978-1-4673-1975-1/12/$31.00 ©2012 IEEE ICIAfS’12

Application Research of an Electric Vehicle DC Fast Charger in Smart Grids

Zuzhi Zhang, Haiping Xu, Lei Shi, Dongxu Li, Yuchen Han Key Laboratory of Power Electronics and Electric Drive,

Institute of Electrical Engineering, Chinese Academy of Sciences, China E-mail: zhangzuzhi@mail.iee.ac.cn

Abstract—Popularization of charging station promotes wide applications of electric vehicle (EV). With the increase in the number of charging station, the problem of grid pollution becomes worse and worse. To improve the utilization rate of EV, the charging hours must be decreased. At present, most EVs start charging instantaneously under almost rated power of the charger. Large-scale EVs charging without adjustment will impact the smart grids. The capability of smart grids will restrict the number of charging station; furthermore, it will affect applications of EV. To solve the above problem, it is necessary to design a smart charger, as an important part of charging station, with unit power factor and with the function of fast charging. This article proposes a novel unit power factor DC fast charger for EV charging stations. This DC fast charger includes two modules: the input module is three-phase PWM rectifier and the output module is the phase-shift full-bridge converter. The two modules are controlled by only one DSP – TMS320F2812, which has abundant outside interfaces, to improve cooperating ability of two converters and the dynamic response of the charger. A prototype of 10 kW Charger is constructed. The experimental results show that the novel DC fast charger has excellent electrical characteristics, and it can be applied to the EV charging stations in smart grids.

Keywords—electric vehicle; unit power factor; DC fast charger; smart grids

I. INTRODUCTION The charger in the fast-charging station is mainly used in

decreasing the charging hours to raise the utilization rate of EV. Therefore, it is necessary to provide a large current to batteries, with high power conversion efficiency. When large numbers of the chargers plug in the power grid, it will induce the power quality degradation [1].

The traditional electric vehicle charging system uses diode rectifier bridges topology cascade with DC-DC. Diode rectifier bridges have some disadvantages: the input current harmonic content is high and absorbs reactive power from the grid, which results in the very low input power factor. Because transmission of reactive power in electric network can lead to network loss and step-down voltage, transmission of a great deal of reactive power necessarily will result in reduction of using efficiency of power energy and severely affect voltage quality. To isolate the power grid from the batteries of electric vehicle, the traditional electric vehicle charging system uses isolated grid-frequency transformer, which is large, heavy and high cost.

To solve the above problems, this paper presents a DC fast charger that consists of three-phase PWM rectifier and the phase-shift full-bridge converter. This DC charger, which is high-frequency PWM charger, makes grid current sinusoidal with low THD and unit power factor. Three-phase PWM rectifier and the phase-shift full-bridge converter can both be used in high power applications. And the latter has an isolated high-frequency transformer. The two modules are controlled by only one DSP – TMS320F2812. With abundant outside interfaces, the DSP makes communication with the Battery Management System (BMS) of EV and the smart grid convenient. And one DSP improves cooperating ability of two converters and the dynamic response of the whole charger, and it reduces costs. To communication with the smart grid, the charger can adjust output power or stop charging to reduce power net load. Therefore the DC charger has lots of advantages that accord with the need of fast-charging station in the smart grid: smart, high power, unit power factor, low harmonic input content, small volume, and light weight [2].

II. ANALYSIS OF THE CHARGING SYSTEM Figure 1 shows the fast-charging system. It is consisted by

three parts: the three-phase PWM rectifier (Voltage Source Rectifier - VSR), the phase-shift full-bridge converter (DC/DC) and the controller.

Figure 1. The block diagram of the charging system

A. The three-phase PWM rectifier The main circuit of a three-phase PWM rectifier (VSR) is

shown in Fig. 2, which is composed of three input filter inductances, a three-phase IGBT bridge and a DC filter capacitor. Va, Vb, Vc are AC power supply voltages. Switch Q1~Q6 constitute the rectifying circuit. Vdc is the output dc-bus voltage.

ICIAfS'12 1569624585

Figure 2. The topology of the three-phase PWM rectifier

The three-phase PWM rectifier with direct current control can get a good current regulation and a good current tracking speed because of the introduction of the current loop control [3]. Figure 3 shows the block of the direct current control. It is a double close-loop control system. The outer is voltage loop control which is to control the output of dc-bus voltage, and the inner is current loop control. The current loop regulates the DQ axis current by PI controller to achieve the unit power factor which is achieved by setting the negative Q axis current reference to zero. R is the equivalent resistance of output.

Figure 3. Direct current control

B. The phase-shift full-bridge converter Figure 4 shows the main circuit of the phase-shift full-

bridge converter.

C2

L2

Q1 Q3

Q2 Q4

D1 D3

D2 D4

T Vo

+

-

Vdc

+

-

Figure 4. The topology of the phase-shift full-bridge converter

In Fig. 4, Vo is the output voltage. T is an isolated high-frequency transformer whose ratio is 1. Switches Q1~Q4

constitute the full bridge circuit. Diodes D1~D4 constitute the rectifying circuit. L2 is the output filter inductance and C2 is the output filter capacitor. The phase-shift full-bridge converter is used to control the process of batteries charging, adopting improved constant current method, which decreases charging current when the voltage of the batteries reaches the reference voltage during the period of normal charging, to optimize the charging process.

The control diagram is shown in Fig. 5. VB, the voltage of the batteries, is used to judge whether the batteries has already been connected. J1 is the program module which judges whether charger meets operating condition. J2 is the module which judges whether the voltage of the batteries reaches the reference voltage to reduce definite charging current. J3 is the module which adjusts output power of the charger or stops charging according to the power net load.

Figure 5. Control diagram of the phase-shift full-bridge converter

The charging control system has single current closed loop. Improved constant current charging method suits the smart grid better than traditional constant current charging, constant voltage charging, and two-step charging which merely meet the characteristics of the batteries without power grid [4]. During the period of normal charging, the charger outputs the reference current at first. When the voltage of EV batteries reaches the reference voltage, the charging current will be reduced with the charging voltage reduced, so repeatedly. When the charging current falls to the 10% preset current, it will stop charging. The normal charging process is shown in solid lines in Fig. 6. The dashed lines show that the charger adjusts output power according to the smart grids. The peak time of power grid is from t1 to t2. At this time, the charger reduces charging current to adjust power system load.

Figure 6. Improved constant current method for batteries

C. The controller Digital signal processor is fast arithmetic operations and

high throughput to handle mathematically intensive algorithms

in real time. TMS320F2812 is accomplished by using the following basic concepts: Harvard architecture, extensive pipelining, dedicated hardware multiplier, special DSP instructions and fast instruction cycle. Therefore, only one DSP controller is used to control both the three-phase PWM rectifier and the phase-shift full-bridge converter. And one DSP controller can decrease the communication time between the two converters sharply to improve cooperating ability of two converters and the dynamic response of the whole charger.

The Event Manager (EV) in the TMS320F2812 is used to control the generation of the PWM. The two EV modules (EVA and EVB) are identical peripherals, which control the two converters, respectively. The CPLD chip is applied to produce PWM signals and protect the whole circuit [5].

III. THE DESIGN OF THE PROGRAM

A. IQmath Library Texas Instruments (TI) TMS320F28x IQmath Library is

collection of highly optimized and high precision mathematical functions for C/C++ programmers to seamlessly port a floating-point algorithm into fixed point code on TMS320F28x devices. These routines are typically used in computationally intensive real-time applications where optimal execution speed and high accuracy is critical. By using these routines, execution speed is considerable faster than equivalent code written in standard ANSI C language. Therefore the TI IQmath library is used in the control program.

B. The control program of the three-phase PWM rectifier

Figure 7. T1 interrupt program

The control program of three-phase PWM rectifier is achieved by Timer 1 (T1) interrupt. The interrupt program flow-process diagram is shown in Fig. 7.The T1 interrupt program contains AD sample, VSR safe model, calculation

model of grid voltage synchronous angle, Clarke transformation, Park transformation, PI control model, inverse Park transformation and calculation of switching time of SVPWM [6].

C. The control program of the phase-shift full-bridge converter The process of batteries charging is achieved by the

charging program, as shown in Fig. 8. At first, the batteries are charged by a constant current if the power grid allows it. When the batteries voltage reaches the preset voltage, the charging current goes down. When the current that smart grid allows (IG) is less than the charging current, the charger reduces the charging current to IG. The charging current rises up when the power grid has recovered. The voltage and charging current of End-of-Charging (EOC) for the batteries is the preset voltage (V*) and the 10% preset current (10%I*), respectively.

Figure 8. The charging program

IV. EXPERIMENTAL RESULT Based on the analysis and design above, using DSP-

TMS320F2812 as main controller, a prototype of an electric vehicle charger module was constructed.

Figure 9. The input voltage and current

Figure 9 are the waveforms of the input voltage and current of the charger. CH1 is the phase A voltage. CH3 is the phase A current. CH4 is the phase B current. The experimental

waveforms show that the phase A voltage and current have same phase and the phase A current leads phase B current by 120 degrees.

Figure 10. The EV charger under 10kW output power

Figure 11. The dynamic waveforms of the charger

Figure 10 is the waveforms of the electric vehicle charger module under about 10kW output power. CH1 is the phase A voltage. CH3 is the phase A current. CH2 is the output voltage of EV charger. CH4 is the output current. The experimental waveforms show that the phase voltage and current have same phase and output voltage and current is steady. Thus the EV charger implements the grid current high sinusoidal and can reach unit power factor. By the power analyzer, power factor can reach 1 and the grid current THD is less than 3% under rated output power.

Figure 11 is the dynamic waveforms of the electric vehicle charger module when charging current is changed from 15A to 5A and from 5A to 15A. CH3 is the phase A current. CH4 is the output current. The experimental waveforms show that the output current-rising-rate 20A/s and the output current-decreasing-rate 20A/s. Thus the EV charger, which is a cascade system that includes two modules: three-phase PWM rectifier and phase-shift full-bridge converter, has good characteristic for dynamic state.

V. CONCLUSIONS Nowadays, as the environmental pollution and energy crisis

is increasing seriously, EV will become more and more popular. The operation of power grid will be more and more impacted by charging behavior of EV. The traditional electric vehicle charging system uses diode rectifier bridges topology cascade with DC-DC, which results in reduction of using efficiency of power energy and severely affects voltage quality. This requires a high power factor smart EV charger to make full use of the grid energy and to charge the batteries fast.

The proposed the DC fast charger for electric vehicle charging station is verified by experiment. The power factor of this charger can reach 1 and THD is less than 3%. The charger module has good characteristic for static and dynamic state, which is suitable to applications of smart grids. Owing to the digital control its work characteristic is excellent, and its control method is easy to improve for the future. Therefore, the DC fast charger is suitable for electric vehicle charging station.

ACKNOWLEDGMENT The author would like to thank the fund and support of the

National Natural Science Foundation of China, Project no (51077122).

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