Elektromagnetische Auslegung eines supraleitenden synchronen Windgenerators mit mittlerer Drehzahl für 13 MW, 600 min-1

A geared mid-speed 24-pole permanent magnet excited rotor of a 6 MW inverter-operated synchronous wind generator with an oil-immersed stator cooling shall be replaced for prototype testing by an electrically excited rotor with a high-temperature superconducting (HTS) exciter winding. The project is funded by the German state, represented and administered by Forschungszentrum Jülich GmbH. So far, the already constructed stator and two prototype HTS exciter coils are manufactured, and the coils are under test at cryogenic temperature. The finally completed superconducting wind generator shall be tested thoroughly at the Flender company test rig, Ruhstorf/Rott, Germany, with stator winding inverter supply and alternatively with diode rectifier supply at variable speed operation. The electromagnetic design for the superconducting rotor at 30 K and 1000 A DC exciter current promises an increase of output power up to 13 MW at unity power factor at rated speed 600 rpm due to the possible high rotor DC current density and the direct oil cooling of the stator winding. The HTS rotor is encapsulated in a rotating stainless-steel cryostat as a vacuum container with a wrapped thermal multi-layer insulation. The residual thermal inflow is extracted via copper bus bars, connected to a liquid neon reservoir (cooling the current leads from warm to cold, too), which is provided by a stationary cold head. The two slip rings with the carbon brushes for DC exciter current supply are air-cooled. The 2D and 3D Finite Element numerical electromagnetic design of the two-layer pancake exciter coils must consider the highly non-linear voltage-current characteristic of the HTS material at a given temperature and magnetic flux density. The air gap field harmonics due to the open stator slots and the additional air-gap magnetic fields of the harmonic stator currents due to the inverter or diode supply cause eddy currents, especially in the electrically conductive cryostat wall. For a proper damping of these magnetic fields to limit AC losses in the HTS material, an additional copper damper must be used at the cryostat surface. The time-step eddy current simulations in the cryostat, in the rotor iron with its non-linear magnetization characteristic, and in the HTS coil material are discussed. The methods to reduce the computation time due to the long electrical time constants, until steady state operation is reached, and due to the necessary refined meshing for the small eddy current penetration depth, are presented. The numerical results of the eddy-current losses at the inverter and diode supply at rated operation and at reduced speed and partial load are reported. These eddy current loss components are used together with the computed thermal inflow to the rotor winding due to heat radiation and conduction to tailor the rotor cooling circuit and the needed size of the cold head and of the cryo-coolers. The expected generator efficiency and the envisaged total active mass show a significantly increased power density at a strongly reduced amount of rare earth material, compared to the state-of-the-art permanent magnet generator solutions.