequency as 1/f. The analytical results are substantiated with available experimental data and the results of a 2D finite element simulation.

Experimental and numerical study of a YBCO pancake coil with a magnetic substrate

A finite element model for a YBCO pancake coil with a magnetic substrate is developed in this paper. An axial symmetrical H formulation and the E–J power law are used to construct the model, with the magnetic substrate considered by introducing an extra time-dependent term in the formula. A pancake coil is made and tested. The measurement of critical current and transport loss is compared to the model result, showing good consistency. The influence of magnetic substrate in the condition of AC and DC current is studied. The AC loss decreases without a magnetic substrate. It is observed that when the applied DC current approaches the critical current the coil turn loss profile changes completely in the presence of magnetic substrate due to the change of magnetic field distribution.

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.

AC Loss Estimation of HTS Armature Windings for Electric Machines

This paper studies 2G high-temperature superconducting (HTS) coils for electric machine armature windings, using finite element method (FEM) and H formulation. A FEM model for 2G HTS racetrack coil is built in COMSOL, and is well validated by comparing calculated ac loss with experimental measurements. The FEM model is used to calculate transport loss in HTS armature windings, using air-cored design. We find that distributed winding used in conventional machine design is an effective way to reduce transport loss of HTS armature winding, in terms of air-cored design. Based on our study, we give suggestions on the design of low loss HTS armature winding.

Measurements and calculations of transport AC loss in second generation high temperature superconducting pancake coils

Theoretical and experimental AC loss data on a superconducting pancake coil wound using second generation (2 G) conductors are presented. An anisotropic critical state model is used to calculate critical current and the AC losses of a superconducting pancake coil. In the coil there are two regions, the critical state region and the subcritical region. The model assumes that in the subcritical region the flux lines are parallel to the tape wide face. AC losses of the superconducting pancake coil are calculated using this model. Both calorimetric and electrical techniques were used to measure AC losses in the coil. The calorimetric method is based on measuring the boil-off rate of liquid nitrogen. The electric method used a compensation circuit to eliminate the inductive component to measure the loss voltage of the coil. The experimental results are consistent with the theoretical calculations thus validating the anisotropic critical state model for loss estimations in the superconducting pancake coil.

Total AC loss study of 2G HTS coils for fully HTS machine applications

The application of HTS coils for fully HTS machines has become a new research focus. In the stator of an electrical machine, HTS coils are subjected to a combination of an AC applied current and AC external magnetic field. There is a phase shift between the AC current and AC magnetic field. In order to understand and estimate the total AC loss of HTS coils for electrical machines, we designed and performed a calorimetric measurement for a 2G HTS racetrack coil. Our measurement indicates that the total AC loss is greatly influenced by the phase shift between the applied current and the external magnetic field when the magnetic field is perpendicular to the tape surface. When the applied current and the external magnetic field are in phase, the total AC loss is the highest. When there is a 90 degree phase difference, the total AC loss is the lowest. In order to explain this phenomenon, we employ H formulation and finite element method to model the 2G HTS racetrack coil. Our calculation agrees well with experimental measurements. Two parameters are defined to describe the modulation of the total AC loss in terms of phase difference. The calculation further reveals that the influence of phase difference varies with magnetic field direction. The greatest influence of phase difference is in the perpendicular direction. The study provides key information for large-scale 2G HTS applications, e.g. fully HTS machines and superconducting magnetic energy storage, where the total AC loss subjected to both applied currents and external magnetic fields is a critical parameter for the design.

High-temperature superconducting (HTS) power cables cooled by helium gas

Abstract This chapter describes the benefits and associated technical challenges of closed-loop cryogenic helium gas circulation for cooling high-temperature superconducting cable systems. The many potential applications of gaseous helium-cooled high-temperature superconducting cables are discussed. The major technical challenges specific to the development of gaseous helium-cooled superconducting power cables are also described. The need for the development of new cryogenic dielectric material systems for realizing medium-voltage power cables is emphasized. Available technology for producing and circulating cryogenic helium gas at sufficiently large mass flow rates needed for power cables is discussed. The chapter concludes with a brief description of ongoing research and development efforts on helium gas-cooled high-temperature superconducting cables.

Study of 2G high temperature superconducting coils: Influence of anisotropic characteristics

This paper focuses on the study of anisotropic characteristic of second generation high-temperature superconductors (HTS), and how it influences the performance of HTS coils. The critical current of 4 sections of a HTS coil is measured and compared. The sectional difference of critical current exists due to an HTS anisotropy. A numerical method is introduced to consider the HTS anisotropy using finite element method. The model shows good consistency with experimental results. The model demonstrates that the anisotropy changes the current distribution inside the coil, and the discrepancy of critical current in different sections is the joint-effect of the anisotropy and shielding current. The paper discusses the possibility to optimize the performance of HTS coils by changing the anisotropic characteristic. Based on the study, strategies to improve the performance of HTS coils are provided.