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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

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Applications of HTS (High Temperature Superconductivity) conductivity 10 6 better than copper current density 10 times larger than in copper windings cheap 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 reduced operate at liquid nitrogen temperature (78K) ceramic materials discovered in 1986 present a modelling challenge because of very highly non-linear characteristics and anisotropic properties of materials, and due to unconventional designs

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HTS transformer built and tested at Southampton 1998/99

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Field plots with and without flux diverters

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HTS transformer built and tested at Southampton 1998/99

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

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Rhyner model: The critical current density J c corresponds to an electric field E c of 100 Vm 1, and c = E c /J c. The power law contains the linear and critical state extremes ( = 1 and respectively). In practice and thus the system is very non-linear. HTS tape subjected to an external magnetic field

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The governing equation: The FD scheme: where C ij = and R= x/ y HTS tape subjected to an external magnetic field

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AC loss as a function of H m (applied peak magnetic field strength)

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Current Field angle Electric field 0o0o 45 o 90 o

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Experimental verification

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Superconducting generators and motors Why ?

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Superconducting generators and motors Losses in conventional and superconducting designs

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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

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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

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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

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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 7 th harmonic field Higher order fields are reduced significantly by the distribution of the phase conductors throughout each phase belt The primary concern is the 5 th harmonic

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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 Machine Design

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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 2D Modelling and Analysis

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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 2D Modelling and Analysis

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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

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Harmonic components of air gap flux and phase voltage 0.02% % % % % % % % % % % Harmonic voltage contribution Actual harmonic Winding factor Sine harmonic magnitude Space harmonic order 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. 3D modelling? Gap field up to 19 th order Flux density (T) Angle (deg)

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

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

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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 3D Modelling and Analysis

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0.03% % % % % % % % % % % harmonic voltage contribution Actual harmonic Winding factor Sine Harmonic magnitude Space harmonic order Angle (deg) Flux density (T) 5 th harmonic voltage causes the most significant problem The undesirable 5 th 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!

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Field Optimisation However mechanical constraint allowed only slight improvement. To reduce the 5 th harmonic, the gap density is reduced at an angle where the 5 th 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 5 th harmonic reduced to 0.55%.

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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 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) 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 Fixed rotation velocity of 3000 rpm Two type of losses: Full-load losses that include the effects of the MMF harmonics of the stator winding. Static and steady-state models Static and steady-state models Transient solution too slow due to low resistance of the cold copper the time constants are very long Transient solution too slow due to low resistance of the cold copper the time constants are very long Modelling of Eddy-Current Loss

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

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

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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)

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

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

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

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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

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Thank you University of Southampton Superconductivity UK, 23 October 2003

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