Testing of a Lightweight Coreless HTS Synchronous Generator Cooled by Subcooled Liquid Nitrogen

A lightweight `coreless´ (100 kW), 2-pole HTS rotor, designed and built at the University of Southampton has been completed and is now in the testing phase of the project. By removing the iron core from the new cold rotor, the weight has been halved in comparison to the previous rotor. The `cold´ components of the rotor assembly make up a sixth of the total weight, which has provided significant time and cost savings in the cool down process. A lightweight doughnut shaped cryostat houses the HTS winding. The winding itself is constructed from sixteen double pancake coils, wound from BSCCO HTS tape. Practical testing of the generator began with a series of stationary tests. The critical current (I_c) of the HTS winding is 77 A at liquid nitrogen temperature (77.4 K) and increases to 156 A, when cooled with liquid air (61.6 K). The air-gap flux density is ~ 0.3 T at 156 A. The machines total harmonic distortion (THD) was predicted to be around 5.4% with the measured THD=5% and this despite using an unoptimized stator. Finally, the synchronous reactance, X_s was evaluated to be 0.24 p.u. This is a 1/5 of the value found for conventional synchronous machines and is largely due to its air-core structure.