Upload
ramiyamin
View
214
Download
0
Embed Size (px)
Citation preview
7/29/2019 2510-gf-011193
1/5
System Architecture of a Modular Direct-DC
PV Charging Station for Plug-in Electric
VehiclesChristopher Hamilton, Gustavo Gamboa, John Elmes, Ross Kerley, Andres Arias,
Michael Pepper, John Shen, and Issa BatarsehSchool of Electrical and
Computer Engineering
University of Central Florida
Orlando, Florida 32816
Email: [email protected]
Abstract Plug-in hybrid electric vehicles (PHEVs) arean emerging technology in the market and are helping tooffset the negative effects of existing transportation methodsthat primarily rely on fossil fuel sources. As PHEVs arebeing introduced into the market, renewable energy sourcessuch as solar power are taking a larger part in theenergy sector. A need for high efficiency battery charging isrequired to decrease the amount of time it takes to chargethese cars in order for them to become a viable meansof transportation. A novel solar carport architecture isproposed that will provide a three port interface to PHEVs,solar panels and the utility grid to create a seamless powerflow between the three ports. Current battery chargersrely heavily on AC/DC conversion from the grid to thecar battery, however a direct DC/DC interface is madein this solar carport thus increasing the overall efficiency.
This paper will prove this concept and show the improvedperformance over available battery charging schemes.
I. INTRODUCTION
In recent years, the improvement in battery technology
has allowed car manufacturers to design more affordable
plug-in electric vehicles. The wide use of electric
vehicles can cause a significant increase in the power
load, especially at peak hours, in local areas. Also, non-
linear charging loads can caused high harmonics and
poor power factor which can significantly affect the
local utility company as well. The implementation of
a PV charging station for plug in electric vehicles has
been an attractive technology since it can optimize thepower consumption at peak hours. Several solar charging
stations have been constructed in [2][4][5][6] with the
intent to offset the load requirements at peak hours.
However, they are based primarily on simulation and/or
fail to compare the overall efficiency.
This paper proposes a plug-in hybrid electric vehicle
(PHEV) solar carport charging station concept featuring a
multi-port power electronic interface among photovoltaic
modules, PHEVs, and the power grid (shown in figure
1). A unique control strategy is implemented, allowing
efficient energy transfer while reducing the conversion
stages between the source and load. The system is
designed to be modular to improve flexibility and allow
for ease of expansion. In the proposed system, a single
modular system will provide charging for two parking
spaces.
Fig. 1. System overview of the proposed multi-port solar carport
The specifications of each DC/DC converter module
will be provided based on design criteria for the
topologies. Preliminary experimental results will also
be presented in this paper showing the efficiency from
PV to car battery. These results will compare thecurrent technology of today based on a DC/AC-AC/DC
conversion scheme with our proposed DC/DC conversion
process. These results will prove the concept of direct
DC/DC car battery charging.
I I . SYSTEM OVERVIEW
Each module consists of four strings of six PV panels.
In this research, the rated power for each of the PV panel
is 200W, thus making the overall power of 1.2 kW per
978-1-4244-5226-2/10/$26.00 2010 IEEE 2510
7/29/2019 2510-gf-011193
2/5
PV string. In normal MPPT operation, the input voltage
the DC/DC converter will see from the panels is 330V-
350V as they were experimentally verified later in this
paper. However, the total open voltage can reach up to
448V per string. Each station also consists of two 4 kW
DC/DC converters for battery charging, one converter
per parking space. Figure 2 illustrates a quick overviewof the proposed modular system for each charging
station. Note the DC/DC charger for the vehicles may
eventually be integrated within each vehicle (on-board).
Finally, the multi-port solar charging station incorporates
a bidirectional converter as the mediator of the module
and the grid.
Fig. 2. Block diagram of the proposed multi-port solar carport
The second phase of the research will include
the design of a bidirectional DC/AC converter. This
bidirectional converter will provide the excess power
from the PV arrays to the grid. This condition is met
when the PV power is greater than then load power.
However, if the PV panels are not providing enough
power to charge the batteries, the control structure ofthe bidirectional DC/AC converter senses the decrease in
bus voltage and supplies the extra power needed by the
vehicle batteries. The proposed control algorithm for the
multi-port solar charging system between photovoltaic
modules, plug-in hybrid vehicles, and the power grid is
shown in figure 2.
III. TOPOLOGY IMPLEMENTATION
Each converter charger and solar DC/DC converter
implemented in this research consists of a synchronous
buck DC/DC converter and the designed values for the
main power components are shown in Table III.
Component Charger Solar
L 47H 650H
Cin 1060F 300F
Cout 600F 1000F
TABLE I
MAIN COMPONENT VALUES FOR THE CHARGER AND SOLAR
DC/DC BUCK CONVERTER
The equivalent DC resistance of the inductor
for the charger DC/DC converter was measured
to be approximately 2.64m. Similarly, the DC
inductor resistance for the solar DC/DC converter is
approximately 81.6m mainly because it contains 72
turns as opposed to the charger inductor where it only
has 10 turns around the core. The equivalent resistance ofthese two inductors under different frequencies is shown
in Table III.
Frequency Rcharger RsolarDC 2.64 m 81.6 m
10kHz 16.5 m 15 m
20kHz 4.25 m 25 m
40kHz 130 m 54 m
80kHz 450 m 1.5
100kHz 600 m 2.3
120kHz 955 m 3.2
140kHz 1.28 4.4
160kHz 1.64 5.6
TABLE II
EQUIVALENT RESISTANCE UNDER DIFFERENT UNDER DIFFERENT
FREQUENCIES FOR THE MAIN INDUCTORS
In order to better understand the results obtained
during testing, it is important to know the range of
inductance under different current conditions. Figure 3
shows that our inductor will remain between 45H [max
load] to 50H [no load]; where the maximum load is at
30A per indictor.
Fig. 3. Charger inductance under different current conditions
Similarly, the main inductor for the solar DC/DCconverter was measured under different current
conditions. Figure 4 shows that the inductance will vary
from 640H [max load]-735H [no load]. Unlike the
charger, the maximum current rating for this inductor
was designed to be 6A.
To reduce switching losses, Zero-Voltage Transition
(ZVT) Pulse-Width-Modulated (PWM) is implemented.
As explained in [3], this technique implements an
auxiliary circuit in parallel with the main power path.
2511
7/29/2019 2510-gf-011193
3/5
Fig. 4. Charger inductance under different current conditions
Several soft switching techniques are used in the industry,
however, a comparison was made in [1] where ZVT-
PWM appeared to be the most desirable since it
combines advantages of both PWM and resonant soft
switching techniques.
In Figure 5, a generalized ZVT-PWM switching cell is
shown. This cell includes the main power switches (S1and S2 assuming ideal case) with an auxiliary switch,
Sr, the auxiliary diode, Dr, the resonant inductor, Lr,
and the resonant capacitor, Cr.
Fig. 5. ZVT-PWM implementation for the Buck DC/DC converters
The designed values for the charger to achieve ZVT
along with their rated values is summarized in Table III.
Component Value Note
Lr 3.3H Irms=60A Isat=84ACr 4.9nF 400VmaxDr 60A 600Vmax
TABLE III
MAIN COMPONENT VALUES FOR THE CHARGER DC/DC BUCK
CONVERTER
Likewise, the designed values for the solar DC/DC
converter to achieve ZVT is summarized in Table III.
For the final paper, these values will be optimized in
order to increase efficiency.
Component Value Note
Lr 4.7H Irms=20A Isat=59A
Cr 4.3nF 400Vmax
Dr 60A 600Vmax
TABLE IV
MAIN COMPONENT VALUES FOR THE SOLAR DC/DC BUCK
CONVERTER
The modes of operation for ZVT are illustrated in
Figure 6. The final paper will include a more detailed
experimental results that confirms ZVT operation.
Fig. 6. Seven modes for the ZVT-PWM implementation
IV. EFFICIENCY RESULTS
The proposed research has undergone some
preliminary efficiency testing to test the power handling
capability of the individual converters as well as theoverall efficiency of the solar carport system. The solar
and charger simulations in PSPICE showed yielded 90%
efficiency when operating without soft-switching and
because of the power level, a soft switching technique
was researched and selected as shown in section II.
Simulation results with the ZVT topology yielded an
efficiency increase of about 5% which is a suitable
specification for the PHEV system. The results shown
below are for the case of ZVT soft-switching.
2512
7/29/2019 2510-gf-011193
4/5
A. Solar Power Stage Testing
The solar power stage showed promising results for
initial testing and when compared with results obtained
from the solar panels, the solar converters will be
operating at high efficiency most of the day due to the
power level distribution during this time.
A grid-tied inverter measured and recorded the powerdelivered to the grid from the panels during a 7 hour
window starting at 7:30 AM and ending at 2:30 PM.
The results from this test are shown below in figure 7.
Fig. 7. Power vs. Time: Power delivered to the grid during morningand mid-afternoon
The results represent 4 strings of 1.2 kW solar panel
units, each of which is to be controlled by a separate 1.2
kW solar power stage operating with MPPT. The results
for the solar power stage testing were obtained during
closed loop operation with only an OVR controller
regulating the output voltage so that efficiency results
could easily be measured. These results are shown in
figure 8.
Fig. 8. Efficiency curve of the solar DC/DC converter operating inoutput voltage regulation
The results shown in figure 8 were taken with the
following: Vin = 330V
Vout = 210V
fsw = 50kHz
Comparing the results of the solar power stage and the
inverter power draw, it can easily be shown that since the
solar converter operates above 93% efficiency at more
than half the rated power and that the panels yield more
than half the rated power from 8:30 AM until the end
of the recorded data, the solar converter operates at high
efficiency levels for most of the day. It should also be
noted that minimal part and layout optimization has been
done and that better results will be extracted over the next
few months.
B. Series Connected Power Stage Results
Testing was also done on the efficiency from the solarpanels to the battery output, which was selected to be 72
V based on common battery voltages in neighborhood
electric vehicles (NEV). The output voltage in the final
system will be selected based on the type of car that
is plugged into the carport. The results in figure 9
show that the solar panel-to-battery efficiency remains
close to 90%. The efficiency test was only up to 1.2
kW because of the rated power of the solar DC/DC
converter, however a thermal camera showed very little
heat dissipation on the parts and the layout and so we
feel comfortable with the power handling capability of
the prototype. Notice that preliminary results show that
the proposed architecture yields a significant increasein efficiency when compared with a typical setup. In
this research, a typical setup is defined as a DC/AC/DC
power transfer before it gets to the vehicle battery. The
converters are currently being optimized and the final
paper will include a more detailed experimental results
that can better illustrate the advantages of the proposed
architecture.
Fig. 9. Efficiency curve of the solar DC/DC converter cascaded withthe charger DC/DC converter (Dash line: typical setup; Solid line:proposed setup
V. SUMMARY
A PHEV solar carport station architecture is proposedin this paper. This architecture along wiht a unique
control algorithm structure eliminates extra conversion
steps which increases overall power transfer efficiency
from the PV arrays to the vehicle battery pack. The
control structure mentioned in section II shows how a
modular system can be made by always monitoring the
bus voltage. The system described allows all PHEVs
connected to the solar charging station for plug-in electric
vehiles to have an equal amount of power if power
2513
7/29/2019 2510-gf-011193
5/5
available from the charging station is limited. In addition,
the solar DC/DC converters behave the same way by
always evenly sharing the power distribution to the
carport. Because the system is modular, it can easily be
expanded thus making the station more affordable and
efficient.
The PV charging station where the proposed algorithmwill be implemented has been completed and it is shown
in Figure 11. Because the solar DC/DC converters always
have the same output, the prototype shown in Figure
10 includes two of the 1.2 kW DC/DC converter with
a common output (bus). This yields an overall power
of 2.4 kW per enclosure. The final paper will include
experimental results of the proposed system architecture
with an optimized ZVT-PWM implememtation. Also, a
more detailed efficiency comparison between DC/AC/DC
charging versus our modular system using the same PV
charging station will be discussed.
Fig. 10. Enclosed first prototype for the solar DC/DC converter (2.4kW total power)
Fig. 11. PHEV carport charging station where the algorithm will beimplemented
REFERENCES
[1] Guichao Hua and F.C. Lee. Soft-switching techniques in pwmconverters. In Industrial Electronics, Control, and Instrumentation,1993. Proceedings of the IECON 93., International Conferenceon, pages 637 643 vol.2, nov 1993.
[2] J.G. Ingersoll and C.A. Perkins. The 2.1 kw photovoltaic electricvehicle charging station in the city of santa monica, california. In
Photovoltaic Specialists Conference, 1996., Conference Record ofthe Twenty Fifth IEEE, pages 1509 1512, may 1996.[3] M.Ld.S. Martins, J.L. Russi, and H.L. Hey. Novel design
methodology and comparative analysis for zvt pwm converterswith resonant auxiliary circuit. Industry Applications, IEEETransactions on, 42(3):779 796, may-june 2006.
[4] D.M. Robalino, G. Kumar, L.O. Uzoechi, U.C. Chukwu, and S.M.Mahajan. Design of a docking station for solar charged electric andfuel cell vehicles. In Clean Electrical Power, 2009 InternationalConference on, pages 655 660, june 2009.
[5] T. Winkler, P. Komarnicki, G. Mueller, G. Heideck, M. Heuer, andZ.A. Styczynski. Electric vehicle charging stations in magdeburg.In Vehicle Power and Propulsion Conference, 2009. VPPC 09.
IEEE, pages 60 65, sept. 2009.[6] Zhang Yu, Minghong Zhang, and Jianning Yang. Design of
energy management systems for mobile power station of electricvehicles. In Information Management, Innovation Managementand Industrial Engineering, 2009 International Conference on,volume 4, pages 250 253, dec. 2009.
2514