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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: chamilton@knights.ucf.edu

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

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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.

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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.

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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

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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

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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.

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