1. High Temperature Superconductivity in Electrical Power Devices:
Applications of
High Temperature Superconductivity
in Electrical Power Devices:
the Southampton perspective
Professor J.K. Sykulski, FIEE, SMIEEE, FInstP
School of Electronics & Computer Science
University of Southampton, UK
University
of Southampton
Superconductivity UK, 23 October 2003

2. (High Temperature Superconductivity)
Applications of HTS
(High Temperature Superconductivity)
ceramic materials discovered in 1986
conductivity 106 better than copper
operate at liquid nitrogen temperature (78K)
cheap technology (often compared to water cooling)
current density 10 times larger than in copper windings
great potential in electric power applications (generators, motors, fault current limiters, transformers, flywheels, cables, etc.), as losses are significantly reduced
present a modelling challenge because of very highly non-linear characteristics and anisotropic properties of materials, and due to unconventional designs

3. HTS transformer built and tested at Southampton 1998/99

4. HTS transformer built and tested at Southampton 1998/99

5. HTS transformer built and tested at Southampton 1998/99

6. HTS transformer built and tested at Southampton 1998/99
Field plots with and without flux diverters

7. HTS transformer built and tested at Southampton 1998/99

8. I Field and current penetration in HTS tape
Diffusion of current density into HTS tape x y
HTS tape I
Flow of transport current through an HTS tape
AC loss as a function of average current density

9. HTS tape subjected to an external magnetic field
Rhyner model:
The critical current density Jc corresponds to an electric field Ec of 100 Vm1, and c = Ec/Jc.
The power law contains the linear and critical state extremes ( = 1 and    respectively).
In practice   and thus the system is very non-linear.

10. HTS tape subjected to an external magnetic field
The governing equation:
The FD scheme:
where
Cij =
and R=x/y

11. HTS tape subjected to an external magnetic field
AC loss as a function of Hm (applied peak magnetic field strength)

12. Field angle 0o
Electric field
Current
45o
90o

13. Experimental verification

14. Superconducting generators and motors
Why ?

15. Superconducting generators and motors
Losses in conventional and superconducting designs

16. Superconducting generators and motors
LTS (Low Temperature Superconductivity) has not been successful in electric power applications
low reliability
high cost
difficult technology
Impact of HTS (High Temperature Superconductivity)
better thermal stability
cheaper cooling
improved reliability

17. Superconducting generators and motors
All conceptual HTS designs and small demonstartors use BSCCO tapes at temperatures between 20K and 30K
at 30K critical fields and currents order of magnitude better than at 78K
it is possible to have a core-less design
But !!!
liquid neon or helium gas needed
increased cost and complexity of refrigeration plant
reduced thermodynamic efficiency
worse reliability and higher maintenance requirements

18. Superconducting generators and motors
Southampton design
100 kVA, 2 pole
cooling at 78 / 81 / 65 / 57 K (liquid nitrogen or air / sub-cooled nitrogen or air)
magnetic core rotor design
- reduces the ampere-turns required by a factor of ten
- significantly reduces fields in the coils
rotor made of cryogenic steel (9%)
10 identical pancake coils made of BSCCO (Ag clad Bi-2223), length of wire approx 10 x 40m

19. Machine Design
Stator
An existing 100kVA stator with 48 slots and a balanced 2-pole 3-phase winding has been used
The pitch of the stator coils ensures that the winding produces very little 7th harmonic field
Higher order fields are reduced significantly by the distribution of the phase conductors throughout each phase belt
The primary concern is the 5th harmonic

20. Machine Design Rotor and field winding
The rotor is made of 9% nickel steel
The core is formed by thirteen plates of various shapes and sizes
The HTS rotor winding is made of silver clad BSCCO-2223 tapes
10 identical coils and each coil has 40 turns
Nominal critical current of >100A at 77K self-field
Each superconducting coil is separated by the flux diverters
The required low temperatures are provided using purpose built closed circuit liquid cryogen cooling system with pipe-network feeding liquid cryogen to the rotor body

21. Machine Design

22. Machine Design

23. Machine Design

24. 2D Modelling and Analysis
In early designs the rotor was made of Invar, but this was rejected due to large difference in thermal expansion coefficient
- Difficult to connect to stainless steel shaft
After thorough investigation, it was decided to use 9% Nickel steel
The 9% Nickel steel is usually produced in plates
- Each plate is 22 mm thick
- Various shapes and sizes
Rotor with Invar design
Rotor with 9% Nickel steel design

25. 2D Modelling and Analysis
The latest design changes:
The HTS coils was reduced to 10 instead of 12 in previous design
Each coil has 40 turns
The plates were made from different thickness

26. 2D Modelling and Analysis
The distribution of the normal field in the HTS coils and the flux potential plot. The flux diverters successfully reduced the normal field to only 0.038T with the air-gap flux at 0.66T.

27. 2D Modelling and Analysis
Gap field up to 19th order
Flux density (T)
Angle (deg)
Harmonic components of air gap flux and phase voltage
0.02% 19
0.01% 17
0.19% 15
0.07% 13
0.39% 11
1.29% 9
0.18% 7
0.17% 5
0.49% 3
100% 1
% Harmonic voltage contribution
Actual harmonic
Winding factor
Sine harmonic magnitude
Space harmonic order
3D modelling?
2D modelling prevents some important features from being investigated:
The effect of the through bolts and their holes.
The leakage flux at the ends of the rotor.

28. 3D Modelling and Analysis
Stator
winding
Stator
HTS field winding
Flux
Diverters
Rotor

29. 3D Modelling and Analysis

30. 3D Modelling and Analysis
The flux density vectors and its distribution

31. 3D Modelling and Analysis
The field over a patch of 180 degree arc and 200mm length at 160mm radius is analysed to extract the harmonics of the air gap flux density

32. 3D Modelling and Analysis
0.03% 19
0.02% 17
0.11% 15
0.09% 13
0.41% 11
0.59% 9
0.18% 7
1.46% 5
0.47% 3
100% 1
% harmonic voltage contribution
Actual harmonic
Winding factor
Sine Harmonic magnitude
Space harmonic order
5th harmonic voltage causes the most significant problem
The undesirable 5th harmonic voltage is higher than predicted in 2D
Total rms harmonic voltage in the 3D model increases from 1.47% to 1.716%
Require further 3D optimisation!
Angle (deg)
Flux density (T)

33. Field Optimisation
To reduce the 5th harmonic, the gap density is reduced at an angle where the 5th harmonic contribution is positive.
Two methods:
(1) Sink the bolts deeper into the core.
(2) Reduce the width as shown in the diagram.
The total rms harmonic voltage improved from 1.46% to 1.35% and the 5th harmonic reduced to 0.55%.
However mechanical constraint allowed only slight improvement.

34. Modelling of Eddy-Current Loss
Two type of losses:
No-load tooth ripple losses due to the distortion of the fundamental flux density wave by the stator slotting.
Full transient non-linear rotating machine
Assumed fixed value of field current (as the cold copper screen prevents changes in reluctance and changes of stator MMF from affecting the value of field current)
Fixed rotation velocity of 3000 rpm
Full-load losses that include the effects of the MMF harmonics of the stator winding.
Static and steady-state models
Transient solution too slow due to low resistance of the cold copper the time constants are very long

35. Modelling of Eddy-Current Loss
No-load losses
Eddy currents occur as 48th time harmonic
Transient losses were estimated and subtracted
Total no-load loss found to be W

36. Modelling of Eddy-Current Loss
Full-load losses
Dominating 5th harmonic (and much smaller 7th)
Losses due to 11th and higher harmonics negligible
Total full-load loss found to be W
(a) DC field
(b) Additional 6th time harmonic field
Contours of vector potential: (a) Non-linear static model and (b) Linear AC model with new current densities defined in each stator slot and incremental permeability data taken from the static model.
Total power loss in the cold region is W.

37. Summary of eddy current losses
No-load losses: W
Full-load losses: W
These losses are released at liquid nitrogen temperature and have to be removed using the inefficient refrigeration system
Each 1W of loss to be removed requires between 15 – 25 W of installed refrigeration power at 78K (a similar figure at 4K would be about 1000 W)

38. Fault Condition Simulation
Full transient non-linear rotating machine model
Losses due to the transient were estimated using a rotating machine simulation
End winding leakage inductance was estimated and added
Fixed time step equivalent to a period for the rotor to pass one stator slot
Simulation was set to run for a period of 2.5 cycles (largest currents occur during this period)
External circuit is connected to finite- element model (to simulate 3-phase short circuit fault condition)

39. Fault Condition Simulation
Results:
Currents in each phase are recorded from each output time-step (curves fitted as shown)
High losses in the stator winding (cause large torque)
Peaks at approximately 1.7 MW
Gradually decrease to steady value as the trapped flux decays

40. Fault Condition Simulation
Results:
Large current also produce large torque
Speed reduces rapidly to % after 50 ms of simulation
Temperature increases to 103K

41. Conclusions
Increasing activity around the world in HTS applications for power devices
All existing demonstrators use HTS tapes at temperatures 20 to 30 K (helium or neon gas)
Southampton design for 78K
Parameters of new tapes improved dramatically
Ability to predict and reduce all ‘cold’ losses of paramount importance to show economic advantages of HTS designs

42. Thank you Superconductivity UK, 23 October 2003 University
of Southampton
Superconductivity UK, 23 October 2003