R.L. Stoll

Design of a 100 kVA high temperature superconducting demonstration synchronous generator

The paper presents the main features of a 100 kVA high temperature superconducting (HTS) demonstrator generator, which is designed and being built at the University of Southampton. The generator is a 2-pole synchronous machine with a conventional 3-phase stator and a HTS rotor operating in the temperature range 57–77 K using either liquid nitrogen down to 65 K or liquid air down to 57 K. Liquid air has not been used before in the refrigeration of HTS devices but has recently been commercialised by BOC as a safe alternative to nitrogen for use in freezing of food. The generator will use an existing stator with a bore of 330 mm. The rotor is designed with a magnetic core (invar) to reduce the magnetising current and the field in the coils. For ease of manufacture, a hybrid salient pole construction is used, and the superconducting winding consists of twelve 50-turn identical flat coils. Magnetic invar rings will be used between adjacent HTS coils of the winding to divert the normal component of the magnetic field away from the Bi2223 superconducting tapes. To avoid excessive eddy-current losses in the rotor pole faces, a cold copper screen will be placed around the rotor core to exclude ac magnetic fields.

Modelling HTc superconductors for AC power loss estimation

AC losses in high-temperature superconductors are modelled as a highly non-linear diffusion process. Two empirical expressions for modelling effective resistivity of the conductor are used. Formulations in terms of both H and E are presented and it is argued, using dimensional analysis, that working with E is numerically more efficient. Typical results calculated using a finite-difference scheme are shown.

High temperature superconducting demonstrator transformer: design considerations and first test results

The paper highlights main features of a 10 kVA single-phase high temperature superconducting (HTS) demonstrator transformer built recently at the University of Southampton. One winding is made of BPSCCO-2223 tape with bulk critical current density of 22A/mm/sup 2/ at 77 K. Flux diverters have been used to reduce the radial component of leakage flux density in the HTS tape. Initial test results are shown with particular emphasis on measurements of ac losses.

A method of estimating the total AC loss in a high-temperature superconducting transformer winding

The paper presents a valuable extension to the new method of estimating total AC loss in solenoidal high temperature superconducting windings of the transformer type, where losses caused by external magnetic field and due to transport current are modeled together. This complex problem is solved using a highly nonlinear 2D finite difference formulation in the superconductor in combination with a linear finite element model representing the rest of the transformer. Comparison with measurements on a real device shows good agreement and validates the method as a practical tool.

Thermometric measurements of the self-field losses in silver sheathed PbBi2223 multifilamentary tapes

Self-field ac losses in Ag sheathed PbBi-2223 tapes were measured using a thermometric method, which determines the losses by measuring the temperature profile of a vacuum insulated sample, with both ends at a fixed temperature. In practice, the samples were placed in a vacuum capsule immersed in LIN bath. By minimizing the bath superheat, thermal emf and heating at the current contacts, a loss induced temperature increase as low as 2 mK was measured using a Si diode thermometer. With a typical sample length of 600 mm, self-held losses between 7/spl times/10/sup -6/ W/m and 4/spl times/10/sup -3/ W/m were measured at different frequencies. The results are in good agreement with both the electric measurement and theoretical calculation. This provides the first independent confirmation that electric measurement with carefully placed voltage loops can give the true losses of the sample.

A new approach to modelling dominant AC loss in HTc superconducting solenoidal windings

The paper presents a novel method of calculating AC loss in solenoidal high temperature superconducting windings of the transformer type. Losses caused by the external magnetic field and due to transport current are modelled together and account is taken of highly nonlinear characteristics of the material. In tape conductors the loss due to the parallel field component is usually small whereas the effect of the normal field component is significant. A nonlinear 2D finite difference formulation is derived for the superconductor and linked with a linear finite element model representing the rest of the transformer and the interaction between coils.

Measurement of Self-field AC Losses in PbBi-2223 Ag-sheathed Tapes

Abstract A technique for measurement of self field AC losses of PbBi-2223 Ag sheathed tapes is presented and preliminary results shown.

Highly non‐linear field diffusion in HTC superconducting tapes

The paper presents an extension to previous work on modelling AC losses in high‐temperature superconducting tapes as a highly non‐linear diffusion process. Following successful formulation for a bulk superconductor the presence of silver in a tape has now been included, using a “sandwich” model, to represent more realistically the practical arrangement. The results of the extended 1‐D model are included and a new 2‐D scheme is described using finite difference formulation. Effects of non‐linearity are emphasised.

Modelling of AC Losses in High-Tc Superconductors with Flux Creep E-J Characteristics

Abstract A numerical model for simulating high-T c bulk material superconducting tapes subject to AC magnetic fields or transport currents, is presented. The model is based on an electromagnetic diffusion process and describes the superconducting tape as a nonlinear conductor with conductivity being a functions of both local magnetic and electric fields, as deduced from the flux creep E-J characteristics of the material. AC losses for various applied fields or transport currents are calculated and discussed.

Magnetic field modelling and calculation of reflected impedance of inductive sensors

Practical aspects of finite-element modeling of magnetic fields and calculating integral parameters such as a reflected impedance in inductive sensing devices are discussed. The electromagnetics of such sensors is briefly explained, and many practical modeling techniques are put forward. Excellent agreement with experiment is reported. >