Oak Ridge National Laboratory’s Research Activities on Wireless Power Transfer Systems
By: Omer C. Onar, Madhu Chinthavali, Steven Campbell, Cliff White, Larry Seiber, Lixin Tang, Chester Coomer, Paul Chambon, John M. Miller, and P. T. Jones
Wireless power transfer (WPT) is a paradigm shift in electric-vehicle (EV) charging that offers the consumer an autonomous, safe, and convenient option to conductive charging and its attendant need for cables. WPT can be fully autonomous due to the vehicle and grid side radio communication systems, and is non-contacting; therefore issues with leakage currents, ground faults, and touch potentials do not exist. It also eliminates the need for touching the heavy, bulky, dirty cables and plugs. It eliminates the fear of forgetting to plug-in and running out of charge the following day and eliminates the tripping hazards in public parking lots and in highly populated areas such as malls, recreational areas, etc.
Furthermore, the high-frequency magnetic fields employed in power transfer across a large air gap are focused and shielded, so that fringe fields (i.e., magnetic leakage fields) attenuate rapidly over a transition region to levels well below limits set by international standards for the public zone (which starts at the perimeter of the vehicle and includes the passenger cabin). The convenience of WPT cannot be overstated. Oak Ridge National Laboratory (ORNL) approach to WPT charging places strong emphasis on radio communications in the power regulation feedback channel augmented with software control algorithms. The over-arching goal for WPT is minimization of vehicle on-board complexity by keeping the secondary side content confined to coil tuning, rectification, filtering, and interfacing to the regenerative energy-storage system (RESS) along with. WPT charging represents the end game in the context of the connected vehicle, wireless communications, and eventually, with in-motion deployment of WPT, the ultimate in electric vehicle operation with unlimited range; dynamic wireless charging. Oak Ridge National Laboratory is working towards more efficient coil designs, power electronics converter developments, communications systems, as well as new control strategies in this field. ORNL WPT programs also define and address any concerns related to personal safety and hazards that may arise. Oak Ridge National Laboratory uses electromagnetic resonance inductive coupling system for wireless charging of electric vehicles.
DOE FOA #667 WPT Program:
ORNL is the lead organization for this activity and partners with General Motors Company, Toyota Motor Corporation, Evatran, Clemson University ICAR Center, Cisco, Duke Energy, and International Rectifier. With OEM, commercialization, communications, grid, and device partners, ORNL meets the aggressive power and efficiency goals for future’s electric vehicles with stationary and in-motion charging capabilities. At the end of the first phase, ORNL demonstrated 6.6kW and 10kW power transfer over 160mm gap with over >90% dc-to-dc efficiency, ~97% coil-to-coil efficiency, and 85% end-to-end (wall outlet to vehicle battery terminals) efficiency. The demonstration used dedicated short range communication (DSRC) systems for vehicle side data monitoring and feedbacks for controlling the grid side units.
The block diagram of the ORNL wireless power transfer system is illustrated in Fig. 1. The grid side unit of the WPT system is composed of an active front-end rectifier (AFER) with power factor correction (PFC), a high frequency power inverter, a high frequency isolation transformer, a tuning capacitor, and the primary coil. On secondary (vehicle) side, the secondary coil in parallel with the tuning capacitor, a diode-bridge rectifier, and a filter capacitor takes place.
Active Front-end Rectifier (AFER) with Power Factor Correction:
In AFER converter, only left leg of the active front-end rectifier is utilized and the right leg acts as a diode phase-leg. The AFER can work as a boost power factor correction circuit and is capable of boosting grid voltage’s peak value up to 10 times (normally 2-3 times). The AFER with PFC can also be interleaved for higher power rating. For high efficiency, low loss, and high switching frequency for reduced current ripples, the APT100MC120JCU2 SiC MOSFET phase-leg modules are utilized. The active front-end rectifier with power factor correction is shown in Fig. 2.
|Figure 1: Block diagram of the ORNL WPT system with the five cascaded power conversion stages.|
The AFER with PFC can also be interleaved for higher power rating. For high efficiency, low loss, and high switching frequency for reduced current ripples, the APT100MC120JCU2 SiC MOSFET phase-leg modules are utilized. The active front-end rectifier with power factor correction is shown in Fig. 2. The converter is operated based on the reference power to be delivered to the vehicle battery. Basically, depending on the battery reference current, a PI controlled is utilized to determine the reference current magnitude from the grid. A phase-locked-loop (PLL) system determines the grid voltage phase angle and the grid reference current is shaped accordingly. An outer PI controller uses the actual grid current and the internally generated reference AC current and determines the switching states as shown in Fig. 3. With the selected architecture, we achieved a power factor of >98% and current total harmonic distortions (THDI) of <5%. In addition, since the AFER very well regulates the primary side DC link voltage, the battery current ripples significantly reduced to about
|Figure 2: Active front-end rectifier with power factor correction.|
|Figure 3: Overall control system block diagram of the AFER with PFC.|
During the operation of the AFER, the top switch is used for the negative half-line cycle of the grid AC voltage and bottom switch is used for the negative half-line cycle of the AC grid voltage. The system parameters are given in Table I.
|fs (PFC switching frequency)||40||[kHz]|
|Ts (Controller sample time)||25||[µs]|
A photograph of the AFER with PFC is shown in Fig. 4 and example characteristic operational waveforms are provided in Fig. 5. In this example test case, the input is 220V AC, the output is 590V DC, when transferring 7.7kW to the load. The factor is about ~0.99 whereas efficiency of the converter varied from 96.5% to 97.4% and THD<5%.
|Figure 5: Experimental test setup of the AFER PFC.|
|Figure 6: Experimental waveforms of the AFER PFC.|
Coupling Coil Design:
Electromagnetic design of WPT coupling coils provides the most fundamental investigation into their performance. At ORNL the WPT team developed couplers based on the magnetic vector potential at a field point due to current flowing in an ideal primary coil conductor. The potential at this field point is defined to lie at the location of the secondary coil. For a coil pair of radius a, assuming infinitesimal conductor radius, and having a coil to coil spacing z, then the radius vector from the primary coil origin to the field point becomes r=. The corresponding vector potential, Aφ, for the case of N1 primary turns and I1 Amps yield a primary excitation of N1I1 amp-turns. This primary excitation is depicted as Idl in Fig. 6 where a1=a2=a for convenience.
|Figure 7: Vector field analysis diagram (analytical construction for coupling coil design)|
High Frequency Inverter Development:
Load conditions; i.e., state-of-charge of the battery and coupling coefficient; i.e., vehicle coil to primary pad gap and any misalignment between transmit and receive coils determine the frequency response of the WPT system. The amount of power transferred to the secondary coil is governed by the switching frequency, duty cycle, and the input voltage of the inverter. This relationship can be expressed as (1) where the HF power inverter rail voltage is Ud0, pulse duty ratio d, and angular frequency .
Although the primary coil voltage can be controlled by the active front end converter to vary the dc rail voltage Ud0, the ultimate objective is to dynamically change the switching frequency and the duty cycle in order to achieve the best operating conditions in terms of efficiency and power transfer. In the ORNL laboratory setting the HF power inverter voltage was adjusted using a power supply. In a commercialized version of this WPT technology a dedicated short range communication (DSRC) link as shown in Fig.1 would be needed. The transmitter side of the DSRC collects the measurement data such as battery voltage, battery current, and battery management system (BMS) messages needed for regulation. The grid-side receiver side of the DSRC channel receives this information for control purposes along with supporting primary side measurements. Then, a DSP based embedded control system determines the switching frequency and the appropriate duty cycle according to the control law being used. The switching signals for the inverter IGBTs are generated by the DSP control algorithm and applied to the HF power inverter gate drives. The control system can also regulate the inverter power based on the reference power commands that can be received through the V2I communications from a smart grid compliant utility.
Experimental Setup and Test Results:
In Fig. 8, the setup of the WPT system with SiC inverter is shown. Fig. 8 (a) shows the primary (bottom) and secondary coil (top); Fig. 8 (b) shows the PFC and HF inverter hardware; Fig. 8 (c) shows HF transformer, tuning capacitors and vehicle side rectifier.
Fig. 8. Primary and secondary WPT coils, tuning capacitors and the vehicle side rectifier (a) and PFC converter integrated with the high frequency power inverter and enclosed in the stationary box (b), detailed view of HF transformer, tuning capacitors and vehicle side rectifier.
With a 6-channel Yokogawa WT3000 power analyzer, the efficiencies of the system power conversion stages are also evaluated. The power conversion efficiencies of each stage are shown in Fig. 9 for two different air gap values that are 137mm and 160mm, respectively. With a NARDA electromagnetic and electric field measurement device, the fields are also measured from 0.8m away from the center of the primary coil while transferring 6.6kW to the load. In this case, magnetic field was measured 0.77µT and electric field measured 3.23V/m. These two field measurements are much lower than the ICNIRP’s international guideline limits. These limits are 6.25µT for magnetic and 87V/m for the electric field.
|Figure 9: Power conversion efficiency of each stage in the WPT system, at 137mm (a) and at 160mm (b).|
This study introduces the components of the ORNL designed wireless power transfer system. According to the results, at 137mm airgap, the efficiency of the AFER, inverter, transformer, coils, and vehicle side rectifier are measured 97.34%, 97.86%, 97.55%, 97.18%, and 98.83% and the overall end-to-end efficiency is 89.25% while delivering 6.6kW to the vehicle battery. At 160mm airgap, the coupling factor reduces. Therefore, more current flows through the magnetizing branch of the WPT equivalent circuit; in other words, more magnetizing current is needed from the inverter. More magnetizing current results in higher reactive power that should be generated by the inverter. This reactive power essentially oscillates between DC bus of the inverter and the primary coil through the series tuning capacitor. Therefore, operation at 160mm airgap results in slight reduction of the efficiencies of the some of the power conversion stages.
- J. M. Miller, O. C. Onar, and M. Chinthavali, “Primary side power flow control of wireless power transfer for electric vehicle charging,” accepted for publication, IEEE Journal of Emerging and Selected Topics in Power Electronics – Special Issue on Wireless Power Transfer, March 2014.
- J. M. Miller, O. C. Onar, C. White, S. Campbell, C. Coomer, L. Seiber, R. Sepe, and A. Steyerl, “Demonstrating dynamic wireless charging of an electric vehicle: The benefit of electrochemical capacitor smoothing,” IEEE Power Electronics Magazine, vol. 1, no. 1., pp. 12-24, March 2014.
- L. Tang, M. Chinthavali, O. Onar, S. Campbell, and J. Miller, “SiC MOSFET based single phase active boost rectifier with power factor correction for wireless power transfer applications,” in Proc., IEEE Applied Power Electronics Conference and Exposition (APEC), March 2014, Fort Worth, TX.
- O. Onar, M. Chinthavali, S. Campbell, P. Ning, C. White, and J. Miller, “A SiC MOSFET based inverter for wireless power transfer applications,” in Proc., IEEE Applied Power Electronics Conference and Exposition (APEC), March 2014, Fort Worth, TX.
- O. C. Onar, S. Campbell, P. Ning, J. M. Miller, and Z. Liang, “Fabrication and evaluation of a high performance SiC inverter for wireless power transfer applications,” in Proc., IEEE Workshop on Wide Bandgap Power Devices and Applications (WiPDA), pp. 125-130, October 2013, Columbus, OH.
- P. Ning, J. M. Miller, O. C. Onar, and C. P. White, “A compact charging system for electric vehicles,” ,” in Proc., IEEE Energy Conversion Congress and Exposition (ECCE), pp. 3629-3634, September 2013, Denver, CO.
- M. S. Chinthavali, O. C. Onar, J. M. Miller, and L. Tang, “Single-phase active boost rectifier with power factor correction for wireless power transfer applications,” in Proc., IEEE Energy Conversion Congress and Exposition (ECCE), pp. 3258-3265, September 2013, Denver, CO.
- O. C. Onar, J. M. Miller, S. L. Campbell, C. Coomer, C. P. White, and L. E. Seiber, “Oak Ridge National Laboratory Wireless Power Transfer Development for Sustainable Campus Initiative,” in Proc., IEEE Transportation Electrification Conference and Expo (ITEC), pp. 1-5, June 2013, Dearborn, MI.
- P. Ning, O. Onar, and J. Miller, “Genetic algorithm based coil system optimization for wireless power charging of electric vehicles,” in Proc., IEEE Transportation Electrification Conference and Expo (ITEC), pp. 1-8, June 2013, Dearborn, MI.
- O. C. Onar, J. M. Miller, S. L. Campbell, C. Coomer, C. P. White, and L. E. Seiber, “A novel wireless power transfer for in-motion EV/PHEV charging,” in Proc., IEEE Applied Power Electronics Conference and Exposition (APEC), pp. 3073-3080, March 2013, Long Beach, CA.
- P. Ning, J. M. Miller, O. C. Onar, C. P. White, and L. D. Marlino, “A compact wireless charging system development,” in Proc., IEEE Applied Power Electronics Conference and Exposition (APEC), pp. 3045-3050, March 2013, Long Beach, CA.
- M. Chinthavali, O. C. Onar, and J. M. Miller, “A wireless power transfer system with active rectification on the grid side,” in Proc., Conference on Electric Roads and Vehicles (CERV), February 2013, Park City, UT.
- J. M. Miller, C. P. White, O. C. Onar, and P. M. Ryan, “Grid side regulation of wireless power charging for plug-in electric vehicles,” IEEE Energy Conversion Congress & Exposition (ECCE), pp. 261-268, September 2012, Raleigh, NC.
- J. M. Miller and O. C. Onar, “Oak Ridge National Laboratory In-motion wireless power transfer system,” 1st Conference on Electric Roads and Vehicles (CERV2012), February 2012, Part City, UT.