Three Phase High Power Integrated Battery Charger for Plugin Electric Vehicles Electrical and Power Electronics Division TM4 Electrodynamic Systems Boucherville, QC, Canada
I. INTRODUCTION Plugin electric vehicles (PEV) are steadily gaining popularity owing to the volatility of fossil fuel prices. The electric drivetrain of a PEV includes a traction motor, its drive inverter and a battery connected to the inverter via a DC link capacitor. An additional AC/DC rectifier and a filter is needed to charge the battery from either a single or three phase source. A typical PEV electric drivetrain is shown in Fig. 1 (a). Conventionally the battery charger is a separate module that can be either on-board or off-board. While off-board chargers can be designed to provide higher charging rates, they are expensive and have large volume and weight. On-board chargers, on the other hand, have to be designed for a smaller volume and weight that limits their charging power [1]. Higher charging power can be achieved by re-using the traction drive and the motor to charge the battery [2]. This arrangement reduces the cost, weight and volume of the charging circuit while providing the capability of charging at a power higher than conventional on-board chargers. The arrangement of integrating the charging function into the traction drive is called an integrated battery charger (IBC) and the structure of a typical IBC is shown in Fig. 1 (b). Since the introduction of IBCs many researchers have proposed various embodiments of the charger. Single phase
AC Grid
DC/AC Inverter AC/DC Rectifier
(a)
M/ React AC Grid
Inverter/ Rectifier
Battery
Keywords—integrated battery charger; plugin electric vehicles; surface mounted permanent magnet motor; level 3 charging;
M
Battery
Abstract—Battery chargers for plugin electric vehicles can be costly and add to the weight and volume of the vehicle when designed to be on-board. Conversely, integrated battery chargers not only re-use the already available components on-board but also provide a higher charging power capability than their dedicated counterparts. Such chargers use the traction motor’s windings as input filter when connected to the three-phase supply. Surface mounted permanent magnet motors have not been used for integrated battery chargers for the apparent risk of rotor movement while charging. This paper demonstrates the use of a surface mounted permanent magnet motor to charge the battery while identifying the limiting factors of its implementation. The maximum torque (70Nm) developed on the rotor while charging is calculated using an FEA model of the motor and is found to be within the limits that a vehicle can withstand. Efficiencies up to 97% with an acceptable THD was achieved with a scaled down power version of the charger.
Battery Charger
Tony Coulombe and Jean-Marc Cyr
Electrical and Computer Engineering Department McGill University Montreal, QC, Canada
[email protected]
Reactor
Syed. Q. Ali, Diego Mascarella and Geza Joos
Battery Charger
(b)
Fig. 1 Main components of an electric drive train a) with a dedicated charger; b) with an integrated battery charger
charging for three phase motors were proposed using one [2-6] or several [7-9] three phase AC motors. These chargers were only capable of low power charging and the second harmonic ripple on the DC link (typical of single phase rectifiers) had to be filtered by the DC link capacitor, which is designed to handle the high frequency ripple of the traction mode of operation. Three phase chargers were proposed for three phase [10-14] and multiphase [15, 16] AC machines. All the proposed solutions were for induction, internal permanent magnet (IPM) or switched reluctance motors. High power solutions based on permanent magnet motor drives were not proposed because of the potential torque production on the always excited rotor. During charging in three-phase IBCs, the motor windings are excited by three-phase currents. This excitation creates a rotating magnetic field around the rotor. The created field has a potential of inducing torque and hence motion in the rotor. The authors of [17] dealt with this potential movement by accessing the midpoint of an IPM motor windings and thus creating an equal and opposite magnetic field that cancel each other out. However, access to the midpoints of the motor windings is a special requirement and is not available in most motors. The rotor of an IPM was allowed to rotate while charging in [18]. Contactors were used to disconnect and reconfigure the machine windings into two isolated winding sets. One set is used to run the motor while the other acts as a generator. This solution also requires a specially constructed machine. The authors of [15] attempted to nullify torque production on the rotor by phase transposition when connecting the IBC to the three-phase supply using induction motors. This arrangement requires special three-winding transformers for its implementation. An IBC implementation based on a surface mounted permanent magnet (SMPM) synchronous motor without special requirements (other than contactors, an off-board
978-1-4673-7637-2/15/$31.00 ©2015 IEEE
On-board
Off-board
Y
AC Isolation Supply Transformer
Ia
Vb
Ib
Vc
Ic
Contactors
Motor
VDC IDC
CDC
Y
mabc Va
Traction Converter
Battery
Fig. 2 The proposed three phase battery charger
isolation transformer and two voltage sensors) is proposed and analyzed in this paper. The paper is organized as follows. Section II explains the configuration of the proposed IBC, section III presents the results of the experiments run on simulations, hardware and FEA, section IV presents an analysis of the results and section V presents conclusions of the study. II. THE THREE-PHASE IBC CONFIGURATION To meet the IEEE 519 [19] and SAE J1772 [20] standards and function as a charger, the IBC system should meet the following set of criteria: 1) draw current from the grid at unity power factor; 2) draw current that has less than 5% THD; 3) regulate the voltage and current (whichever required) at the DC output; 4) ensure the rotor does not move during the charging operation; 5) provide isolation from grid. The system design meets the mentioned requirements and is shown in Fig. 2. The system can operate in either of the two modes of its operation namely traction and charging. In traction mode the contactors configure the motor windings in a star (or delta) connection that are powered by the battery via a standard two level voltage source inverter. The modeling, control and operation of a traction drive based on a three phase SMPM synchronous motor is well documented [21] and is not discussed further in the paper. A. AC Supply and Isolation In charging mode, the contactors disconnect the three-phase windings from each other and connect each winding to the respective phase of the three-phase supply. The system is designed for a three-phase supply 208VLL rated at 80A per phase. Therefore the maximum power that can be drawn from the outlet is 28.8kW. The supply is isolated using a transformer as a part of the electric vehicle supply equipment (EVSE). The transformer has to be rated at the maximum power of the three-phase outlet (28.8kW). In this design the off-board EVSE only consists of an isolation transformer which can be eliminated if the isolation is provided by the supply infrastructure. B. The Motor Drive The motor in the setup is modeled as a constant three phase mutual inductance as it will only consist of the phase leakage self- and mutual-inductance. The inductances can be considered constant because of the following two reasons: 1) change in inductance with respect to the rotor position is negligible for SMPM motors; 2) the rotor is expected to
remain stationary. The rotor is expected to remain stationary because the windings are excited instantaneously at the line frequency (60Hz in this case). Besides, traction motors are not designed to have the features required to start at line frequency. Vibrations in the rotor, however, can be expected due to two reasons: 1) rotor trying to catch the rotating magnetic field created by the three phase currents in the motor windings; 2) rotor reacting to the switching frequency current ripple. The vibration amplitude due to the first reason is minimized, albeit with increased intensity, in high torque motors because they conventionally have a high number of poles. However, the torque vibrations developed should be taken into account by the mechanical drivetrain designers. The second source of vibration can be minimized by switching at a higher frequency. The traction inverter is operated as a unity power factor rectifier that transfers power from the supply to the DC link. The phase current sensors and the DC link voltage sensor required to control the inverter as a rectifier are available for the traction mode of operation. However, two extra voltage sensors will be needed to synchronize to the supply and to control the input current. The charging power, however, is limited by the traction drive (converter and motor windings) ratings. For traction applications the drives are rated higher than the defined ‘level 1’ (1.9kW) and ‘level 2’ (19.2kW) power. Therefore, the proposed system is capable of providing a higher charging power in the range of ‘level 3’ (≥20kW) [22]. C. Filter Inductance The inductance provided by the machine windings has an impact on the following parameters of the charging operation: 1) Current Ripple or THD In order to meet the THD requirement of the grid as defined by the grid connection standards, it has to be made sure that the inductance provided by the motor winding is enough to limit each current harmonic to its limit. Therefore, the maximum current harmonic produced with the available inductance L can be calculated as follows max
(1)
where h is the harmonic number, Vh is the magnitude of the hth voltage harmonic, ω is the base grid frequency and Ih is the maximum magnitude of the hth current harmonic defined by
Vg
TABLE I. SYSTEM PARAMETERS
VR
3 phase input supply
IL XL
Voltage
208 VLL
Phase current
80A
Fig. 3 Charger and grid model with filter inductance
Isolation transformer
the standard. The harmonics should be calculated for all the harmonic groups as mentioned in the standards.
Power rating
30kVA
Voltage
208V/208V
2) Power Delivery Capacity at Unity Power Factor Referring to Fig. 3 the maximum power delivery capacity of the converter due to the inductance provided by the motor windings can be calculated by calculating the maximum current that the rectifier can control. The current is given by
Leakage impedance
0.03pu
Battery and DC Link
(2)
Vd
Idcmax
VDC
+ Idref -
PI
+ md +-
500μF
Nominal Power/Peak Power
37kW/60kW
Voltage Rating
400Vdc
Phase inductance / poles
260μH (0.07pu) / 10
(3)
The converter output is synchronized with the supply input voltage using a phase locked loop (PLL). Using the angle information from the PLL the ‘abc’ voltage and current components are converted to their direct (‘d’) and quadrature (‘q’) components. The control structure (shown in Fig. 5) has two fast inner current control loops and a slower outer DC voltage control loop. The current control loops control the input current to be in phase with the input voltage. This is done by controlling Iq at zero and Id at the value corresponding to the input power required to charge the battery. (a) 200
Va
100
Ia
0 -100 -200 0
0
0.01
0.02
0.03
0.04
0.05
(b) 400
θg Id
ωgL
Iq
Iqref
dq
ωgL
+
PI
+ ++ m q Vq
Fig. 5 Control loop for the proposed battery charger
abc
mabc
Idc
395
Vdc
390
100 90 80
385
70
380
60
375 0
Current (A)
PI
+-
DC Link capacitor
where, is the supply voltage, is the voltage at the converter terminals, is the input current, and are the phase resistance and inductance matrices respectively.
Voltage (V)
VDCref
55kWh
r
Voltage (V), Current (A)
E. Modeling and Control The IBC uses the phase windings of a SMPM synchronous motor as the input reactance to the three phase active rectifier (traction inverter) connected to the battery via a DC link. The active rectifier is operated such that it controls the grid current to be in phase with the grid phase voltage i.e. the power is drawn at unity power factor. The converter can be modeled as:
400V
Capacity
SMPM Synchronous Motor Ratings
The maximum current drawn is, therefore, limited by the maximum voltage that the rectifier can produce at its AC terminals and that depends on DC link voltage. D. DC Link Capacitor and Battery The capacitor is designed to absorb the worst case battery current ripple for much higher currents during traction operation, therefore, it is capable of absorbing the battery current ripple during charging operation that takes place with unity power factor and fixed modulation index (for constant current charging). However, the voltage at which the capacitor is charged during traction mode of operation is lower than that required during charging. The DC link voltage needs to be higher than the battery terminal voltage for charging current to flow. The magnitude of the current depends on the voltage drop across the battery terminal impedance. This requirement has to be met for any charger, therefore, is not an additional requirement for the IBC.
Nominal Battery Voltage
50 0.02 0.03 0.04 0.05 Time (s) Fig. 4 Simulation results for constant current charging mode a) voltage and currents at the transformer output; b) DC link voltage and charging t 0.01
Voltage (V), Current (A)
The DC can be controlled to be a constant current (CC) source or a constant voltage (CV) source. For CV charging mode the error between the DC link voltage and its reference is used to produce a reference for Id. The output of the PI controller for the DC link voltage control is limited at the maximum current that the converter can provide. If this limit is the current that corresponds to the charging current required by the battery in the CC charge mode, it naturally adjusts the operation into the conventional CC-CV charging.
(a) 100
B. Hardware Results A scaled down version of the system was setup for hardware experiments. The setup was run for 650W with 60VLL and 6A at the input (primary windings) of a 208Δ/240Y 3kVA transformer translating to 73 VLL and 5A at the output (secondary windings). The inverter used was a MiniSkiip 8 Power Board by Semikron. The motor used was a 37kW SMPM synchronous machine (MΦtive-A by TM4[23]). The battery charging operation was emulated by dissipating the drawn power across a 25Ω resistance connected to the DC link that had a capacitance of 1.5mF. Experiments were conducted using different switching frequencies for CC charging. The transformer secondary voltage (filtered for clarity) and current for the experiment run at 20kHz is shown in Fig. 8(a) and the output DC link voltage and the DC current through the
0 -50 -100 0
0.01
0.02
(a)
15 12.5 I 10 dc 7.5 5 2.5 0 0.05 V
dc
0.01
C. Finite Element Analysis Results A finite element model of the motor was developed in MagNET by Infolytica to analyze the torque developed on the shaft while charging. Two dimensional transient simulations with motion were performed by exciting the windings with the current that was recorded with the scaled down power hardware experiments for Fig. 9 (a) (with 20kHz switching frequency) and (b) (with 8kHz), and with the full-scaled power simulation experiments for Fig. 9 (c) (with 20kHz). (a) 100 Efficiency (%)
I
a
0 -50 0.03
0.04
0.05
95
90
15 12.5 I 10 dc 7.5 5 2.5 0 0.05
0.01
0.02 0.03 Time (s)
0.04
Fig. 8 Hardware results for constant current charging mode with 20kHz switching frequency a) input voltage and currents at the transformer secondary side; b) DC link voltage and charging current
8
12
16
20
40 THD (% )
dc
4
(b)
V
Current (A)
Voltage (V)
(b) 150 125 100 75 50 25 0 0
0.05
resistance is shown in Fig. 8(b). Similar input and output results for operation with 8kHz switching frequency are shown in Fig. 6(a) and Fig. 6(b) respectively. The THDs of the input current and the efficiencies (ratio of DC link power to transformer primary power) of the process are shown in Fig. 7. The control algorithm was implemented on OP4500 by OPAL-RT [24]. The execution time step of the control structure was 10µs.
a
pk
0.02
0.04
0.02 0.03 0.04 Time (s) Fig. 6 Hardware results for constant current charging mode with 8kHz switching frequency a) input voltage and currents at the transformer secondary side; b) DC link voltage and charging current
V
7A
0.01
0.03
Current (A)
Voltage (V)
150 125 100 75 50 25 0 0
100
-100 0
a
I
(b)
A. Simulation Results A simulation model was developed with the parameters shown in Table I. Experiments were run, with a simulation time step of 10µs, for constant current charging at 28.8kW that corresponds to the maximum of 80A input current. The voltage and current at the secondary side of the transformer are shown in Fig. 4(a). The output DC link voltage and the battery charging current shown in Fig. 4(b). The battery is charged at a constant current of 74Adc when the DC link voltage is 392V. The total input current THD is found to be around 3.5% which is below the allowed limits.
50
V
pk
a
III. RESULTS
Voltage (V), Current (A)
7A
50
30 20 10 0
4
8 12 16 20 Frequency (kHz) Fig. 7 Results of the experimental setup conducted using different swithcing frequencies a) efficiency of each experiment; b) input current THD of each experiment
(a) Torque (Nm)
10 5 0 -5 -10 0
0.01
0.02
0.03
0.04
0.05
(b) Torque (Nm)
10 5 0 -5 -10 0
0.01
0.02
0.03
0.04
0.05
(c) Torque (Nm)
100 50 0 -50 -100 0
0.01
0.02 0.03 0.04 0.05 Time (s) Fig. 9 Torque developed on the rotor shaft for the charging operation with a) low power hardware experiment with 20kHz switching frequency; b) low power hardware experiment with 8kHz; c) full power simulation experiment with 20kHz
IV. DISCUSSION An analysis and discussion on the results presented in Section III are presented in this section. A. THD and Efficiency Simulation results show that the inductance of the 37kW motor used is enough to meet the standards in the constant current charging mode and the control loop developed is capable of controlling the input current in phase with the input voltage. A motor with lower inductance would require a higher switching frequency or additional inductance to meet the allowable THD limits. The input current THD, shown in Fig. 7 (b), exhibits a decreasing trend with the switching frequencies. This is expected as increased switching frequency decreases the current ripple that reduces the THD. However, in the experimental setup, the least achievable THD at the output power was 13%, which is greater than that allowed by the standards. This is due to the scaled down version of the setup. Nevertheless, it is shown from simulations that when the system operates at full power, the THD will be less than 5%. It should also be noted that when the charging shifts from constant current to constant voltage mode, the power drawn from the supply reduces. This reduction in power causes the input current fundamental to go down, therefore increasing the THD. The efficiencies of the hardware experiments are shown in Fig. 7 (a). The highest achieved efficiency is at 8kHz (97%) and the least is at 16kHz (94%) switching frequency
operation. Although the difference in efficiencies do not exhibit a particular trend with operation at different switching frequencies, two conflicting trends can be attributed to two different sources of losses: vibrational and switching losses. The vibrational losses are expected to increase when the switching frequency is decreased. As shown in Fig. 8 and Fig. 6, reduced switching frequency results in higher current ripple that results in a higher torque ripple which increases the high frequency vibrations on the rotor. As shown, the high frequency torque vibrations in Fig. 9 (a) are less than those in Fig. 9(b). The efficiencies observed with the scaled-power operation might be different from those observed with full-scaled power operation. The vibrational losses are expected to increase with higher currents due to the resultant torque vibrations. The switching losses are also expected to increase with a higher DC link voltage and charging current. Higher charging current will also incur higher conduction losses. All these factors will result in a decreased efficiency at full-scale power. The DC link voltage shown in Fig. 8(b) and Fig. 6(b) exhibit a higher ripple because the charging power was dissipated across a resistor. With an actual battery load, the battery voltage would limit the voltage and the current ripple on the DC link. B. Switching Frequency Implications Higher switching frequency results in increased switching losses. For higher power charging, therefore, the switching frequency should be reduced. Switching frequency reduction results in increased THD. With the limited inductance provided by the machine windings, a compromise has to be reached between acceptable efficiency and acceptable THD. A lower switching frequency is possible for a motor with a higher winding inductance to achieve similar THD. Since high torque motors have a higher leakage inductance, it makes them favorable for high power IBCs. C. Torque on the Rotor Shaft It can be seen in Fig. 9 that a sinusoidal and zero averaged torque at the fundamental frequency of the input current will develop on the stationary shaft when the battery is being charged through the motor windings. Apart from the fundamental component of the torque there will be a torque ripple that is produced by the input current ripple. For fullscale power charging the torque is expected to reach a peak of 70Nm as shown in Fig. 9 (c).The mechanical drivetrain has to be designed to withstand the torque developed on the rotor. The drivetrain inertia may be large enough to absorb the developed torque without letting the shaft rotate. However, if it is not, then the brake should be designed to keep the shaft from rotating. D. Magnet Heating and Losses The balanced three phase current in the motor windings creates a rotating magnetic field around the rotor. Since the rotor remains stationary, the magnets mounted on the rotor see the full peak to peak variation of magnetic field fundamental. Since this fundamental is at a low frequency (60 Hz), its effect
should be negligible. However, at higher power levels, this fundamental may become high enough to generate enough losses in the magnet that they start to heat up and demagnetize. The charging power, therefore, should be limited below the level at which the magnets start to heat up and demagnetize. V. CONCLUSION The paper presented an integrated battery charger based on a SMPM synchronous motor drive. The motor was successfully used as an input inductance for a unity power factor battery charger while the rotor remained stationary. Efficiencies of 97% were recorded for the scaled-down hardware implementation of the converter (expected to be lower for full-scaled charging power). Input current THD is achievable (as per simulations) at full-scaled power charging. Vibrations on the rotor observed were within the limits that the mechanical drivetrain can withstand. ACKNOWLEDGMENT The authors would like to thank the Natural Science and Engineering Research Council of Canada (NSERC) and the industrial partners TM4 Electrodynamic Systems, Linamar and Infolytica Corporation for their support under the Automotive Partnership Canada (APC) project. REFERENCES [1]
[2] [3]
[4] [5] [6] [7]
A. Khaligh and S. Dusmez, "Comprehensive Topological Analysis of Conductive and Inductive Charging Solutions for Plug-in Electric Vehicles," IEEE Trans. Veh. Technol., vol. 61, pp. 34753489, 2012. W. E. Rippel, "Integrated Traction Inverter and Battery Charger Apparatus," US 4920475, Apr. 24, 1990. M. Milanovic, A. Roskaric, and M. Auda, "Battery Charger Based on Double-Buck and Boost Converter," in Proc. IEEE Int. Symposium on Industrial Electronics (ISIE), Bled, Slovania, 1999, pp. 747-752 vol.2. L. Solero, "Nonconventional On-board Charger for Electric Vehicle Propulsion Batteries," IEEE Trans. Veh. Technol., vol. 50, pp. 144-149, 2001. G. Pellegrino, E. Armando, and P. Guglielmi, "An Integral Battery Charger With Power Factor Correction for Electric Scooter," IEEE Trans. Power Electron., vol. 25, pp. 751-759, 2010. D. Thimmesch, "An SCR Inverter with an Integral Battery Charger for Electric Vehicles," IEEE Trans. Ind. Appl., vol. IA-21, pp. 1023-1029, 1985. W. E. Rippel and A. G. Cocconi, "Integrated Motor Drive and Recharge System," US 5099186, Mar. 24, 1992.
[8] [9]
[10] [11]
[12]
[13] [14]
[15]
[16] [17] [18]
[19] [20] [21] [22]
[23] [24]
S. Seung-Ki and L. Sang-Joon, "An Integral Battery Charger for Four-Wheel Drive Electric Vehicle," IEEE Trans. Ind. Appl., vol. 31, pp. 1096-1099, 1995. S. Q. Ali, D. Mascarella, and G. Joos, "Integrated Battery Charger for Delta Connected Machines in Plug-in Hybrid Electric Vehicles," in IEEE Canadian Conf. on Electrical and Computer Engineering (CCECE), Toronto, Canada, 2014, pp. 1-6. A. G. Cocconi, "Combined Motor Drive and Battery Recharge System," US 5341075, Aug. 23, 1994. S. Kinoshita, "Electric System for Electrical Vehicle - has Power Converter which Rectifies AC Power to DC Power so that DC Power is Regenerative to the Secondary Battery," US 5504414, Apr. 2, 1996. F. Lacressonniere and B. Cassoret, "Converter Used as a Battery Charger and a Motor Speed Controller in an Industrial Truck," in European Conf. on Power Electronics and Applications (EPE), Dresden, Germany, 2005, pp. 7 pp.-P.7. L. De Sousa and B. Bouchez, "Combined Electric Device for Powering and Charging," 0221363 A1, Sep. 15, 2011. S. Haghbin, S. Lundmark, M. Alakula, and O. Carlson, "An Isolated High-Power Integrated Charger in Electrified-Vehicle Applications," IEEE Trans. Veh. Technol., vol. 60, pp. 4115-4126, 2011. I. Subotic, E. Levi, M. Jones, and D. Graovac, "Multiphase Integrated On-Board Battery Chargers for Electrical Vehicles," in European Conf. Power Electronics and Applications (EPE), Lille, France, 2013, pp. 1-10. I. Subotic, N. Bodo, E. Levi, and M. Jones, "On-board Integrated Battery Charger for EVs Using an Asymmetrical Nine-Phase Machine," IEEE Trans. Ind. Electron., vol. PP, pp. 1-1, 2014. S. Lacroix, E. Laboure, and M. Hilairet, "An Integrated Fast Battery Charger for Electric Vehicle," in IEEE Vehicle Power Propulsion Conf. (VPPC), Lille, France, 2010, pp. 1-6. S. Haghbin, K. Khan, S. Zhao, M. Alakula, S. Lundmark, and O. Carlson, "An Integrated 20-kW Motor Drive and Isolated Battery Charger for Plug-in Vehicles," IEEE Trans. Power Electron., vol. 28, pp. 4013-4029, 2013. "IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems," IEEE Std 519, 2014. "Surface Vehicle Recommended Practice - SAE Electric Vehicle and Plug in Hybrid Electric Vehicle Conductive Charge Coupler " SAE J1772, 2012. R. Krishnan, Electric Motor Drives: Modeling, Analysis, and Control. Upper Saddle River, NJ: Prentice Hall, 2001. S. Dusmez, A. Cook, and A. Khaligh, "Comprehensive Analysis of High Quality Power Converters for Level 3 Off-board Chargers," in IEEE Vehicle Power and Propulsion Conf. (VPPC), Chicago, IL, 2011, pp. 1-10. TM4 Electrodynamics. (2015, Mar. 10). MOTIVE A [Online]. Available: http://tm4.com/products/previous-production/motive-a/ OPAL-RT. (2015). OP4500 Simulator: RT-LAB / RCP / HIL System [Online]. Available: http://www.opal-rt.com/newproduct/op4500-simulator-rt-lab-rcp-hil-system
