Study On Key Technical Issues Of A Synchronous Generator With HTS Amrature Windings

Because superconductor has properties of high current density and low loss,the electric rotating machines can improve the power density and efficiency then reduce the volume and weight by replacing copper with superconductor.Recently the main trend of superconducting motor research is to increase the gap field by using DC excitation superconducting windings.Furthermore,the power density can be further improved if the armature winding also use superconducting coils,which would make a fully superconducting rotating machine.But the AC loss problem is the main obstacle of developing superconducting armature windings thus there were few relevant studies.Considering the frequency dependency of AC loss,it suggests that the AC loss can greatly drop off when the superconducting windings operate at low frequency conditions,such as large wind turbines.So the superconducting armature windings have great potential at low frequency conditions.In this dissertation,for studying the feasibility of low frequency HTS amrature winding,a synchronous generator with a PM rotor and an HTS armature winding was developed,of which the rated power/revolution speed/frequency is 2.5 kW/300 rpm/10 Hz.Taking account of the difficulty of cryogenic techniques,the working temperature of the HTS armature winding is set around liquid nitrogen temperature.Another technical exploration is using conduction cooling method to cool down the stator and HTS armature windings,which would make the cryogenic structure simplified and easy to promote in future designs.The reliability of the conduction cooling method is also a key research topic.The HTS armature coil is wound by Bi-2223/Ag tapes.In design stage,the mechanical,thermal and electromagnetic properties of HTS coils were seriously considered.The steady cryogenic temperature of HTS stator is 82.1 K after cooling process,and during no-load running test the temperature increment of HTS stator is 1.3 K.These two facts indicate that conduction cooling method can sufficiently cool down the HTS stator and suppress temperature increment caused by iron loss.The load test data show that the output properties fit well with simulation results,and the generator can operate at rated power ststus for short time.This generator can still operate steadily for long time until the output power/armature current(effective value)reaches 1.6 kW/12.6A.Combining the temperature measurement data and basic principle of heat transfer,the heat leakage,heat production,temperature distribution and variation of HTS stator was analyzed and calculated in theory.The calculation results fit well with the experiment data.Comparing the AC loss of HTS coils at static state and coil temperature change during machine running period,the AC loss of HTS coils during machine running period is estimated.The results show that when the AC loss of HTS coil is lower than 1.5 W,the HTS generator can run steadily for long time.From the tests and analysis,we can conclude that applying HTS armature windings at low frequency is feasible and conduction cooling method is also available.At last,based on developing experience of the former HTS generator,we try to design the electromagnetic structure of a 15 kW/150 rpm/10 Hz fully HTS generator.The local flux leakage on HTS coils is optimized to guarantee the safe operation of HTS coils.And the armature load is also improved to reflect the merit of high current density of HTS conductors.