Development of a brushless HTS exciter for a 10 kW HTS synchronous generator

Abstract

HTS synchronous generators, in which the rotor coils are wound from high-T c superconducting wire, are exciting attention due to their potential to deliver very high torque and power densities. However, injection of the large DC currents required by the HTS rotor coils presents a technical challenge. In this paper we discuss the development of a brushless HTS exciter which operates across the cryostat wall to inject a superconducting DC current into the rotor coil circuit. This approach fundamentally alters the thermal load upon the cryogenic system by removing the need for thermally inefficient normal-conducting current leads. We report results from an experimental laboratory device and show that it operates as a constant voltage source with an effective internal resistance. We then discuss the design of a prototype HTS-PM exciter based on our experimental device, and describe its integration with a demonstration HTS generator. This 200 RPM, 10 kW synchronous generator comprises eight double pancake HTS rotor coils which are operated at 30 K, and are energised to 1.5 T field through the injection of 85 A per pole. We show how this excitation can be achieved using an HTS-PM exciter consisting of 12 stator poles of 12 mm YBCO coated-conductor wire and an external permanent magnet rotor. We demonstrate that such an exciter can excite the rotor windings of this generator without forming a thermal-bridge across the cryostat wall. Finally, we provide estimates of the thermal load imposed by our prototype HTS-PM exciter on the rotor cryostat. We show that duty cycle operation of the device ensures that this heat load can be minimised, and that it is substantially lower than that of equivalently-rated conventional current leads.

Design Improvisation for Reduced Harmonic Distortion in a Flux Pump-Integrated HTS Generator

“…In the present study with the flux pump, the same trend was observed and the non-uniform voltage profile observed across the HTS stator (see Figure 3) is similar to that of Bumby et al [16]. Based on the previous reported results [9], it is well established that the flux pump can be successfully used as an alternative to DC excitation for the field system in a generator.…”

J e (4.2 K, 31.2 T) beyond 1 kA/mm2 of a ~3.2 μm thick, 20 mol% Zr-added MOCVD REBCO coated conductor

“…REBCO coated conductors have also been demonstrated in a magnet with an overall 42.5 T field generated by a 11.3 T REBCO insert coil in a 31.2 T background field 5 , a 26 T superconducting magnet 6 , and the concept design of a 100 T magnet 7 . Moreover, prototype coils for rotation machinery applications 8 , superconducting magnetic energy storage 9 , magnetic resonance imaging and nuclear magnetic resonance 10–12 , and high energy physics 13, 14 are being extensively investigated. A critical need for high field magnet R&D is the substantially increased performance and reduced cost, and aggressively pursuing the development of superconducting magnets 15 .…”

The dynamic resistance of YBCO coated conductor wire: effect of DC current magnitude and applied field orientation

“…HTS field windings in rotating machines experience an applied ripple field whilst carrying a DC current. The resulting dynamic resistance leads to heat dissipation which may impose a substantial heat load on the cryogenic system [6][7][8]. Dynamic resistance is also critical to the design and operation of HTS flux pumps, as it sets a limit to the maximum achievable current which such devices can deliver [9][10][11][12][13].…”