2017 Twelfth International Conference on Ecological Vehicles and Renewable Energies (EVER)

Design and Modeling of V2G Inductive Charging

System for Light-Duty Electric Vehicles
Yassine Benomar, Mohamed El Baghdadi, Omar Hegazy, Yang Yang, Maarten Messagie, Joeri Van Mierlo
Vrije Universiteit Brussel (VUB), ETEC Dept. & MOBl Research Group, Pleinlaan 2, Brussels, 1050, Belgium
Yassine. Benomar({iJvub. ac. be

Abstract-Vehicle-to-Grid (V2G) is an emerging infrastructures able to transmit power from the vehicle to
technology resulting from the emergence of Electric the grid. The latter is commonly denoted as vehicle-to-
Vehicles (EVs) and Renewable Energy Sources (RES). The grid or V2G. Furthermore, this technology offers
energy stored in battery packs of EVs are a solution to the decentralized energy transition as each EV could
intermittency of RES. Therefore, design and control design
potentially supply energy back to the grid.
of charging infrastructure for EVs are of high importance
Methods for energy replenishment of EVs are battery
for the development of V2G. This paper presents a wireless
charging infrastructure based on Inductive Power Transfer
swapping, conductive charging and wireless charging. The
(IPT) for semi-fast charging of light-duty (LD) EVs. The first one can realize relatively short energy restore times as
Power Electronic Converters (PECs) and the applied the whole battery pack is exchanged with a charged one.
control strategies are modelled by using The main difficulty to the application of this technology lies
MATLAB/Simulink. Simulation results show that G2V and however in standardization and state of health estimation of
V2G are feasible with the proposed charging system, with batteries [3]. Conductive charging emerged as the most
an overall efficiency of 92 %. common charging method. It consists of a galvanic
connection of the charging infrastructure, through a
Keywords-Vehicle-to-Grid (V2G), Inductive connector, with the EV. This galvanic connection can be
charging, Inductive Power Transfer (IPT), Wireless isolated or not, depending on the charger topology. The
Power Transfer (WPT), Electric Vehicles (EVs), Control plug-in charging method, despite its widespread use, still
Strategies presents some electrical and mechanical hazards related to
the cable and handling ofthe power connector [3], [4]. Most
of the inductive chargers for EV application rely on
I. INTRODUCTION inductive power transfer (lPT), which is based on magnetic
Nowadays many concerns exist due to environmental, coupling of the EV with the charging infrastructure through
resonance. It offers multiple advantages over plug-in
economic, and political issues related to fossil fuels. This
systems, [4] and [5] present some:
has led governments and industries, to shift their attention
towards renewable energy sources. The European Galvanic isolation;
example for this is the Renewable Energy Directive Convenience: Charging of the EV can start after the
(2009/28/EC) which targets the supply of 20% of the vehicle is parked over the charging infrastructure, without
EU's energy needs to be renewable by 2020 [1]. However, additional action of the driver. Furthermore, cumbersome
one of the main characteristics of renewable energy and sometimes dirty cables are not needed anymore;
sources is intermittency, which constitutes an additional
energy management challenge to engineers. A remedy to Weather proof: The charging infrastructure can be
built underground, thus reducing exposure to rainy, snowy
this issue is present in another technology aiming to
or freezing environments and all ensuing problems;
reduce greenhouse gases and improve air quality, which
are the electric vehicles (EVs). The development of this • Anti-vandalism: Inductive charging infrastructures
technology is also backed by Clean Power for Transport are less prone to vandalism such as copper theft due to their
(CPT) by the European Commission [2]. A characteristic limited visibility and accessibility (underground) and
of EV s is the large on board energy storage in the shape • Low risk of hazards: Potential tripping hazards
of a battery pack. This energy capacity can be used as present in conductive charging systems, due to cables, are
buffer to cope with the intermittency issue of renewable eliminated.
energy sources, which induces the need for EV charging

978-1-5386-1692-5/17/$31.00102017 European Union

 These advantages, aside from increasing safety, also subject of comparative studies in literature [10], [11],
inspire future perspectives. The result of implementing [12], [13].

c'p
inductive chargers regularly along roads would be Off board ! On board
beneficial to EV s by downsizing of batteries, and allowing
longer lifetimes by avoiding deep cycling.
Different contributions have been made in recently on
q '-AC-'-AC-H-" P "j"q '-A-C-H''D--
design and control of bidirectional wireless power transfer
[6], [7], [8], with experimental validation up to 6.6 kW [9]. Fig. 2: Power conversions performed by an inductive charger
However, these contributions focus mainly on the wireless based on IPT, for which a Series-Series compensation topology is
power transfer and do not take into account design and adopted.
control of a grid and battery pack interface. This paper The resonance frequencies of such IPT systems are
presents a newly developed inductive semi-fast charging usually in the 104 Hz range. Therefore, at primary side,
system intended for semi-fast charging of light-duty (LD) starting from mains supply, frequency boosting is
EVs at 22kW. In this article, the power electronics converter necessary to excite the LC-circuits. On board of the
(PEC) topology of the complete infrastructure, starting from vehicle, rectification of the current from the secondary
the grid interface up to the vehicle's battery pack, is coil is necessary to supply the battery pack when
presented and control strategies for Grid-to-Vehicle, G2V, charging. The necessary power conversions are illustrated
as well as Vehicle-to-Grid, V2G, operation are designed on Fig. 2.
and validated using MA TLAB/Simulink.
The typical structure of the power train of a LD EV is
The organization of the final paper is as follow: Section
depicted on Fig. 3 and is composed of a three-phase
II presents the PEC topology of the inductive charger,
inverter driving the electric motor, starting from a 400 V
followed by the implemented control strategy in Section III.
Section IV illustrates and discusses the simulation results, DC-link voltage. The application of an IPT charging
and conclusions are made in Section V. system to such vehicles considers the use of an on board
DC-DC converter. Light-duty EVs are typically equipped
[I. SYSTEM DESCRIPTION with such converters in order to reduce the battery pack
voltage.
Inductive charging for EV charging applications is
based on magnetic coupling of two coils, one belonging to
the charging infrastructure, from now on called the primary
coil, and one belonging to the EV, from now on called the
secondary coil. After juxtaposition of the two coils,
magnetic coupling and power transfer is possible. One of
the main challenges engineers are faced with when dealing
with inductive charging, is the loose magnetic coupling of
the primary and secondary coil. This looseness is mainly Fig. 3: On board architecture of an IPT charging system for LD
caused by the high reluctance of the relatively large airgap vehicles.
(typically 100-300mm) between the two coils. To realize the power conversions on Fig. 2, different
Accordingly, this causes high magnetizing current, which PECs are used. The developed topology depicted on Fig.
results in high winding losses and thus lower power
transfer efficiency.
This problem has been solved by introducing
resonance to the system. Compensation capacitors are
connected at secondary side to enhance power transfer
capability, while at primary side, compensation capacitors
are placed to reduce VA ratings ofthe supply [6], [7]. The
design of these capacitors is such that the resonance
frequency of the resulting primary and secondary LC-
circuits coincides. Depending on how the capacitors are
connected, in parallel or in series, four basic
compensation topologies can be distinguished: Series-
Series (SS), Series-Parallel (SP), Parallel-Series (PS) and
Parallel-Parallel (PP). These topologies have been the
 Off board On board
400 V 700 V 400 V 400 V 150 V
32A

Fig. 4: Power electronic converter topology for a 22 kW Inductive charging infrastructure.

4 has been developed for a SS compensated primary and a constant DC-link voltage VdC1 over DC-link capacitor
secondary coils. A Six-Switch converter is used as grid Cd. To this end, Voltage oriented grid current control with
interface, and generates a constant voltage of 700 V. load current compensation is applied. The control law is
Thereafter, a bidirectional DC-DC converter working in implemented in a synchronous dqO frame, and the
Buck mode in G2V lowers this voltage to 400 V. The modulation technique is modelled as a gain Kc , with a
same power semiconductors operate as a DC-DC boost time delay TO, expressed by GpWM(S) (1).
converter when running in V2G. On both primary and
secondary sides, a single-phase full-bridge inverter (1)
excites the resonant circuit at the transmitting side, and The voltage buck stage from 700 V to 400 V is
rectifies the current at receiving side. Finally, the on board regulated with an inductor current control loop embedded
bidirectional DC-DC converter lowers the voltage from in an outer DC-link voltage control loop. The transfer
400 V to the required battery pack voltage of ISO V. functions (2) and (3) in Fig. 6 are derived from the small-
The parameters of the resonant circuit (Fig. 5) have signal model of a DC-DC converter. In G2V operation,
been optimally designed in [14] for 22kW wireless DC-DC buck converter is considered, while when
charging of Light-Duty EVs. The values of these performing V2G, the small-signal model of a DC-DC
parameters are listed in Table 1, together with the boost converter is considered. The symbol r stands in
resonance frequency f, the mutual inductance M and the equation (3) for the equivalent series resistance (ESR) of
magnetic coupling factor k. capacitor Cl. Control law Ge(s) (4) is implemented to
compensate the low-frequency gains and to improve the

3E
phase margin [15]. The considered load for the control
design is modelled by an equivalent resistance Reql, taking
into account the nominal values of V de2 and the charging
power (5).
Fig. 5: Equivalent system model for a SS compensated resonant
circuit. Vdc1
Gid primary (S) = - L (2)
s 1
Table 2: Parameters for a SS compensated 22 kW WPT system
[13] RL1
L1 (~H) 603 Gvi BOOST(S) = L (3)
1+S-1
L z (~H) 160 RL1

C1 (~F) 0.097
Cz (~F) 0.365 (4)
R1 en) 0.132
R z en) 0.179
M (~H) 43.5 (5)
k(-) 0.14
f (kHz) 21 At battery side, a battery current reference is derived
III. CONTROL STRATEGY from a desired charging power, using an online battery
pack voltage measurement. This current reference is fed
The control strategy of the considered charging to a battery current control loop, implementing DC-link
system is represented in Fig. 6. First, at grid side, pulse- voltage compensation, followed by an inductor current
width modulated (PWM) Six-Switch converter maintains
 Grid side Battery side

Vbat

Voltage Oriented Grid Current Control Battery Current Control

Vdc3*

Vdc3

PWM

,,r------------------ ------ --- ------------- ---~

DC-link Voltage Control ! 'u' • '----_ _ _ _ _ _ _---'

L ___________________________________________ _

B
Battery Current Reference

Pbat*
Ibat*
Vba' _ _ _-I -;- 1 - - - - -

Fig. 6: Power Electronics Converter topology and control strategy for a bidirectional charging system based on Inductive Power
Transfer.
control loop. In G2V mode, voltage boosting is applied,
therefore the transfer function Gidl(S), expressed by (6), is (9)
derived from small-signal modeling. In V2G mode,
Gid2(S) (7) is considered. Finally, the DC-link capacitor C2 The DC-DC converters' passive components are
and the battery pack are modelled with the transfer listed in Table 2.
Table 2: DCIDC converters passive components
function Grts) (8), with equivalent load resistance Req ,
defined by (9). Ll (mH) 1.6
L2 (mH) 0.2
Vb at
G d1 (S) = -
I sL
(6) C1 (~F) 17.8
C2 (mF) 2.0

(7)
IV. SIMULATION RESULTS

(8) 1) Grid-to- Vehicle

 i8 ig 1100. 5001
The PEe topology on Fig. 6 is simulated for charging
of a 150 V / 20 kWh Lithium-Ion battery. The charging .00 •..... ' . . .•................ .•........•....:........ .•... .• 500
0 ••••••• 0
power reference is set to 22 kW. Accordingly, a battery ,., 0 0 ;,

;ri '. •• •• •• ;ri

ro ro
current of 137 A is achieved on average, with a ripple of '"co '"c0
II App (ampere, peak-to-peak), as depicted on Fig. 7(a).
-100 '"
0.2
, ,
0.2001
, -500
(}', -100 0.2 0.2001 -500 (}',
u u

Given a battery voltage of 165 V (Fig. 7(b)), 2l.9 kW Time[s]

Time rs1
(a) (b)
charging is achieved at battery side.
On grid side, 23.8 kW is drawn on average by the 400 ~

active rectifier (Fig. 8). The overall efficiency from grid c 200 ~
~:J 1'1
to battery lies around 92.0 % for this topology. U o ~
<!) <!)

Furthermore, unity power factor is achieved as can be '" -200 ~

'" -100 L-'-~~~-'--'--~~'--'--'
0.05
.<: .<:
D-
seen in Fig. 9( c). Regarding the resonant circuit, Fig. 9(a) D-
0.1 0.15
and 9(b) show that both the primary and secondary sides Time[s]

of the circuit are in resonance. (c)

Fig. 9: Grid-to-Vehicle Power Transfer: (a) Primary side of the
resonant circuit: voltage (red), current (blue); (b) Secondary side
160 r::=:=======:::T'[Tf=Z=rl of the resonant circuit: voltage (red), current (blue); (c) Mains
supply: Phase voltage (red) and phase current (blue).
~140

~
~ 120 ~ ~ ~,
2) Vehicle-ta-Grid
'"
.~
-5100 Vehicle-to-Grid power transfer is simulated for a
'"
~ battery with identical specifications, i.e. a 150 V /20 kWh
"' 80
Lithium-Ion technology. The battery discharge power
60
o 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
reference is set to 22 kW, which results in an average
Time [51 battery current of 152 A, with a ripple of II App (ampere,
(a)
peak-to-peak), as depicted on Fig. 10(a). Given a battery
voltage of 156 V (Fig. lOeb)) 23.7 kW discharging power
159.6 is achieved at battery side.
.
2: 159.4
:l'
~ 159.2

'"
J'!
~ 159

158.8

0.05 01 0 15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Time [5]

(b)
Fig. 7: Grid-to-Vehicle Power Transfer: (a) Battery charging
0,05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
current; (b) Batteryvoltage; (c) AC grid power (red) and DC Time [51
battery power (blue); (c) Battery State of Charge. (a)

o 0,05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
005 01 0.15 0.2 0.25 0.3 035 0.4 045 05 Time [s1
Time [51
(b)
Fig. 8: Grid-to-Vehicle Power Transfer: AC grid power (red) Fig. 10: Vehicle-to-Grid Power Transfer: (a) Battery charging
and DC battery power (blue). current; (b) Batteryvoltage; (c) AC grid power (red) and DC
battery power (blue); (c) Battery State of Charge.
 proposed to achieve 22 k W charging. The topology is then
modelled and control design is performed at grid and
battery side. The complete end-to-end power electronics
interface is modeled and simulated in Matlab/Simulink,
and performances and efficiency of the WPT system and
its control system are evaluated.
Simulation results show that both grid-to-vehicle
(G2V) and vehicle-to-grid (V2G) wireless power transfer
can be realized for light-duty electric vehicles. The same
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 efficiency of92 % is observed for G2V and V2G, together
Time[s]
Fig. 11: Vehic1e-to-Grid Power Transfer: AC grid power (red) and with similar impact on grid stability and power quality.
DC battery power (blue). The losses taken into account are all conduction losses in
the system, including real passive elements' parasitics and
power semiconductor devices losses.
500 ~ This study has shown that V2G is feasible on wireless

r mmm
. :00 1
"E (j)

~
OJ
.l'l charging infrastructures, with minor modifications to the
(5 0 0
~
0 >
~
control design. Future work includes the extension of such
E'" '"
E 8 ... : ... : . . . 8 charging methods to heavy-duty EV s with special
~ -100 '---"--~~~~'-' -500 ~
0.2 0.2001
~ -100 ..
0.2 0.2001
• -500 ~
attention to interoperability of the infrastructure between
Timers] Time [sl different vehicle categories. Losses models for each
(a) (b) converter will also provide more accurate data on the
~ 100 11. 11. 11. 11. 11. 11. 11. 400 2: efficiency of the entire charging system.
C 1\ 1\ 1\ 1\ 1\ 1\ 200 ~
~ ~ Acknowledgments
c3 D O:§!
~ v Vv Vv V Vv v VV VV-200 ~ We acknowledge Flanders Make for the support to our research
-100 .. . -400 group.
0.05 0.1 0.15
Time [sl REFERENCES
(c)
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