Capacitively Coupled Power Transfer Systems Concepts & Opportunities

Transcription

1 Capacitively Coupled Power Transfer Systems Concepts & Opportunities Dr. Dan Ludois Assistant Professor, Electrical & Computer Engineering University of Wisconsin Madison University of Illinois Urbana Champaign November 2, 2015

2 University of Wisconsin

3 A Little Group History Founded in 1981 Prof. Don Novotny and Prof. Tom Lipo 4 corporate sponsors Today ~90 corporate sponsors (~$15k buy in) Flexible money, compliments federal grants Individual sponsored projects case by case 22 M.S. Students on campus 49 Ph.D. Students on campus Prof. Don Novotny (Emeritus) 26 Distance Learning Graduate Students off campus 34 years in power & energy research power electronics, electric machines, grid applications MS Degrees granted: 364 PhD Degrees granted: 147 Prof. Tom Lipo (Emeritus)

4 Tenured Faculty Prof. Bob Lorenz Physics-based control and estimation methodologies Design for sensing, loss minimization, dynamic power conversion and observerbased sensor replacement Volt-second based control approaches Shaping fields for power semi sensor integration Prof. Tom Jahns Electric machines, especially PM machines Power conversion and control for distributed generation Microgrids Battery energy storage Prof. Giri Venkataramanan Application of wide bandgap devices Power converters for energy applications End-user power quality Microgrids Distributed generation for sustainable power

5 Tenure Track Faculty Prof. Yehui Han Resonant converters High frequency magnetic components Battery equalizers Multi-level converters Switched capacitor converters Integrated modular drives Prof. Bulent Sarlioglu Converters using wide bandgap devices Electric machines High-speed electric machines and controls Axial flux machines Power converters for energy applications Prof. Dan Ludois Multi-level converters Wireless power transfer Capacitive power transfer New wound-field machine topologies Integrated L-C filters Electrostatic machines

6 Research Projects Converters Controls Machines Devices & Multi-Phys Integ Power Sys

7 Why Wireless Power? Connector free Increased reliability No compatibility issues Freedom of motion Galvanic isolation Access enclosed spaces

8 Wireless Power Transfer Large gap, coreless, inductive coupling V in Primary Electronics B Secondary Electronics V out Small gap, ferrous core, inductive coupling (gapped transformers) V in Primary Electronics B B Secondary Electronics V out Magnetic field coupler Electrostatic coupling, i.e. capacitive coupling V in Primary Electronics E E Electric field coupler Secondary Electronics V out

9 Capacitive Power Transfer (CPT) CPT is the use of electric fields to transfer power and/or data across a boundary CPT provides galvanic isolation CPT eliminates physical electrical contact (no connectors) Electrically possible via: High frequency power electronics Surface Area Dielectric materials E V in Primary Electronics Secondary Electronics V out E Electric field coupler

10 CPT Fundamentals What are the underlying physics that govern CPT? How do we push power though a simple CPT system? i C Primary Primary A E v C E Gap, d C A r d Secondary o Secondary R load

11 Calculate v C for a sinusoidal i C : Typically C is small, ~10s -100s pf High reactance Results in high voltage CPT Fundamentals Cont. j C If the gap field exceeds its dielectric strength, breakdown will occur! v C i c Calculate E by subbing in: The gap electric field: C E A r v d C d o j A r o ic

12 CPT is about A/Hz (charge) Electric field is proportional to Amps per Hertz [A/Hz] has units of Coulombs Electrostatic counterpart to [V/Hz] in magnetic systems Maximize power transfer capability Higher frequencies for a given current Lower current higher voltage loads Large amounts of surface area E Increase gap relative permittivity above unity v d C j A r o ic Keep plates as close together as possible Air limit is E air = 3.0 kv/mm, design limit Small gaps: greater area per unit volume

13 CPT Potential Advantages Materials Displace the use of Copper windings Displace the use of Iron Displace the use of Litz wire coils These are replaced with: Thin Aluminum foil surfaces Shielding For small gaps with wide surface area, E fields largely cancel outside the gap

14 CPT Beginnings Nikola Tesla using the electric field of a parallel plate capacitor to drive fluorescent tube lighting, at Columbia University NY, May 20 th online:

15 Low Power Applications Wahab, et al 1997 Byungcho, et al 2004 Culurciello and Andreou 2006 Piipponen, et al 2007

16 Low Power Charging Pads Chao Liu, et al; Pad charging systems are transitioning from concept to commercialization. All pads pictured are CPT

17 Low Power Vehicle Charging Battery charging for mobile robots Robot backs up to a wall with imbedded coupling plates Strip or ribbon form factor A.P. Hu, et al; 2008

18 Brush & Slip Ring Replacement L. Chao, A. P. Hu, and N. K. C. Nair, "Coupling study of a rotary Capacitive Power Transfer system," in Industrial Technology, ICIT IEEE International Conference on, 2009, pp D. C. Ludois, K. Hanson, and J. K. Reed, "Capacitive power transfer for slip ring replacement in wound field synchronous machines," in Energy Conversion Congress and Exposition (ECCE), 2011 IEEE, 2011, pp

19 Kilowatt Scale CPT 1kW to load, 90.1% efficient 200kHz, 300V input 12nF Coupling Cap ~200in 2 Al foil ~4mil Polypropylene dielectric Jiejian Dai; Ludois, D.C., "Single Active Switch Power Electronics for Kilowatt Scale Capacitive Power Transfer," in Emerging and Selected Topics in Power Electronics, IEEE Journal of, vol.3, no.1, pp , March 2015

20 Large Gap Higher Power CPT Fei Lu; Hua Zhang; Hofmann, H.; Mi, C., "A Double Sided LCLC Compensated Capacitive Power Transfer System for Electric Vehicle Charging," in Power Electronics, IEEE Transactions on, vol.30, no.11, pp , Nov kW system, large gap, 1MHz, 7.2 kvrms tank A. Kumar, S. Pervaiz, C. K. Chang, S. Korhummel, Z. Popovic, and K. K. Afridi, Investigation of power transfer density enhancement in large air gap capacitive wireless power transfer systems, in 2015 IEEE Wireless Power Transfer Conference (WPTC), 2015, pp kw Scale Vehicle Concept Large air gap high power CPT systems are emerging, but serious safety concerns remain Low coupling capacitance requires High Voltage & Higher Frequency 614 V/m for MHz and falls with increasing frequency as 823.8/f V/m for MHz IEEE Standard for Safety Levels With Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 khz to 300 GHz, IEEE Std C Revis. IEEE Std C , pp. 0_1 238, 2006.

21 Survey of Published Work Jiejian Dai; Ludois, D.C., "A Survey of Wireless Power Transfer and a Critical Comparison of Inductive and Capacitive Coupling for Small Gap Applications," in Power Electronics, IEEE Transactions on, vol.30, no.11, pp , Nov Compare CPT & IPT >100 publications with empirical results Observations Similar power levels CPT<1mm gap IPT>1mm gap Similar power density Frequencies differ

22 Survey of Published Work Cont. Observations Continued Coupling area is similar Power frequency growing Moore s law like Advances due to technologies like wideband gap devices Optimization Packaging

23 CPT Research Community Thrusts Increase power levels via: Advanced power electronics Wide band gap semiconductors for high frequency Minimal active components Active and passive control techniques Modular and pixilated architectures Surface area management Surface (plate) alignment issues Gap maintenance Flexible & Conformal Surfaces Minimize Gap Dielectric Materials (Solid & Fluid)

24 Applications to Explore OK, so where can we deploy CPT? Application 1: Slip Ring Replacement for Wound Field Synchronous Machines Application 2: Conformal Bumper for Wireless Vehicle Charging Both projects funded by DOE EERE Program

25 PM Machines for EV Traction Commercial & societal detractions of permanent magnet synchronous machines (PMSMs) Rare earth PMs are significant fraction of EV motor cost Rare earth PM market is volatile Rare earth PM extraction and refinement environmentally hazardous Rare earth PMs are largely single source from a foreign power

26 WFSMs for EV Traction PMSM s operational detractions in a traction application PMs have a fixed flux level, non variable, always on ; safety concerns during inverter faults. Interior PMSMs typically operate with negative d-axis current (especially during field weakening operation); Power factor lowered because of the reactive current Traction inverter oversized to supply reactive current Increased losses in inverter and stator (ohmic) Wound Field Synchronous Machines (WFSM) stand to overcome the limitations of PMSMs via electromagnets

27 Rotor Excitation Techniques Brushes Brushless Exciter Rotary transformer CPT 7.5kW generator

28 IPT vs CPT Coupling for Machines Basic idea: replace PMs with electromagnets Brusa To Inverter Why CPT for this application? Mechanical robustness (no windings, no ferrite, continuous surface) Potential ease of manufacturing

29 Class E 2 Converter Class E amplifier and rectifier, class E 2 kw capable, 500kHz 1MHz switching, 1200V SiC switches Requires 2 5 nf of coupling capacitance for C1, C2

30 Interleaved Configurations

31 Journal Bearing Coupling Journal style hydrodynamic bearing (GET THE GAP SMALL!) Air gap space is constrained by OD & ID Light weight oil working fluid Simple construction, bidirectional rotation possible Windage losses lend this configuration to smaller diameters

32 Journal Bearing Coupling Sleeve journal version Rotor rings are anodized AL Stator Rings are brass Stator rings retained by silicone sleeve Designed for 1800rpm Thermal image shows max temp of 36C. Hagen, S.; Knippel, R.; Jiejian Dai; Ludois, D.C., "Capacitive coupling through a hydrodynamic journal bearing to power rotating electrical loads without contact," in Wireless Power Transfer Conference (WPTC), 2015 IEEE, vol., no., pp.1-4, May 2015

33 Journal Capacitance vs. Speed.. Sleeve journal version Hydrodynamic operation established Capacitance asymptotically approaches steady state Coupling capacitance suitable for kw scale power transfer Track tank frequency for speed sensing

34 10kVA Generator Set Test Demo CPT on a machine 2 nd prototype sleeve coupling, all AL Generator: 2 pole 10kVA Field Voltage Generator Output A simple CPT coupling solution, but What if we want more capacitance per volume?

35 Axial Flux Rotating Capacitors stator plate Air Rotor Plate Air rotor plate Air Hub Flexure Air Axially stack thin plates Spiral grooves on stators channel air into gaps Air bearing action is established Thin plates and flexures allow contouring Stator Plate Ludois, D.C.; Erickson, M.J.; Reed, J.K., "Aerodynamic Fluid Bearings for Translational and Rotating Capacitors in Noncontact Capacitive Power Transfer Systems," in Industry Applications, IEEE Transactions on, vol.50, no.2, pp , March-April 2014

36 Use Analytical techniques to get close Dial it in with finite element Finite element challenging, aspect ratio Multi physics cross coupling Need for further optimization Thrust Equations: Spiral Groove Hydrodynamics 3 4,, 1,,,,,, cot cot 1,,,, 1 1

37 Solid Rendering of Plates Spiral groove thrust bearing design, air is working fluid Cascade as many plates as needed 100mm diameter, 50 micron gap, 5 nf realized for C1 & C2

38 Rotor Plate Rendering Φ 100mm Φ 60mm 0.016in. thick 3003 O Aluminum sheets Hard anodized beyond flexures Torque transmitted through featured I.D. and nylon 6/6 alignment pins 3003 O Resistivity 3.649E 8 [Ohm m] Yield Strength [Mpa] 6061 T6 Resistivity 4.066E 8 [Ohm m] Yield Strength [Mpa]

39 Stator Plate Rendering Φ 113mm Φ 60mm 0.016in. thick 3003 O Aluminum sheets Designed as outwardly pumping spiral groove bearing Supported on flexure beams at OD Φ 85mm

40 Stator & Rotor Plates Prototype hydroflex coupling plates Stator left, Rotor right Outward pumping groove pattern on stator

41 Rotary Capacitor Dyne Stand air gap (25 micron) Smallest air gap maintained to date. (previously )

42 Plate Capacitance vs. Speed 3E E 09 Measured Capacitance [F] 2E E 09 1E 09 5E Percent Rated Speed 3 rotors sandwiched between 4 stators, lift off functionality Capacitance drops 32% between 0 and 10 krpm (speed sense) Adjust flexure design to compensate for pressure

43 Integration into EV Traction Integration in progress with a 70kW, 3500rpm base, 12000rpm max, WFSM Machine rotor and stator (above) Capacitive coupling plates (left)

44 Application 2: EV Charging Can we extend rotating capacitors concepts to EV Charging? What do we do about the gap? Large gaps result in high voltage between transmitter and receiver Let s keep the small gap, viable for fleet applications Mechanical tricks for small gaps & pads Confine the field for safety, EMI, etc.

45 No Surface is truly flat! Like the rotation systems, use the dielectric to mechanically guide Make the surfaces flexible and compressible! A conformal bumper/pad

46 Conformal Bumper Docking On the vehicle Vehicle body Receiver metal foil On the Wall Insulation/dielectrics Transmitter metal foil Foam Wall

47 Rigid vs. Conformal Pad

48 Corbin Sparrow Implementation Aluminum foil strips adhered to car Identical strips on bumper station 4 foam rubber backing for station Polyethylene insulation Aside from foam thickness, all attributes match the benchtop model

49 EV Bumper Capacitance EV: C2 EV: C The EV conformal bumper capacitance was sized to match the bench Coupling capacitance was comparable Power electronics development on bench, then vehicle deployment

50 Measured Performance 500kHz operation, ~400V on coupling caps Parameter Input Class E 2 on EV 250V 4.73A (1183W) Class E 2 on bench 240V 4.83A (1159W) Output 173.9V 5.98A (1040W) 185.4V 5.76A (1068W) Efficiency 88% 92%

51 Loss Distribution Proximity and skin effects Current crowding at edges of plates Distance corresponds to loss discrepancy between EV and Bench Future versions optimized for higher frequency & power 1MHz +, 4 6kW, different electronics architecture

52 Conclusions / Take Aways CPT can deliver kilowatts of power For small gaps, CPT is as power dense as IPT IPT makes more sense than CPT for large gaps CPT can be efficient, >90% CPT Enabling technologies High frequency power electronics Hydrodynamic surfaces (rotation) Conformal surfaces (stationary)

53 Thank You! Q&A