Bidirectional Contactless Power Transfer System Expandable from Unidirectional System

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Imogene Nicholson
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1 Bidiectional Contactless Powe Tansfe ystem Epandable fom Unidiectional ystem oichio Naadachi*, higeu Mochizui*, ho aaino*, Yasuyoshi Kaneo*, higeu Abe*, Tomio Yasuda** *aitama Univesity, aitama, Japan **Technova nc., Toyo, Japan Abstact Contactless powe tansfe (CPT) systems with pimay seies and seconday paallel capacitos (P topology) ae useful fo chaging electic vehicles. Howeve, P topology is not suitable fo bidiectional powe tansfe. This pape intoduces a novel P topology fo bidiectional CPT systems that is ealized by adding an inducto and an invete to the P topology. The 3W and 6mm gap bidiectional test esults show that the efficiency of is 9.5%, and the efficiency of is 93.4%, which ae appoimately equal to the efficiency of the unidiectional P topology system of 93.%.. NTODUCTON The development and commecialization of plug-in hybid electic vehicles (PHs) and electic vehicles (Es) ae being actively pusued due to envionmental concens and ising oil pices. Both PHs and Es cuently equie a physical connection to powe supplies by electic cables fo battey chaging. n ecent yeas, contactless powe tansfe (CPT) systems have been developed as a new battey chaging method in which the battey can be chaged simply by paing the cas in a pedefined position. Although CPT systems have been eseached etensively, most of these systems ae unidiectional gid (home) to vehicle () powe tansfe systems [-4]. Cuent chaging methods using electic cables (with a focus on lage E batteies) have been eseached etensively fo use in bidiectional powe tansfe systems, which can also tansfe powe fom the vehicle to the home (H o ). These systems use the E batteies as enegy stoage fo homes, movable powe supplies, and emegency powe supplies. imilaly, CPT systems should also accommodate bidiectional powe tansfe. Madawalaa et al. [5] poposed a point-to-multipoint bidiectional CPT system that allows fo bidiectional powe tansfe by opeating the pimay and seconday invetes and contolling the phase between these invetes. They used a CPT system with pimay paallel and seconday paallel capacitos (PP topology). The othe method fo contolling the powe is by opeating only the tansmitte side invete and using the eceive side invete as a ectifie. As fo the capacito topology, seies and seies capacitos ( topology) [6, 7] and seies and paallel capacitos (P topology) ae applicable. The topology can easily tansfe the powe bidiectionally due to its symmetical cicuit. Howeve, the topology has immittance convete chaacteistics [8]; that is, if the input voltage is constant, the output cuent is constant. This is not suited fo eeping the battey voltage constant duing the final stage of chaging. On the othe hand, the P topology has the ideal tansfome chaacteistics: if the input voltage is constant, the output voltage is constant. This is suited fo pecise chage contol of i-ion batteies. Moeove, the seconday voltage of the tansfome is ept low compaed to that fo the topology, since it is desiable to avoid high voltages inside a ca. Howeve, in, the squae voltage wavefom of the invete causes a poblem: a lage hamonic cuent flows into the paallel capacito at the seconday side. n this pape, we popose a novel P (P plus ) topology that esolves this poblem and has good chaacteistics. The P topology achieves bidiectional powe tansfe by adding an inducto ( ) to the P topology and by changing the C value slightly. The P topology allows bidiectional powe tansfe with high efficiency and simplifies the contol of invetes by opeating only one side of the invete. The P topology also has an ecellent featue whee the pimay and seconday voltages become equal between and if the same ated powe is tansfeed at maimum efficiency. This means that the capacity (KA) of the bidiectional system s invete is almost equal to that of the unidiectional system s invete and ectifie.. BDECTONA CONTACTE POWE TANFE YTEM A. ystem configuation Figue (a) shows a schematic diagam of the conventional unidiectional CPT system using the P

2 (a) Unidiectional CPT system Figue. Equivalent cicuit of P topology. (b) Bidiectional CPT system Figue. CPT systems. topology. The 5Hz/6Hz AC powe is conveted to highfequency 5Hz~5Hz AC powe by the invete at the pimay side, tansfeed to the seconday side though a contactless powe tansfome, and then conveted into DC powe to chage the battey. Figue (b) shows the poposed bidiectional CPT system using the P topology. The etension pocedue is as follows: Change the ectifie in the pimay side to a bidiectional PWM ectifie. Change the C value in the pimay side slightly. Add an inducto ( ) to the seconday side. Change the ectifie in the seconday side to an invete. The pimay and seconday invetes can be used as ectifies by tuning all switching devices (e.g., BT and FET) off. By opeating the tansmitting side invete as a nomal invete and the eceiving side invete as a ectifie, this system can contol the powe tansfe diection. B. P topology (seies and paallel esonant capacito topology) Figue shows a detailed equivalent cicuit fo the P topology. t consists of a T-shaped equivalent cicuit in which the pimay seies capacito C, the seconday paallel capacito C P, and a esistance load have been added. The battey and ectifie located in the vehicle ae appoimated as, whose value is detemined as = N /P, whee N is the seconday voltage, and P is the seconday powe. The pimay values ae conveted into seconday equivalent values by a tun atio (a=n /N ) and ae epesented by the pime symbol. The winding esistances (' and ) and the feite coe loss (' ) ae consideably lowe than the mutual and leaage eactance (', ', and ) at the esonant fequency. Theefoe, ',, and ' can be omitted in the cicuit fo the analysis. To achieve esonance with the self-eactance of the seconday winding ω, which is equivalent to adding a mutual eactance ' (=ω l' ) and a leaage eactance (=ω l ), the seconday paallel capacito C P is given by P () CP The pimay seies capacito C' ( = a C ) is detemined as C The input-output chaacteistic of the P topology can be epessed by the F mati as () ΙΝ ab N N a b (3) N / ab N N Equation (3) indicates that the equivalent cicuit fo a tansfome with these capacitos is the same as an ideal tansfome with a voltage atio of ab at the esonant fequency. The b is almost equal to the coupling coefficient of the contactless tansfome. The voltage atio between the pimay and seconday sides ( N / N ) can be set to any value by selecting a (=N /N ) coesponding to. Howeve, in the P topology, is not allowed because of the lage hamonic cuent flowing though C P. C. P topology (seies and paallel esonant capacito plus topology) The P topology is a novel cicuit topology that can be obtained by adding an inducto to the P topology and solves the issue of hamonic cuent. Figue 3 shows a detailed equivalent cicuit fo the P topology. The values of C and C P ae detemined by (6) and (7) to achieve esonance with the self-eactance of the connected side winding. (6) C P (7) CP Hee, and ae the self-inductances of the pimay and seconday windings, espectively. ince (7) is the same as (), the value of C P does not need to be changed if it is used in the P topology. The value of the added inducto is given by (8) to be the same value as (8) When C, C P, and ae detemined as (6)-(8), and the input-output chaacteistics of the P topology in can be epessed by an F mati as

3 N N N ΙΝ / b N N a ab ab (9) Equation (9) is the same as (3). Thus, the input-output chaacteistics of the P and P topologies ae epessed by the same F mati, which is equivalent to an ideal tansfome with a voltage atio of ab. D. Condition fo maimum efficiency opeation n the P topology, should be appopiately selected to opeate at maimum efficiency. The efficiency of the contactless powe tansfome η (=P N /P N ) is epessed by equation (9) because is sufficiently smalle than and to be ignoed in this analysis. N N N () Hee, the esistance of inducto (s) is ignoed as well. The elationship between,, N, and N is given by N N () Equation () can also be epessed as, N N N (3) Equation (4) is obtained by substituting ' N = N /b developed fom (9) and (3) into (). b (4) Equation (4) shows that η depends on the value of. The maimum efficiency in (η ma ) is obtained when = ma is epessed by ma (5) ma is given by ma (6) imilaly, in, the efficiency of the contactless powe tansfome η (=P N /P N ) is epessed by N N N (7) The elationship between,, N, and N is given by N N (8) Equation (8) can also be epessed as, N N N (9) Then, equation () is obtained by substituting (9) into (7). () Thus, the maimum efficiency in (η ma ) and the value of ma, which maimizes η, ae epessed by ma () ma ma a () n the case of, the value of should be selected as = ma fom equation (6), and in the case of, the value of should be selected as = ma fom equation (). ince the value of is also detemined by the load voltage and the load powe P as = /P, the tansfome is usually designed to be ma = = /P. n the case of a bidiectional system, it is desiable that ma and ma have the almost same value. E. Powe chaacteistic at maimum efficiency opeation in both and The bidiectional CPT system equies high efficiency and a compaable voltage level in both powe diections ( and ). n the following, we pesent the elationship between the output powe, the pimay voltage ( N ), and the seconday voltage ( N ) at the maimum efficiency. Equations (5) and (6) can be epessed by (4) and (5) using and as defined in (3).,, M (3) ma (4) (a) (b) Figue 3. Equivalent cicuit of P topology.

4 ma TABE. ma AND η ma OF EACH TOPOOY P() P() P() b a ' N,' N -' ' ' ' ± α - P N, N η ma b Figue 4. egulation of voltage atio added eactance. ma η ma ma (5) imilaly, in, () and () can be epessed by (6) and (7), ma (6) ma (7) whee ' = /a. Consideing (6) and (), the elationship between ma and ma is given by ma ma ma. b b a b (8) When the efficiency is maimum in, the seconday voltage N(), the output powe P (=P N ), and ma satisfy the following elationship: ma N() P. (9) On the othe hand, the equivalent esistance in with the output powe P (=P N ) in is epessed as follows: N P abn P (3) Fo the case in which P = P and N = N(), is deived as follows: abn ab ma ma P (3) Equation (3) shows that the pimay and seconday voltages do not change between and if the same ated powe is tansfeed at maimum efficiency. This is an ecellent featue of the P topology and means that the capacity (KA) of the bidiectional system s invete is almost equal to the capacity of the unidiectional system s invete and ectifie. ma and η ma in each diection epessed using and ae shown in Table. The values of η ma ae found to be equal in the two topologies (P and P). Note that the a appoimation epessed by equation (3) is used to calculate ma and η ma fo the P topology. (3) F. Adustment of the seconday voltage by inseting C and The voltage atio ( N / N ) can be adusted easily by changing the self-inductance of the seconday winding. This means that the voltage atio can be adusted by inseting C o between the seconday winding and C P. Figue 4 shows a detailed equivalent cicuit fo the P topology when C o ae inseted. Hee, α is the eactance of C o. When the values of C P and ae detemined as (33), the input-output chaacteistics η ma and ma in each diection can be epessed by (34)-(37) P (33) C P N N ma ab c ma N c N ab (34) (35) c ma, ma (36) c (37) Hee, c is the step up o step down atio defined by (37). When is inseted to incease, α is positive. On the othe hand, when C is inseted to decease, α is negative. Equations (34)-(37) show that the voltage atio ( N / N ) can be also changed by c. n addition, η ma does not change based on whethe o C is inseted.. EXPEMENTA EUT n ode to veify the pefomance of the poposed P topology, a 3W bidiectional CPT test was conducted. n addition, a 3W unidiectional CPT test with the P topology was also caied out to compae its pefomance with that of the P topology. n the test, the invete fequency was 5Hz, and the value of was selected to

(a) (b) Figue 5. Epeimental cicuit. TABE. EXPEMENTA EUT (NOMA POTON) P() [Ω] 7.5 5 ma [Ω].9 8..9 N [].6 73. 84. N [].5 93.5 83. N/ N... P N [W] 339 367 334 P N [W] 369 384 384 η [%] 9.5 93.4 93.

5 (a) (b) Figue 5. Epeimental cicuit. TABE. EXPEMENTA EUT (NOMA POTON) P() [Ω] ma [Ω] N [] N [] N/ N... P N [W] P N [W] η [%] η ma [%] N N 4A N 4A N N N 4A N 4A N Figue 6. Photgaph of tansfome Time [s] (a) Time [s] (b) TABE. PAAMETE OF TANFOME Feq [Hz] 5 [µh] 44.8 ap [mm] 6 [µh] 6. N C [µf].7 N 4 C P [µf] [µh] 6. maimize the efficiency in the P ( and ) and P topologies. A. A contactless powe tansfome and epeimental cicuit Figue 5 shows the epeimental cicuits fo each diection ( and ). Figue 6 shows the 3W tansfome used in the bidiectional CPT test [9]. Table shows the tansfome paametes at the nomal position (a gap length of 6mm and no misalignment). B. Chaacteistics of nomal position Fist, the esult of the 3W bidiectional CPT test in the nomal position is descibed. The test condition was a gap length of 6mm with no misalignment. The test esults ae shown in Table and Figue 7. Table shows the test esults fo the P s, the P s, and. The efficiency in the P topology (η o η ) was appoimately 93% when the output powe was 3 W, which is nealy equal to the P topology (η P ) efficiency of 93.%. The eason why the efficiency of was highe than the 4A 4A N N N N Time [s] (c) P topology () Figue 7. Epeimental wavefom of P topology. 9 calculated value calculated value P calculated value epeimental esult epeimental esult P epeimental esult 8 3 [ Figue 8. Tansfome efficiency with load change.

6 TABE. EXPEMENTA EUT WTH CHANE N POTON AND A AP [mm] position y [mm] gap [mm] diection [Ω] N [] N [] N/ N P N [W] P N [W] η [%] P() P() gap [mm] [mm] y [mm] (a) Change in gap length (b) Change in fowad diection (c) Change in lateal diection Figue 9. Efficiency with change in position and ai gap. P() efficiency of the P topology is consideed to be measuement eo. Figue 7 shows the input and output voltage and cuent. The pimay and seconday voltages wee almost coheent, which demonstates that the P topology has the chaacteistics of an ideal tansfome in both diections. n Figue 7(a), thee ae C oscillations aound the instances of voltage switching. This oscillation appeas when the cuent N becomes discontinuous. C. Chaacteistics with esistance-load change Figue 8 shows the epeimental efficiency when is changed and the cuves of calculated value ae deived fom (4) and (). Hee, the value of is calculated fom the input voltage N, N of the full-bidge ectifie and the powe unning though it. ince the theoetical efficiency ignoes the coe loss, the theoetical efficiencies ae highe than the epeimental efficiencies, and the diffeence coesponds to the coe loss. Both efficiencies show the same chaacteistics. Duing battey chaging, the battey voltage is almost constant. Theefoe, Figue 8 epesents the change in efficiency when the chaging powe is changed. D. Chaacteistics with change in gap length and misalignment n the contactless powe tansfe system fo Es, misalignment of the tansfome due to the dive s sill and the change in the gap length fom the ca's weight is inevitable. The coupling facto depends on the elative position between the pimay and seconday tansfomes, and the efficiency deceases with a decease in. Table shows the test esults when a change in gap length and fowad / lateal misalignment occued. Hee, is the fowad diection, and y is the lateal diection of the ca. Figue 9 shows the efficiency when the gap length and misalignment ae changed. The efficiency of the P topology is almost the same as that of the P topology E. Fequency chaacteistics of P topology Because CPT systems have a esonance cicuit, it is necessay to now the chaacteistics of the cicuit when the invete fequency is changed. Figue shows the fequency chaacteistics of the impedance of the P cicuit with load fom the output teminal of the invete. Hee, the impedance is calculated fom the measued paametes of the tansfome. f the fequency is changed in the ange of ±% (±Hz), the absolute value of the impedance changes slightly in both and.

7 Z [deg] Z [deg] Z feqency [Hz] 5 5 (a) Z feqency [Hz] EFEENCE [] Yuichi Nagatsua, N. Ehaa, Y. Kaneo,. Abe, and T. Yasuda Compact Contactless Powe Tansfe ystem fo Electic ehicles PEC-appoo pp () [] M. Chigia, Y. Nagatsua, Y. Kaneo,. Abe, T. Yasuda, and A.uzui mall-ize ight-weight Tansfome with New Coe tuctue fo Contactless Electic ehicle Powe Tansfe ystem ECCE-PHOENX pp. 6-66() [3] M. Budhia,. Covic, and J. Boys Development of a ingle-ided Flu Magnetic Couple fo Electic ehicle PT Chaging ystems, EEE Tans. nd. Electon., ol. 6, No., Januay 3, pp [4]. Covic and J. Boys, nductive Powe Tansfe, Poc EEE, ol., No. 6, June 3, pp [5] U. K. Madawala and D. J. Thimawithana, A Bidiectional nductive Powe nteface fo Electic ehicles in ystems, EEE Tans. nd. Electon., ol. 58, No., pp () [6] C.-. Wang,. Covic, and O. H. tielau eneal tability Citeions fo Zeo Phase Angle Contolled oosely Coupled nductive Powe Tansfe ystems in Poc. EEE ECON, vol.,, pp [7] Y. Jang, and M. M.Jovanovic A Contactless Electical Enegy Tansmissin ystem fo Potable- Telephone Battey Chages EEE Tans. nd. Electon., ol. 5, No. 3, June 3, pp [8] H. ie, and T. Yabuuchi High-Fequency Constant-Cuent Powe upply in Noncontact Enegy Tansfe ystem Using mmittance Conve Electical Engineeing in Japan, ol. 58, No. 3, 7, pp [9] H.Taanashi, Y. Kaneo,. Abe, and T.Yasuda "A age Ai ap 3 W Wieless Powe Tansfe ystem fo Electic ehicles" ECCE, Noth Caolina, pp (ep 5- ) (b) Figue. Fequency chaacteistics of Z. CONCUON We poposed a novel P topology fo a bidiectional contactless powe tansfe system. The P topology is epandable fom the P topology by adding an inducto to the seconday side, changing the pimay capacito value, and eplacing the seconday ectifie with an invete. The input / output chaacteistics of the P topology ae equal to those of an ideal tansfome with a tun atio of b fo the P topology. The powe tansfe contol enables bidiectional powe tansfe ( and ) with high efficiency at the ated powe. The 3W bidiectional test esults show that the efficiency of the P topology in is 9.5%, and the efficiency of the P topology in is 93.4%, which ae appoimately equal to the efficiency of the unidiectional P topology of 93.%. The poposed P topology can be used not only fo vehicles but also fo othe CPT systems. We hope that this will allow a wide ange of applications of bidiectional CPT technology.

THE UNIVERSITY OF NEW SOUTH WALES. School of Electrical Engineering & Telecommunications

THE UNIVERSITY OF NEW SOUTH WALES. School of Electrical Engineering & Telecommunications THE UNIESITY OF NEW SOUTH WAES School of Electical Engineeing & Telecommunications EE97 POWE EETONIS FO ENEWABE AND DISTIBUTED GENEATION EXAMINATION Session (Supplementay Exam) TIME AOWED: 3 hous TOTA