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  • IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 12, DECEMBER 2013 5391

    Battery Charger for Electric VehicleTraction Battery Switch Station

    A. Kuperman, Member, IEEE, U. Levy, J. Goren, A. Zafransky, and A. Savernin

    AbstractThis paper presents the functionality of a commer-cialized fast charger for a lithium-ion electric vehicle propulsionbattery. The device is intended to operate in a battery switchstation, allowing an up-to 1-h recharge of a 25-kWh depletedbattery, removed from a vehicle. The charger is designed as adual-stage-controlled ac/dc converter. The input stage consists ofa three-phase full-bridge diode rectifier combined with a reducedrating shunt active power filter. The input stage creates an un-controlled pulsating dc bus while complying with the grid codesby regulating the total harmonic distortion and power factoraccording to the predetermined permissible limits. The outputstage is formed by six interleaved groups of two parallel dcdcconverters, fed by the uncontrolled dc bus and performing thebattery charging process. The charger is capable of operatingin any of the three typical charging modes: constant current,constant voltage, and constant power. Extended simulation andexperimental results are shown to demonstrate the functionalityof the device.

    Index TermsBattery charger, electric vehicle (EV), powerconverters, power quality.

    I. INTRODUCTION

    THE traction battery is undoubtedly the most critical com-ponent of an electric vehicle (EV), since the cost andweight as well as the reliability and driving range of the vehicleare strongly influenced by the battery characteristics [1]. Mod-ern rechargeable lithium batteries, which are, by far, the mostpower or energy dense among modern batteries, are commonlyused in traction applications [2]. The high energy/power contentrequires appropriate battery management to ensure safety andoptimal performance. In particular, proper recharging is essen-tial in order to utilize the full capacity of the battery pack andpreserve its nominal lifetime [3][6].

    There are two common types of vehicle battery chargers. Theonboard (often referred to as slow or low power) charger islocated on board. The propulsion battery is recharged via theslow charger, plugged into a charging spot, while the vehicleis at parking lot [7][14]. The offboard (so-called fast or highpower) charger is located at the battery switch station (BSS).The battery must be removed from the vehicle to be recharged

    Manuscript received March 12, 2012; revised June 21, 2012 andNovember 3, 2012; accepted November 24, 2012. Date of publicationDecember 12, 2012; date of current version June 21, 2013.

    A. Kuperman is with the Hybrid Energy Sources R&D Laboratory,Department of Electrical Engineering and Electronics, Ariel University Centerof Samaria, Ariel 40700, Israel (e-mail: alonku@ariel.ac.il).

    U. Levy, J. Goren, A. Zafransky, and A. Savernin are with GamatronicElectronic Industries Ltd., Jerusalem 97774, Israel (e-mail: info@gamatronic.co.il).

    Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

    Digital Object Identifier 10.1109/TIE.2012.2233695

    via the fast charger (FC) [15], [16]. The slow charger usuallyoperates at 0.150.25-C rates, while the FC rate may typicallyreach 2-C rates, i.e., while charging a 25-kWh battery; the slowcharger supplies 34 kW, while the FC peak power is typically3050 kW.

    The typical concept of EV includes urban driving only, wherethe full battery charge is sufficient for medium-range routes of50100 miles. Recharging is accomplished by plugging the carinto charge spots placed at different city locations throughoutthe day and at drivers home during the night. Recently, aparadigm shift toward closing the gap between EV and con-ventional vehicles has occurred, forcing the infrastructure tosupport EV intercity driving as well. The following conceptof BSS was developed: When out of charge, the EV batterycan be replaced at a BSS, allowing nearly uninterrupted long-range driving. The replacement process takes 24 min, similarto the duration of conventional refueling process [17]. The near-empty battery, removed from a vehicle at the BSS, is rechargedby an FC to be available as quickly as possible for the nextcustomer. The charging time is obviously crucial, affecting thebattery stock. For example, assuming 4-min battery replace-ment time, 15 vehicles per hour may be processed by eachservice lane. If battery charging time is 1 h, the minimumstock of 15 batteries per lane should be present at the station.Reducing the charging time obviously reduces the stock as well.It is worth noting that, since there is no human involvementin the fast charging process, galvanic isolation is usually notrequired to be present in an FC.

    The FC is basically a controlled ac/dc power supply, drawingthe power from the three-phase ac utility grid, converting it todc and injecting it into the traction battery [18]. In order tocreate a feasible solution, the FC must both satisfy the gridcode in terms of power factor (PF) and harmonic content fromthe utility side and support lithium-ion charging modes fromthe battery side. Since the BSS usually contains multiple FCs,its impact on the distribution grid is very significant, as shownby previous research [19][25]. Therefore, the input stage ofthe FC usually performs PF correction (PFC) according to theregulation requirements in addition to rectification. It can beaccomplished by employing either an active rectifier [26][28],or a diode rectifier combined with a PFC circuit [29]. The well-known single-phase PFC approach, utilizing an uncontrolledrectifier followed by a full-rating boost dcdc converter [30],is unsuitable for the three-phase diode rectifier case. However,it can be modified by splitting the three-phase rectifier intoeither two single-phase legs followed by two independent PFCconverters [31] or three single-phase - or Y-connected stages[32], [33]. Alternatively, a more elegant approach employs a

    0278-0046/$31.00 2012 IEEE

  • 5392 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 12, DECEMBER 2013

    shunt-connected active power filter (APF) at the uncontrolledrectifier input, supplying the reactive current to the diode rec-tifier, thus achieving both near-unity PF and near-zero totalharmonic distortion (THD) by letting the utility to supply theactive current only, which is in phase with the utility voltageand of the same shape. The use of either one three-phase[34][37] or three single-phase [38][40] APF configurationsis potentially feasible for implementing a three-phase PFCstage. The additional advantage of the approach is the fact that,because of the shunt connection, the APF rating is less than one-third of the bridge rectifier rating, since the APF supplies thereactive and harmonic power only, while the series-connectedPFC converter rating is equal to the load kilovoltampere rating.The resulting loss reduction is an extremely desirable featurefor an FC since the dissipated heat must be removed from theBSS by means of a cumbersome ventilation system, whosecomplexity is proportional to the amount of the heat to beremoved.

    From the battery side, a conventional lithium-ion batterycharging is characterized by two main phases: constant current(CC) and constant voltage (CV). Recently, constant power (CP)charging became popular in large vehicle battery packs [41].Hence, the charger output stage (typically consisting of dcdcconverters) must be capable of operating as either a current orvoltage source. Alternatively, it can be operated as a voltagesupply with dynamic current limitation. CP mode is usuallyachieved by operating as a current source, constantly varyingaccording to the power profile. Moreover, the charger outputcurrent ripple should be kept as low as possible in order toprevent undesired influence on the battery chemistry. The well-known solution, allowing splitting the load power betweenmultiple modules in order to reduce both the conduction lossesand current ripple, is interleaving [42][45]. Interleaving em-ploys parallel operation of converters, whose output current isequally shifted with respect to others such that, when summed,the current ripples partially cancel each other, creating a lowripple total output current. In addition, interleaving also reducesthe implementation challenge of designing a single full-ratingconverter by using several lower rating converters instead at theexpense of somewhat increased hardware cost, volume/weight,and more complex control circuitry.

    This paper describes the development of a 50-kW commer-cial FC, employed in the first generation of BSS in Israel.Rather than presenting a novel topology, the main goal ofthis paper is to present a successful industrial application ofwell-known power electronics concepts. The charger employsa three-phase diode rectifier combined with three single-phaseAPFs as the input stage and twelve buck dcdc converters,separated into six interleaved groups as the output stage. Thepower stage of the charger operates as a programmable volt-age supply with controllable dynamic current limitation. Thecharger draws power from the three-phase utility grid and isable to charge lithium-ion batteries within the voltage rangeof 230430 V by supplying currents up to 125 A. The controlstage of the FC supports the required communication protocolsboth of the battery and the Control and Management Server(CMS) via controller area network (CAN) bus and Ethernet,respectively.

    TABLE IMAJOR SPECIFICATIONS OF THE TRACTION BATTERY

    Fig. 1. FC output performance envelope.

    The rest of this paper is arranged as follows. Section IIcontains an overview of a typical vehicle traction battery.Section III presents the FC design requirements, relevant to thispaper. The topology of the charger power

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