Design and Control of Grid-connected Converter in
Bi-directional Battery Charger for Plug-in Hybrid
Electric Vehicle Application
Xiaohu Zhou, Srdjan Lukic, Subhashish Bhattacharya, Alex Huang
Future Renewable Electric Energy Delivery and Management (FREEDM) Systems Center, North Carolina State University
1017 Main Campus Drive, Suite 2100 Raleigh, NC 27695, USA
xzhou5@ncsu.edu, smlukic@ncsu.edu, sbhattacharya@ncsu.edu, aqhuang@ncsu.edu
Abstract—A new bi-directional power converter for Plug-in
Hybrid Electric Vehicles (PHEV) is proposed based on a typical
household circuitry configuration. This converter can achieve
three major functions: battery charger mode, vehicle to grid
mode (V2G) and vehicle to home mode (V2H), which are the
main topics of integration of PHEVs with the grid. The detailed
converter design is presented. An improved AC/DC controller is
proposed in order to achieve low input current harmonics for the
charger mode. The Proportional resonant+harmonics selective
compensation method is utilized for the V2G mode, and capacitor
current feedback and proportional resonant control methods are
adopted for the V2H mode. Compared with conventional PI
controllers, the proposed controllers greatly enhance the gridconnected converter’s performance in the aspects of low
harmonics output and robustness against background noise.
grid or loads. This imposes a requirement on the battery
charger of the PHEV: it should not only charge the battery
efficiently but also follow IEEE standard 1547-2003[1], the
interconnection requirement and testing guideline for
distributed generators.
Keywords-plug-in hybrid electric vehicle;bi-directional battery
charger; grid-connected converter; vehicle to grid;
II. BI-DIRECTIONAL CHARGER SYSTEM
CONFIGURATION AND GRID-CONNECTED CONVERTER
I.
INTRODUCTION
At present, plug-in hybrid electric vehicles (PHEV) are an
area of much interest for researchers because of their attractive
properties of reducing gasoline usage and lower greenhousegas emissions when compared to conventional vehicles.
PHEVs accomplish this by using a higher-capacity battery pack
that can be recharged using power supplied by electric utility to
extend the all-electric drive range of the vehicle. In addition,
bi-directional power electronics can be used to operate the
PHEV as a distributed generator(DG) to supply power to the
In this paper, a bi-directional grid-connected converter is
proposed to implement the integration of PHEV with a
household electric system. The system configuration and three
operational modes principles are described and an analysis of
the converter’s passive components is presented. The
implementation of improved control methods is described for
the converter’s three modes to achieve better performance
compared to a conventional PI controller.
In figure 1 the infrastructure of a PHEV’s integration with a
household electric system is shown. The bi-directional charger
is the interface between the grid and PHEV; it has two stages: a
grid-connected converter, and a DC/DC converter. This
proposed grid-connected converter can operate in three modesthe first of which is a battery charger: the converter uses
different IGBT bridges based on 120V or 240V input voltage
to converter the ac power to dc and use the dc/dc converter to
charge the battery. According to the current limitation of circuit
branch at home 10kW for 240V input will be more than
adequate. The grid-connected converter will control the power
Figure.1 Topology of the proposed bi-directional battery charger.
978-1-4244-2601-0/09/$25.00 ©2009 IEEE
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factor to unity and regulate the DC bus voltage and control the
input current harmonics to be low. The second mode is
“Vehicle to Grid” (V2G) mode: the converter operates as the
DG inverter to control its output current to be in phase with the
grid voltage to feed real power back to the grid. The third mode
is “Vehicle to Home” (V2H): the converter serves as a UPS to
supply critical loads at a home when the grid has failure.
Moreover, because vehicles are inherently easily moved,
PHEVs can be a movable power source for use in other
applications.
In the United States’ electrical distribution scheme, one
house receives input power from a split-phase distribution
transformer that converts 13.2kV to a split-phase 240V/120V.
The center-tapped transformer supplies 120V to normal home
loads and 240V to heavy duty appliances. In order to fit the
household circuitry configuration, a split-phase three-leg
converter is used as a grid-connected converter. Compared with
the traditional H-bridge, the center point of three-leg converter
is tapped to the third leg instead of the middle point of the DC
capacitors. The remaining two legs of the converter have the
same modulation method as an H-bridge. This converter has
the following advantages over the traditional split-capacitor Hbridge converter: 1) no DC capacitor voltage balance issue; 2)
smaller output filter size; 3) smaller DC bus current ripple; 4)
higher utilization of DC bus voltage [2].
ripple. By manipulating (3) and integrating the instantaneous
power for a half cycle, the ripple energy is:
T
EC =
U 2 I 2 ω 2 L2 I 4
+
U I
ω LI
4
4
sin 2ωtdt =
+
ω
4
4
2 2
2
∫
0
2 2 4
(4)
From the ripple energy stored in capacitor we can derive
the correlation between DC capacitor, DC bus voltage ripple
and input inductor, given by equation(5) and graphically
presented in figure 2.
U 2 I 2 ω 2 L2 I 4
+
4
4
C=
2 ⋅ U dc ⋅ ΔU dc ⋅ ω
(5)
Set the DC bus voltage ripple can not exceed more than 5%
of the nominal DC bus voltage; the DC capacitor value is
selected as 1.7mF.
correlation of voltage ripple, input inductor and dc capacitor
-3
x 10
6
A.
GRID-CONNECTED CONVERTER PASSIVE
COMPONENTS DESIGN
5
dc capacitor(F)
III.
Dc bus capacitor
When designing the dc bus capacitor, the major
disadvantage of a single phase PWM rectifier is the secondorder harmonic on the DC bus, which needs a fairly large bus
capacitor to smooth the DC voltage. Considering this capacitor
an ‘energy buffer’ between input AC power and output DC
power, the capacitor value can be calculated and chosen based
on its stored energy. Assuming the converter has unity power
factor, the input power is:
Pin = uin × iin =
UI UI
−
cos 2ωt
2
2
(1)
2
0
3
250
2
200
-3
x 10
150
1
100
50
0
input inductor(H)
0
voltage ripple(Volt)
Figure.2 Correlation of voltage ripple, input inductor and dc capacitor.
Input/output filter
The filter inductor in the input/output filter is designed
based on the current ripple on that inductor. At any given time,
the ripple current can be calculated based on worst case ripple
current.
(2)
The energy first passes through the input inductor and then
the H-bridge finally charges the DC capacitor. Without
considering devices power loss, the energy stored in the DC
capacitor is the difference between the input energy and the
energy stored in inductor:
PC = Pin − PL =
3
1
B.
Instantaneous power stored in the input inductor is:
1
2
PL = ∂ ( L ( I sin ωt ) ) ∂t = ω LI 2 sin ωt cos ωt
2
4
UI UI
−
cos 2ωt − ω LI 2 sin ωt cos ωt
2
2
(3)
The DC component in (3) is supplied to the DC output,
while the left second-order components would charge and
discharge the capacitor which leads to the DC bus voltage
U DC ⋅
I pk =
U sin ωt
U sin ωt
)
⋅ (1 −
U DC
U DC
2L ⋅ fs
(6)
Here, UDC is the bus voltage with voltage ripple, Usinωt is
the instantaneous value of AC input voltage at the positive
cycle, and fs is the switching frequency. Based on equation (6)
the correlation between the DC bus voltage, input inductor and
current ripple is described the by 3-D drawing in figure 3.
1717
second-order harmonic on the DC bus voltage, the feature of
single phase power flow.
correlation of input inductor, current ripple and DC bus voltage
Vgrid
DC bus voltage (V)
440
ig
Vdc
420
400
380
ig
Vdc
360
15
5
10
4
Figure.4 Conventional AC/DC controller.
3
5
2
1
0
current ripple (A)
0
-3
x 10
input inductor (H)
Figure.3 Correlation of current ripple, input inductor and dc bus voltage
The inductor value is chosen to be 0.75mH and the ripple
current is 6.6A which is around 10% of the peak output
current (58.9A). To calculate the filter capacitor, the LC filter
is to damp the harmonics of the output voltage. Equation (7)
shows it can achieve better performance with higher LC value.
However, the output capacitor value could not be too large
otherwise too much of power will be stored in the capacitor. It
is normally said that less than 10% of the rated power could be
stored in the capacitor. The filter capacitor value is calculated
and chosen as 50uF. The power stage components and
parameters are listed in table I.
2
⎛ 1 ⎞ 1
Cf = ⎜
⎟ ⋅
⎝ 2π f res ⎠ L
(7)
Output power rating
DC Link Voltage
DC Capacitor
AC filter inductor
AC filter capacitor
In figure 5, the proposed control method is to use an
internal voltage reference and a 2nd order notch filter to
eliminate these two pollution sources. To solve background
harmonics, an internal voltage source reference is generated to
supply a phase reference to the current loop. This internal
voltage reference uses a Phase Lock Loop (PLL) to catch the
phase information of the grid voltage. If the PLL operates
properly there will be no harmonic distortion from the grid
voltage. Since the proposed charger has bi-directional working
capability, a PLL is required in both of charger mode and V2G
mode so it is reasonable to have a PLL in the controller instead
of using the phase information coming from the grid input
voltage directly.
It is very difficult to eliminate the second-order harmonic on
the DC bus voltage. A solution is to use a notch filter to
eliminate second-order harmonic on the feedback voltage
signal. By adjusting the cutoff frequency of the notch filter to
120Hz and quality factor Q to 10, the filter can achieve a high
attenuation at 120Hz in the voltage feedback signal.
In the current loop, a Proportional+ Resonant (PR)[3]
controller is utilized. The PR controller is more effective in
stationary frame than a PI controller at achieving zero steadystate error and enhances the reference tracking capability. By
setting the resonant frequency to 60Hz, PR controller can
sharply damp other frequency variables to ensure output
follow 60Hz input reference perfectly.
Table I: power stage components in experimental setup
IV.
Vgrid i *
g
Vdc*
10kW (5kW for each
120V phase)
400V
2mF
0.75mH
50uF
ig
IMPROVED CONTROL METHOD FOR CHARGER
MODE
As shown in figure 4 the conventional single phase AC/DC
control uses the outer voltage loop to generate the magnitude
reference for the inner current loop and the magnitude is
multiplied with the phase reference supplied directly by the
grid voltage. This brings two major pollution sources to the
control loop: background harmonics from grid due to the
phase reference of the input current being a copy of the grid
voltage, furthermore if any harmonic pollution exists, it will
become the reference for the current loop. This problem is
exacerbated in the household level since the grid voltage is
heavily distorted and often doesn’t have very good power
quality. The other source of control loop pollution is the
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Vgrid
Vdc
Vgrid i *
g
Vdc*
Vdc
ig
Figure.5 Improved AC/DC controller.
Vdc
The PR current controller Gc(s) is defined as:
Gc ( s ) = K p + K i ⋅
2ωc s
s + 2ωc s + ω 02
even the current generated by the conventional method may be
allowed. However, this current will be a potential issue to the
power quality of the whole power system with a large
penetration of the PHEVs in the coming future.
(8)
2
Table II: harmonics components of the grid voltage
Here Kp determines the dynamic response of the system Ki
adjusts the phase shift between the output and the reference,
ωc is the cutoff frequency which is much smaller than ω0, and
ω0 is the resonant frequency which is set to 376.8 rad/s in this
case.
To better show the advantages of the proposed control
method, simulations of both control methods were carried out.
The simulation uses the data from the measurement to emulate
the grid voltage with the low order harmonics components.
The low order harmonics percentage and phase is shown in
Table II and the total harmonics of the grid voltage is 3.71%.
200
Input current
grid voltage
150
100
50
Harmonics order
3rd
5th
7th
9th
11th
13th
V.
percentage
0.76%
2.57%
1.33%
0.20%
0.81%
0.55%
phase
73.0º
214.1º
0.1º
40.3º
208.3º
168.5º
CONTROL OF V2G MODE AND V2H MODE
In V2G mode, a PR+HC (proportional resonant+selective
harmonics compensation) controller[4] is used the controller’s
goal is to control the output current without being affected by
the grid voltage and effectively reduce the low order harmonics
on the output current. The selective harmonics compensation
method cascades several resonant blocks which are tuned to
resonate at the desired low-order frequencies to compensate
low-order harmonics in the output current.
0
Gh ( s ) =
-50
∑
K ih
h = 3,5,7
2ωc s
s + 2ωc s + (hω0 ) 2
(9)
2
-100
Vdc
-150
-200
0.3
0.31
0.32
0.33
0.34
0.35
iout
Vgrid
0.36
Time
Figure.6 Output current with conventional controller under emulated
grid voltage.
*
iout
iout
200
input current
grid voltage
∑
150
100
Figure.8 V2G mode controller block.
50
0
12.00%
-50
10.00%
-100
8.00%
-150
PI
6.00%
-200
0.3
PR+HC
0.31
0.32
0.33
0.34
0.35
0.36
Time
4.00%
2.00%
Figure.7 Output current with the improved controller under emulated
grid voltage.
0.00%
Figure 6 and 7 show the comparison of the input current
waveform between the improved control method and the
conventional control method clearly demonstrating the
improved control method’s immunity to grid background
harmonics. The THD of the input current in figure 6 is 3.90%
and the input current THD in figure 7 is 1.2%. There is no
much bigger difference between the two current harmonics,
1kW
2kW
3kW
4kW
5kW
6kW
7kW
8kW
9kW
10kW
Figure.9 Two controller output current harmonics comparison from
1kW to 10kW.
The control block for V2G mode is shown in figure 8. The
current reference coming from the power command that should
be the combination of the grid real power demand for a vehicle
1719
and the state of charge of the battery pack in the PHEV. A
single phase PLL is used to obtain grid voltage phase
information and the current controller is PR+HC.
PR+HC
PI
0. 60%
0. 50%
0. 40%
in figure 11, in three-leg topology there are two output filter
capacitor feedback loops and each loop controls one halfbridge IGBT. The capacitor current is sent into the control loop
after passing through a low pass filter to remove the high
frequency components of the current to prevent the pollution of
the current loop. A PR controller is used in the voltage loop to
achieve zero steady-state error for the output voltage. Figure 12
shows the dynamic response of output voltage with a load
transient change from 0 to 10kW, and figure 13 shows the
output voltage with a 9kW non-linear load.
0. 30%
0. 20%
output voltage
load current
load transient
0. 10%
400
0. 00%
300
3r d
5th
7th
9th
200
11th
100
Figure.10 Two control methods output current low-order harmonics
comparison.
0
-100
-200
The PR+HC and PI controllers were evaluated by
comparing of low-order harmonics at 10kW output in figure 10
and also by THD comparison from 1kW to 10kW in figure 9.
From results shown it can be concluded that PR+HC can
greatly attenuate dominant low-order harmonics and the results
also verify that it can also reduce THD effectively, but in
figure.10, the 9th harmonic with PR+HC is slightly higher.
When the grid-connected converter operates in V2H mode,
it functions as a UPS. So without considering the load types,
the output voltage should keep ideally sinusoidal. Among the
UPS control methods, capacitor current feedback control can
achieve better voltage output with a low cost current
transformer to sense the current from the output capacitor [5].
This method has the advantages over the load current feedback
method that the inner loop is always running with the stable
reference under any type of loads or dynamic change. In the
other hand the load current feedback method may have the
difficulty to design the compensator to meet the different load
conditions because of the changing current reference especially
at the no-load condition.
ic A
VoA
-300
-400
0.05
0.06
0.07
0.08
0.09
0.1
Time
0.11
0.12
0.13
0.14
0.15
Figure.12 Load transient from 0 to 10kW.
output voltage
load current
non-linear load
400
300
200
100
0
-100
-200
-300
-400
0.1
0.11
0.12
0.13
0.14
0.15
Time
0.16
0.17
0.18
0.19
0.2
Figure.13 Non-linear load 9kW (diode bridge).
From the above results we can see the controller can achieve
good performance including sinusoidal voltage output and fast
response speed. More different load types have been tested and
results are listed in table III.
Table III: 240V output voltage THD performance
i cB
VoB
Cf s
VrefA
ic A
VoA
VoA
Cf s
VrefB
VoB
ic B
VoB
Figure.11 V2H mode controller block.
In this mode, PR and capacitor current feedback control
methods are used together. The control block diagram is shown
0
58.9
Output
Voltage
THD
0.61%
0.37%
Output
Current
THD
0
0.37%
103
3.92%
67.51%
41.8
1.22%
1.39%
Different Loads
Type
Current
Peak
No load
Full resistance
load 10kW
Diode bridge
RC load
One phase no
load and the
other phase 5kW
RC load
The figure 14 and 15 shows the waveforms of the two 120V
output voltage with different types of load: in figure 14 one
phase is loaded with 5kW non-linear load and the other phase
1720
is running with no load. In figure 15, one phase is fully
operated with 5kW resistive load and the other phase is
connected with 5kW inductive load. Between these two cases,
the worst output voltage THD is 4.91% at non-linear load.
Thus, we can see the capacitor current feedback with PR
controller can achieve very good performance for the phase
voltage.
200
output voltage at no load
150
100
50
0
-50
-100
-150
-200
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.2
200
output voltage at non-linear load
VI.
CONCLUSION
A new grid-connected bi-directional converter for the
battery charger in PHEVs is proposed in this paper. The
converter system infrastructure and operational principles are
illustrated. Converter power stage components are analyzed
and designed. Improved controllers have been designed and
utilized to achieve better performance compared to
conventional PI controllers in Charger, V2G and V2H modes.
A PR controller with an internal voltage reference and notch
filter was used to improve the input current harmonics for the
charger mode. The PR and selective harmonics compensation
method achieves good rejection of dominant harmonics in V2G
mode. Finally capacitor current feedback method and PR
controller guarantee low THD of output voltage for different
types of loads in V2H mode. The proposed converter can
greatly improve the performance of PHEV’s integration with
power grid.
150
100
ACKNOWLEDGMENT
50
0
-50
-100
-150
-200
0.1
0.11
0.12
0.13
0.14
0.15
Time
0.16
0.17
0.18
0.19
0.2
Figure.14 120V output voltage waveforms with one 5kW non-linear
load and the other no load.
200
output voltage
load current
150
This work was partially supported by the National Science
Foundation, Award number: EEC-08212121 and this work is a
part of an ongoing project in collaboration of the FREEDM
systems centre (Future Renewable Electrical Energy Delivery
and Management) with ADAC (Advanced Diagnosis
Automation and Control) Lab and ATEC (Advanced
Transportation Energy Center), North Carolina State
University, USA).
100
REFERENCES
50
0
[1]
-50
-100
with Electric power Systems, IEEE, pp.27,2003.
-150
-200
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.2
200
output voltage
load current
150
[2]
Jin Wang, Peng, F.Z, J. Anderson, A. Joseph, and R. Buffenbarger,
“Low cost fuel cell converter system for residential power generation,”
IEEE Trans. Power Electron., vol. 19, no.5, pp. 1315-1322, Sep. 2004.
[3]
D.N. Zmood, D.G. Holmes, “Stationary frame current regulation of PWM
inverters with zero steady-state error,” IEEE Trans. Power Electron, vol. 18,
no.3, pp. 814-822, May, 2003.
[4]
C. Lascu, L. Asiminoaei, I. Boldea, and F. Blaabjerg, “Frequency response
analysis of current controllers for selective harmonic compensation in active
power filters,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 337–347, Feb.
2009.
[5]
M.J. Ryan, W.E. Brumsickle, and R.D. Lorenz, “Control topology options for
single-phase UPS inverters,” IEEE Trans. Ind. Appl., vol. 33, pp. 493–501,
Mar./Apr. 1997.
100
50
0
-50
-100
-150
-200
0.1
IEEE 1547-2003, IEEE Standard for Interconnecting Distributed Resources
0.11
0.12
0.13
0.14
0.15
Time
0.16
0.17
0.18
0.19
0.2
Figure.15 120V output voltage waveforms with one resistive load and
the other inductive load.
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