Comparison among Chargers of Electric Vehicle Based on Different Control Strategies

doi:10.4236/epe.2013.54B265

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Energy and Power Engineering, 2013, 5, 1398-1403

doi:10.4236/epe.2013.54B265 Published Online July 2013 (http://www.scirp.org/journal/epe)

Comparison among Chargers of Electric Vehicle Based on

Different Control Strategies*

Pengxin Hou, Chunlin Guo, Yubo Fan

State Key Laboratory for Alternat e El ectrical Power Sy stem with Renewable E n ergy Sources,

North China Electric Power University, Beijing, China

Email: [email protected]

Received March, 2013

ABSTRACT

The charger of electric vehicle is a power electronic device which consists of rectifying de vices and DC-DC conver ters.

This nonlinear diode rectifier circuit has low power factor and high harmonic content. In order to improve power factor

and reduce the harmonic distortion rate of the AC side current, single-phase non-controlled rectifier charger needs to

install the active power factor correction device. A piece of power system analysis software which is called PSCAD is

used in modeling of an EV charger which contains Boost-APFC. By means of simulation and analysis, differences of

APFC characteristics between the hysteresis current control mode and average current control mode which has an in-

fluence on the power grid are compared. The consequence of simulation shows that the two control strategies achieve

power factor correction and harmonic reduction requirements; Boost type power conversion circuit employs the average

current control mode is better, which has following features: relatively faster settling time of the output voltage, rela-

tively smaller overshoot, lower current harmonic distortion rate on AC side, lower switching frequency and better con-

trol effect.

Keywords: Electric Vehicle Charger; APFC; Control Strategy; Power Factor; Harmonic

1. Introduction

Environmental and energy consumption problems have

become the focus of the public throughout the world.

Electric vehicle, as a high-speed developing transport,

has an unparalleled advantage on energy conservation,

and reduction in pollutant emission. As the energy supply

facilities of Electric vehicle, charging station is the es-

sential part in the development of electric vehicle [1].

One of the main equipments of the electric vehicle is

charger that is an electrical and electronic device consisting

of rectifier, DC/DC converter and etc [2]. Due to the no-

nlinear structure of the charging system, a large number

of harmonic current components are produced that makes

harmonic noise of the circuit increase and power factor

decrease. The conventional approach is to add filters at

the input of the rectifier circuit, but the cost of the

method is so high and the volume and weight is too large

[3]. Therefore, in order to enhance the performance of

the on-board charging system, active power factor

correction (APFC) device need to be installed to improv e

the power factor of the system input side and reduce

harmonics.

Aiming at low-power single-phase uncontrolled recti-

fying charger, a simulation model of the Boost converter

circuit is builds in this thesis with the utilization of elec-

tromagnetic transient simulation tool PSCAD / EMT-DC

[4], studying two different typical strategies separately

used for controlling circuit, and making a comparative

analysis of the input voltage and current waveforms,

output voltage, AC side current harmonic distortion and

etc.

2. EV Charger Simulation Model

EV charger rectifier filter circuit is composed b y the rec-

tifier diode and the energy storage element filter capaci-

tor, and the nonlinear element leads to the conduction

angle of the rectifying element less than 180 degrees.

When the input peak voltage exceeds the voltage across

the filter capacitor, the rectifier diode has electric current

to flow through, which results in severe distortion of the

input AC current waveform [5]. Figure 1 shows an AC

side voltage and current waveforms, the green curve is

the voltage waveform, and the blue curve is the current

waveform. Current was pulse-like, which leads the input

AC voltage and current to generate the additional phase

shift. And the input side contains a lot of higher harmon-

*This work is supported by: National High Technology R&D Program

of China (863Program) (2012AA050804), Key Project of the National

Research Program of China (2011BAG02B14), National High Tech-

nology R&D Program of China (863 Program) (2011AA05A109).

P. X. HOU ET AL. 1399

ics, resulting in the low level of system input power fac-

tor, which make the input power factor measured at least

0.7. When circuit in the open loop state, the output volt-

age is unstable, especially during the beginning period,

the voltage values have serious overshoot. The introduc-

tion of the power factor correction circuit can promote

the operating characteristics of the charging machine and

improve the input power factor of the charging system,

also can reduce the impact and polluti o n o n t he grid [6].

3. APFC Control Circuit Simulation Model

3.1. Operating Characteristic of Boost-PFC

Circuit

Boost-PFC circuit is shown as Figure 2. To meet the

requirements of input current harmonics, the adoption of

Boost circuit takes account of the two functions realize-

tion----power factor correction and raising the voltage

[7].

Boost APFC circuit has the following characteristics

[8]:

1) Output voltage is greater than the input voltag e peak,

can achieve the purpose of raising the voltage.

2) Power factor correction to be completed by the

reasonable control strategy;

3) Input inductor can reduce the input filter requirements

to prevent the impact of the grid on the main circuit high

frequency transient.

4) The input current is continuous, EMI small, RFI

low;

(a) The voltage waveform on Ac side

(b) The ac current waveform on Ac side

Figure 1. EV charger AC voltage and current waveform.

Figure 2. Boost - PFC circuit structure diagram.

5) Easy to drive the power switch, the potential of its

reference endpoint is 0V.

In the Boost circuit, there are two main mode: Dis-

continuous Conduction Mode (DCM) and Continuous

Conduction Mod e (CCM).

The control mechanism of Continuous conduction

mode (CCM) is: according to the feedback quantity of

the sampling output voltage and output current , the

amount of deviation between quantity and feedback are

calculated. Through the judgment of deviation between

voltage and current to gain charger charge state, the

output pulse width control and the adjustment of switch-

ing time of power devices can realize closed loop control

for the charging system output quantity;

The continuous conduction mode (CCM)) has three

main control strategies: hysteretic current mode, peak

current mode, average current mode[9].

The current benchmark of three kinds of control strat-

egies are double half-wave sinusoidal voltage. Using

different control strategy to make the inductance current

change with the input voltage waveform, the input cur-

rent is close to sinusoidal wave to have the same phase as

the voltage wave to achieve power factor correction

3.2. Hysteresis Current Control Mode

Figure 3 shows the PSCAD hysteresis current mode

control circuit. Hysteresis loop logic comparator was

introduced in the contro l circuit, g enerating the h ysteresis

band, in order to control the inductor current waveform.

By the inductor current limit changes in the hysteresis

band, so that the input current could change as the

reference current change as the same time, ultimately

following the input voltage chang es.

In the hysteresis current model, the switch open time

constant, off time change, switching cycle is changed

[10]. Changes in the load have a great influence on the

switching frequency. The contradiction between switch-

ing frequency and the hysteresis width directly affect the

performance .of the system .Accuracy and delay of the

comparator and other factors easily because the zero

current dead zone, thus circuit needs to be compensated

to solve the problem [11].

P. X. HOU ET AL.

1400

3.3. Average Current Control Mode Table 1 shows the comparative analysis of the two

different control modes.

Figure 4 shows the PSCAD average current mode control

circuit. By using the current error amplifier, the control

circuit could deal with average of the switching frequency,

and then use the slope comparison of the waveform

generates a switching control signal. By comparison

between the inductor current and the reference current,

making errors equalized, controlling change of the input

current, so that the input current eventually follows the

input voltage[12]. The feedb ack of control strategy is the

input current, the amount charged is the average value of

the input current, and the current loop has a higher gain

bandwidth, good transient characteristics. The average

current is not sensitive to the noise and the switching

frequency is fixed[13].

4. The Simulation Results and Comparative

Analysis

Through the simulation of Boost-PFC circuit, the system

input voltage and current waveform can be measured in

the input. Two control methods both can achieve the

power factor correction and reduce harmonic [14]. Fig-

ure 5 shows the input side voltage and current waveform

under th e avera ge cu rrent mo de, AC side curren t phas e is

according to the source voltage.

Figure 6 gives power factor of the hysteresis current

mode and average current mode, two power factor is

close to 1, and the stable time and overshoot time of av-

erage current mode is short.

Figure 3. Hysteresis current control mode.

Figure 4. average current mode control.

P. X. HOU ET AL. 1401

Table 1. Comparative analysis of the two different control

modes.

Control method hysteresis current

control mode average current

control mode

Detect current inductive current inductive current

operating mode CCM all

switching frequency Variable stable

noise sensitive Insensitive

add notes logic control current error

amplification

Figure 5. The input side voltage and current waveform of

the average current mode.

(a) The power factor of hysteresis current mode

(b) The power factor of average current mode

Figure 6. The power factor of hysteresis current mode and

average current mode.

Figure 7 gives the current distortion rate (THD

) of

hysteresis current mode and average current mode. The

current distortion rate of averag e current mode is smaller

than that of hysteresis current mode, which is only 4.2%,

and that of hysteresis current mode is to 5.4%.

Figure 8 gives the output voltage waveform of the

hysteresis current mode and average current mode. As

can be seen from the graph, the overshoot and smaller

stable time of average current mode is small, the over-

shoot is only 0 and stable time is 0.35 s. Table 2 presents

the comparison result of APFC performance for two dif-

ferent control modes.

Through the simulation study of two kinds of different

control mode, the following conclusion can be obtained:

Hysteresis current control relies on logic control dur-

ing the process, hysteresis loop width and switch fre-

quency interact with each other. Current error need to be

average amplification processed under average current

mode, which increase the complexity of a circuit.

Through the computer simulation analysis, average cur-

rent mode has the following advantages: relatively faster

output voltage stability time, higher power factor, rela-

tively smaller overshoot and AC current harmonic distor-

tion rate.

(a) The current distortion rate (ITHD) of hysteresis current mode

(b) The current distortion rate (ITHD) of and average current mode

Figure 7. The current distortion rate (ITHD) of hysteresis

current mode and average current mode.

P. X. HOU ET AL.

1402

(a). The output voltage waveform of hysteresis current mode

(b). The output voltage waveform of average current mode.

Figure 8. The output voltage waveform of the hysteresis

current mode and average current mode.

Table 2. APFC working performance of two different con-

trol modes.

APFC performance

hysteresis current

control mode average current

control mode

The overshoot of output

voltage (%) 9.3 0

Stable time of the outpu t

voltage (s) 0.4 0.35

THD

I(%) 5.4 4.2

PF 0.997 0.998

According to the above performance advantage, the

active power factor correction circuit of off-board charger

adopts average current control mode for power factor

correction can achieve better control effect.

5. Acknowledgements

1) After the installation of an active power factor cor-

rection circuit, the power factor of the circuit of EV

charger has been improved, and the current harmonic

distortion of the AC side reduced. The control circuit of

Boost-PFC power factor correction has made the input

AC current waveform smoothness, which contains a

small high-frequency components; AC current zero

smooth, no large discontinuity; small overshoot starting

when the output DC voltage can be controlled within

range between 5% and 10%, and the start-up time is

faster; a good stability of the DC output can be achieve d,

and self-excited is not produced in various conditions;

input current is sinusoidal, and the same phase between

input voltage and the input current, and the power factor

of the circuit is greatly improved, PF can achieve more

than 0.99.

2) Load change has a great influence on the switching

frequency in hysteretic current mode. The contradiction

between switching frequency and the hysteresis width

directly affect the performance of the system. When the

input supply voltage is closing to 0, the difference be-

tween the two reference signals is small. Considering th at

the comparator accuracy and delay factors could easily

cause zero current dead band problems, therefore com-

pensation for the circuit has been a solution. For the av-

erage current mode, the current loop has a higher gain -

bandwidth, good transient characteristic, therefore THD

and EMI are very small. The simulation analysis shows

that the settling time of the output voltage is faster,

overshoot of output voltage is smaller, the switching

frequency is lower, and the power factor correction can

be completed without increasing the slope compensation

circuit.

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