Dynamic Wireless Power Transfer System for Electric Vehicles to Simplify Ground Facilities - Real-time Power Control and Efficiency Maximization -

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1 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-5 EVS29 Symposium Montréal, Québec, Canaa, June 9-22, 26 Dynamic Wireless Power Transfer System for Electric Vehicles to Simplify Groun Facilities - Real-time Power Control an Efficiency Maximization - Katsuhiro Hata, Daita Kobayashi, Takehiro Imura, Yoichi Hori The University of Tokyo, 5--5, Kashiwanoha, Kashiwa, Chiba, Japan, hata@hflab.k.u-tokyo.ac.jp Abstract This paper focuses on ynamic wireless power transfer for electric vehicles an proposes a vehicle-sie control metho for real-time power control an efficiency maximization. The propose control strategy an controller esign are presente base on a real-time estimation of the mutual inuctance between a transmitter an a receiver. Simulations an experiments emonstrate that the propose metho can achieve the maximum efficiency an the esire power simultaneously. Keywors: wireless charging, ynamic charging, EV (electric vehicle), efficiency, power Introuction Wireless power transfer (WPT) is one of the hottest research topic in transportation applications [, 2]. In particular, a ynamic wireless power transfer (DWPT) system for electric vehicles (EVs) has gathere attention to exten the cruising istance of EVs an to reuce the size of the energy storage system [3] [6]. Its groun facilities are mainly compose of power source, high-frequency inverters, transmitters, an so on. As they are applie to rugge roaways over long istances, power control an efficiency maximization of wireless charging are esirable to be achieve on the vehicle sie without signal communication. Although previous research has propose simultaneous control methos of power control an efficiency maximization on the vehicle sie [7, 8], they have not been applie to the DWPT system. For maximizing the transmitting efficiency in the DWPT system, the mutual inuctance between a transmitter an a receiver has to be estimate from the vehicle sie. In this paper, an estimation metho consiering the vehicle-sie control is propose an applie to the simultaneous power an efficiency control. The effectiveness of the propose metho is verifie by simulations an experiments. 2 Wireless Power Transfer via Magnetic Resonance Coupling 2. Circuit analysis The transmitter an the receiver coils are shown in Figure. They are compensate by resonance capacitors for WPT via magnetic resonance coupling [9], which can achieve a highly efficient mi-range transmission an robustness to misalignment. In this paper, a series-series (SS) compensate circuit topology is use an its circuit iagram is shown in Figure 2 []. The transmitter an the receiver are characterize by the inuctances, 2, the series-resonance capacitances C, C 2, an the internal EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle Symposium

2 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ cm 2 cm Receiver 2 cm 4 cm Transmitter Figure : Coils. Table : Parameters of Coils. Transmitter Receiver Resistance R, R 2.95 Ω.6 Ω Inuctance, µh 2.6 µh Capacitance C, C 2 63 pf 2 pf Resonant frequency f, f 2.4 khz 99.7 khz Coil gap mm Mutual inuctance m 36.3 µh (no misalignment) Coupling coefficient k.22 (no misalignment) V I C R C m 2 I 2 R 2 2 V 2 R e e W e e t W Power source Transmitter an Receiver oa Figure 2: Equivalent circuit of WPT system. >Z > Figure 3: R vs. η an P. resistances R, R 2, respectively. m is the mutual inuctance between the transmitter an the receiver. These parameters are expresse in Table. If power source angular frequency is esigne as follows: ω = C = 2 C 2, () the transmitting efficiency η an the transmitting power P can be obtaine as follows []: η = (ω m ) 2 R (R 2 + R ){R R 2 + R R + (ω m ) 2 } (2) P = (ω m ) 2 R {R R 2 + R R + (ω m ) 2 } 2 V 2, (3) where V is the RMS value of the primary voltage an R is the loa resistance. When V equals to V, η an P are expresse in Figure 3. In orer to maximize the transmitting efficiency η, R has to be given as follows []: R ηmax = { (ω m ) R 2 } 2 + R 2. (4) R Then, the transmitting power P is etermine by R ηmax. As a result, the esire power cannot be achieve only using R optimization when the transmitting efficiency η is maximize. 2.2 System configuration In orer to achieve the maximum efficiency an the esire power simultaneously, the vehicle is equippe with two power converters, i.e. Half Active Rectifier (HAR) an the DC-DC converter [7, 8]. The circuit iagram of the DWPT system is shown in Figure 4. The groun facility consists of voltage source V S an an inverter, which generates a square wave voltage with resonance angular frequency ω. The transmitting power P is rectifie by the HAR an the charging power P is controlle by the DC-DC converter. These control strategies an controller esign methos are escribe below. EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 2 Symposium 2

3 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-53 P in I C R m R2 C 2 I 2 P I c P V S V 2 V 2 V c S r C c S 2 E Power Source Transmitter an Receiver Half Active Rectifier DC-DC Converter Battery Figure 4: Circuit iagram of the DWPT system. P I c P P =, I c = P V high V 2 V c V 2 = V c V c * C c C c V low (a) Rectification moe (b) Short moe T r T s t Figure 5: Operation moes of Half Active Rectifier. Figure 6: Waveform of the DC link voltage. 3 Efficiency Maximization by Half Active Rectifier 3. DC link voltage control The HAR regulates the DC link voltage V c for efficiency maximization. V c control is achieve using two operation moes of HAR, which are shown in Figure 5 [2]. When the lower arm MOSFETs are off-state, HAR is operate as the rectification moe. If the MOSFETs are turne on, HAR becomes the short moe an the receiver is shorte. Assuming the transmitting power P is larger than the loa power P, V c is increase uring the rectification moe. On the other han, V c is ecrease uring the short moe because P is cut-off an P is supplie by the DC link capacitor. Therefore, the waveform of V c can be epicte in Figure 6. If the upper boun V high an the lower boun V low are efine as follows: V high = V c + V (5) V low = V c V (6) where V c is the reference value of the DC link voltage an V is the hysteresis ban, V c is kept within the esire range. 3.2 Efficiency maximization In orer to maximize the transmitting efficiency, the loa resistance R, which is expresse in Figure 2, has to satisfy eq. (4) uring the rectification moe. If V c is given as follows: V cηmax = R2 R ω m R R 2 + (ω m ) 2 + R R 2 V S, (7) R is equate to R ηmax an the transmitting efficiency η can be maximize [3]. On the other han, uring the short moe, the transmitting power P is rastically ecease because R is minimize. As a result, losses uring the short moe are assume to be negligible to losses uring the rectification moe in this paper. Therefore, V c is etermine only by V cηmax. EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 3 Symposium 3

4 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-54 P S (t) r V cηmax r r S 2 E V cηmax E V cηmax E (a) Simplifie DC-DC converter (b) S : ON, S 2: OFF (c) S : OFF, S 2: ON Figure 7: Circuit iagram of the DC-DC converter. 3.3 Mutual inuctance estimation For tracking the maximum efficiency in the DWPT system, the mutual inuctance m has to be estimate to obtain V cηmax only using the vehicle-sie information. From the circuit equations of the DWPT system, the estimation equation of m can be given as follows [4]: ˆ m = V ± V 2 4R I 2 (V 2 + R 2 I 2 ) 2I 2 ω. (8) Although eq. (8) has two solutions, the solution with a positive sign is use in this paper. Assuming the RMS primary voltage V is constant an given to simplify groun facilities, m can be estimate from the vehicle sie. The RMS seconary voltage V 2 an the RMS seconary current I 2 are calculate from the DC link voltage V c an the rectifie DC current I c using Fourier series expansions. In orer to reuce the estimation error ue to the sensor noise, recursive least square (RS) filter is applie. From eq. (8), output y[i] an regressor φ[i] are etermine as follows: y[i] = V + V 2 4R I 2 [i] (V 2 [i] + R 2 I 2 [i]) (9) φ[i] = 2I 2 [i]ω. () RS filter is use to estimate m statistically by upating ˆ m [i], y[i] an φ[i] with following equations. ˆ m [i] = ˆ m [i ] + φ[i]p [i ] λ + φ[i] 2 P [i ] ϵ[i] ϵ[i] = y[i] φ[i]ˆ m [i ] P [i] = {P [i ] φ[i]2 P [i ] 2 } λ λ + φ[i] 2 P [i ] () where λ is forgetting factor. The initial values are given as ˆ m [] = an P [] =. In orer to use the effective values for the estimation, the RS filter upates ˆ m [i], y[i] an φ[i] only uring the rectification moe of the HAR. If the HAR is operate as the short moe, I c equals to an the estimate value becomes incorrect. Therefore, the RS filter has to be improve accoring to the operation moe of the HAR. 4 Power Control by the DC-DC converter 4. Moeling of the DC-DC converter The DC-DC converter controls the loa current for battery charging. Assuming the DC link voltage V c is controlle to V cηmax by the HAR, the circuit iagram of the DC-DC converter can be expresse in Figure 7 (a). E is the battery voltage, is the inuctance of the reactor coil an r is the internal resistance of the reactor coil an the battery. In this paper, the DC-DC converter moel is obtaine by the state space averaging metho. Assuming the DC-DC converter is operate in the continuous conuction moe, its circuit iagram in each switching moes is expresse in Figure 7 (b) an (c). EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 4 Symposium 4

5 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-55 V c I c Mutual inuctance estimation ^ m V cηmax calculation * ^ V cηmax + Equilibrium point calculation - FF controller C PI (s) FB controller + D + DC-DC converter Figure 8: Block iagram of the propose control. From the circuit equation, the state equation of Figure 7 (b) is given as follows: t (t) = r (t) E + V cηmax. (2) Also, the state equation of Figure 7 (c) is escribe as follows: t (t) = r (t) E. (3) When (t) is efine as the uty cycle of the upper arm MOSFET S, the state space moel of the DC-DC converter is obtaine as follows: t (t) = r (t) E + V cηmax (t). (4) In orer to apply the linear control theory to the controller esign, this moel is linearize aroun an equilibrium point. When I an D are efine as the equilibrium point, (t) an (t) are expresse as follows: (t) = I + (t) (5) (t) = D + (t), (6) where (t) an (t) are the microscopic fluctuations aroun the equilibrium point. By substituting eq. (5) an eq. (6) in eq. (4), the linearize DC-DC converter moel is given as follows: t (t) = r (t) + V cηmax (t). (7) Therefore, the transfer function from (s) to (s) is obtaine as follows: P i (s) = (s) (s) = V cηmax s + r. (8) 4.2 Controller esign Figure 8 shows the block iagram of the propose control. The feeforwar controller is the same as the equilibrium point calculation, which is given by the constraint of the DC-DC converter. From ˆV cηmax, which is calculate from ˆ m, an the reference value of the loa current, the equilibrium point is obtaine as follows: I = (9) D = E + ri ˆV cηmax. (2) EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 5 Symposium 5

6 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-56 Motor Receiver DSP Seconary circuit Battery Inverter Transmitter (a) Whole picture of the DWPT system. (b) Half Active Rectifier an the DC-DC converter. Figure 9: Experimental setup. Table 2: Simulation an experimental conitions. Parameter Value Parameter Value Power source voltage V S 8 V Battery voltage E 6 V Operating frequency f khz Reactor resistance r.3 Ω Hysteresis ban V.5 V Reactor inuctance µh Carrier frequency f c 2 khz DC link capacitance C c 2 µf The feeback controller is esigne by the pole placement metho. As P i (s) is the first-orer system, we apply a PI controller C P I (s), which is expresse as follows: C P I (s) = sk P + K I. (2) s If close loop poles are given by a multiple root ω cl, the gains are obtaine as follows: K P = 2ω cl r V cηmax (22) K I = ω cl 2 V cηmax. (23) In this paper, V cηmax an the gains are calculate by assuming the nominal value of m is 3 µh. 5 Simulation an Experiment 5. Experimental setup The experimental setup is shown in Figure 9. The system configuration is the same as Figure 4. The receiver is riven by the motor to simulate motion of the vehicle. The inverter is operate only while the receiver is above the transmitter to prevent huge power losses. Simulation an experimental conitions are expresse in Table 2. The forgetting factor λ of the RS filter was set to.95 an the estimate mutual inuctance ˆ m was upate only uring the rectification moe of the HAR. The reference value of the DC link voltage ˆV cηmax was calculate from ˆ m an the reference value of the loa current was set to. A. 5.2 Simulation In the simulations, the change in m was simulate by a sine wave. Its minimum an maximum values were set to 2 µh an 4 µh. EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 6 Symposium 6

7 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-57 DC current I c [A] (a) DC current I c Mutual inuctance m [µh] actual m estimate m (b) Mutual inuctance m DC link voltage V c [V] actual V 4 cηmax estimate V cηmax V c (c) DC link voltage V c Transmitting efficiency η η max η () Transmitting efficiency η oa current I [A] (e) oa current Figure : Simulation results without the propose control. DC current I c [A] (a) DC current I c Mutual inuctance m [µh] actual m estimate m (b) Mutual inuctance m DC link voltage V c [V] actual V cηmax estimate V cηmax V c (c) DC link voltage V c Transmitting efficiency η η max η () Transmitting efficiency η oa current I [A] (e) oa current Figure : Simulation results with the propose control. Figure shows the simulation results without the propose control. In this simulation, the HAR was operate as only the rectification moe an the uty cycle of the DC-DC converter was equate to.95. From Figure (b), ˆ m is closely matche with the actual m. However, the transmitting efficiency η is ecrease from the maximum efficiency because the DC link voltage V c cannot be regulate to V cηmax. Furthermore, the loa current cannot be controlle to. Figure shows the simulation results with the propose control. The close loop poles of the propose control were place at -2 ra/s. Although m was estimate only uring the rectification moe of the HAR, ˆ m accors with the actual m as shown in Figure (b). From Figure (c) an (), V c is regulate aroun V cηmax an η is maximize uring the rectification moe. In aition, Figure (e) inicates that the loa current control can be achieve. EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 7 Symposium 7

8 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-58 e / e e e, > D > > e e s e s e s s s e e (a) DC current I c (b) Mutual inuctance m (c) DC link voltage V c e e / > > e e e e () DC to DC efficiency η c (e) oa current Figure 2: Experimental results without the propose control. e / e e e, > D > > e e s e s e s s s e e (a) DC current I c (b) Mutual inuctance m (c) DC link voltage V c e e / > > e e e e () DC to DC efficiency η c (e) oa current Figure 3: Experimental results with the propose control. 5.3 Experiment In the experiments, the receiver was riven at km/h an ˆ m was compare to the actual m, which was measure by an CR meter (NF Corp., ZM237). The DC to DC efficiency η c inclues not only the transmitting efficiency but also the converter efficiency because it was measure by the DC voltages an currents on each sies. Therefore, improvement of system efficiency is verifie in the experiments. EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 8 Symposium 8

9 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-59 Figure 2 shows the experimental results without the propose control. The HAR an the DC-DC converter were operate at the same conition as the simulation without control. From Figure 2 (b), ˆ m an the actual m are closely matche. Although ˆ m has a short-time elay, ˆV cηmax is near by the actual V cηmax as shown Figure 2 (c). However, the transmitting efficiency cannot be maximize because V c is not regulate to V cηmax. Furthermore, Figure 2 (e) inicates that cannot be controlle unless the propose control is applie. In the case of with control, the DC-DC converter starte power control when V c reache V cηmax. The close loop poles of the propose control were place at - ra/s. Figure 3 shows the experimental results with the propose control. Although the error of ˆ m is larger than without control, V c can be controlle aroun V cηmax as shown in Figure 3 (c). From Figure 3 (), η c uring the rectification moe of the HAR is increase compare to without control. In aition, Figure 3 (e) shows that can be controlle to. If the close loop poles of the propose control are optimize, it is possible to suppress the current ripple ue to the fluctuation of V c. 6 Conclusion This paper propose a simultaneous control metho of real-time power control an efficiency maximization base on improve mutual inuctance estimation from the vehicle sie. Its control strategy an controller esign methos were presente. The effectiveness of the propose metho was verifie by the simulations an the experiments. Future works are to propose an efficiency maximization metho consiering losses uring the short moe of HAR an to esign an optimal controller for the propose control. Furthermore, the propose metho is implemente to an actual DWPT system using an EV. Acknowlegments This work was partly supporte by JSPS KAKENHI Grant Number an 5H2232. References [] G. A. Covic an J. T. Boys, Moern trens in inuctive power transfer for transportation application, IEEE Journal of Emerging an Selecte Topics in Power Electronics, vol., no., pp. 28 4, Mar. 23. [2] S. i an C.C. Mi, Wireless power transfer for electric vehicle applications, IEEE Journal of Emerging an Selecte Topics in Power Electronics, vol. 3, no., pp. 4 7, Mar. 25. [3] J. Shin, S. Shin, Y. Kim, S. Ahn, S. ee, G. Jung, S. Jeon, an D. Cho, Design an implementation of shape magnetic-resonance-base wireless power transfer system for roaway-powere moving electric vehicles, IEEE Transactions on Inustrial Electronics, vol. 6, no. 3, pp , Mar. 24. [4] J. M. Miller, O. C. Onar, C. White, S. Campbell, C. Coomer,. Seiber, R. Sepe, an M. Chinthavali, Demonstrating ynamic charging of an electric vehicle: the benefit of electrochemical capacitor smoothing, IEEE Power Electronics Magazine, vol., no., pp. 2 24, Mar. 24. [5] K. ee, Z. Pantic, an S. M. ukic, Reflexive Fiel Containment in Dynamic Inuctive Power Transfer Systems, IEEE Transactions on Inustrial Electronics, vol. 29, no. 9, pp , Sep. 24. [6]. Chen, G. R. Nagenra, J. T. Boys, an G. A. Covic, Double-couple systems for IPT roaway applications, IEEE Journal of Emerging an Selecte Topics in Power Electronics, vol. 3, no., pp , Mar. 25. [7] G. ovison, M. Sato, T. Imura, an Y. Hori, Seconary-sie-only simultaneous power an efficiency control for two converters in wireless power transfer system, in 4st Annual Conference of the IEEE Inustrial Electronics Society (IECON), 25, pp [8] K. Hata, T. Imura, an Y. Hori, Dynamic wireless power transfer system for electric vehicle to simplify groun facilities - power control an efficiency maximization on the seconary sie -, in Proc. 3st Annual IEEE Applie Power Electronics Conference an Exposition, 26, pp EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle 9 Symposium 9

10 Worl Electric Vehicle Journal Vol. 8 - ISSN WEVA Page WEVJ8-52 [9] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, an M. Soljacic, Wireless power transfer via strongly couple magnetic resonance, Science Express on 7 June 27, vol. 37, no. 5834, pp , Jun. 27. [] T. Imura an Y. Hori, Maximizing air gap an efficiency of magnetic resonant coupling for wireless power transfer using equivalent circuit an Neumann formula, IEEE Transactions on Inustrial Electronics, vol. 58, no., pp , Oct. 2. [] M. Kato, T. Imura, an Y. Hori, New characteristics analysis consiering transmission istance an loa variation in wireless power transfer via magnetic resonant coupling, in IEEE 34th International Telecommunications Energy Conference (INTEEC), 22, pp. 5. [2] D. Gunji, T. Imura, an H. Fujimoto, Basic stuy of transmitting power control metho without signal communication for wireless in-wheel motor via magnetic resonance coupling, in Proc. The IEEE/IES International Conference on Mechatronics (ICM), 25, pp [3] M. Kato, T. Imura, an Y. Hori, Stuy on maximize efficiency by seconary sie control using DC-DC converter in wireless power transfer via magnetic resonant coupling, in Proc. The 27th International Electric Vehicle Symposium an Exhibition (EVS), 23, pp. 5. [4] D. Kobayashi, T. Imura, an Y. Hori, Real-time coupling coefficient estimation an maximum efficiency control on ynamic wireless power transfer for electric vehicles, in Proc. IEEE PES Workshop on Emerging Technologies; Wireless Power, 25, pp. 6. Authors Mr. Katsuhiro Hata receive his B.E. egree in electrical engineering from Ibaraki National College of Technology, Ibaraki, Japan. He receive his M.S. egree in Frontier Sciences from The University of Tokyo in September 25. He is currently working towar a Ph.D. egree at the Grauate School of Engineering with the University of Tokyo. His research interests are mainly on wireless power transfer via magnetic resonant couplings. Mr. Daita Kobayashi receive his B.S. egree in applie physics from Tokyo University of Science in 24. He is currently working towar a M.S. egree at the Grauate School of Frontier Sciences with the University of Tokyo. His research interests are mainly on wireless power transfer via magnetic resonant couplings. Dr. Takehiro Imura receive his B.S. egree in electrical an electronics engineering from Sophia University, Tokyo, Japan. He receive his M.S. egree an Ph.D. in Electronic Engineering from The University of Tokyo in March 27 an March 2 respectively. He is currently a Specially Appointe Associate in the Grauate School of Engineering in the same university. Dr. Yoichi Hori receive his Ph.D. in electrical engineering from The University of Tokyo, Japan, 983, where he became a Professor in 2. In 28, he move to the Department of Avance Energy, Grauate School of Frontier Sciences. Prof. Hori was the recipient of the Best Paper Awar from the IEEE Transactions on Inustrial Electronics in 993, 2 an 23 an of the 2 Best Paper Awar from the Institute of Electrical Engineers of Japan (IEEJ). He is the Chairman of the Motor Technology Symposium of the Japan Management Association. EVS29 International Battery, Hybri an Fuel Cell Electric Vehicle Symposium