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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
DESIGN BI-DIRECTIONAL CHARGER FOR PHEV
APPLICATION
CHAUDHARI TEJAL A., BARIYA CHETAN K., UPADHYAY CHETAN D.
Department Of Electrical Engineering D College Of Engineering, Gujarat
Technological University,Ahmedabad–380015, Gujarat, India.
[email protected]
ABSTRACT:- Because of the environmental consideration “Go green” factor, and the automotive industry is
going through a major restructuring, and automakers are looking for new generations of hybrid vehicles
called plug-in hybrid electric vehicles (PHEVs). In the event that PHEVs become more available and the
number of PHEVs on the road increases. So Plug-in Hybrid Electric Vehicles (PHEVs) has been gaining
popularity. But here the major issue comes into account is the battery charging. Here, a battery charging
system based on a three level ac-dc converter, bidirectional dc-dc converter. It has been shown that with
direct duty cycle calculation technique that unity power factor is achieved and Total Harmonic Distortion
(THD) is minimized. The simulation has been performed using SIMULINK. The graphs are presented to
show the battery charging/discharging and the converter characteristics
Keywords- PHEV Battery, DC To DC Converter, DC To AC Inverter, Battery Control Strategy.
I. INTRODUCTION:
Today’s electrical grid has much inefficiency that is
both costly and wasteful. Some of these issues are
simply a result of the fluctuations in demand that
occur each day in addition to the need for voltage and
frequency regulations. When the demand placed on
the grid exceeds the capacity of the base-load power
plants, peak power plants, and sometimes spinning
reserves, must be turned on. During periods of low
demand, the electricity usage drops below the output
of the base-load power plants, and all the unused
energy is wasted. With the increase in demand for
environment-friendly automobile, plug-in hybrid
electric vehicle (PHEV) becomes a preferred choice
as automotive industries are focusing on hybrid
electric vehicle (HEV) and electric vehicle (EV)
development. A plug-in hybrid electric vehicle can be
defined as any hybrid electric vehicle which contains:
a higher capacity of battery storage system used for
powering the vehicle and a battery charger for
recharging the battery system from an external outlet
and has an ability to drive in all-electric range
without consuming gasoline [1],[2][3]. The
conversion from hybrid electric vehicle to plug-in
hybrid electric vehicle can be done by adding a high
energy density battery pack in order to extend the allelectric-range (AER).Increased market penetration of
PHEVs along with a vehicle-to-grid (V2G) network
that enables coordinated charging could significantly
reduce problems with electrical demand. This huge
potential for improving the efficiency of the grid by
utilizing idle storage capacity in PHEVs can be
unlocked through a bi-directional interface. The bidirectional charger will need to function smoothly in
both directions.
The battery pack of PHEV must be able to store
energy from external charging as well as from
regenerative braking and preferably be able to supply
stored energy back to the utility if necessary. PHEV
requires an AC outlet charging system for charging
the battery. AC-DC converters are used in a number
of applications such as power supply, household
electric appliances, battery charger, etc [4][5][6].
Depending on the switching frequency they are
classified as converters with low switching frequency
and those with high switching frequency.
Conventional uncontrolled rectifiers and line
commutated phase controlled rectifiers so far have
dominated the AC to DC power conversion. Such
converters have inherent drawbacks such as
harmonics in the input current and output voltage;
low input power factor especially at low output
voltage since these conventional rectifiers draw
nonsinusoidal currents from the grid. Since power
devices demand reactive power in addition to active
power, a charger with low power factor increases
burden on the utility system. On the other hand,
harmonics have a negative effect in the operation of
the electrical system and therefore, an increasing
attention is paid to their mitigation and control. The
problems due to harmonics in conventional rectifiers
have resulted in the establishment of standards such
as IEC 61000-3-2. Thus, a PHEV battery charger
with a Power Factor Correction (PFC) based AC-DC
converter is desirable [7],[8]. Voltage source
converter or synchronous link converter is the best
solution under such situation, which has both
rectification and regeneration capability. The input
current in these converters flows through the inductor
which can be wave shaped with appropriate current
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ELECTRICAL ENGINEERING
mode control. These converters have high efficiency
and inherent power quality improvement at the ac
input and dc output.
In this paper a three level ac-dc converter
and bi-directional dc- dc converter have been
implemented for a PHEV battery charger. In Fig. 1
the block diagram of the battery charger is shown. In
section II. Analysis of the proposed battery charger in
section III. Converter controller design in section IV.
Battery charging technique are described in section
V. Simulation and results are shown and finally in
section VI conclusion.
Fig.2 bidirectional Buck-Boost, Buck and Boost
converter
Figure1. Block diagram of battery charger for plug-in
hybrid electric vehicle
II. ANALYSIS OF THE PROPOSED BATTERY
CHARGER
A. DC-DC Buck, Boost and Buck-Boost converter:
In order to understand a Bidirectional Full bridge
converter, it is important to study the Buck converter,
Boost converter and Buck-Boost converter principle
of operation before. Fig.2 illustrates different DC-DC
converters:
Boost operation:
The current direction is from Vo to Vd and Vd < Vo.
During the period that M2 is conducting, ton = DTs,
inductor L is charged and when M2 is off
(toff =
Ts - ton ) inductance current will be discharged
through M1. Fig. 2 shows the Boost converter
scheme.
Bidirectional Buck-Boost Operation:
By combining two above converters a two quadrant
converter is obtained that can operate bidirectional.
From Vd to Vo in Buck and from Vo to Vd in Boost
mode. Changing the position of L and M1 changes
Buck-Boost converter to Boost-Buck converter and
that is the only difference between the Buck-Boost
and Boost-Buck converter. (It should be noted that
bidirectional Buck-Boost converter topology is
different from conventional unidirectional BuckBoost converter topology.
B. DC-DC CONVERTER:
The bidirectional dc-dc converter shown in Fig. 3 is
used for the battery charging topology. There are two
switches S1 and S2 and two diodes D1 and D2. When
the battery is charged by the dc-dc converter, switch
S1 and diode D2 conduct current alternately. The
inductor current IL is positive in this case. When the
battery is being discharged switch S2 and D1 conduct
current alternately. During this time the inductor
current IL is negative[9],[10].
Figur.3 Bi-directional DC-DC converter
The model equations of the system are derived below.
During the system equations when state switch S 1 is on
and S2 is switched off, based on Kirchhoff’s laws:
By inspection, when S1 is 'on' and S2 is 'off', the voltseconds passing through the inductor (L) is
(Vin - Vout)ton
(1)
When S1 is 'off' and S2 is 'on', the diode is
conducting and the volt-seconds applied to inductor
(L) is
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ELECTRICAL ENGINEERING
Vouttoff
(2)
Hence to meet the volt-seconds equality on
inductor(L) we get
(Vin - Vout)ton = Vouttoff
Therefore
(3)
(4)
Where ton/(ton+toff ) is defined as
and thus above
equation simplifies
Vout = Vin
(5)
III. CONVERTER CONTROLLER DESIGN:
This section focuses on the PI controller design of acdc converter and dc-dc converter. The ac-dc
converter is connected to dc-dc converter whose
output is connected to the
battery such that it charges the battery when the state
of charge of the battery goes below 85%. The dc-link
voltage of the ac-dc converter is maintained at 500V.
The primary objective is to regulate the dc bus
voltage within a narrow band thus proportional
integral (PI) controller is the obvious design as the
voltage control loop need not be very fast in
response. Fig. 4 shows the schematic of the voltage
control design. Linear controller has been designed in
the following section, which explains the choice of
the values of Kp and Ki. Also in the later part of this
section the PI controller for dc-dc converter is also
designed with explanation of the choice of values of
Kp and Ki for pulse charging technique.
(9)
Applying the Kirchhoff's voltage law, the inductor is
in series with the input voltage of the converter. The
voltage across the capacitor is Vout. The input voltage
is sum of inductor voltage and battery voltage (i.e,
output voltage).
u(t) = Vin + Vout = L
+
(10)
=
(11)
And applying the Kirchhoff’s current law to circuit in
Fig 3. The source current is the sum of current
passing through the capacitor and output current.
(12)
Where
=
(13)
From the state space model of the buck converter
during switch S1 is on and switch S2 is off. When the
output is voltage.
(14)
From above
,
Figure 4. Control strategy for dc link voltage.
During the Switch S1 On and S2 Off: The voltage
across the inductor during S1 is on and S2 is off is
(i.e, switch S2 is open and S1 is closed)
VL = Vin - Vout
(6)
where
Vin is the rectifier output voltage i.e, input voltage of
the buck converter, and Vout is the output voltage of
the converter i.e, input voltage of the battery.
Let us consider current passing trough the inductor is
x1(t), current passing through the capacitor is Ic ,
output voltage is x2(t) and input voltage of the
converter is u(t) (in Fig .3).
Voltage across the inductor is
During the Switch S1 is Off and S2 is On: The
voltage across the inductor during S1 is off and S2 is
on is(i.e, switch S1 is open and S2 is closed)shown in
Fig.3
VL = -Vout
Applying the Kirchhoff’s voltage law to the circuit
shown in Fig 3, sum of inductor voltage and output
voltage is equal to zero.
VL + Vout = 0
(15)
L
+
=0
(16)
=
(17)
Applying the Kirchhoff’s law to circuit, the current
passing through inductor is the sum of current
passing through capacitor and output current.
x1(t) = Ic + Iout
(18)
(8)
Current Passing through the capacitor is
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(20)
(31)
Where
(32)
From the above equation we can get the transfer
function the Buck Converter is given in for the
current controller. The values of R, L and C will be
substitute in the transfer function.
(21)
From above
,
During the battery voltage as a output total state
space model of the buck converter is
A = A1 + A2(1 - )
where A1=A2
Therefore
By using the Ziegler Nicholas method of tunning, the
control parameters are K and i And from this
transfer function we can get the values of Kp and Ki
for the PI controller during constant current charging
are respectively Kp= 0.1335 and Ki= 3
And the transfer function the Buck Converter is given
in for the voltage controller is given by,
(22)
A=
B = B1 + B2(1 - )
Where
Similarly during the constant voltage charging we
get,
Kp= 2.160 and Ki= 3 [11-15].
,
Therefore
And therefore
where C1=C2
Total state space model of the converter the voltage
as a output is
(23)
IV. BATTERY CHARGING TECHNIQUE:
As mentioned earlier the battery is charged by a dc-dc
converter. The type of battery used for this system is Liion. The battery has a nominal voltage of 200 V and an
initial state of charge (SOC) of 90 percent. The battery
model used here is a detailed model available in
Simulink block set. The battery with a voltage factor of
116% and nominal voltage of 200V has a fully charged
voltage of 232V.
and
A.Constant Current Method:
That gives
=A
+B
Initially the voltage across the battery is small, in that
situation charging the battery with constant current
than the voltage grows up in the battery. The battery
charges at a constant current to a set voltage
threshold (Stage 1 shown in Fig 5.).
(24)
(25)
y(t) = Cx(t)
(26)
Now take the Laplace transform
sX(s) = AX(s) + BU(s)
y(s) = CX(s)
output
y(s) = C[sI - A]-1BU(s)
(27)
(28)
(29)
Substituting the values of A,B and C
U(s)
(30)
The transfer function of the system with as output
voltage is
Figure5: Charging stages of a battery
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B. Constant Voltage Method:
As the battery saturates, the current drops and
maintains voltage constant (Stage 2 in Fig 5). With
increasing the voltage in battery the charging current
would be decrease. As charging continuously the
voltage would increase further, it may occur over
voltage across the battery. Over voltage can be
avoided with liming the voltage.
C. Trickle Charging Method:
After charging the battery 100%, the voltage and
current both maintains constant for trickle charging.
Trickle charging characteristics and self discharge of
the battery (Stage 3). Batteries can be charged
manually with a commercial power supply featuring
voltage regulation and current limiting. Charging a
12-volt battery would require a voltage setting of
14.40V. The charge current for small lead-acid
batteries should be set between 10% and 30% of the
rated capacity.
Observe the battery temperature, voltage and current
during charge. Charge only at ambient temperatures
and in a ventilated room. Once the battery is fully
charged and the current has dropped to 3% of the
rated current, the charge is completed. After full
charge, remove the battery from the charger. If oat
charge is needed for operational readiness, lower the
charge voltage to about 13.50V. Most chargers
perform this function automatically[16].
In the first mode battery is charged through the
bidirectional charger. Battery is charged by constant
voltage, constant current methods. This procedure
and simulation results are shown in above battery
charging model.
Mode-II:
Now all battery power is provided from the grid.
Also, sometimes battery power is needed to be
discharged. During charging mode dc-ac inverter is
operated as PWM ac-dc converter to obtain dc link
voltage and battery is charged by bi-directional
converter. On the other hand in case of discharge
mode the power is followed through bi-directional
dc-dc converter and the power which is followed by
battery is given to grid by dc-ac inverter to grid.
Simulation Result Bidirectional Battery Charger:
Fig.7. Battery voltage
V.SIMULATION AND RESULTS
PSIM6.0 was used for simulating the converter-battery
system. Simulation is used for verifying the battery
charging control technique and the bi-directional
characteristics of the converter. The input voltage at the
ac side is 240V rms. The voltage source converter and
dc-dc converter are bi-directional in nature.
Fig.8.Capacitor(C3)voltage
Fig.9. Battery current
Fig.6 simulation of the bi-directional charger
The bi-directional charger operates in two modes:
Mode-I:
Fig.10.Capacitor (C3) current
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Fig.11. DC link Capacitor (C2) Voltage
Fig.12.DC link Capacitor (C2) Current
Simulation is used for verifying the battery charging
control
technique
and
the
bi-directional
characteristics of the converter. The input voltage at
the ac side is 110V dc. The voltage source converter
and dc-dc converter are bi-directional in nature.
Fig.7and Fig.8 shows the battery voltage and
capacitor voltage, Fig.9& Fig.10 shows the battery
voltage and capacitor currents and Fig.11 & Fig.12
shows dc link voltage and current waveform which
has given to ac grid through DC-AC three level
inverter.
VI.CONCLUSION:
A battery charger for the plug-in hybrid vehicle has
been simulated using a three level ac-dc and bidirectional dc-dc converter. The converter system
connected to a single phase ac supply draws unity
power factor from the grid which complies with the
standard. And charging controller has been designed
for the battery charging and vehicle to grid concept
has been demonstrated in the simulation results. It
has to manage the charging and the is charging of the
battery, and must therefore provide several flexible
adjustment functions (e.g., load voltage, charging
current).and by this arrangement the power from the
battery is transfer to grid.
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