Power Flow Control of Magnetic Resonance Wireless Charing forHybrid Energy Storage System of Electric Vehicles Application

Katsuhiro Hata1†, Xiaoliang Huang1, Yoichi Hori1

1Graduate School of Frontier Sciences, The University of Tokyo, Chiba, Japan(Tel: +81-4-7136-3873; E-mail: hata@hflab.k.u-tokyo.ac.jp)

Abstract: Battery and supercapacitor (SC) hybrid energy storage system (HESS) has been widely known for solvinga power density problem of a conventional battery storage system for electric vehicles (EVs). Wireless power transfer(WPT) is a promising solution for the EV charging. In this paper, the frame of wireless charging is given for EV applica-tions and a charging power control method for the HESS to achieve the constant current charging of the main battery isproposed. A small power test equipment is implemented and the effectiveness of the charging power control is verified bythe experiment. This result shows the importance of the SC bank for the WPT charging system and the need for optimizedpower coordination control of the HESS.

Keywords: electric vehicle, hybrid energy storage system, wireless charging, charging power control

1. INTRODUCTION

Energy storage systems (ESSs) have been a major re-search area in electric, hybrid electric and plug-in hybridelectric vehicles (EVs, HEVs, and PHEVs). A batteryand supercapacitor (SC) hybrid energy storage system(HESS) is designed to achieve high power density andhigh energy density in only one system to satisfy the re-quirement of an EV power train system.

Considering two energy sources among different typessuch as fuel cells, batteries, SCs and so on, the aim isto achieve high efficiency, high performance, and easycharging of ESSs for EV applications. By applying theHESS to EVs, the battery life can be extended and ac-celeration performance can be improved [1, 2]. More im-portantly, efficiency of energy recovery from regenerativebraking can be increased based on the characteristics ofthe SC charging [3].

Wireless power transfer (WPT) can simplify charg-ing operations of EVs. Inductive power transfer andmagnetic resonance coupling have been two mainstreammethods for applying WPT to an EV charger. WPT viamagnetic resonance coupling can achieve a highly effi-cient mid-range transmission [4]. This characteristics issuitable for a personal EV in terms of a transmitting dis-tance and a misalignment between a transmitter in theground and a receiver under the vehicle body. Due toa repetitive charging and a dynamic charging for EVs,WPT can reduce the size of the HESS [5] and extend thedriving range of EVs [6].

In this paper, the frame of wireless charging for theHESS is given for EV applications. The principle of thepower flow regulation to achieve high energy efficiencyand to reduce the size of the HESS is discussed. Thecharging power control of the WPT charger to achievethe constant current charging of the main battery is pro-posed. A small power test equipment is implemented andthe effectiveness of the charging power control is verifiedby experiment.

† Katsuhiro Hata is the presenter of this paper.

Advanced

Motion Control

Energy System

Management

Vehicle

Control

Unit

Motor

Control

Unit

In Wheel Motor

In Wheel Motor

R

INVERTER

L

INVERTER

WPT Charging

Rectifier +

Converter

Main

Battery SC Bank

Bidirectional

DC-DC

Converter

Control

Signal Flow

Information

Signal Flow

Power Flow

Charging Power Source

DC BUS

Fig. 1 EV frame powered by HESS with WPT charger.

2. EV STRUCTURE

Fig. 1 shows the EV structure powered by the HESSwith the WPT charger. All power devices of the EV arelinked by the DC bus and the DC bus voltage is regu-lated by the main battery. When the power train systemrequires high power output, the boost converter is com-monly connected to the battery. On the other hand, in thesmall scale EV, the battery can be linked to the DC busdirectly. Then, the power flow between the battery, theSC bank, the power train system, and the wireless charg-ing system can be assumed as the current flow of the DCbus. As a result, the current distribution determines thepower flow and it is the main control objective for ev-ery power converter interface. Therefore, the power flowcontrol can be considered the same as the current controlif the DC bus voltage is robust with respect to the outputcurrent to the motor traction system and to the chargingcurrent from the wireless charging system regarded dis-turbances.

IWPT

VWPT

CWPT

LWPT rWPT

E

IDC

LmC

1C

2R2

L2

R1

L1V1

Power source Transmitter and receiver Rectifier DC-DC converter Hybrid energy storage system

V2

I1 I

2

VS

VDCV

SC

LSC rSC

ISC

P

CDCCSC

d

Fig. 2 The whole frame of HESS with WPT charger.

In order to satisfy the power requirements of the EVpower train system, frequency decoupling is used as asimple and effective power sharing method [7]. TheSC bank provides high frequency output power, whilethe battery output power becomes smooth. As a result,the maximum output peak power of the ESS can be in-creased. Additionally, the battery stress can be mitigatedand the battery life can be extended naturally.

Setting the SC bank to absorb all regenerative en-ergy can increase the total energy efficiency in compari-son with the conventional battery storage system becausethe SC equivalent series resistance is much lower thanthe battery one. In addition, the SC bank can be oper-ated as a bidirectional energy interface [3]. Consideringthe dynamic charging, it can absorb the charging energyfrom the WPT side and provide the constant power to thepower train system at the same time. Therefore, the ad-vantages of the SC are remarkable and will increase thewhole system efficiency.

The whole frame of the HESS with the WPT chargeris shown in Fig. 2. In this paper, the power train sys-tem is neglected as this is a fundamental study. WPT viamagnetic resonance coupling uses LC resonance in thetransmitter and the receiver. The power source operatingfrequency is set to the resonance frequency. The trans-mitting power charges the HESS and its power level iscontrolled by a DC–DC converter on the WPT side. Thecharging power can be distributed by a power converterinterface for the SC bank. The objective is to achieve theconstant current charging of the main battery.

3. WIRELESS CHARGING

3.1. WPT via magnetic resonance coupling

WPT is a promising solution for EV charging. Aseries-series (SS) circuit topology of WPT via magneticresonance coupling is used and its equivalent circuit isshown in Fig. 3 [8]. The transmitter and the receiverare characterized by the inductancesL1, L2, the series-resonance capacitancesC1, C2, and the internal resis-tancesR1, R2 respectively. The equivalent resistanceRL includes the power converter interface for the WPTcharger. This study focuses on fundamental waves of theprimary voltageV1 and the secondary voltageV2 to ana-lyze the transmitting efficiency and the charging power.

RL

V11

Power source Load

V21

I1

I2

C1

C2R

2R1 L

1-L

m

Lm

Transmitter and receiver

L2-L

m

Fig. 3 Equivalent circuit of wireless power transfer.

The primary voltageV1 and the secondary voltageV2

can be assumed as rectangular waves and the primary cur-rentI1 and the secondary currentI2 can be approximatedas sinusoidal waves in the SS circuit topology of WPTvia magnetic resonance coupling [9]. From Fourier se-ries expansion, a fundamental primary voltageV11 and afundamental secondary voltageV21 are expressed as fol-lows:

V11 =2√2

πVS (1)

V21 =2√2

πVWPT . (2)

If the power source angular frequencyω0 satisfies eq. (3),a voltage ratioAV and a current ratioAI between thetransmitter and the receiver are given as eq. (4) and (5).

ω0 =1√L1C1

=1√L2C2

(3)

AV =V21

V11= j

ω0LmRL

R1R2 +R1RL + (ω0Lm)2(4)

AI =I2I1

= jω0Lm

R2 +RL(5)

From eq. (4) and (5), the transmitting efficiencyη is ex-pressed as follows:

η =(ω0Lm)2RL

(R2 +RL)R1R2 +R1RL + (ω0Lm)2(6)

and the charging powerP is described as follows:

P =(ω0Lm)2RL

R1R2 +R1RL + (ω0Lm)22V11

2. (7)

0

2

4

6

8

10

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000

Ch

agin

g p

ow

erP

[kW

]

Tra

nsm

itti

ng e

ffic

ien

cy η

Equivalent resistance RL [Ω]

η

P(100V)

P(200V)

Fig. 4 Equivalent resistance vs. transmitting efficiencyand charging power.

Table 1 Parameters of transmitter and receiver.

Primary coil Secondary coilResistanceR1, R2 1.15Ω 1.20ΩInductanceL1, L2 636µH 637µHCapacitanceC1, C2 4000 pF 3994 pFResonance frequencyf1, f2 99.8 kHz 99.8 kHzMutual inductanceLm 80.8µHCoupling coefficientk 0.127Transmitting gapg 200 mmOuter diameter 448 mmNumber of turns 56 turns

Fig. 4 shows the equivalent resistanceRL versus thetransmitting efficiencyη and the charging powerP . Theparameters of the transmitter and the receiver are listed inTable 1.η is maximized ifRL is given as follows:

RLηmax =

√R2

(ω0Lm)2

R1+R2

. (8)

On the other hand,P is maximized ifRL satisfies thefollowing equation.

RLPmax =(ω0Lm)2

R1+R2 (9)

Then, the maximum powerPmax is given as follows:

Pmax =V11

2

4R1

(1 +

R1R2

ω0Lm

) . (10)

If we can assumeR1R2 ≪ ω0Lm, the maximum powercan be approximated as follows:

Pmax ≃ V112

4R1. (11)

Therefore, if the fundamental primary voltageV11 is suf-ficient and the desired power is smaller thanPmax, thecharging powerP can be controlled by the equivalent re-sistance conversion.

3.2. Charging power control of WPT chargerIn order to achieve the constant current charging of the

main battery, the charging power should be controlled.However, the current control of the DC–DC converteron the WPT side destabilizesVWPT in Fig. 2. This isbecause the WPT charger is connected to the constant

0

2

4

6

8

10

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000

Ch

argin

g p

ow

er

P [

kW

]

Tra

nsm

itti

ng e

ffic

ien

cy η

Fundamental secondary voltage V21 [V]

η(100V)

η(200V)

P(100V)

P(200V)

Fig. 5 Fundamental secondary voltage vs. transmittingefficiency and charging power.

power load and the dynamics ofVWPT has a unstablepole [10]. Therefore, this paper uses the voltage controlof the DC–DC converter on the WPT side for the charg-ing power control [9, 11].

From circuit equations in Fig. 3, the secondary currentI2 can be calculated. Then, the charging powerP can begiven as follows:

P =ω0LmV11V21 −R1V21

2

R1R2 + (ω0Lm)2. (12)

Fig. 5 shows the fundamental secondary voltageV21

versus the transmitting efficiencyη and the chargingpowerP . The parameters of the transmitter and the re-ceiver are described in Table 1. IfV21 can be controlled,the charging powerP can also be controlled. Then, interms of the transmitting efficiency,V21 should be limitedbelowV21Pmax, which maximize the charging powerPand is expressed as follows:

V21Pmax =ω0Lm

2R1V11. (13)

From eq. (12), the reference voltageV21∗, which obtains

the desired powerP ∗, is given as follows:

V21∗ =

(ω0LmV11

2R1

)−√(

ω0LmV11

2R1

)2

− R1R2 + (ω0Lm)2P ∗

R1. (14)

From eq. (2), this is replaced with the DC reference volt-ageVWPT

∗, which is expressed as follows:

VWPT∗ =

π

2√2V21

∗. (15)

As for the voltage control, the DC-DC converter modelis analyzed and the control system is designed by the poleplacement method [11]. Then, the equilibrium point ofthe DC–DC converter is defined according to the desiredpowerP ∗ and the currentIWPT . As a result, the dutycycled of the DC–DC converter can be obtained and thecharging power control can be achieved.

(a) Transmitter andreceiver.

Digital signal processor (DSP)

Power supply for control circuit

Current

sensor

Voltage

sensor

Half-bridge circuit

Gate driver

(b) DC–DC converter.

Fig. 6 Experimental equipment.

0

5

10

15

20

0 10 20 30 40

Char

gin

g p

ow

er P

[W

]

Fundamental primary voltage V11 [V]

P*=5W

P*=10W

Fig. 7 Experimental result of charging power control.

4. EXPERIMENT

The experimental equipment is shown in Fig. 6. Theparameters of the transmitter and the receiver are listedin Table 1. The DC–DC converter was controlled by adigital signal processor. The battery voltage was 12 VandVWPT was controlled by the DC–DC converter.

The experimental result is shown in Fig. 7. Whenthe fundamental primary voltageV11 is insufficient, thecharging powerP is limited by eq. (10). This prob-lem can be avoided by designing the minimum value ofV11. On the other hand, when theV11 is enough large butthe desired powerP ∗ is lower, P ∗ cannot be achievedbecause the minimum value ofVWPT andP was deter-mined by the battery voltage. If the surplus power can beabsorbed into the SC bank, the constant current chargingof the main battery can be achieved.

5. CONCLUSION

This paper proposed the charging power controlmethod for the HESS to achieve the constant currentcharging of the main battery. Experimental result showedthe importance of the SC bank for WPT charging and theneed for the optimized power coordination control of theHESS with the WPT charger.

In future works, the WPT charging of the HESS usingthe power coordination control will be experimented andthe optimization of the charging power based on informa-tion of the HESS will be discussed.

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