Stephen Paul Ashworth

Alternating current (ac) losses in conductor-on-rounded-core (CORC) cables of YBCO high-temperature superconducting (HTS) tapes are a significant challenge in HTS power applications. This study employs two finite element analysis (FEA) models to investigate the contributions from different ac loss components and provide approaches for reducing ac losses in cables. An FEA model based on the T-A formulation treats the cross section of thin superconducting layers as 1D lines and, therefore, only can predict the ac loss generated by the perpendicular magnetic field. In contrast, the model based on H-formulation can be performed on the actual 2D rectangular cross section HTS tapes to provide the total ac losses generated by magnetic fluxes penetrating from both the edges and surfaces of HTS tapes, although this model requires more computing time and memory. The 1D and 2D simulation models were validated by cross comparing the results from both models and by comparing sub-section and full cross section models. Subsequently, two models relate cable design and operational parameters to the surface and edge losses of a two-layer CORC cable by considering the (1) relative contributions of edge and surface losses to the overall ac losses; (2) effect of the current distribution between inner and outer HTS layers on ac losses; (3) impact of the tape alignment on ac losses in each HTS layer; (4) influence of the thickness of HTS layers on ac losses; (5) effect of size and number of inter-tape gaps on ac losses; and (6) contribution frequency on the ac losses. The research results given in this paper are therefore not only valuable to suggest strategies for reducing ac loss in multi-layer cables but also for developing more accurate and effective methods to calculate ac loss in CORC HTS cables.

A free electron laser (FEL) can be used to produce high intensity X-rays, or other monochromatic radiation for a number of purposes. Two primary components, in terms of size and expense, of an FEL system are an electron accelerator and undulator. Reducing undulator period without reducing its magnetic field would allow a reduction in the electron beam energy and potentially reduce the total cost and complexity of FELs. In this paper, finite-element modeling and experimental approaches were used to investigate the feasibility of using high-temperature superconducting (HTS) conductor to create tunable, liquid nitrogen cooled undulators with shorter periodicity. A prototype undulator with three periods was fabricated and tested. Hiperco 50 A structures with machined grooves were used as the magnetic cores. Commercial YBCO tapes were used to wind the magnetizing coils in the grooves of the magnetic core. The undulator was tested and measured in subcooled liquid nitrogen at 65 K. Peak magnetic fields of 0.77 T with a periodicity of 14 mm were attained. Our simulations indicate that by using optimally processed magnetic core and HTS wires, fields of 1 T or higher can be achieved.

Measurements of AC losses in high temperature superconducting (HTS) coils wound from two different types of YBa2Cu3O7 (YBCO) coated conductors are reported. AC loss measurements by different arrangements of voltage loops and pick-up coils were investigated to propose accurate and convenient techniques to measure the AC losses in HTS coils, especially for large coils with the measurement signals significantly higher than the input range of typical lock-in amplifiers. A new and simple sub-cooling technique with an open liquid nitrogen bath was developed to measure AC losses in the sample coils at variable temperatures between 65 and 77 K. The temperature dependence of the losses in these coils was qualitatively explained based on the data on transport and magnetization AC losses in isolated tapes at variable temperatures.

In this paper, we investigate the effectiveness of alternative designs to reduce the AC losses of high temperature superconducting (HTS) power transmission cables. The idea behind these designs is to undermine the edge effect, which is one of the main factors contributing to AC losses in HTS power cables made of coated tapes. The edge effect, which arises from the presence of gaps between the tapes, results in large normal components of magnetic field near the edges of the tapes and in turns leads to a current distribution with higher density near the edges. To perform our investigation we use a numerical technique developed in our previous work which allows us to consider the helical configuration of the tapes. Through numerical simulations we assess the effectiveness of two overlapped designs, i.e. a cyclic overlapped design and an anticyclic overlapped design, in reduction of AC losses in single layer HTS power cables made of coated tapes. Simulation results show that AC losses can be reduced by about 70% as compared with a typical single layer cable.

This paper presents our new experimental facility and techniques to measure ac loss and current distribution between the layers for High Temperature Superconducting (HTS) cables. The facility is powered with a 45 kVA three-phase power supply which can provide three-phase currents up to 5 kA per phase via high current transformers. The system is suitable for measurements at any frequency between 20 and 500 Hz to better understand the ac loss mechanisms in HTS cables. In this paper, we will report techniques and results for ac loss measurements carried out on several HTS cables with and without an HTS shielding layer. For cables without a shielding layer, care must be taken to control the effect of the magnetic fields from return currents on loss measurements. The waveform of the axial magnetic field was also measured by a small pick-up coil placed inside a two-layer cable. The temporal current distribution between the layers can be calculated from the waveform of the axial field.

Long distance transmission of electrical power with superconducting cables is likely necessary for energy conservation and effective utilization of renewable energy sources. The performance and cost of such superconducting lines is as significantly influenced by cryogenic issues as by superconductor performance. One significant cryogenic issue is that in the usual method of cooling using sub-cooled cryogen flow there is a limited cable length before the cryogen needs to be re-cooled. This adds complexity and cost to the cable system. Here we address this problem by utilizing the latent heat of the cryogen without the complication of multi-phase flow. The cryogen is distributed to the superconducting components by spraying it through small holes in a pressurized line. The pressurized liquid exiting the holes turns into mixed liquid and vapor with a temperature near the boiling point of the cryogen at the pressure of the space surrounding the superconducting components. The pressure in the space surrounding the superconducting components is then kept near atmospheric by maintaining short distances to a vent. The sprayed liquid accumulates but rapidly vaporizes in response to the heat load, providing even cooling power at a fixed temperature for the entire length of the line. Our work indicates that it may be possible to implement a cooling system with much simplified cryogenic stations at the cable ends and allowing cable lengths of up to 100 km with no intermediate cooling stations. (C) 2011 Elsevier Ltd. All rights reserved.

The combination of noninductive winding and the two-in-hand tape configuration has been found to be very effective in reducing the AC loss of coils of RABiTS (TM) YBCO coated conductor for fault current limiting (FCL) applications. In this study, a noninductive coil composed of 11 turns of two-in-hand RABiTS T YBCO tape was designed and constructed for characterizing and investigating the AC loss, critical current and inductance. The AC losses in this coil were also calculated by finite element modeling (FEM) using the COMSOL Multiphysics package. With the magnetic field dependent permeability and ferromagnetic loss of the substrate material taken into account, the simulation results are in good agreement with the experimental data. The simulations therefore were employed to study the effect on the AC loss behavior of the coil of the width of YBCO tape and of the spatial separation between the coil turns to suggest a more effective coil design.

In this paper, we introduce a very fast method to compute the current distribution in helically wound thin conductors when one or many of them are arranged in a symmetrical manner to form a single-layer power cable. The method relies on two different approaches to find the magnetic vector potential due to helically wound current sheets. By invoking relevant symmetry arguments associated with the geometry of the problem and neglecting the thickness of the tape conductors, we show that this 3-D problem can be reduced to a computationally small 1-D problem whose domain lies along the half-width of any of the constituting conductor. As a consequence, the proposed method is very efficient in terms of computational time, and it is more accurate than many previous 2-D methods that cannot take into account the twist pitch. Since the nonlinear resistivity of the superconducting material can easily be treated with this method, it can be used to find current and field distributions, as well as ac losses in high-temperature superconductor coils and cables made of coated tapes. To verify the validity of the proposed method, we performed experimental measurements of ac losses in two configurations of solenoid-type cables made of a sample of YBCO-coated conductor tape. Excellent agreement was observed between the experimental data and the simulation results.

In a three-phase tri-axial cable, the magnetic interaction between the phases makes the loss measurement by an electromagnetic method very complex. We developed the theoretical background showing that three-phase AC loss measurements by the electromagnetic method are, in principle, possible. We then implemented this theory in practical measurements on a 3 m long, tri-axial cable fabricated from RABiTS (rolling-assisted biaxially textured substrate) coated conductor. Initially, the proposed measurement technique was implemented in the simpler cases when the three cable phases are 180 degrees out of phase, i.e. (0 degrees, 180 degrees, 360 degrees) or (0 degrees, 360 degrees, 180 degrees) rather than (0 degrees, 120 degrees, 240 degrees) as in a traditional three-phase system. For these cases, the currents in the phases are either in phase (360 degrees phase difference) or anti-phase (180 degrees phase difference). These are essentially single-phase measurements with only one transport current used as the supply for all three phases. This simplification allowed us to use both the established and the proposed techniques to measure the total losses of the cable for these cases. An excellent agreement between the two measurement methods confirmed the validation of our proposed measurement technique. The proposed measurement method was then employed to measure the total AC losses in a cable when it operates in the true three-phase mode. We believe that these data represent the first electromagnetic three-phase AC loss measurements.

This paper presents a new finite-element simulation model for computing the electromagnetic properties and AC losses in systems of YBCO (yttrium barium copper oxide) conductors on roll assisted biaxially textured substrates (RABiTS). In this model, the magnetic field dependent permeability and ferromagnetic loss of the substrates in RABiTS YBCO tapes are taken into account. The simulations were employed to simulate the AC loss in stacks of two parallel connected YBCO tapes. The simulation results are compared with the experimental data to check the validity of the simulation model. The result reveals an effective way of significantly reducing AC loss in YBCO tapes by stacking two RABiTS YBCO coated conductors with the appropriate relative tape orientation.