An Innovative Isolated Bidirectional Soft-Switched Battery Charger For ...

International Review of Electrical Engineering (I.R.E.E.), Vol. xx, n. x

An Innovative Isolated Bidirectional Soft-Switched Battery Charger For Plug-In Hybrid Electric Vehicle Seyyedmilad Ebrahimi1 , Farid Khazaeli2 , Farzad Tahami3 , Hashem Oraee4

Abstract –Plug-In hybrid electric vehicles could be connected to the electrical grid to be recharged. To ease the charging process, one solution could use an on-board charger to charge the electric vehicle battery. This charger should be able to be connected to a conventional outlet for convenience reasons and moreover, it should be a grid friendly charger in order not to pollute the electrical network. Since most of the electric vehicle related projects are at their initial phases, keeping up with the pace and developing new technologies and proposing innovations and concepts will help growing the industry. In this regard, a new integrated bidirectional isolated soft-switched plug-in hybrid electric vehicle battery charger has been proposed in this paper, which utilizes a phase-shift controlled dual bridge series resonant DC/DC converter. Finally, the performance of the proposed charger has been investigated and verified in PSIM software environment. Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved.

Keywords:Plug-In Hybrid Electric Vehicles; Integrated Bidirectional Soft-Switched Isolated Battery Charger; PFC; Dual Bridge Series Resonant Converter

Nomenclature Phase „a‟ voltage of power supply. Phase „b‟ voltage of power supply. Phase „c‟ voltage of power supply. Phase „a‟ voltage of inverter input port. Phase „b‟ voltage of inverter input port. Phase „c‟ voltage of inverter input port. Resistance of line per-phase. Inductance of line per-phase. Phase „a‟ line current. Phase „b‟ line current. Phase „c‟ line current. The d-component of three-phase transformed power supply voltages. The q-component of three-phase transformed power supply voltages. The d-component of three-phase transformed inverter input port voltages. The q-component of three-phase transformed inverter input port voltages. The d-component of three-phase transformed line currents. The q-component of three-phase transformed line currents.

Manuscript received January 2014, revised January 2014

The angular frequency. The dc voltage of DBSRC input port. The dc voltage of DBSRC output port. The input port current of DBSRC. The output port current of DBSRC. A constant value. Power flow between the ports of DBSRC. Turns ratio of the DBSRC transformer. Characteristic Resistance. The inductance of series resonant tank. The capacitance of series resonant tank. Ratio of switching/resonant frequency. Switching frequency. Resonant frequency. Normalization factor. Phase-shift between the two square waveforms of input and output port of DBSRC. Maximum power to/from the grid. RMS value of phase to neutral voltage. Reference voltage vector for SVM. DC-Link voltage.

Copyright © 2014 Praise Worthy Prize S.r.l. - All rights reserved

Seyyedmilad Ebrahimi, Farid Khazaeli, Farzad Tahami, Hashem Oraee

I.

Introduction

Constantly rising fuel prices, exhaustible oil reserves and environmental consciousness have caused alternative transport solutions to be considered. In the past, electric cars have not been noteworthy alternatives for traditional internal combustion engine cars due to poor batteries. Lately, batteries and electric vehicle industries have developed to become a reasonable alternative for gasoline powered cars. During the last years, it sounds hybrid electric vehicle (HEV) and electric vehicle (EV) technology to provide an effective solution to help the fuel economy with better performance and lower emissions, compared to conventional vehicles [1]. According to [2], more than 60 percent of automobiles driven on roads would be Plug-In Hybrid Electric Vehicles (PHEVs) up to 2050. A plug-in hybrid electric vehicle is a hybrid vehicle with a storage system that can be recharged by connecting to an external electric power source. At the moment, batteries are the most preferable energy storage units but super-capacitors seem to have great chances to be substituted instead of batteries. The charging time and lifetime of a battery depend on how it is charged and discharged. In the near future, the increasing appearance of EVs will have some consequences on the electric distribution system, like harmonic pollution and effects of low power factor of the charger. Therefore, the PHEV battery charger should be able to be connected to a conventional outlet for convenience reasons and moreover, it should be a grid friendly charger in order not to pollute the electrical network. The most common charger topologies include an AC/DC converter with power factor correction (PFC) followed by an isolated dc/dc converter in order for regulating possible mismatches of battery and dc-link voltage levels [3]. More advanced technologies offer a bidirectional battery charger so that the electric vehicle could be able to inject its stored battery energy to the electrical grid if needed, known as V2G (vehicle to grid) operation for future smart grid. In recharge operation mode, the battery charger should meet the related standards, such as safety, reliability, EMC and harmonics requirements [4]. The detailed PHEV battery charger requirements are listed in [5]. Of great importance, the power factor should not be less than 0.95 and the total harmonic distortion (THD) of input current should be no more than 20% at rated load. In inverter operation mode of depletion process, the bidirectional battery charger roles as grid connected inverter and all the inversion standards, such as the IEEE 1547 standard, should be satisfied by the bidirectional converter [4]. Some single-phase and three-phase AC/DC and DC/DC topologies are reported to be used in PEHV battery charger structure in [6-8]. Most of the existing and introduced battery chargers for PHEV applications are unidirectional and do not have galvanic isolation which is vital in terms of safety issues as well as compatibility of voltage levels, in addition they need a dedicated charging system hardware. In this paper, a new integrated


bidirectional isolated soft-switched plug-in hybrid electric vehicle battery charger has been proposed which benefits a soft-switched bidirectional series resonant dc/dc converter. This paper is organized as follows. In Section II, the topology of the proposed PHEV battery charger has been described. In Section III and IV the front-end AC/DC and rear-end DC/DC converters have been analyzed and appropriate models are derived for control purposes. In Section V a common battery‟s charging profile has been presented for designing a suitable charge algorithm. At last, the performance of the proposed battery charger has been investigated and validated through PSIM software simulation results, in Section VI.

II.

Topology Description of The Proposed Battery Charger

In this paper, a dual bridge series resonant converter (DBSRC) is utilized to serve as the DC/DC battery charger rear-end converter. This topology provides bidirectional power flow capability as well as isolation benefiting the high-frequency transformer used in structure. One of the outstanding advantages of resonant converters is the possibility of soft-switching operation of semiconductor devices which is noteworthy dealing with high current levels to charge the batteries. A bidirectional three-phase PFC boost converter is used as the front-end AC/DC converter to rectify the ac input while providing power factor correction which is controlled by SVM modulation scheme. Fig. 1 shows the schematic of the proposed PHEV battery charger. The three-phase phase inverter/rectifier uses the same drive inverter of motor and the whole battery charger uses the same system hardware of traction mode, therefore no extra electrical component is needed for charging process. The three-phase structure of the integrated battery charger eliminates the second harmonic of battery current, which is unavoidable for single-phase PFC structures, leading to an increase in battery life. To design the corresponding controllers for the two cascaded stages, the two AC/DC and DC/DC converters are analyzed and modeled.

Figure 1.Schematic of the proposed PHEV integrated battery charger

International Review of Electrical Engineering, Vol. xx, n. x


III. Modeling and Analysis of The FrontEnd AC/DC Three-Phase Boost converter To obtain unity input power factor, the three inductor currents should be controlled to be in-phase with the three-phase input voltages. These three-phase currents are transformed into d-q axes and a space-vector modulation scheme is utilized to appropriately control the three-phase rectifier/inverter such that the iq component is forced to be zero in order for achieving power factor correction operation. To model the threephase AC/DC converter, power supply voltages are represented by and notations are used for inverter input voltages. The following equations lead to the d-q model of inverter.

IV.

Modeling and Analysis of The RearEnd DC/DC Dual Bridge Series Resonant converter

Fig. 3 illustrates the topology of the dual bridge series resonant converter. One of the most common methods for controlling the resonant converters is the frequency control. However, this scheme suffers from complexity of control circuit and difficulty to optimize the design of magnetic elements of the circuit.

Cr

V1

Lr

1

n

u2

u1

V2

Figure 3.Schematic of the Dual Bridge Series Resonant Converter

{ Transforming (1) into d-q reference frame and using Laplace transformation, d-q equations appear as (2):

Another advantageous control method is to control the phase-shift between square-wave voltages of the two ports of DBSRC. According to (4), the power flow between the two ports could be simply controlled by varying the phase-shift between the two square wave-forms.

(

{

⁄ )

Where: The terms could be treated as disturbances whose effects can be annihilated using an appropriate PI controller. Therefore, the inverter‟s simplified d-q model could be represented by (3):

√ {

{ {

The Schematic diagram of the current control loops are depicted in Fig. 2. Idref also results from the beforehand voltage control loop.

Idref

Iqref=0

+

+

Current Controller

Current Controller

Vd

The following equations will lead to the low-frequency model of the converter, which is well suited for the battery charger with a gradual dynamic. The thorough high-frequency model of the DBSRC could be found in [9].

-1/(Ls+R)

Id Vq

-1/(Ls+R)

Iq

By linearizing for an operation point:

-

(̅̅̅

̅ ̂ ̂)

(̅̅̅ ̅

̂) ̂

̅ ̂ ̅

̂

Figure 2.Current control loops of the Three-Phase PFC Boost Converter




Since the control parameter ̂ is small, the following assumptions are made: ̂ , ̂ ̂ Substituting (9) in (8): ̂

̅̅̅

̅

̂

̅

̂

̂

̅

̂

should have a minimum in order to be able to generate a definite three-phase set of voltages. The minimum dclink voltage should be such enough for the worst-case maximum negative power injection to the electrical grid. Assuming a series impedance of for the 380 V-line, the sinusoidal reference voltage vector is calculated as (12):

And in a same manner: ̂

̅

V.

̅

Battery Modeling

Fig. 5 and equation (13) describe the relationship between Vdc and Vref.

To charge the battery, it is vital to have an insight to the battery‟s charging profile, so as to obtain an appropriate battery model and devise the corresponding charge algorithm. The most forthcoming battery type for PHEV applications is Lithium-Ion (Li-Ion) whose charging profile is depicted in Fig. 4 [10].

f Vre

60

2/3Vdc Figure 5.Minimue dc voltage to generate a sinusoidal reference voltage

√

Figure 4.Charging profile of lithium-ion batteries

The charge cycle consists of two parts: constant current charging (CCC) segment in which a constant current is flown into the battery so that the battery voltage goes up to a definite value, and the constant voltage charging (CVC) segment in which the battery voltage is kept constant by allowing a sufficient current flowing into the battery. Therefore, the battery is modeled by a constant current source during the (CCC) segment and it is intended to control the battery voltage, and it is modeled by a constant voltage source whose current (or power) is intended to be controlled to follow a reference value.

VI. Performance Evaluation of The Proposed PHEV battery Charger To verify the performance of the proposed battery charger, simulations have been carried out in PSIM software. The Li-Ion battery-pack voltage is chosen to be 250 volts with a maximum required charging power of 5 kilo watts. Using SVM technique the DC-link voltage


Using the assumed power and voltage values a minimum dc-link voltage of 540 V yields. As the proposed charger shares the same system hardware of traction mode, dynamic considerations of the vehicle in traction mode should be taken into account as well. For a more quick dynamic performance of the electric vehicle in traction mode, higher dc-link voltages are preferred. Hence, a typical dc-link voltage of 750 volts is chosen. The switching frequency is 50 kilo hertz. Because of the voltage levels, IGBT type semiconductors are chosen. To achieve ZCS operation for IGBTs for the whole charging power range, the parameter M is chosen to be unity. Moreover, the resonant tank should operate below its resonance frequency [11], so that F=0.9 is used. The resonant tank is designed as follows. According to (4):

(

{

{

{

(

⁄ )

⁄ )

{ (

⁄ )



The charger performance is evaluated for each charge cycle segments. 

Constant Voltage Charging (CVC)

The battery has been replaced by a constant voltage source and the power flow is controlled to follow the reference value. Fig. 6 shows the positive power during the charging process which is well following the command reference. The corresponding d-q current wave-forms of the three-phase rectifier/inverter are also sketched in Fig. 7 for realization. The dc-link voltage is depicted in Fig. 8 which is set to be constant 750 volts. As illustrated, it is well maintained for a step power rise of 500 watts. To verify the power factor correction capability of the proposed topology, three-phase input currents and phase „a‟ voltage/current wave-forms are sketched in Fig. 9. The input power factor and THD of the input currents are summarized in Table 1. In order to corroborate the ZCS operation of the semiconductor IGBTs, appropriate scales of voltage and currents of the resonant tank input and output port switches are illustrated in Fig. 10. As depicted, semiconductor currents go negative before to be turned-off which means the reverse-parallel diodes of IGBTs conduct the flowing current leading to ZCS operation and eliminating switching losses.

Figure 8.DC-Link voltage during the charging process

Figure 9.Three-phase input currents (above diagram) and phase „a‟ voltage/current with PFC operation for positive power (below diagram)

TABLE 1 INPUT PF AND THD OF INPUT CURRENTS (POSITIVE POWER) Parameters

Values

THD (ia current) THD (ib current) THD (ic current)

5.4 % 5.4 % 5.3 %

Input Power Factor

0.999

Figure 6.Positive power into battery and its reference value

Figure 10.Appropriate scales of voltage and currents of the resonant tank input and output port switches confirming ZCS operation of IGBTs

Figure 7.Three-Phase rectifier/inverter d-q currents during the charging process


The corresponding performance indices are also shown for the negative power flow operation in which the battery‟s stored energy is injected into the electrical grid.



The negative power during the depletion process and its reference value, the rectifier d-q currents, the dc-link voltage and the input voltage/current wave-forms with PFC operation are shown in Figs. 11-14 respectively. The input power factor and THD of the three-phase input currents are also summarized in Table 2.

TABLE 2 INPUT PF AND THD OF INPUT CURRENTS (NEGATIVE POWER)



Parameters

Values

THD (ia current) THD (ib current) THD (ic current)

7.9 % 7.7 % 7.6 %

Input Power Factor

0.997

Constant Current Charging (CCC)

The battery has been replaced by a constant current source and its voltage is controlled to follow the reference value. Fig. 15 shows the battery voltage which is well following the command reference. Figure 11.Negative power from battery and its reference value

Figure 15.Battery voltage and its reference value during the CCC segment. Figure 12.Three-Phase rectifier/inverter d-q currents during the depletion process

VII. Conclusion

Figure 13.DC-Link voltage during the depleting process

Figure 14.Input three-phase currents (above diagram) and phase „a‟ voltage/current with PFC operation for negative power (below diagram)


The bidirectional battery charger is the most key power component for plug-in function of a PHEV. Most of the existing and introduced battery chargers for PHEV applications are unidirectional and do not have galvanic isolation which is vital in terms of safety issues as well as compatibility of voltage levels, in addition they need a dedicated charging system hardware. Since most of PHEV related projects are at their initial phases, keeping up with the pace and developing new technologies and proposing innovations and concepts will help growing the industry. In this paper, a new integrated bidirectional isolated soft-switched plug-in hybrid electric vehicle battery charger has been proposed which benefits a softswitched bidirectional series resonant DC/DC converter. The proposed charger shares the traction mode hardware for charging process and no additional component is required. Moreover, the three-phase structure of the aforementioned charger eliminates the second harmonic of battery current, which is unavoidable for single-phase PFC structures, leading to an increase in battery life. Modeling and analysis of the front-end AC/DC and the rear-end DC/DC converters were presented. Moreover, appropriate performance of the whole battery charger has been investigated and validated by simulation results in PSIM software environment.



Authors’ information

Acknowledgements The authors would like to thank Mr. Ramin Mirzahosseini for his critical and extensive comments that led to a transparent and a useful representation of this paper.

References [1]

[2]

[3]

[4]

[5]

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Sharif University of Technology. Sharif University of Technology. Sharif University of Technology. 4 Sharif University of Technology. 2 3

Seyyedmilad Ebrahimi received his B.Sc. degree in Electrical Engineering from the School of Electrical and Computer Engineering, Sharif University of Technology, Tehran, Iran in 2010. He is now graduated from M.Sc. program in Electrical Engineering from Sharif University of Technology, Tehran, Iran. His field of interest is power electronics, application of power electronics to power systems and control of power systems.

Farid Khazaeli received his B.Sc. degree in Electrical Engineering from Amirkabir University of Technology, Tehran, Iran. Now, he has graduated with M.Sc. degree in Electrical Engineering at Electrical Engineering department of Sharif University of Technology, Tehran, Iran. His research interests include Modeling and control of power system dynamics with particular interest in control of grid connected renewable electrical energy systems, electric drives and power electronics.

Farzad Tahami received the B.S. degree from the Ferdowsi University of Mashhad, Mashhad, Iran, in 1991, and the M.S. and Ph.D. degrees from the University of Tehran, Tehran, Iran, in 1993 and 2003, respectively, all in electrical engineering. He is currently an Assistant Professor at Sharif University of Technology. His current research interests include electric motor drives, modeling and control of power electronic converters, soft switching resonant converters, and vehicle system dynamics.

Hashem Oraee received the Ph.D. degree in electrical machines from the University of Cambridge, Cambridge, U.K., in 1984. He is currently a Professor of electrical engineering with the Sharif University of Technology, Tehran, Iran. His research interests include electrical energy conversion and power quality.
