Flux Pump Based Field Excitation for a High Temperature Superconducting Synchronous Generator
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High-Temperature Superconductors (HTS), first discovered in 1986, have proven to be a potential replacement for their low-temperature counterparts, the Low-temperature superconductors (LTS). HTS materials exhibit the superconducting behaviour (zero resistance against current) at a practically realizable high temperature above absolute zero (>30K). Applications like Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR), which require very high magnetic fields, are finding HTS to be a viable alternative to LTS. Research and development of second generation high temperature superconducting (HTS) wires and tapes in the last decade has made HTS based coils and wires a possibility which is suitable for power applications like transmission cables, transformers, generators and motors. The HTS power applications are expected to capture a substantial amount of market in few years to come-owing to their high power density capability. An HTS generator is one such potential application. Although HTS generators and motors have been designed since quite some time, the designs are still evolving, and the materials in use in these applications are still being researched and the applications are yet to be commericially established. Flux pump offers a unique way of magnetizing a superconductor. In a type 2 HTS flux pump, a quasi-dc emf is induced across the stator of the flux pump thus making it a potential source. By the application of flux pump to a rotating machine, there is a fair amount of success in avoiding the thermal link between cryogenic and non-cryogenic environments, with this the associated thermal loading on the cryogenic system is significantly reduced. Such a system design involves a lot of critical design factors. This thesis aims at investigating the effects of flux pump based pulsating field excitation and the stator design effects on the Total Harmonic Distortion(THD) of the induced voltage in a 10 kW proof-of-concept HTS rotor synchronous generator. This thesis focuses on the performance of an HTS generator integrated with mechanically rotating HTS flux pump. The experiments are conducted to investigate for flux pump as a viable alternative to DC excitation of the field in a synchronous generator. A mechanical HTS flux pump applies a homo-polar magnetic field to an HTS tape acting as the stator and is soldered to the HTS coil of the rotor; this arrangement avoids a physical, thermal link between cryogenic and non-cryogenic environment, and has been proven feasible. Experiments were conducted on two different types of flux pump systems, to study the influence of a rotating homo-polar magnetic rotor on a type 2 HTS tape. The HTS tape acting as the stator in both the types of flux pumps was excited at different frequencies and current measured through field using a hall sensor. The V-I characteristics for various frequencies at different separation distances between the rotor magnets and HTS stator in both the types of flux pump were analysed and were found to be consistent and followed a similar trend in both the cases. A Finite Element Model (FEM) of the proof-of-concept flux pump integrated 10 kW HTS generator, modelled geometrically in SOLIDWORKS, was developed in ANSYS. The boundary conditions, time stepping and meshing, was carried out to provide a reasonable accuracy to carry out the THD performance analysis. Different winding configurations and geometrical placements were also modelled to improve the THD performance; such a trial with the actual system being uneconomical. This research has provided some benchmark results towards the development of an MW range flux pump integrated HTS wind turbine generators. In conclusion, this thesis reports: (1) The experimental study carried out on two different types of HTS flux pumps, designed to study the magnetisation of Yttrium Barium Cupric Oxide (YBCO) tape using a mechanically rotating homopolar magnet system. Consistent V-I characteristics were observed for both the types. (2) Finite Element Modelling (FEM) based analytical study of the harmonic performance of a flux pump integrated proof-of-concept HTS generator. FEM based analysis of the synchronous generator incorporating such a flux pump was carried out for focusing on the stator design and THD performance of the synchronous generator. The results indicate that flux pump can be utilized as an alternative for DC field excitation in HTS rotating machinery, the thesis also contributes towards the practical analysis of proof-of-concept flux pump integrated 10 kW HTS Generator being developed by Robinson Research Institute, VUW. The flux pump operation has been studied by various expert groups, our work on a warm rotor flux pump model based study of a flux pump integrated HTS generator is first of its kind. This research work fills the gap between the machine design aspects and an innovative application of flux pump technology and thus aids in optimized design of the machine.