O. Tsukamoto

Study of 10 MW-Class Wind Turbine Synchronous Generators With HTS Field Windings

We study the electro-magnetic design of 10 MW-class wind turbine generator with high temperature superconducting field winding by using the FEM analysis. The design examples are presented and the generator characteristics are investigated. The 10 MW-class HTS wind turbine generator is considered to be feasible from the stand point of the electro-magnetic design. In this study, 8-pole and 12-pole generators are investigated. From the results, it is considered that the 8-pole design is preferable compared with the 12-pole design. The analysis also reveals the small synchronous reactance of the HTS wind turbine generator. Therefore, proper protection is necessary for the large short circuit current.

Numerical analysis of AC losses in high T/sub c/ superconductors based on E-j characteristics represented with n-value

A numerical code for the electromagnetic analysis of high T/sub c/ superconductors by finite element method has been developed. The E-j characteristics of superconductor are represented with the n-value. The equivalent conductivity of superconductor is determined as a function of electric field, and Ohms law is used as the constitutive equation. First, the current and magnetic flux distributions in the infinite slabs of superconductor exposed to a parallel external magnetic field are analyzed. The influence of n-value on AC loss and the frequency dependence of AC loss are studied. The AC loss in the infinitely-long superconductor tapes exposed to the external magnetic field is calculated numerically to study the influence of n-value.

Influence of longitudinal magnetic field on thermomagnetic instabilities in a.c. superconducting cables

Abstract As the critical current of a single a.c. superconducting wire cannot be more than several tens of amperes, such single wires (strands) are bundled and twisted to make cables with a large current capacity. As the result of the twisting and bundling, a longitudinal magnetic field parallel to a strand axis is produced in the cables and windings by the transport current. It is shown theoretically and experimentally in this paper that the a.c. longitudinal magnetic field influences the current distribution in multifilamentary superconductors with an a.c. transport current and can induce thermomagnetic instabilities in them. A longitudinal magnetic field of the order of 10 −1 T is easily produced in these cables. It is shown that thermomagnetic instabilities induced by the longitudinal magnetic field in the cables cause a.c. quench current degradation in multiply-twisted a.c. superconducting cables. A way of twisting to decrease these instabilities is also proposed.

AC losses in a type II superconductor strip with inhomogeneous critical current distribution

Analytical formulae derived by Brandt and Indenbom (1993 Phys. Rev. B 48 12893–906) and Norris (1970 J. Phys. D: Appl. Phys. 3 489–507) are often used to calculate the magnetization and AC transport current losses in HTS strip conductors, respectively. In these formulae, homogeneous distribution of critical sheet current density σc in the strip is assumed. However, it is considered that σc distributions are inhomogeneous in actual HTS strips and that the inhomogeneous σc distributions cause deviations of the measured AC loss data of actual HTS strips from those formulae. A semi-analytical method to calculate AC transport current and magnetization losses is derived for a type II superconductor strip with inhomogeneous distribution of σc in the direction of the strip width. The method is derived modifying the analysis of Brandt et al. The validity of the semi-analytical method is shown by comparing the results calculated by this method with those calculated by the Norris and Brandt formulae and by a different method of our previous work and also with experimental data. Moreover, it is shown that the deviation of the measured data from the Norris and Brandt models can be estimated by assuming proper σc distributions.

Influence of AC external magnetic field perturbation on trapped magnetic field in HTS bulk

Abstract Electric motors and actuators are most promising future applications of HTS bulks. The HTS bulks in these electric machines are exposed to AC magnetic field perturbations. The perturbations cause AC losses in the bulks and also affect the magnetic field trapped in the bulks. In a previous work we measured AC losses in a YBCO bulk subject to the AC magnetic field and found the loss characteristics well followed the Bean model. In this work we experimentally studied the influence of the AC perturbations of the external magnetic field on the trapped field in a YBCO bulk. The experimental results showed that the application of the AC magnetic field decreased the trapped magnetic field even to zero. In this paper, the experimental results are presented and a reason for the decay of the trapped magnetic field is discussed based on the Bean model.

Influence of current re-distribution and thermal diffusion among strands on stability of superconducting cables against local disturbances

Quench of multi-strand superconducting cables is often initiated by a local disturbance that produces a normal zone in one or a few strands in the cable. Once the normal zone is produced, current re-distribution occurs as the current bypasses the normal zone via contact conductance between the strands and/or at the ends of the cable. Heat generated in the normal zone also diffuses between strands. A numerical analysis was performed to investigate stability property of multi-strand cables against local disturbances. The influence of the current re-distribution and thermal diffusion between strands was studied. The insulation of strands affects the current re-distribution. When the transport current is small, removing the insulation of strands substantially increases the minimum quench energy against local disturbances. On the contrary, when the transport current is large, the insulation of strands has little influence on the minimum quench energy. In this case, current re-distribution cannot improve the stability because of little current margin. >

R&D of a 500 m superconducting cable in Japan

Super-ACE project started to research and develop the superconducting AC power equipment as a national project in 2000 fiscal year. This project is R&D of the basic technologies of high temperature superconducting (HTS) cable, HTS fault current limiter (FCL), HTS magnet for reactor and transformer. Main subjects of the cable are to develop an HTS conductor rated 3 kA, cooling technology of a 500 m HTS cable of 700 A rated current, and the analysis of integrated HTS power system. The paper describes the background of this project, its target, major results of studies, and testing schedule of 500 m cable.

Control strategies for multiple parallel current-source converters of SMES system

Multiple structured current-source converters are applied for a superconducting magnetic energy storage (SMES) system. Suitable control method and control block diagram are proposed by taking much use of the advantage of this multiple structure. A multi-modular pulse width modulation (PWM) control strategy of current-source converters for the purpose of higher efficiency and less harmonic distortion is developed with an idea for practical application. A design of a multi-reduced instruction set computer (RISC) controller is presented to put the proposed control blocks into practice. The results of the digital simulation by Electromagnetic Transients Program (EMTP) and the experimental results of the test module are provided to validate the proposed control method of SMES power conditioning.

Decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work it was observed that trapped magnetic field in a YBCO bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. To study a reason for decay we investigated temperature rise of a YBCO bulk subject to the AC external magnetic field by use of thermocouples. A case was observed that the temperature rise of the bulk reached almost the critical temperature. This result shows that the decay of the trapped magnetic field was related to the temperature rise due to AC losses in the bulk.

Copper stabilization of YBCO coated conductor for quench protection

The required amount of copper stabilizer to protect a YBCO conductor coil from damage caused by a hot spot due to a quench is studied in this paper. YBCO coated conductors which are made by deposition of thin YBCO film on high resistance metal substrates are highly resistive when they are quenched. Therefore, stabilization and quench protection are more important for YBCO conductors than Bi/Ag sheathed tapes which have low resistance silver matrix. In the work we numerically calculated maximum temperature rise of a YBCO conductor quenched by a local disturbance during the sequence of quench detection and energy dump. In the analysis, necessary amount of copper to keep the maximum temperature of the conductor below a threshold value is calculated for cases of conduction cooled coils of dry windings. Based on the analysis, optimum conductor design is discussed to protect the coil from a quench reliably.