Wei Xian

Design and Test of a Superconducting Magnetic Energy Storage (SMES) Coil

This paper presents an SMES coil which has been designed and tested by University of Cambridge. The design gives the maximum stored energy in the coil which has been wound by a certain length of second-generation high-temperature superconductors (2G HTS). A numerical model has been developed to analyse the current density and magnetic field distribution and calculate the AC losses during the charge and discharge process of the coil. A cryostat has been designed and a test of the I-V curve measurement of the coil has been accomplished. In addition, the power electronics control of the SMES coil has been simulated.

The design, magnetization and control of a superconducting permanent magnet synchronous motor

This paper describes in detail the method of magnetization of a superconducting permanent magnet synchronous motor. The rotor of the motor consists of 60 superconducting pucks, which are magnetized by two additional copper windings. The pulse field magnetization (PFM) method is considered and the resulted distribution of the magnetizing flux linkage from the rotor is not a perfect sine wave in the air gap, which leads to a large torque ripple and harmonics of the stator currents. In order to suppress the torque ripple, an iterative learning control (ILC) algorithm is used in addition to the former field-oriented control method. The results show the ILC algorithm can largely reduce the torque ripple.

Minimize frequency fluctuations of isolated power system with wind farm by using superconducting magnetic energy storage

Wind power generation as one of the most popular renewable energy applications is absorbing more and more attention all over the world. However, output power fluctuations of wind farm due to random variations of wind speed can cause network frequency and voltage flicker in power systems. The power quality consequently declines, particularly in an isolated power system such as the power system in a remote community or a small island. This paper proposes an application of superconducting magnetic energy storage (SMES) to minimize output fluctuations of an isolated power system with wind farm. The isolated power system is fed by a diesel generator and a wind generator consisting of a wind turbine and squirrel cage induction machine. The control strategy is detailed and the proposed system is evaluated by simulation in Matlab/Simulink.

Control and Operation of a High Temperature Superconducting Synchronous Motor

We have built a four-pole high temperature superconducting (HTS) permanent magnet synchronous motor (PMSM) in our lab. At this stage, the HTS PMSM uses two 2G HTS racetrack coils, which are YBCO wires, type 344 from AMSC, and four conventional copper coils as stator windings. 75 YBCO bulks are mounted on the surface of the rotor. After the pulsed field magnetization system had been developed and tested in our lab in 2011, the rotor can trap a four-pole magnetic field. This makes HTS bulks possible for motor application, other than HTS coils. The HTS PMSM can successfully run at a low speed of around 150 rpm for an initial test. This paper states theoretical and practical works on the HTS PMSMs operation including HTS motor drive development and its application.

Pulsed Field Magnetization of a High Temperature Superconducting Motor

As we known, the high temperature (77 K) superconducting (HTS) motor is considered as a competitive electrical machine by more and more people. There have been various of designs for HTS motor in the world. However, most of them focus on HTS tapes rather than bulks. Therefore, in order to investigate possibility of HTS bulks on motor application, a HTS magnet synchronous motor which has 75 pieces of YBCO bulks surface mounted on the rotor has been designed and developed in Cambridge University. After pulsed field magnetization (PFM) process, the rotor can trap a 4 poles magnetic field of 375 mT. The magnetized rotor can provide a maximum torque of 49.5 Nm and a maximum power of 7.8 kW at 1500 rpm.

Magnetization Process of an HTS Motor and the Torque Ripple Suppression

This paper describes in detail the magnetization process of a synchronous motor, constructed with High Temperature Superconducting (HTS) elements on the rotor. The Pulse Field Magnetization (PFM) method is considered to magnetize the HTS rotor as a four-pole trapped flux magnet. A 2D model is built for the magnetization process. During the machine operation, intolerable torque ripples occur, which are mainly caused by the irregular field distribution from the magnetized HTS rotor. In order to suppress the large torque ripples, an iterative learning control method is used in addition to the traditional field-oriented control method.

A Novel Design of Thermally Actuated Magnetization Flux Pump for High Temperature Superconducting Bulks

High temperature superconductors, such as melt-processed YBCO bulks, have great advantages on trapping strong magnetic fields in liquid nitrogen. To enable them to function well, there are some traditional ways of magnetizing them, in which the YBCO bulks are magnetized instantly under a very strong source of magnetic field. These ways would consume great amounts of power to make the superconductors trap as much field as possible. Thermally Actuated Magnetization (TAM) Flux pump has been proved a perfect substitution for these expensive methods by using a relatively small magnet as the source. In this way, the field is developed gradually over many pulses. Unlike conventional flux pumping ways, the TAM does not drive the superconductor normal during the process of magnetization. In former experiments for the flux pump, some fundamental tests were done. In this paper, the experiment system is advanced to a new level with better temperature control to the thermal waves moving in the Gadolinium and with less air gap for the flux lines sweeping through the superconductor. This experiment system leads to a stronger accumulation of the magnetic field trapped in the YBCO bulk. We also tried different ways of sending the thermal waves and found out that the pumping effect is closely related to the power of the heaters and the on and off time.

Thermally Actuated Magnetization Method in High Temperature Superconductor Bulks

It is particularly attractive for High Temperature Superconductors (HTS), such as the melt-processed YBCO single-grain bulks, to trap strong magnetic fields at cryogenic temperatures. With the flux density orders of magnitude much greater than the rare earth magnets, the YBCO bulks can be formed as small and as compact as them. As a result, YBCO bulks are used as magnets in magnetic bearings, MRI scanners and motors, etc. The trapped field by the YBCO magnet is decided by the magnetization, which normally includes three different ways. However, the traditional ways to magnetize the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it. In this paper, we will describe a technique which facilitates the creation of the high magnetic field and utilize a normal permanent magnet in stead. By using this rare earth permanent magnet which is not as strong, this technique involves the superconducting flux pump concept, which means that a much larger field being trapped by the superconductor bulk is caused by a small field repeatedly applied to it. To achieve such pumping effect, an intermedia material is necessary and in the related experiments, Gadolinium is used as such an important thermally actuated material. Traveling magnetic field through it will magnetize the YBCO in such a way that the flux density will be accumulated step by step on the surface on the bulk.

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In order to design a High Temperature Superconducting (HTS) machine that is able to operate safely and reliably, studies on the characterization of Second Generation (2G) HTS tapes are of paramount importance. This paper presents an experimental setup to measure critical current of 2G HTS tapes in high DC magnetic fields (up to 5 Tesla) with an AC current ripple superimposed, as well as various temperatures ranging from 25 K to 77 K. The 2G tape measured is the SGS12050 coated conductor made by SuperPower. The critical current is measured by a flux vector with reference to the widest sample face from 0 to 90 degrees in 10 degree steps. Smaller steps are required close to 0°. A Variable Temperature Insert (VTI) is utilized to control temperature change.

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A high temperature superconducting (HTS) permanent magnet synchronous motor (PMSM) is designed and developed in Cambridge University. It is expected to become cost competitive with the conventional PMSM owing to its high efficiency, high power density, high torque density, etc. The structure and parameters of HTS PMSM are detailed. Both AC losses by transport current and applied filed in stator armature winding of HTS PMSM are also analyzed. Computed and simulated results of the characteristics of the HTS PMSM and conventional PMSM are compared. The improvement on stability of direct torque control (DTC) on the HTS PMSM is estimated, and proved by simulation on Matlab/Simulink.