Development of 1 kW Class Novel HTS Rotating Machine Operated by Contactless Superconducting Field Exciter

Abstract

This dissertation deals with the design, fabrication, and performance testing of a prototype machine for the world's first implementation of a new type high-temperature superconducting rotating machine (HTSRM), which is charged and operated by the application of a contactless excitation technique with a rotary-type HTS flux pump based on a permanent magnet (PM). HTS conductors can provide a large current stably without attenuation of transport current by exhibiting the inherent characteristic of "zero resistance" at higher temperatures and magnetic fields than low-temperature superconductors. Thus, HTS conductors are highly suitable for largescale rotating machineries that require a high energy density, such as low-speed and high-torque rotating machines. Some examples of these are direct drive-type generators for off-shore wind turbines or propulsion motors for electric ships. Among HTS conductors, rare-earth barium copper oxide tape, which is a second generation (2G) coated conductor, provides high current density to rotor-field coils (FCs). This enables the manufacturing of machines with smaller volumes and lower weights compared with conventional rotating machinery of equal capacity. However, to achieve these objectives, electrical and mechanical connections are required between a direct-current (DC) power supply (PS) at room temperature and an FC at cryogenic temperature through a slip-ring/brush set and metal current leads, constituting the contact-type field excitation device. The use of such contact-type excitation device has the following technical limitations. 1. Mechanical noise and vibration in a slip-ring/brush set decrease the system-reliability, -economics, and -productivity owing to frequent maintenance requirement. 2. Thermal losses in metal current leads decrease the thermal stability of HTS coil owing to local temperature rise, and decease in system efficiency due to the increased cryogenic cooling load and corresponding cooling cost. 3. External DC power sources decrease the system-efficiency, -economics, and maintainability because of the use of additional DC PSs. Therefore, the development of a contactless excitation system is required to fundamentally solve the abovementioned problems of conventional contact-type excitation system. Thus, a contactless superconducting field exciter (CSFE) with a 2G HTS flux pump is very promising for realizing a more economic HTSRM with high efficiency and stability for various industrial applications, compared with mechanical contact excitation. By using this exciter, the slip-ring/brush set, metal current lead, and external PS in the contact excitation system can be eliminated by the onboard exciter system that can generate and supply its DC power inside the HTS rotor. The HTS flux pump applied as a field exciter can completely block the thermal intrusion from the conventional contact-type field excitation devices at room temperature and strongly suppress the internal Joule heating during current excitation. Thus, this apparatus enables high efficiency, strong reliability, and cost saving in superconducting rotating machine systems by removing the mechanical connection between the contact excitation system at room temperature. The general technical advantages of CSFE with the flux pump are as follows. 1. To provide high system-efficiency and -economics by omitting the metal current leads, slip-ring/ brush assembly, and external power source. 2. To provide high electrical and thermal stability of HTS FCs by perfectly eliminating heat intrusions through metal current leads. 3. To considerably reduce refrigeration cost by almost completely eliminating thermal losses in metal current leads. 4. To provide high-durability, -economic efficiency, -maintainability, and -productivity of system by perfectly eliminating noise and vibration from mechanical frictions at slip sing/ brush assembly. Although this ability of HTS flux pump is expected to enable the commercial application of the HTSRM because of the abovementioned technical advantages, any practical demonstration of an HTS noncontact excitation system in rotary superconducting applications, specifically, HTS rotating machines, has not yet been conducted or reported on. Therefore, this dissertation presents a novel structure design for HTSRM equipped with CSFE. This exciter originates from the socalled rotating magnet-based HTS flux pump and is a noncontact PS for superconducting DC magnets. The novelty of the core technologies proposed in this dissertation and the corresponding advantages are as follows. 1. An integrated rotor structure with HTS field poles of rotating machine and DC power generation unit of CSFE; Increased cooling efficiency by simplifying cryogenic cooling structure. 2. A structure that does not need additional rotor drive for generating time-varying magnetic field, which is required to excite DC power; Increased system-maintainability and -efficiency by simplifying system structure. 3. A structure with a cylindrical time-varying magnetic field supply unit at room temperature; Increased controllability of charged current by readily controlling the number of PMs at room temperature. 4. Novel structures (toroidal-head and magnetic back plate) and winding method (serial multiple winding with no junction) in CSFE rotor; Increased non-contact charge current within limited rotor size. In dissertation, to verify and evaluate the feasibility of CSFE for practical operation of HTSRM, an HTS rotor with an integrated DC power source in field exciter and HTS FCs was designed, fabricated, and tested. The prototype HTSRM with 1 kW class capacity was mechanically, thermally, and electromagnetically designed and analyzed using numerical design process. The current pumping mechanism using external time-varying magnetic field was introduced to develop CSFE. The fabricated CSFE was experimentally tested on full-scale HTS FCs to analyze and evaluate the contactless current charge characteristics and its performances, respectively. The test results showed that although it was partially passive, current control is possible within a certain range by changing the number of PM, rotating speed, and air-gap length. In addition, the self-controlling behavior in charged current by CSFE was observed in the thermal transient state in which the liquid cryogen is insufficient, and it is considered that the reliability in current supply and the thermal stability in HTS FCs can be improved. Finally, motor-generator setup was built based on the developed rotating machine system. A non-contact excitation performance was successfully confirmed in generator operation, and its charge behaviors were experimentally analyzed. The test results verified that the developed field exciter can improve the system efficiency by approximately 1% by considerably decreasing the excitation loss, compared with the field charge using metal current leads. Further, an output of 1086 W and field current of 101 A were achieved at a rated rotating speed of 400 rpm. Moreover, the design results from the 2D finite element analysis were 90% consistent with the experiment results under the same operating conditions. Total harmonic distortion of HTSG was measured at the 1% level, which experimentally confirmed that the pulsation of the non-contact charging current by the flux pump is not reflected in the output of HTSG. For charge behaviors of 100-Aclass CSFE, it is suggested that the developed field exciter is suitable for applications requiring constant speed operation because it operates like a permanent magnet generator or motor within a certain speed range.