Applications of High Temperature Superconductivity in Electrical Power Devices:the Southampton perspective

Professor J.K. Sykulski, FIEE, SMIEEE, FInstP

School of Electronics & Computer ScienceUniversity of Southampton, UK

Superconductivity UK, 23 October 2003

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

HTS transformer built and tested at Southampton 1998/99

HTS transformer built and tested at Southampton 1998/99

HTS transformer built and tested at Southampton 1998/99

HTS transformer built and tested at Southampton 1998/99

HTS transformer built and tested at Southampton 1998/99

IHTS tapeFlow of transport current through an HTS tapeField and current penetration in HTS tapeDiffusion of current density into HTS tapeAC loss as a function of average current density

The critical current density Jc corresponds to an electric field Ec of 100 Vm1, and c = Ec/Jc. The power law contains the linear and critical state extremes ( = 1 and respectively). In practice 10 - 20 and thus the system is very non-linear. HTS tape subjected to an external magnetic field

The governing equation:HTS tape subjected to an external magnetic field

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

CurrentField angleElectric field0o45o90o

Experimental verification

Superconducting generators and motorsWhy ?

Superconducting generators and motorsLosses in conventional and superconducting designs

Superconducting generators and motorsLTS (Low Temperature Superconductivity) has not been successful in electric power applicationslow reliabilityhigh costdifficult technologyImpact of HTS (High Temperature Superconductivity)better thermal stabilitycheaper coolingimproved reliability

Superconducting generators and motorsAll conceptual HTS designs and small demonstartors use BSCCO tapes at temperatures between 20K and 30Kat 30K critical fields and currents order of magnitude better than at 78Kit is possible to have a core-less designBut !!!liquid neon or helium gas neededincreased cost and complexity of refrigeration plantreduced thermodynamic efficiencyworse reliability and higher maintenance requirements

Superconducting generators and motorsSouthampton design100 kVA, 2 polecooling 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 coilsrotor made of cryogenic steel (9%)10 identical pancake coils made of BSCCO (Ag clad Bi-2223), length of wire approx 10 x 40m

Machine DesignStator An existing 100kVA stator with 48 slots and a balanced 2-pole 3-phase winding has been usedThe pitch of the stator coils ensures that the winding produces very little 7th harmonic fieldHigher order fields are reduced significantly by the distribution of the phase conductors throughout each phase beltThe primary concern is the 5th harmonic

Machine DesignRotor 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 tapes10 identical coils and each coil has 40 turns

Nominal critical current of >100A at 77K self-fieldEach superconducting coil is separated by the flux divertersThe required low temperatures are provided using purpose built closed circuit liquid cryogen cooling system with pipe-network feeding liquid cryogen to the rotor body

Machine Design

Machine Design

Machine Design

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 shaftAfter thorough investigation, it was decided to use 9% Nickel steelThe 9% Nickel steel is usually produced in plates- Each plate is 22 mm thick- Various shapes and sizes Rotor with Invar designRotor with 9% Nickel steel design2D Modelling and Analysis

The latest design changes:The HTS coils was reduced to 10 instead of 12 in previous designEach coil has 40 turns The plates were made from different thickness2D 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.2D Modelling and Analysis

2D Modelling and Analysis2D 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.

3D modelling?

3D Modelling and Analysis

3D Modelling and Analysis

3D Modelling and AnalysisThe flux density vectors and its distribution

3D Modelling and AnalysisThe 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

3D Modelling and Analysis5th harmonic voltage causes the most significant problemThe undesirable 5th harmonic voltage is higher than predicted in 2DTotal rms harmonic voltage in the 3D model increases from 1.47% to 1.716%Require further 3D optimisation!

Field OptimisationHowever mechanical constraint allowed only slight improvement. To reduce the 5th harmonic, the gap density is reduced at an angle where the 5th harmonic contribution is positive. (1) Sink the bolts deeper into the core.Two methods:(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%.

Modelling of Eddy-Current LossNo-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 rpmTwo type of losses: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

Modelling of Eddy-Current Loss Eddy currents occur as 48th time harmonic Transient losses were estimated and subtracted Total no-load loss found to be 0.264 WNo-load losses

Modelling of Eddy-Current Loss Dominating 5th harmonic (and much smaller 7th) Losses due to 11th and higher harmonics negligible Total full-load loss found to be 2.319 WFull-load lossesContours 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. (a) DC field (b) Additional 6th time harmonic field Total power loss in the cold region is 2.583 W.

Summary of eddy current lossesNo-load losses: 0.264 WFull-load losses: 2.319 WThese losses are released at liquid nitrogen temperature and have to be removed using the inefficient refrigeration systemEach 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)

Fault Condition Simulation 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)Full transient non-linear rotating machine model External circuit is connected to finite-element model (to simulate 3-phase short circuit fault condition)

Circuit data: Power supply = 0Phase angle = 0Resistance = 0.029 ohmInductance = 0.125 mHCapacitance = 0Circuit length = 325 mmCircuit type = Filament

Conductor data:Turns = 3Resistance/mm=0

Terminal short circuit

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External circuit

HTS Generator

Fault Condition Simulation 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 decaysResults:

Fault Condition Simulation Large current also produce large torqueSpeed reduces rapidly to 19.45% after 50 ms of simulation Temperature increases to 103K Results:

Conclusions Increasing activity around the world in HTS applications for power devicesAll existing demonstrators use HTS tapes at temperatures 20 to 30 K (helium or neon gas)Southampton design for 78KParameters of new tapes improved dramaticallyAbility to predict and reduce all cold losses of paramount importance to show economic advantages of HTS designs

Thank youSuperconductivity UK, 23 October 2003