Dynamic Wireless Power Transfer System for Electric Vehicle to Simplify Ground Facilities - Power Control Based on Vehicle-side Information -

Transcription

1 EVS8 KNTEX Korea May Dynaic Wireless Power Transfer Syste for Electric Vehicle to Siplify Ground Facilities - Power Control Based on Vehicle-side nforation - Katsuhiro Hata Takehiro ura Yoichi Hori The University of Tokyo 5--5 Kashiwanoha Kashiwa Chiba Japan hata@hflabku-tokyoacjp Abstract Electric vehicles (EVs) have environental advantages and the capacity for advanced otion control However EVs need to be charged frequently due to their liited ileage per charge A dynaic wireless power transfer (WPT) syste for EVs can extend their cruising distance and reduce the size of their energy storage syste However when being applied to rugged roadways over long distances it is iportant to siplify ground facilities as uch as possible While it is practical for a static syste to control both side using counication a dynaic syste for EVs should be controlled only on the vehicle-side To ipleent a suitable control syste this paper focuses on vehicle-side control for achieving a required power and proposes a control ethod based on road-side voltage estiation using only vehicle-side inforation Conventional ethods have proposed voltage control using a DC-DC converter on the vehicle-side while road-side voltage is regulated to obtain a reference value However this causes ground facilities to becoe coplicated due to a need for a feedback syste The proposed ethod estiates roadside voltage therefore eliinating the need for its regulation As a result ground facilities can be siplified The estiation equation is based on the equivalent circuit of a WPT syste and expressed as a function of vehicle-side voltage and current The reference value and the equilibriu point of the DC-DC converter can be obtained by the estiated road-side voltage Therefore the power control syste with a voltage controller can be designed The estiation equation and the power control are verified by experients These results suggest that the proposed ethod can achieve the required power without controlling the road-side or counicating between a vehicle and ground facilities Keywords: Wireless power transfer Magnetic resonant coupling Dynaic charging syste Priary voltage estiation Power control ntroduction Electric vehicles (EVs) have not only environental advantages but also the capacity for advanced otion control Their electric otors have the advantages of a faster and ore accurate toque response over internal cobustion engines [] However EVs need to be charged frequently due to their liited ileage per charge t is iportant to ake a charging network and to reduce the burden on the user EVS8 nternational Electric Vehicle Syposiu and Exhibition

2 C R C - - R V V R Power source Transitter and Receiver oad Figure : Equivalent circuit of wireless power transfer via agnetic resonant coupling Wireless power transfer (WPT) can itigate coplicated charging operations by eliinating the use of wiring n recent years a dynaic WPT syste for oving EVs has gathered attention []- [8] t can extend the cruising distance of EVs and reduce the size of the energy storage syste of EVs However when being applied to rugged roadways over long distances it is iportant to siplify ground facilities as uch as possible n addition highly efficient transission and a stable supply of power have to be achieved regardless of a change in position of the receiver which is equipped in a vehicle WPT via agnetic resonant coupling can achieve a highly efficient id-range transission and it has robustness to isalignent of the transitter and the receiver [9] [] The transitting efficiency and charging power are deterined not only by the paraeters of the transitter and the receiver but also by the load [] [] The load condition can be controlled by using a DC-DC converter on the vehicle-side [3]-[6] Previous research [7] has proposed voltage control on the vehicle-side for efficiency axiization However this control ethod has to regulate the road-side voltage and coplicates ground facilities This paper proposes a control ethod based on road-side voltage estiation using only vehicleside inforation The proposed ethod can control the charging power regardless of the road-side voltage therefore eliinating the need for its regulation As a result ground facilities can be siplified Wireless Power Transfer via Magnetic Resonant Coupling nput/output characteristics at resonance frequency This paper uses a series-series (SS) circuit topology of WPT via agnetic resonant coupling and its equivalent circuit is shown in Figure [8] The transitter and the receiver are coposed of the inductances the series-resonance capacitances C C and the internal resistances R R is the utual inductance between and V and are the root-ean-square voltage and current on the priary-side which is the road-side V and stand for the root-ean-square voltage and current on the secondary side which is the vehicle-side R is the load resistance The transitter and the receiver are designed to satisfy the equation which is expressed as follows: () C C where ω is the power source angular frequency Then the voltage ratio A V and the current ratio A between the secondary-side and the priary-side are expressed as follows: R AV j R R R R A j R R () (3) As a result the transitting efficiency η is described as follows: R R R R R R R (4) Furtherore the load power P is given as follows: AV P V (5) R EVS8 nternational Electric Vehicle Syposiu and Exhibition

3 Transitting fficiency η oad power P [W] Transitting efficiency η oad power P [W] Table : Characteristics of transitter and receiver Priary side Secondary side Resistance R R 5 Ω Ω nductance 636 μh 637 μh Capacitance C C 4 pf 3994 pf Resonant frequency f f 998 khz 998 khz Outer diaeter 448 Nuber of turns 56 turns 8 η P 5 8 η P oad resistance R [Ω] Figure : Transitting efficiency and load power versus load resistance Secondary voltage V [V] Figure 3: Transitting efficiency and load power versus secondary voltage The transitting efficiency and the load power are deterined by the paraeters of the coil the resonance frequency and the load resistance Figure shows the transitting efficiency and the load power versus the load resistance Then the coil paraeters are indicated in Table and the utual inductance is 88 μh The aplitude of V is V and its frequency is 998 khz When the transitting efficiency is axiized the load resistance R ηax is expressed as follows: R ax R R R (6) The axiu load power is obtained when the load resistance R Pax is given as follows: R R (7) Pax R Voltage control on vehicle side Figure 3 shows the transitting efficiency and the load power versus the secondary voltage under the sae conditions as Figure Fro these figures the equivalent load resistance is increased in response to the increase in the secondary voltage The secondary voltage V ηax which axiizes the transitting efficiency is described as follows: V R V ax R R R R R (8) n order to achieve axiu efficiency a secondary voltage control syste should be designed to satisfy eq (8) Power control can also be achieved by the secondary voltage control However it is effective only if the secondary voltage is controlled below the axiu secondary voltage V ax which is expressed as follows: V ax V R (9) Then the equivalent load resistance goes to infinity and the voltage ratio A V becoes saturated [8] The axiu power is obtained when the secondary voltage V Pax is given as follows: V V V () Pax ax R EVS8 nternational Electric Vehicle Syposiu and Exhibition 3

4 Ground facility Receiver DC-DC converter M Power source Transitter Rectifier Battery Electric vehicle Motor drive Figure 4: Syste configuration for power control on vehicle-side C R C - - R V V V Power source Transitter and Receiver Rectifier Figure 5: Circuit diagra of wireless power transfer syste n order to achieve the required power P * the secondary voltage control syste is designed Figure 4 shows the syste configuration for power control on the vehicle-side The DC-DC converter can control the output voltage of the rectifier As a result the secondary voltage can be also controlled For efficient transission it is iportant to define the operating range of the secondary voltage to be below V Pax As a result the reference value of the secondary voltage V * can be expressed as follows: V V V Pax Pax R R P R () Then V * includes inforation of the priary voltage V However it is undesirable to require counication between a vehicle and ground facilities and to regulate the priary voltage Therefore this paper proposes an estiation ethod of the priary voltage using only vehicleside inforation 3 Priary Voltage Estiation 3 Circuit analysis with phasors Assuing that the secondary voltage control is designed properly the circuit diagra of the WPT syste can be indicated in Figure 5 When the SS circuit topology is used the secondary current can be assued to be a sinusoidal wave oscillating at the resonant frequency with its phase advanced 9 degrees fro the priary voltage [9] f it is assued that the diodes conduct according to the secondary current the secondary voltage becoes a square wave which has the sae aplitude as the DC voltage V the sae phase as the secondary current and the resonant frequency Therefore the phasor of the secondary voltage is given by a Fourier series expansion and expressed as follows: V j V () EVS8 nternational Electric Vehicle Syposiu and Exhibition 4

5 v i P * Equilibriu point calculation D i V * + - C PD (s) Δd + + DC-DC Converter v Figure 6: Block diagra of power control At the resonant frequency the circuit equation can be expressed as follows: V R j j R V (3) Fro eq () eq (3) is transfored as follows: R j j R j V V (4) Therefore the phasors of the priary current and the secondary current are expressed as follows: RV V R R V RV j R R (5) (6) 3 Estiation equation of priary voltage Fro eq (6) the average current fro the rectifier to the DC-DC converter is expressed as follows: V RV R R (7) f the utual inductance does not change drastically the priary voltage is obtained by the estiation equation which is described as follows: RV R R ˆ V (8) Then the DC voltage V is already obtained for the secondary voltage control As a result the current sensor is only needed as an additional sensor to easure the inflowing current 4 Power Control on Vehicle side 4 Control strategy Power control is ipleented by secondary voltage control using the DC-DC converter on the vehicle-side The block diagra of the power control is shown in Figure 6 n order to apply linear control theory to the secondary voltage control an equilibriu point of the DC-DC converter has to be defined properly Figure 7 shows the block diagra of the equilibriu point calculation The equilibriu point which achieves the required power P * is defined as V * and D Firstly V * has to be deterined to satisfy a constraint on the charging power of WPT n order to obtain V * the priary voltage has to be estiated and the estiation equation requires the DC voltage v and the inflowing current i By the equilibriu point calculation the secondary voltage controller can be designed and the required power P * can be achieved EVS8 nternational Electric Vehicle Syposiu and Exhibition 5

6 v Priary ^ V i voltage estiation P * Constraint on Charging power V * Equilibriu point calculation D Figure 7: Block diagra of equilibriu point calculation 4 Modeling of DC-DC converter Figure 8 shows the circuit configuration of the DC- DC converter where E is battery voltage is inductance of the reactor coil r is internal resistance of the reactor coil and the battery C is capacitance of the DC-link capacitor v is DClink voltage and i is the average current flowing into the battery Assuing that the resonant frequency of WPT is uch higher than the switching frequency of the DC-DC converter i is defined as the average current flowing into the DC-link capacitor This paper expresses the plant odel of the DC- DC converter using the state space averaging ethod [] Then the DC-DC converter is operated in a continuous conduction ode When d(t) is defined as the duty cycle of the upper switch the state space odel of the DC-DC converter is indicated as follows: d x t Ad t x t Bu t (9) dt v t cx t () r d t A: B : dt C C c : t t i E xt : u t : v i t This plant odel shows that the DC-DC converter is a non-linear syste n order to use linear control theory we linearize the state space odel around the equilibriu point which is given as V and D The linearized odel is expressed as follows: d t t t dt x Ax Bu () v t cx t () r D V A: B : D C C C c : t t x : X x X i t : t : V x v t u t : U u t U D d t : t : u i t The equilibriu point has to satisfy constraint equations which are indicated as follows: V D (3) ED r D (4) By the linearization of eq (7) Δi is also expressed as follows: i 8 R v R R (5) EVS8 nternational Electric Vehicle Syposiu and Exhibition 6

7 Therefore the linearized plant odel can be transfored as follows: d x t Ad t x t Bu t (6) dt v t cx t (7) r D A : 8 D R C CR R V B: c : C x t : X x t X i t : t : V x v t u t : U u t U: D u t : d t Fro this odel the transfer function fro Δd(s) to Δv is given as follows: v bs b Pv s d s as a r 8 R a : C R R 8 rr a : D C R R : r DV b b C C (8) 43 Definition of the equilibriu point The equilibriu point is defined to achieve the required power P * V * is calculated by a reference value of the secondary voltage V * which is obtained by eq () and a Fourier series expansion Then the priary voltage is obtained by the estiation equation which is expressed as eq (8) is deterined by the paraeters of the WPT circuit and obtained by eq (7) and D is given by eq (3) and (4) i v C SW SW Figure 8: Circuit configuration of DC DC converter Therefore the equilibriu point can be defined as follows: ˆ * V V V P (9) ˆ V RV R R E E 4rV i r (3) D (3) V (3) D By substituting fro eq (9) to eq (3) into eq (8) the transfer function ΔP v (s) can be calculated As a result the feedback controller for the secondary voltage control can be designed using linear control theory f a PD controller is applied as the feedback controller we can design the PD gain using the pole placeent ethod The PD controller is expressed as follows: PD K s s D C s K P K s 5 Experient D (33) 5 Experiental setup The wireless power transfer syste for power control is shown in Figure 9 n this paper the otor drive syste is neglected as this is a fundaental study The experiental equipent is shown in Figure The characteristics of the transitter and the receiver are indicated in Table and the specification of the DC-DC converter is E EVS8 nternational Electric Vehicle Syposiu and Exhibition 7

8 C R R C i d SW V V v C i r SW E Power source Transitter and Receiver Rectifier DC-DC converter Battery Figure 9: Wireless power transfer syste for power control on the vehicle-side Digital signal processor (DSP) Current sensor Voltage sensor Power supply for control circuit DC-link capacitor Gate driver (a) Transitter and receiver (b) DC-DC converter Figure : Equivalent circuit of wireless power transfer syste Half-bridge circuit Battery nductor Table : Specification of DC-DC converter Battery voltage E nternal resistance r nductance Capacitance C Carrier frequency f c V 8 Ω 5 μh 33 μf khz Table 3: Mutual inductance between transitter and receiver in each transitting distance Transitting distance [] Mutual inductance [μh] expressed in Table The switching frequency of the DC-DC converter was set to khz which is uch lower than the resonance frequency of the WPT Therefore the flowing current into the DClink capacitor can be used as the average value The transitting distances were set to and 3 Then the utual inductance between the transitter and the receiver for each transitting distance were easured by a CR eter (NF Corporation ZM37) These values are shown in Table 3 The power source consisted of a function generator (AFG3B Tektronix) and a high-speed bipolar aplifier (HSA44 NF Corporation) and its frequency was set to 998 khz 5 Priary voltage estiation n the experient of the priary voltage estiation the aplitude of the priary voltage V was tuned to V or 3 V at each condition The DC-DC converter was replaced with an electronic load (PZ4W KKUSU) to siulate the constant voltage on the DC-link The priary voltage was calculated by the DC voltage V and the DC current which were easured by a power analyzer (PPA553 Newtons4th td) EVS8 nternational Electric Vehicle Syposiu and Exhibition 8

9 Measured DC current [A] Measured DC current [A] Estiated priary voltage [V] Estiated priary voltage [V] Estiated priary voltage [V] Estiated priary voltage [V] DC voltage V [V] (a) V = V DC voltage V [V] (b) V = V (enlarged view) DC voltage V [V] (c) V = 3 V DC voltage V [V] (d) V = 3 V (enlarged view) Figure : Experiental result of priary voltage estiation DC voltage V [V] (a) V = V 3 4 DC voltage V [V] (b) V = 3 V Figure : Measured DC current at the experient of priary voltage estiation The experiental result is shown in Figure At any transitting distance the value calculated by eq (7) and its true value are closely atched The enlarged view indicates a trend which the estiated priary voltage is decreased as the DC voltage is increased This trend becoes strong according to the increase in the transitting distance Figure indicates that the change in the DC current was increased at long distance transission However the error of the estiated priary voltage which is shown in Figure is within the allowable range As a result the priary voltage estiation can be achieved and used for power control for wireless charging EVS8 nternational Electric Vehicle Syposiu and Exhibition 9

10 Charging power P [W] Charging power P [W] Charging power P [W] Charging power P [W] Charging power P [W] Charging power P [W] 6 5 w/o control w/ control w/o control w/ control Priary voltage V [V] (a) P * = 5 W at transission 3 4 Priary voltage V [V] (b) P * = 5 W at transission 5 w/o control w/ control w/o control w/ control Priary voltage V [V] (c) P * = 5 W at transission 3 4 Priary voltage V [V] (d) P * = W at transission 5 5 w/o control w/o control w/ control 4 w/ control Priary voltage V [V] (e) P * = W at 3 transission 3 4 Priary voltage V [V] (f) P * = W at 3 transission Figure 3: Experiental result of power control 53 Power control n order to verify power control based on the priary voltage estiation secondary voltage control with the equilibriu point calculation was designed and deonstrated However in this paper the feedback controller was not designed and the DC-DC converter was controlled by only D Therefore secondary voltage control was designed by only the feedforward controller Without the control the equilibriu point was defined to satisfy the required power P * when the aplitude of the priary voltage was set to V The experiental result of the power control is shown in Figure 3 When the aplitude of the priary voltage was equated to the design value the charging power satisfied P* regardless of the equilibriu point calculation based on the priary voltage estiation However when a change in EVS8 nternational Electric Vehicle Syposiu and Exhibition

11 the priary voltage was occurred without the control P* was not achieved On the other hand by using the equilibriu point calculation based on the priary voltage the secondary voltage control achieved P* at the broader operating range f P* is not achieved this is caused by a constraint on the charging power of the WPT circuit or a liitation of the duty cycle of the DC-DC converter However the charging power which was obtained by the proposed ethod was uch closer to the required power P* than without the control Therefore it was verified that the power control based on the priary voltage estiation was effective for the dynaic WPT syste which does not have to regulate road-side voltage 6 Conclusion This paper proposed a control ethod based on the priary voltage estiation using only vehicle-side inforation to siplify ground facilities on a dynaic WPT syste Experients verified that the priary voltage estiation was achieved and the power control using the equilibriu point calculation based on the priary voltage estiation is effective for the dynaic WPT syste which can be siplified and not be required to regulate the road-side voltage n future works the feedback controller for secondary voltage control which achieves power control regardless of road-side voltage will be ipleented to reduce the steady-state error Furtherore the transient response characteristics of power control and its stability will be discussed References [] Y Hori Future vehicle driven by electricity and control-research on four-wheel-otored UOT electric March EEE Transactions on ndustrial Electronics vol 5 no 5 pp Oct 4 [] M Yilaz V T Buyukdegirenci and P T Krein General design requireents and analysis of roadbed inductive power transfer syste for dynaic electric vehicle charging in Proc EEE Transportation Electrification Conference and Expo pp -6 [3] S Raabe and G A Covic Practical design considerations for contactless power transfer quadrature pick-ups EEE Transactions on ndustrial Electronics vol 6 no pp4-49 Jan 3 [4] K Throngnuchai A Hanaura Y Naruse and K Takeda Design and evaluation of a wireless power transfer syste with road ebedded transitter coils for dynaic charging of electric vehicles in ProcThe 7th nternational Electric Vehicle Syposiu and Exhibition 3 pp -5 [5] S ee B Choi and C T Ri Dynaics characterization of the inductive power transfer syste for online electric vehicles by aplace phasor transfor EEE Transactions on Power Electronics vol 8 no pp Dec 3 [6] J Shin S Shin Y Ki S Ahn S ee G Jung S Jeon and D Cho Design and ipleentation of shaped agnetic-resonance-based wireless power transfer syste for roadway-powered oving electric vehicles EEE Transactions on ndustrial Electronics vol 6 no 3 pp 79-9 Mar 4 [7] G R Nagendra Chen G A Covic and J T Boys Detection of EVs on PT highways EEE Journal of Eerging and Selected Topics in Power Electronics vol no 3 pp Sep 4 [8] K ee Z Pantic and S M ukic Reflexive field containent in dynaic inductive power transfer systes EEE Transactions on power Electronics vol 9 no 9 pp Sep 4 [9] A Kurs A Karalis R Moffatt J D Joannopoulos P Fisher and M Soljacic Wireless power transfer via strongly coupled agnetic resonance Science Express on 7 June 7 vol 37 no 5834 pp Jun 7 [] T ura T Uchida and Y Hori Flexibility of contactless power transfer using agnetic resonance coupling to air gap and isalignent for EV World Electric Vehicle Association Journal vol 3 pp - [] M Kato T ura and Y Hori New characteristics analysis considering transission distance and load variation in wireless power transfer via agnetic resonant coupling in Proc EEE 34th nternational Telecounications Energy Conference pp -5 [] S i and C C Mi Wireless power transfer for electric vehicle applications EEE Journal of Eerging and Selected Topics in Power Electronics vol PP no 99 pp -4 Apr 4 [3] Y Moriwaki T ura and Y Hori Basic study on reduction of reflected power using DC/DC converters in wireless power transfer syste via agnetic resonant coupling in Proc EEE 33rd nternational Telecounications Energy Conference pp -5 [4] K iura N Hoshi and J Haruna Experiental discussion on inductive type contactless power transfer syste with boost or buck converter connected to rectifier in Proc EEE 7th EVS8 nternational Electric Vehicle Syposiu and Exhibition

12 nternational Power Electronics and Motion Control Conference vol 4 pp [5] K Takuzaki and N Hoshi Consideration of operating condition of secondary-side converter of inductive power transfer syste for obtaining high resonant circuit efficiency EE Japan Transactions on ndustry Applications vol 3 no pp Oct (in Japanese) [6] H shihara F Moritsuka H Kudo S Obayashi T takura A Matsushita H Mochikawa and S Otaka A voltage ratio-based efficiency control ethod for 3 kw wireless power transission in Proc The 9th Annual EEE Applied Power Electronics Conference and Exposition 4 pp 3-36 [7] M Kato T ura and Y Hori Study on axiize efficiency by secondary side control using DC-DC converter in wireless power transfer via agnetic resonant coupling in Proc The 7th nternational Electric Vehicle Syposiu and Exhibition 3 pp -5 [8] T ura and Y Hori Maxiizing air gap and efficiency of agnetic resonant coupling for wireless power transfer using equivalent circuit and Neuann forula EEE Transactions on ndustrial Electronics vol 58 no pp Oct [9] D Gunji T ura and H Fujioto Fundaental research of power conversion circuit control for wireless in-wheel otor using agnetic resonance coupling in Proc 4th Annual Conference of the EEE ndustrial Electronics Society 4 pp [] D Takei H Fujioto and Y Hori oad current feedforward control of boost converter for downsizing output filter capacitor in Proc 4th Annual Conference of the EEE ndustrial Electronics Society 4 pp Authors Mr Katsuhiro Hata received his BE in electrical engineering fro baraki National College of Technology baraki Japan in 3 He is currently working toward a MS degree at the Graduate School of Frontier Sciences with the University of Tokyo His research interests are ainly on wireless power transfer via agnetic resonant couplings Dr Takehiro ura received his BS in electrical and electronics engineering fro Sophia University Tokyo Japan He received his MS degree and PhD in Electronic Engineering fro The University of Tokyo in March 7 and March respectively He is currently a research associate in the Graduate School of Frontier Sciences in the sae university Dr Yoichi Hori received his PhD in electrical engineering fro The University of Tokyo Japan 983 where he becae a Professor in n 8 he oved to the Departent of Advanced Energy Graduate School of Frontier Sciences Prof Hori was the recipient of the Best Paper Award fro the EEE Transactions on ndustrial Electronics in 993 and 3 and of the Best Paper Award fro the nstitute of Electrical Engineers of Japan (EEJ) He is the Chairan of the Motor Technology Syposiu of the Japan Manageent Association EVS8 nternational Electric Vehicle Syposiu and Exhibition