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Battery Charger for Electric Vehicle Traction Battery Switch Station A. Kuperman, Member, IEEE, U. Levy, J. Goren, A. Zafransky and A. Savernin Abstract—The paper presents the functionality of a commercialized fast charger for a lithium-ion electric vehicle propulsion battery. The device is intended to operate in a battery switch station, allowing an up-to one hour recharge of a 25kWh depleted battery, removed from a vehicle. The charger is designed as a dual stage controlled AC/DC converter. The input stage consists of a three phase full bridge diode rectifier combined with a reduced rating shunt active power filter. The input stage creates an uncontrolled pulsating DC bus while complying with the grid codes by regulating the total harmonic distortion and power factor according to the predetermined permissible limits. The output stage is formed by six interleaved groups of two parallel DC-DC converters, fed by the uncontrolled DC bus and performing the battery charging process. The charger is capable of operating in any of the three typical charging modes: Constant Current, Constant Voltage and Constant Power. Extended simulation and experimental results are shown to demonstrate the functionality of the device. Index Terms—Battery charger, electric vehicle, power quality, power converters.
I. INTRODUCTION
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HE traction battery is undoubtedly the most critical component of an electric vehicle (EV), since the cost and weight as well as the reliability and driving range of the vehicle are strongly influenced by the battery characteristics [1]. Modern rechargeable lithium batteries, which are by far the most power or energy dense among modern batteries, are commonly used in traction applications [2]. The high energy/power content requires appropriate battery management to ensure safety and optimal performance. In particular, proper recharging is essential in order to utilize the full capacity of the battery pack and preserve its nominal lifetime [3]–[6]. There are two common types of vehicle battery chargers. The on-board (often referred to as slow or low power) charger is located onboard. The propulsion battery is recharged via the slow charger, plugged into a charging spot while the vehicle is at parking lot [7]–[14]. The off-board (socalled fast or high power) charger is located at the Battery Copyright (c) 2009 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to
[email protected]. A. Kuperman is with the Hybrid Energy Sources R&D Laboratory, Ariel University Center of Samaria, Dept. of Electrical Engineering and Electronics, Ariel 40700, Israel (Tel. +972-526943234; fax: +97239066238, email:
[email protected]). U. Levy, J. Goren, A. Zafransky, A. Savernin are with Gamatronic Electronic Industries Ltd., Jerusalem 97774, Israel (email:
[email protected]).
Switch Station (BSS). The battery must be removed from the vehicle to be recharged via the fast charger [15], [16]. The slow charger usually operates at 0.15-0.25C rates, while the fast charger rate may typically reach 2C rates, i.e. while charging a 25kWh battery; the slow charger supplies 3-4kW while the fast charger peak power is typically 30-50kW. The typical concept of EV includes urban driving only, where the full battery charge is sufficient for medium-range routes of 50-100 miles. Recharging is accomplished by plugging the car into charge spots placed at different city locations throughout the day and at driver’s home during the night. Recently, a paradigm shift towards closing the gap between EV and conventional vehicles has occurred, forcing the infrastructure to support EV intercity driving as well. The following concept of BSS was developed: when out of charge, the EV battery can be replaced at a BSS, allowing nearly uninterrupted long range driving. The replacement process takes 2-4 minutes, similarly to the duration of conventional refueling process [17]. The near-empty battery, removed from a vehicle at the BSS, is recharged by a fast charger (FC) to be available as quickly as possible for the next customer. The charging time is obviously crucial, affecting the battery stock. For example, assuming 4 minutes battery replacement time, 15 vehicles per hour may be processed by each service lane. If battery charging time is 1 hour, the minimum stock 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 involvement in the fast charging process, galvanic isolation is usually not required to be present in a FC. The FC is basically a controlled AC/DC power supply, drawing the power from the three phase AC utility grid, converting it to DC and injecting it into the traction battery [18]. In order to create a feasible solution, the FC must both satisfy the grid code in terms of power factor and harmonic content from the utility side and support lithium-ion charging modes from the battery side. Since the BSS usually contains multiple FCs, its impact on the distribution grid is very significant, as shown by previous research [19]-[25]. Therefore, the input stage of the FC usually performs power factor correction (PFC) according to the regulation requirements in addition to rectification. It can be accomplished by either employing an active rectifier [26] [28], or a diode rectifier combined with a power factor correction (PFC) circuit [29]. The well-known single phase PFC approach, utilizing an uncontrolled rectifier followed by a full-rating boost DC-DC converter [30] is unsuitable for the three-phase diode rectifier case. However, it can be modified by splitting the three-phase rectifier into either two singlephase legs followed by two independent PFC converters [31]
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or three single phase Δ or Y connected stages [32], [33]. Alternatively, a more elegant approach employs a shunt connected active power filter (APF) at the uncontrolled rectifier input, supplying the reactive current to the diode rectifier, thus achieving both near unity power factor (PF) and near zero total harmonic distortion (THD) by letting the utility to supply the active current only, which is in phase with the utility voltage and of the same shape. The use of either one three phase [34]–[37] or three single phase [38]-[40] APF configurations are potentially feasible for implementing a three phase PFC stage. 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 the reactive and harmonic power only, while the series connected PFC converter rating is equal to the load kVA rating. The resulting loss reduction is an extremely desirable feature for a FC since the dissipated heat must be removed from the BSS by means of a cumbersome ventilation system, whose complexity is proportional to the amount of the heat to be removed. From the battery side, a conventional lithium-ion battery charging 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 DC-DC converters) must be capable of operating either as a current or voltage source. Alternatively, it can be operated as a voltage supply with dynamic current limitation. CP mode is usually achieved by operating as a current source, constantly varying according to the power profile. Moreover, the charger output current ripple should be kept as low as possible in order to prevent undesired influence on the battery chemistry. The well-known solution, allowing splitting the load power between multiple modules in order to reduce both the conduction losses and current ripple is interleaving [42][45]. Interleaving employs parallel operation of converters, whose output current is equally shifted with respect to others such that when summed, the current ripples partially cancel each other, creating a low ripple total output current. In addition, interleaving also reduces the implementation challenge of designing a single full rating converter by using several lower rating converters instead at the expense of somewhat increased hardware cost, volume/weight and more complex control circuitry. The paper describes the development of a 50kW commercial FC, employed in the first generation of BSS in Israel. Rather than presenting a novel topology, the main goal of the paper is to present a successful industrial application of well-known power electronics concepts. The charger employs a three phase diode rectifier combined with three single phase APFs as the input stage and twelve buck DC-DC converters, separated into six interleaved groups as the output stage. The power stage of the charger operates as a programmable voltage supply with controllable dynamic current limitation. The charger draws power from the three phase utility grid and is able to charge lithium-ion batteries within the voltage range of 230 - 430V by supplying currents up to 125A. The control stage of the FC supports the required communication protocols both of the battery and the Control and Management Server
(CMS) via CAN bus and Ethernet, respectively. The rest of the paper is arranged as follows: Section II contains an overview of a typical vehicle traction battery. Section III presents the FC design requirements, relevant to the paper. The topology of the charger power stage and the appropriate control circuitry and simulation results are described in Section IV. Experimental waveforms as well as some of the qualification test results are presented in Section V. The paper is concluded in Section VI. II. FAST CHARGER REQUIREMENTS The FC, described in the paper, was designed to charge 355V, 70A traction batteries formed by 96S2P connection of 3.7V, 35Ah lithium-manganese spinel cells. The main electrical specifications of the battery are summarized in Table I. TABLE I MAJOR SPECIFICATIONS OF THE TRACTION BATTERY
Technology Capacity Nominal voltage Nominal Energy Full discharge voltage Full charge voltage Maximum charge rate
Lithium Manganese Spinel 70 Ah 355V 25kWh 240V 403V 2C (50kW)
A. Output stage In order to charge such a battery, the FC must be able to operate in the full range of the possible battery voltages. In addition, power cable voltage drop should be taken into account. Hence, the maximum output voltage design requirement was set to 430 volts. The maximum charging current was limited by the battery manufacturer to 125 amperes for safety reasons, leading to the charger output performance envelope requirement, shown in Fig. 1. B. Input stage The charger was designed to draw the power from 380– 415 VRMS 50Hz three phase grids with neutral and protective earth connections. Since typical grid operators provide the AC power with 10% accuracy, the charger must be capable of functioning in the range of 342–457 VRMS input line voltages. The minimum consumer PF allowed by the Israel Electric Company in Israel is 0.92. Note that devices with active power correction usually operate with near unity displacement factor (DF), therefore the power factor is affected mostly by the total harmonic distortion (THD) according to the following well known relation, PF
DF 1 THD 2
.
(1)
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III. SYSTEM OVERVIEW
Fig. 1. FC output performance envelope
In case the DF is kept near unity, the theoretical maximum allowed value of THD is 42.6%. However, since the rated input current exceeds 16A, the charger emissions must comply with the IEC61000-3-4 standard, leading to the THD upper limit of 15%. C. Additional requirements The efficiency requirements are dictated mostly by the cooling ability of the BSS, as mentioned above. The air conditioning system of the BSS under study was designed to remove up to 2.5kW of heat from each FC. Therefore, when supplying full rated power, the chargers are allowed to operate with minimum efficiency of 95%. For de-rated operation, the efficiency requirement may be reduced down to 92% at 10kW. The physical size requirements are typically derived from BSS space limitations. The target weight of the charger was 70kg and the upper limit of the dimensions was set to 500 x 650 x 600mm in order to support the BSS infrastructure. These demands led to the minimum gravimetric and volumetric power densities of 720 W∙kg-1 and 0.26 W∙cm-3.
Fig. 2. FC system level block diagram
The system-level block diagram of the proposed solution is presented in Fig. 2. On the signal level, the FC communicates with traction battery BMS and CMS terminal via CAN and Ethernet buses, respectively. The charger supports both master and slave modes charging. In the master mode, the charger performs either CC-CV of CP charging according to the CMS commands while monitoring the battery condition. In the slave mode, the battery manages the charging process by sending current/voltage/power requests to the charger. The CMS is the highest level management layer of a BSS, performing administration and billing tasks in addition to the ability of limiting or completely shutting down the FC fleet power in case of a safety issue or according to the utility grid operator request. In the charger under study, both communication protocols are realized by Zilog eZ80F91 microcontroller with the assistance of Gridconnect RS232CAN adaptor since eZ80F91 does not support CAN bus directly. Zilog microcontroller is a bidirectional gateway between the CMS, battery BMS and the Atmel Atmega 128 microcontroller, which is in charge for the power management and monitoring of the charger power circuitry. The APF and Buck control boards are independent and based on fully analog controllers. On the power level, there are low and high voltage links between the charger and the battery. The high voltage link transfers the charging power while the low voltage (12V) low power link supplies power to the battery contactors. Note that the 12V power supply, located inside the FC is used to power both charger and battery contactors. The battery contactors are used to isolate the high-voltage power pack from the environment when the battery is not located in the vehicle or charging.
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IV. THE OUTPUT STAGE The DC-DC stage of the charger consists of 6 interleaved cells of 2 parallel 4.5KVA buck converters [47], [48]. A single cell structure is shown in Fig. 5. The converters are of basic buck topology with an enhanced switch, consisting of a parallel connection of IGBT and MOSFET. A small delay between the turn on and turn off instants of the transistors allows relatively high switching frequency operation of 50KHz and significant losses reduction. Detailed operation of the combined switch is out of the current paper scope and will be reported in a separate paper.
Both ripple reduction and frequency increase lead to THD reduction and electromagnetic compatibility (EMC) filter requirements loosening. The voltage at the input of the DC-DC stage is the rectified input voltage, shown in Fig. 6 (for 400VRMS grid). Since buck topology is employed at the output stage, the minimum value of the rectified voltage must be higher than the maximum output DC voltage in order to ensure undistorted operation. The minimum value of the rectified voltage is given by 6 VRMS , (2) VIN min 2 hence the global minimum of the rectified voltage (neglecting the voltage drop of the diode bridge) is VINmin = 419V for VRMS = 342V and the requirement of maximum charge output voltage of 430V cannot be met. As a result, the modification of the maximum output voltage requirement shown in Fig. 7 was proposed and accepted by the customer. It is worth noting that 410V output is sufficient to charge the mentioned battery in most cases if the power cable is of a reasonable length and cross-section area since during CV and CP charging stages the current diminishes and cable voltage drop reduces proportionally with the current. inductor current [A]
Additional feature of the battery contactors is precharging of the vehicle inverter DC link capacitor upon battery connection. The charger contactors serve similar purposes since the charger DC-DC stage contains output capacitors which must be carefully pre-charged prior to the battery connection in order to prevent excessive inrush currents. When a charging process is terminated, the charger output voltage is usually higher than 400V. After the battery is disconnected, the output capacitors should be discharged to a low voltage because of safety reasons. This is accomplished by a bleeder circuit, discharging the output capacitors quickly below 50V. The bleeder and the 12V contactor power supply are both operated by the Atmega 128 microcontroller.
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The PWM signals of the two converters in the same cell are synchronized while the PWM signals of two adjacent cells are 60 degrees shifted in order to implement time based interleaving, as shown by PSIM simulation results in Fig. 4. While the ripple of cell output current iOn is around 6A (n = 1…6), the FC output current ripple is reduced as 0.7A as a result of interleaving. In addition, the ripple frequency is multiplied by six and as a result its influence on the battery current further reduces because of the low-pass characteristics of the battery internal impedance [49]. The DC-DC stage input current ripple is much improved as well, as shown in Fig. 5, leading to the electromagnetic emissions reduction as follows. Since basic buck topology is used, the input cell current is highly discontinuous, dropping to zero each time the switch is open. However due to interleaving the input current ripple is significantly reduced and its frequency is multiplied by six as well. The input current of the DC-DC stage loads the diode bridge; hence the current ripple is polluting the mains since the APF stage is unable to suppress high frequency harmonics.
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Fig. 6. Simulation results. Rectified voltage at the DC-DC stage input
for the first version of the FC. A three-phase APF employment is currently being developed for the future versions of the device. The diagram of the input stage phase R is shown in Fig. 9. The diode bridge is represented by a nonlinear current source iRD, which is supplied by both the mains and the APF. The main idea is forcing the APF to supply the reactive and harmonic content of the nonlinear current, leaving the mains to supply the fundamental harmonic only. The APF consists of a controlled full bridge with a DC bus capacitor (since it does not provide real power to the diode bridge), connected to the AC bus via an inductor. Although the diode bridge is represented by a current source, its actual behavior resembles a constant power load since it drives a battery through DC-DC converter stage. Hence the worst case bridge rectifier output current, occurring when the mains voltage is minimal, is shown in Fig. 10.
Fig. 7. Modified maximum output voltage envelope
The architecture of the DC-DC stage control circuitry is shown in Fig. 8. The analog control board receives from the Atmega 128 microcontroller reference voltage command in case of CV operation or reference current command in case of CC or CP operation, senses the real values of the appropriate voltage/current and creates PWM commands to switch drivers. The control algorithm has a conventional dual loop structure, where the slow outer loop controls the charger output parameter (voltage or current, according to the operation mode) and the inner current mode control (CMC) loop controls the inductor currents of the individual converters to follow the outer loop generated reference. The interleaving is achieved by shifting the clocks used by CMC. The PWM signal, created by the current loop is split into two time delayed signals to the drivers of the combined switch transistors. Actual currents, voltages and cell temperatures are continuously monitored by the Atmega 128 microcontroller, which can shut cells down in case of malfunction and perform corresponding output power de-rating. Control loops design and components selection issues are omitted for the sake of brevity. V. THE PFC STAGE The FC input stage consists of a three phase diode rectifier and three single phase APFs [50]. The reason of using three single phase APFs instead of a single three-phase APF is the fact that a single-phase APF module was developed earlier by the company for another application and was found suitable
Fig. 8. DC-DC stage control circuitry diagram
Fig. 9. Input stage R phase diagram
In order to understandably describe the constant power load effect, the output stage of the FC was represented in the simulation by a 50kW constant power load and the high frequency harmonics (Fig. 5) were neglected to prevent confusion. For the R phase, the mains voltage and diode bridge current are (3) vRM (t ) 2VRMS sin(t ) and
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iRD (t ) I n sin(nt n ),
(4)
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iRF (t ) I1 sin 1 cos(t ) I n sin( nt n ).
(6)
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constant. Moreover, according to (7) there is a following tradeoff between the DC link voltage level and filter inductor: in order to prevent the switching frequency leakage, the inductor value should be as high as possible; however a high inductor leads to the high frequency harmonics compensating ability deterioration since the DC link voltage increase is limited by the capacitor voltage rating. The tradeoff values are eventually determined by PF and THD design requirements. If no solution is available satisfying all the constraints, an LC filter instead of a single inductor may be considered. However in the present case study a single inductor solution turned out to be sufficient. Time and frequency domain simulation results of the input stage performance are shown in Figs. 12 and 13, respectively. The frequency domain results clearly demonstrate that the mains current consists of the fundamental harmonic only and the rest of the bridge current is supplied by the APF.
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Fig. 10. Simulation results. Worst case bridge rectifier output current
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Note that the according to Fig. 11, where the phase R voltage and input current of the bridge rectifier are shown, the DF of the diode bridge current is actually unity (φn = 0); hence the APF must supply harmonic content only, given by the second term of the right hand side of (6), to the diode bridge. The desired APF current is created by impressing the following voltage at the filter terminals, (7) vRF (t ) n LRF I n sin(nt ). 2 n2
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In order to realize (7), the DC link voltage of the APF should be kept above the absolute maximum of the mains voltage because of the buck structure of the circuit. In addition, since there are internal switching and conduction losses in the APF, some amount of active current should be drawn by the filter from the mains to maintain the DC link voltage nearly
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Fig. 13. Simulation results. Input stage current spectra
The bridge per-phase current rating determined for the worst case of mains voltage is around 88.9ARMS (assuming lossless conversion), while the APF current rating is 26.5ARMS, i.e. the shunt connected APF rating is about 30% of the load rating, as expected.
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The APF control circuitry architecture is presented in Fig. 14. The dual loop analog control structure consists of the outer slow voltage loop, keeping the DC link voltage at the predetermined level, set by the Atmega 128 microcontroller. The voltage controller generates mains current amplitude reference command, which is multiplied by the mains voltage sinusoidal template to create the main current reference. The fast inner loop forces the mains current to follow the reference by generating the appropriate duty cycle command to the PWM logic circuitry. The APF operates at 17KHz switching frequency employing unipolar PWM technique. APF currents, voltages and temperatures are monitored by the Atmega 128 microcontroller for safety reasons. Control loops design and components selection issues are omitted for the sake of brevity.
(a) Front view Fig. 14. Input stage control circuitry diagram
VI. EXPERIMENTAL RESULTS The photography of the designed FC is shown in Fig. 15. The weight and the physical dimensions of the device are 67kg and 495 x 650 x 598mm, respectively, complying with the design requirements. The charger was tested under both 35kW and 50kW charging powers. Sample input and output waveforms of a 35kW CP charging mode are presented in Figs. 16 and 17, respectively. It can be concluded that the input current is nearly sinusoidal (the THD content is observable in the shape, moreover note that the switching frequency components were filtered out to improve readability) and in phase with the mains voltage. The waveform was obtained by measuring the phase current without an EMI filter connected and filtering it in software by a filter with a similar (theoretically) frequency response. The non-periodic spikes are just noise traces while the periodic ones are the distortions at switching instants. As to the output stage, the output current and voltage (and thus the power) are stable and possess low ripple. The PF, THD and efficiency for two different output powers and upper and lower limits of the input voltages are presented in Figs. 18 and 19. According to the results, the PF is kept near unity and the THD is within permissible limits of 15%. The efficiency is well above the lower limit of 95%. It can be concluded that the FC complies with all the design requirements. (b) Rear view Fig. 15. FC pictured
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Fig. 19. Experimental results. PF, THD and efficiency at 50kW charging. Solid – VRMS = 342V, dashed – VRMS = 457V
Fig. 16. Experimental results, 230VRMS mains, 35kW charging. Upper: mains voltage (phase R); Lower: mains current.
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VII. CONCLUSION A 50kW fast charger design for a lithium-ion electric vehicle traction battery was presented in the paper. The device is capable of supplying up to 50kW charging power to any battery, operating in 240 – 430V voltage range in either constant current, voltage or power mode. The charger topology may be referred to as a two-stage controlled rectifier. The input stage consists of a three phase full bridge rectifier combined with a reduced rating active power filter (three single stage power filters are actually employed). The input stage creates an uncontrolled DC bus while complying with the grid codes by keeping the THD and power factor within the permissible limits. The output stage is formed by six interleaved groups of two DC-DC converters, reducing the input and output current ripples. Two independent control boards are employed: active filters control circuitry and the DC-DC control circuitry. The former is operated according to the predetermined grid interfacing behavior, while the operation of the latter is dictated either by preprogrammed charging sequence or by the requests from the Battery Management System. The designed device performance is shown to comply with main design requirements and extended simulation/experimental results are presented.
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VIII. REFERENCES 10
[1]
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Fig. 18. Experimental results. PF, THD and efficiency at 35kW charging. Solid – VRMS = 342V, dashed – VRMS = 457V
[3]
[4]
[5] [6]
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[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
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[19]
[20]
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