AbstractThe development of various technologies in recent years provides confidence in the ability to create new-generation electric power applications such as full superconducting electric machines. High temperature superconductor (HTS) bulks or stacks of coated conductors (CCs) have emerged as a promising technology to provide extremely strong magnetic fields by trapping persistent currents. HTS coils are great candidates for higher current capacity and lower losses. They can enable more efficient devices with compact volumes and higher energy efficiencies. A prototype machine has been previously created showing that trapped field magnets and coils have superb benefits to be used as field poles on the rotor and as the armature windings of a motor/generator, respectively, for HTS rotating electric machines.
The key issues for the practical application of such HTS stacks or coils in HTS rotating electric machines are providing high trapped field magnets using in-situ magnetisation methods and mitigating the production of alternating current (AC) losses. Pulsed field magnetisation (PFM) is an efficient method to activate and magnetise HTS stacks using pulses in the order of milliseconds with relatively reduced size and expense of facilities. However, the heat generated during the dynamic process limits the trapped field of stacks. Furthermore, using HTS armature windings in time-varying currents and/or magnetic fields gives rise to unavoidable AC losses on account of the movement of magnetic flux vortices in the superconducting material.
This thesis focuses on investigating the magnetic flux dynamics during PFM especially for trapped field magnets (TFMs) of stacks, and the interpretation of AC losses mitigation methods. Both TFM and AC losses will be analysed using numerical simulations and experimental work.
The original achievements of this research and which are presented in this thesis can be summarised as follows:
1. Modelling finite element models (FEMs) in 2D with realistic laminated structures using H-formulation and T-A formulation. The models of stacks (with and without magnetic substrates) based on commercially available HTS tape were magnetised by PFM in simulations by coupling an electromagnetic model and a thermal model, which were validated by experiments. The results of the H-formulation and T-A formulation models using zero field cooling (ZFC) magnetisation methods were also compared.
2. Exploring a time-saving 2D thermal-coupled model using T-A formulation for laminated-structure model, which is compared with a previous H-formulation model in respect to the distribution of magnetic field, current density and temperature variation in different material layers.
3. Effects of related simulated parameters, magnetism and thicknesses of corresponding substrates on the trapped fields of stacks with and without magnetic substrates were analysed for better HTS tape selection.
4. Experimental validation comparing stacks with and without magnetic substrates was completed using vortex-type PFM. The PFM for stacks with different ramp-down times was tested and the results were compared. The PFM involved with temperature monitoring and sensor arrays for a stack with a magnetic substrate was achieved, which helped to comprehend and confirm the thermal-effect magnetisation and magnetic field distribution.
5. Striated (multifilament) and non-striated superconducting coils were wound. Testing and comparing the critical current and transport AC losses of striated and non-striated coils using both experimental and simulated methods was successfully achieved.
|Date of Award||16 Sept 2020|
|Supervisor||Xiaoze Pei (Supervisor) & Manuchehr Soleimani (Supervisor)|
- High temperature superconductors
- Superconducting machine
- Trapped field magnets
- AC losses
- Numerical modelling