Superconducting electrical machines

Superconducting Electrical Machines

Bulk Superconductivity Group, Department of Engineering

Dr Mark Ainslie

Royal Academy of Engineering Research Fellow

4th IOP Superconductivity Summer School, Oxford, UK, July 2016

Presentation Outline

• Electrical machines

• Three-phase & rotating fields

• Types of machines

• Synchronous, induction machines

• Superconducting electrical machines

• Case studies/examples with technical challenges & results

• Use of high temperature superconducting (HTS) wire

• Use of bulk HTS materials

B S G 4th IOP Superconductivity Summer School 2016


• Approx. one third of electricity consumed by industry [1]

• Approx. two thirds of this consumed by electric motors [2]

• Using superconductors can increase electric / magnetic loading of an electric machine

• Torque proportional to these + active volume

• Higher current density, higher magnetic field increased torque/power density reduced size & weight

• Lower wire resistance lower losses & higher efficiency / better performance

• Bulk superconductors >> permanent magnets

[1] International Energy Agency, “2013 Key World Energy Statistics”, http://www.iea.org/publications/freepublications/publication/KeyWorld2013.pdf [2] ABB, “ABB drives and motors for improving energy efficiency,” 2010


http://www.iea.org/publications/freepublications/publication/KeyWorld2013.pdf

Electrical Machines

• Electrical machines = motors & generators

• Motors: Convert electrical power mechanical power

• Generators: Mechanical power electrical power

• Huge range of sizes, power ranges & applications

• Sizes: Nanometres up to 10s of metres

• Power ranges: µW up to GW


Electrical Machines - Applications

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Washing machine

Hydroelectric generator

Extractor fan

Wind power generation

Computer fan

Synchronous generator Hobby motors

4th IOP Superconductivity Summer School 2016

Presenter

Presentation Notes

Superconductors offer a good solution for medium to large scale machines. There is a critical or “break-even” size, which is smaller for higher operating temperatures, due to the cryogenics / refrigeration

Electrical Machines

• Generally operate via interaction of current-carrying conductors & magnetic fields

• Various classifications based on how this interaction occurs:

• Alternating Current (AC) / Direct Current (DC)

• Synchronous machines

• Induction machines

• Hysteresis machines

• Switched reluctance machines


Electrical Machines

• Generally operate via interaction of current-carrying conductors & magnetic fields

• Various classifications based on how this interaction occurs:

• Alternating Current (AC) / Direct Current (DC)


• Induction machines

• Hysteresis machines

• Switched reluctance machines

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Most widely used machines Usually “three-phase AC”


Three-Phase Systems & Rotating Fields

• Three-phase is a common method of AC electric power generation, transmission & distribution

• Economical & efficient polyphase system

• Good compromise between complexity, no. of conductors, power transmitted & cost



• Allows generation of a rotating magnetic field

• Basis for three-phase generators & motors

• Balanced three-phase currents flowing in a balanced three-phase winding produce a rotating field of constant magnitude!






Phase Angle (α) Current (I) A 0° I cos ωt

B –120° I cos (ωt – 120°)

C –240° I cos (ωt – 240°)






Phase Angle (α) Current (I) A 0° I cos ωt

B 120° I cos (ωt – 120°)

C –120° I cos (ωt – 120°)






Electrical Machines – Synchronous Machines


• MWs – 100s of MW, used in most large scale power generation, fixed speed applications in mills, factories, etc.

• Rotor rotates in synchronism with line frequency/stator rotating field

• ns [rpm] = 60 f / p f = line frequency, p = no. of pole pairs

e.g., n = 60*50/1 = 3000 rpm

• Frequency of induced generator voltage ↔ rotor/machine rpm


Electrical Machines – Synchronous Machines


• Rotating rotor winding can be

• DC field winding = direct current (“excitation”) fed into winding

• Requires slip rings/brushes

• Can also use permanent magnets

• More costly (economics), but no slip rings/brushes required (maintenance)

• Varying excitation for reactive power compensation not possible


Electrical Machines – Synchronous Condensers

• Synchronous condensers

• Most industrial loads are inductive (“lagging”) by nature

• Draws excess current larger capacity equipment, more line losses, penalties from electricity supply companies

• Varying excitation of rotor windings (under- or over-excited) absorbs or supplies reactive power (VARs)

• Can drive the mechanical load & improve power factor*

*Power factor = ratio of real power to apparent power. A low power factor draws more current for same amount of useful (real) power transferred.


Electrical Machines – Induction Machines

• Induction (asynchronous) machines

• Induction motor is known as the workhorse of industry

• Simple construction, low cost, robust, variable speed possible, long operating lifetime

• Induction generators less utilised (lower efficiency)

• Used in wind turbines, small hydro power generation



• Rotor rotates asynchronously with stator rotating field

• Rotor can be wound or “squirrel cage”

• Slip, s = (ns – nr) / ns

• No load, nr ≈ ns

• Slip increases with load/torque

• Variable speed operation with electronic control

• Torque-speed curves



• Rotor rotates asynchronously with stator rotating field

• Rotor can be wound or “squirrel cage”

• Slip, s = (ns – nr) / ns

• No load, nr ≈ ns

• Slip increases with load/torque

• Variable speed operation with electronic control

• Torque-speed curves



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X1: stator leakage reactance R1: stator winding resistance RI: iron loss resistance Xm: magnetising reactance X2: rotor leakage reactance (referred) R2/s: rotor winding resistance (referred)

Electromagnetic torque:

Maximise power dissipated in R2/s component

Torque-speed curves:



• Over many decades, various superconducting machines shown to be technically feasible over wide range of power ratings

• First attempted in the 1960s, replacing copper windings with LTS

• Although improved efficiency (about 1%) was expected, the main rationale was the size/weight reduction

• Operated at liquid helium temperature (4 K)

• Complexity & cost of 4 K cryogenics prohibitive

• Large AC losses in armature winding unacceptable heat load

• Only DC field winding feasible

• Stationary room-temperature armature + rotating SC field winding (cooled) + electromagnetic shield to attenuate AC field



• Discovery of HTS materials in 1987 renewed enthusiasm

• Expectation that materials could be exploited at higher temperatures, e.g., 77 K

• Reduced capital costs for & complexity of refrigeration system

• Refrigeration efficiency ~100 times greater than at 4.2 K

• Carnot efficiency (ideal): 1 W heat = 2.8 W @ 77 K 68 W @ 4.2 K






• Refrigeration efficiency ~100 times greater than at 4.2 K







• Refrigeration efficiency up to 100 times greater than at 4.2 K


• Multiplication factor incl. cryocooler inefficiency: 20-50 @ 77 K




• Liquid nitrogen is inexpensive (cheaper than milk!), inert, easy to use & store, & readily available

• Improved thermal properties:

• Assuming Cu stabiliser, specific heat increases

• Critical heat flux (nucleate film boiling) much higher for 77 K than 4 K

• For 2G HTS (YBCO), improved in-field critical current density


In-Field Performance of Various Materials

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Data from NHFML: http://fs.magnet.fsu.edu/~lee/plot/plot.htm


Superconducting Electrical Machines – AMSC

• American Superconductor (now AMSC) HTS ship propulsion motors

• U.S. Navy moving towards all-electric ship systems incl. propulsion

• Requirements difficult to achieve using conventional technology

• 2001-2004: 5 MW / 230 rpm [1]

• 2003-2008: 36.5 MW / 120 rpm [2]

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[1] G. Snitchler et al., IEEE Trans. Appl. Supercond. 15 (2005) 2206–2209. [2] B. Gamble et al., IEEE Trans. Appl. Supercond. 21 (2011) 1083–1088.



• ROTOR (AMSC)

• HTS field winding at 32 K

• BSCCO-2223 (1G) wire

• Helium gas cooled, external cryocooler module

• EM shield

• STATOR (ALSTOM)

• Dielectric oil cooled Litz wire

• Air-gap winding, non-magnetic support structure


Presenter

Presentation Notes

Rotating superconducting rotor with cryostat. Thermally isolated and shielded electromagnetically from stationary, conventional stator.


• Rotor integrated with stator at Alstom for full factory testing (2003)

• Full load, full speed testing completed • Operated for 21 hours at

Center for Advanced Power Systems, FSU (2005)

• Achieved specified performance & power ratings under full operating conditions

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5 MW, 230 rpm HTS Motor (left) with 2.5 MW load motor (right)

at ALSTOM, UK



• Extended to 36.5 MW HTS motor

• 14:1 increase in torque over 5 MW machine

• Passed full power tests by end of 2008

• Achieved specific target of 75 metric tonnes


Superconducting Electrical Machines – Other Efforts

• Japanese Super-GM program, 70 MW-class superconducting generators (LTS)

• Siemens developed a 380 kW motor, extended to a 4 MVA generator

• Sumitomo Electric 30 kW motor for electric passenger car

• Converteam 1.7 MW hydroelectric power generator

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SIEMENS SUMITOMO CONVERTEAM


Superconducting Induction Machine

• High temperature superconducting induction-synchronous machine (HTS-ISM)

• Developed at Kyoto University

• Target: electric vehicle drive motor

• Replace rotor windings of squirrel cage induction machine with HTS materials



• Current induced in rotor winding at slip frequency, sf

• s = 1 (standstill) s 0 (near synchronous speed)

• Large current induced when starting flux-flow resistivity (E/J) large starting torque

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Test bench for Kyoto University’s HTS-ISM, including load

(permanent magnet) motor



• Current induced in rotor winding at slip frequency, sf

• s = 1 (standstill) s 0 (near synchronous speed)

• Large current induced when starting flux-flow resistivity (E/J) large starting torque



• As motor accelerates, s and rotor resistance reduces small resistance at low slip

• At s = 0, magnetic flux linked between rotor bars is trapped (induced persistent current) synchronous torque at zero slip

• Hence, coexistence of synchronous and slip modes robust against overload conditions, dynamic switching between modes


Bulk High Temperature Superconductors

• Can be utilised in machines in three ways:

• Flux shielding (reluctance)

• Flux pinning (hysteresis)

• Flux trapping (trapped flux)

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A large, single grain bulk superconductor



• RELUCTANCE MOTOR

• Difference in permeability in direct (‘easy’ path) & quadrature (‘difficult’)

• Rotor aligns itself with direct axis

• Torque, T, proportional to difference of flux in direct & quadrature axes

• T, SC machine > conventional machine (magnetic & non-magnetic interleaving)

B S G Barnes et al. Supercond. Sci. Technol. 13 (2000) 875-878

d axis

q axis



• RELUCTANCE MOTOR • Based on flux shielding property

• Combines bulk SC + ferromagnetic material

• Bulk SC shields flux, reinforcing ferromagnetic material

• Can show flux pinning / hysteresis motor-like behaviour

• Disadvantages

• Increase in torque only up to + ~1/3 of conventional

• Use of iron limits flux density < 2 T; usually much less than, but up to 1 T air gap field

B S G Kovalev et al. Supercond. Sci. Technol. 15 (2002) 817-822



• HYSTERESIS MOTOR

• Based on flux pinning property

• Normal hysteresis motor has ferromagnetic rotor torque produced by interaction between stator/driving field + magnetisation of rotor by this field

• Like induction motor, ‘slippage’ (lag) between driving field & rotor

• Lag independent of speed, so constant torque from start-up to synchronous speed



• HYSTERESIS MOTOR

• Type II superconductor exhibits magnetic hysteresis = pinned flux lines

• Approaching synchronous speed/steady state, behaves like synchronous motor

• Main magnetic field must be produced by stator windings

• Low operating power factor, efficiency, torque density



• Conventional magnets (NdFeB, SmCo) limited by material properties

• Magnetisation independent of sample volume

• Bulk HTS trap magnetic flux via macroscopic electrical currents

• Magnetisation increases with sample volume

• Trapped field given by

Btrap = A µ0 Jc d

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A large, single grain bulk superconductor



• Demonstrated trapped fields over 17 T

• 17.24 T at 29 K 2 x 26.5 mm YBCO Tomita, Murakami Nature 2003

• 17.6 T at 26 K 2 x 25 mm GdBCO Durrell, Dennis, Jaroszynski, Ainslie et al. Supercond. Sci. Technol. 2014

• Significant potential at 77 K • Jc = up to 5 x 104 A/cm2 at 1 T • Btrap up to 1 ~ 1.5 T for YBCO • Btrap > 2 T for (RE)-BCO

• Record trapped field = 3 T at 77 K • 1 x 65 mm GdBCO

• Nariki, Sakai, Murakami Supercond. Sci. Technol. 2005

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Stack of 2 x GdBCO samples that achieved 17.6 T at 26 K


Rotating Machines Using Bulk HTS

• Recent topical review in SUST • An overview of rotating machines

with high-temperature bulk superconductors

• D Zhou, M Izumi et al. Supercond. Sci. Technol. 25 (2012) 103001

• Overview of bulk HTS machine development • Tokyo University of Marine

Science and Technology (TUMSAT) & other groups Comparison of radial

and axial machines


Bulk HTS Axial Flux Motor

• Axial gap, trapped flux-type motor

• Advantages:

• Higher torque/power density

• Compact ‘pancake’ shape

• Better heat removal

• Adjustable air gap

• Multi-stage machines possible

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TUMSAT prototype motor for ship propulsion


Presenter

Presentation Notes

Multi-stage = multi-stator, multi-rotor machines


• Uses stator coils to magnetise HTS bulks with pulsed field

• Cooled using liquid nitrogen

• Dual purpose: magnetising coils, then armature winding

• Closed cycle neon thermosyphon system

• Includes cryo-rotary joint

• Cryogen from static condenser to rotating rotor plate with bulk HTS

• Allows cooling of bulks HTS down to below 40 K

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Schematic diagram of TUMSAT prototype motor




Magnetisation of Bulk Superconductors

• Three magnetisation techniques:

• Field Cooling (FC)

• Zero Field Cooling (ZFC)

• Pulse Field Magnetisation (PFM)

• To trap Btrap, need at least Btrap or higher

• FC and ZFC require large magnetising coils

• Impractical for applications/devices

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ZFC FC


Pulsed Field Magnetisation of Bulk Superconductors

• Achieving in-situ magnetisation is crucial for trapped-flux-type rotating machines

• PFM technique = compact, mobile, relatively inexpensive

• Issues = Btrap [PFM] < Btrap [FC], [ZFC]

• Temperature rise ΔT due to rapid movement of magnetic flux

• Record PFM trapped field = 5.2 T at 29 K (45 mm diameter Gd-BCO) [Fujishiro et al. Physica C 2006]

• Many considerations:

• Pulse magnitude, pulse duration, temperature, number of pulses, shape of magnetising coil(s)

• Dynamics of magnetic flux during PFM process


Presenter

Presentation Notes

Lots of work being done to optimise PFM technique!

Future Views & Prospects

• Significant body of work 2G HTS (RE)BCO coated conductor, MgB2

• Most designs have focused on isolated, cryogenic rotor + conventional stator

• Low ac loss conductor and/or improved winding/machine design

• All-cryogenic / all-superconducting solutions with unprecedented power densities

• Will reduce complexity, improve reliability

• Cost still a major issue as identified by large-scale projects

• Appropriate infrastructure & knowledge required for large-scale manufacture

• Superconducting materials & cryogenic/vacuum systems need to be available on an industrial level


Future Views & Prospects

• Superconductor Science & Technology ‘Focus Issue on Superconducting Rotating Machines’

• Contributions on a variety of topics:

• Novel topologies (claw-pole, homopolar machines)

• Wind turbines (MgB2 field winding & ac loss analysis)

• Bulk-based machines (TUMSAT review)

• HTS stator (BSCCO) thermal analysis

• Brushless HTS-PM exciter for rotating DC field winding (flux pump)

• Magnetic gears (HTS conductors)

Available online: http://iopscience.iop.org/0953-2048/focus/Focus-on-Superconducting-Rotating-Machines


Presentation Outline

• Electrical machines

• Three-phase & rotating fields

• Types of machines

• Synchronous, induction machines

• Superconducting electrical machines

• Case studies/examples with technical challenges & results

• Use of high temperature superconducting (HTS) wire

• Use of bulk HTS materials
