Superconducting Rotating Machines

Transcription

1 Superconducting Rotating Machines Dr Mark Ainslie EPSRC Early Career Fellow EUCAS 2017 Short Course: Superconducting Power Applications Geneva, 17 September 2017 Bulk Superconductivity Group, Department of Engineering

2 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) conductors Use of bulk HTS materials

3 Electrical Machines

4 Electrical Machines Electrical machines = motors & generators Motors: Generators: Convert electrical power mechanical power Mechanical power electrical power Huge range of sizes, power ranges & applications Sizes: Power ranges: Nanometres up to 10s of metres µw up to GW

5 Electrical Machines Approx. one third of electricity consumed by industry [1] Approx. two thirds of this consumed by electric motors [2] Electric motors & systems they drive are the single largest electricity end-use [3] 43-46% of global electricity consumption About 6 Gt of CO 2 emissions [1] International Energy Agency, 2013 Key World Energy Statistics, 2013 [2] ABB, ABB drives and motors for improving energy efficiency, 2010 [3] International Energy Agency, Energy-Efficiency Policy Opportunities for Electric Motor Driven Systems, 2011 [1] & [3] available from

6 Electrical Machines Applications Washing machine Extractor fan Computer fan Hydroelectric generator Synchronous generator Wind power generation Hobby motors

7 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

8 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 Most widely used machines Usually three-phase AC Hysteresis machines Switched reluctance machines

9 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

10 Three-Phase Systems & Rotating Fields 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!

11 Three-Phase Systems & Rotating Fields Allows generation of a rotating magnetic field Basis for three-phase generators & motors Phase Angle (α) Current (I) a 0 I cos ωt b 120 I cos (ωt 120 ) c 240 I cos (ωt 240 ) Balanced three-phase currents flowing in a balanced three-phase winding produce a rotating field of constant magnitude!

12 Three-Phase Systems & Rotating Fields Allows generation of a rotating magnetic field Basis for three-phase generators & motors Phase Angle (α) Current (I) a 0 I cos ωt b 120 I cos (ωt 120 ) c 240 I cos (ωt 240 ) Balanced three-phase currents flowing in a balanced three-phase winding produce a rotating field of constant magnitude!

13 Three-Phase Systems & Rotating Fields

14 Three-Phase Systems & Rotating Fields

15 Electrical Machines Synchronous 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 n s [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

16 Electrical Machines Synchronous 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

17 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.

18 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

19 Electrical Machines Induction Machines Rotor rotates asynchronously with stator rotating field Rotor can be wound or squirrel cage Slip, s = (n s n r ) / n s No load, n r n s Slip increases with load/torque Variable speed operation with power electronic control Torque-speed curves

20 Electrical Machines Induction Machines Rotor rotates asynchronously with stator rotating field Rotor can be wound or squirrel cage Slip, s = (n s n r ) / n s No load, n r n s Slip increases with load/torque Variable speed operation with power electronic control Torque-speed curves

21 Electrical Machines Induction Machines X 1 : stator leakage reactance R 1 : stator winding resistance R I : iron loss resistance X m : magnetising reactance X 2 : rotor leakage reactance (referred) R 2 /s: rotor winding resistance (referred) Torque-speed curves: Electromagnetic torque: Maximise power dissipated in R 2 /s component

22 Superconducting Electrical Machines

23 Superconducting Electrical Machines Synchronous (radial) machine P = π 2 ത B ҧ JD 2 Lω General machine P = 1 m π 1 + K Φ m 1 2 K ek i K p λ 2 0 BAf mech D²Lη Using superconductors can increase magnetic / electric 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] S. Huang et al., A General Approach to Sizing and Power Density Equations for Comparison of Electrical Machines, IEEE Trans. Ind. Appl (1998) 92-7

24 Superconducting Electrical Machines 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 Improved efficiency (about 1%) was expected, but 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

25 Superconducting Electrical Machines Optimal configuration for ac superconducting machines from studies using LTS conductors in the 1960s & 70s Superconducting dc field winding (rotating) External excitation current supply ROTOR Liquid helium coolant High vacuum cryogenic insulation Normally-conducting eddy current shielding system (conducting shell) STATOR Normally-conducting armature winding (stationary) Cooling system rejecting heat at ambient temperature Laminated back-iron flux shield

26 Superconducting Electrical Machines 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 = K K

27 Superconducting Electrical Machines 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 = K K

28 Superconducting Electrical Machines 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 up to 100 times greater than at 4.2 K Carnot efficiency (ideal): 1 W heat = K K Multiplication factor incl. cryocooler inefficiency: 77 K

29 Superconducting Electrical Machines Potential savings in weight / volume increased power / torque density Particularly attractive for applications where size / weight is a significant cost driver: Wind power generation, electric aviation, electric ship propulsion Advantages only appear over critical, break-even size or rating due to cryogenic system(s) I.e., where the refrigeration penalty (size, weight, cost) becomes negligible Critical / break-even size reduces with increasing operating temperature

30 Superconducting Electrical Machines Discovery of HTS materials in 1987 renewed enthusiasm! Liquid nitrogen is inexpensive, 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

31 In-Field Performance of Various Materials Data from NHFML:

32 Superconducting Electrical Machines HTS Conductors

33 Superconducting Electrical Machines AMSC American Superconductor (now AMSC) HTS ship propulsion motors U.S. Navy moving towards allelectric ship systems incl. propulsion Requirements difficult to achieve using conventional technology : 5 MW / 230 rpm [1] : 36.5 MW / 120 rpm [2] [1] G. Snitchler et al., IEEE Trans. Appl. Supercond. 15 (2005) [2] B. Gamble et al., IEEE Trans. Appl. Supercond. 21 (2011)

34 Superconducting Electrical Machines AMSC 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

35 Superconducting Electrical Machines AMSC 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 5 MW, 230 rpm HTS Motor (left) with 2.5 MW load motor (right) at ALSTOM, UK

36 Superconducting Electrical Machines AMSC 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 1/2 size 1/3 weight of traditional copper-based propulsion systems BSG

37 Superconducting Electrical Machines Other Efforts Japanese Super-GM program, 70 MW-class superconducting generators (LTS) AMSC developed a 8 MVAR (1800 rpm) synchronous condenser Siemens 380 kw (1500 rpm) motor, extended to a 4 MVA (3600 rpm) generator Sumitomo Electric 30 kw motor for electric passenger car Converteam 1.7 MW (214 rpm) hydroelectric power generator SIEMENS SUMITOMO CONVERTEAM

38 Superconducting Induction Machine High temperature superconducting inductionsynchronous machine (HTS-ISM) Developed at Kyoto University Target: electric vehicle drive motor Replace rotor windings of squirrel cage induction machine with HTS materials

39 Superconducting Induction Machine 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 Test bench for Kyoto University s HTS-ISM, including load (permanent magnet) motor

40 Superconducting Induction Machine 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

41 Superconducting Induction Machine 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

42 Superconducting Electrical Machines Bulk Superconductors

43 Bulk High Temperature Superconductors Can be utilised in machines in three ways: Flux shielding (reluctance) Flux pinning (hysteresis) Flux trapping (trapped flux) A large, single grain bulk superconductor

44 Bulk High Temperature Superconductors RELUCTANCE MOTOR Difference in permeability in direct ( easy path) & quadrature ( difficult ) d axis q axis Rotor aligns itself with direct axis Torque, T, proportional to difference of flux in direct & quadrature axes T, SC machine > conventional machine (magnetic & nonmagnetic interleaving) Barnes et al. Supercond. Sci. Technol. 13 (2000)

45 Bulk High Temperature Superconductors 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 Kovalev et al. Supercond. Sci. Technol. 15 (2002)

46 Bulk High Temperature Superconductors 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

47 Bulk High Temperature Superconductors 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

48 Bulk High Temperature Superconductors 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 A large, single grain bulk superconductor

49 Bulk High Temperature Superconductors 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 B trap = k µ 0 J c a a = radius; k = geometric constant (Biot- Savart law) Typical trapped magnetic field profile above a bulk superconductor

50 Bulk HTS Trapped Fields Trapped field measurements tell us the potential of a sample as a strong, permanent magnet Demonstrated trapped fields over 17 T T at 29 K 2 x 26.5 mm YBCO Tomita, Murakami Nature T at 26 K 2 x 25 mm GdBCO Durrell, Dennis, Jaroszynski, Ainslie et al. Supercond. Sci. Technol. 2014

51 Bulk HTS Trapped Fields Significant potential at 77 K J c = up to 5 x 10 4 A/cm 2 at 1 T B trap up to 1 ~ 1.5 T for YBCO B trap > 2 T for (RE)BCO, neutron-irradiated YBCO Record trapped field = 3 T at 77 K 65 mm GdBCO Nariki, Sakai, Murakami Supercond. Sci. Technol. 2005

52 Rotating Machines Using Bulk HTS Comparison of radial and axial machines Radial-type bulk machine

53 Rotating Machines Using Bulk HTS Axial-type bulk machine Comparison of radial and axial machines

54 Bulk HTS Axial Flux Motor Axial gap, trapped fluxtype motor Advantages: Higher torque/power density Compact pancake shape Better heat removal Adjustable air gap Multi-stage machines possible *Izumi et al. Tokyo University of Marine Science and Technology (TUMSAT) TUMSAT* prototype motor for ship propulsion

55 Bulk HTS Axial Flux Motor 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 Schematic diagram of TUMSAT prototype motor

56 Bulk HTS Axial Flux Motor

57 Magnetisation of Bulk Superconductors Three magnetisation techniques: Field Cooling (FC) Zero Field Cooling (ZFC) Pulse Field Magnetisation (PFM) To trap B trap, need at least B trap or higher FC and ZFC require large magnetising coils Impractical for applications/devices ZFC FC

58 Pulsed Field Magnetisation PFM technique: compact, mobile, relatively inexpensive Issues: B trap [PFM] < B trap [FC], [ZFC] Temperature rise ΔT due to rapid movement of magnetic flux Record PFM trapped field: K Top surface of 45 mm diameter Gd-Ba-Cu-O Fujishiro et al. Physica C 2006 Record trapped field by FC: K Centre of 2 x 25 mm diameter Gd-Ba-Cu-O Durrell et al. Supercond. Sci. Technol. 2014

59 Pulsed Field Magnetisation Many considerations for PFM: Pulse magnitude, pulse duration, temperature(s), number of pulses, type of magnetising coil(s), use of ferromagnetic materials Dynamics of magnetic flux during PFM process Multi-pulse PFM: effective in increasing trapped field/flux Fujishiro et al. Physica C 2006 Zhou et al. Appl. Phys. Lett. 2017

60 Superconducting Electrical Machines Future Outlook

61 Future Views & Prospects Significant body of work 2G HTS (RE)BCO coated conductor, MgB 2 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

62 Future Views & Prospects Bulk-based Machines Magnetisation techniques Improved in-situ PFM 5-7 Tesla trapped fields Magnetic field stability + demagnetisation Time-varying fields in real machine environments demagnetisation? Mechanical properties of bulks + mechanical design of machines Limiting factor for very high field applications (> 7-9 T); brittle ceramics Development of numerical models Crycooler + cryostat technology Efficiency, reliability, cost, redundancy; complete engineering solution required

63 Recommended Further Reading Superconductor Science & Technology Focus Issue on Superconducting Rotating Machines (2016) Novel topologies (claw-pole, homopolar machines) Wind turbines (MgB 2 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: Superconducting-Rotating-Machines

64 Recommended Further Reading J. R. Bumby, Superconducting rotating electric machines (monographs in electrical and electronic engineering), Oxford: Clarendon Press (1983) K. S. Haran et al., High Power Density Superconducting Machines Development Status and Technology Roadmap, Supercond. Sci. Technol. (accepted for publication) Bulk-based machines: D. Zhou et al., An overview of rotating machine systems with high-temperature bulk superconductors, Supercond. Sci. Technol. 25 (2012) Y. Zhang et al., Melt-growth bulk superconductors and application to an axial-gaptype rotating machine, Supercond. Sci. Technol. 29 (2016)

65 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) conductors Use of bulk HTS materials