AC Converter for PHEV's and EV - IEEE Xplore

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Abstract - This paper describes an inductive charging system based on bidirectional ac-dc-ac converter for Plug-in Hybrid and Electric Vehicles.
A New Concept for Bidirectional Inductively Coupled Battery Charging System based on ACDC-AC Converter for PHEV’s and EV’s using Fuzzy Logic Approach Ezhil Reena Joy.T.P, Kannan Thirugnanam and Praveen Kumar

Indian Institute of Technology Guwahati/Electronics and Electrical Engineering, Guwahati-781039, India, E-mail: [email protected], [email protected] and [email protected] Abstract - This paper describes an inductive charging system based on bidirectional ac-dc-ac converter for Plug-in Hybrid and Electric Vehicles. Unlike traditional full bridge converters for high frequency generation, the proposed new topology has the advantage of simple circuit with reduced number of switches and diode for inductive charging systems. Thus, number of components, switching stress, losses and control complexity is reduced. Based on the qualitative description of the charging system, a fuzzy controller is developed to regulate the current flow in battery during charging and discharging process. The entire charging system is designed and simulated for a maximum power handling capacity of 200KW. Keywords — Inductive charging system, Electric vehicle, Fuzzy logic Controls Grid-to-Vehicle and Vehicle to Grid.

I.

INTRODUCTION

Inductive coupling is a method of transferring power magnetically rather than by direct electrical contact [1]-[4]. Recent developments in power electronics have enabled the use of inductive coupling in Electric Vehicle (EV) charging systems across large air-gaps [1],[3]-[4]. By utilizing, appropriate power electronic converters in Inductively Coupled Battery Charging System (ICBCS), EV batteries can either receives energy from the grid or supply back to the grid. This unique aspect of bidirectional power flow is defined as Vehicle-to-Grid (V2G) and Grid-to-Vehicle (G2V) capability [4] An off-vehicle bidirectional ICBCS employs two stage high-frequency converters an ac-dc and dc-ac full bridge inverter to generate high frequency current in the primary track/coil [3]-[4]. However, extra semiconductor switches can be costly and bulky, which also increases power losses, electromagnetic interference effect, switching stress and control complexity [2]. Recently, researches have been investigating the use of direct ac-dc-ac converter for inductive coupling system to generate high frequency ac current from an ac input[2], [5]-[7]. At present, matrix converters are commonly used to generate high frequency ac

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from 50Hz power supply [5]-[7]. However, a matrix converter has to employ multiple bidirectional semiconductor switches for this conversion. In [2], a bidirectional ac-ac converter based on free oscillation and energy injection control has been proposed for inductively coupled system. However, this work describes only unidirectional operation and restricted for low power applications. Another important area in inductive coupling systems is its controllers. The most widely used controller for different applications is PI controller. Conventional PI controllers are designed using mathematical equations based on the small signal model of converters, as they are simple to implement and easy to design [8]. However, these models cannot be used in ICBCS, as EV batteries have different battery ratings and it requires higher order mathematical equation. The uncertain behavior of EV’s batteries causes the PI controller, sensitive to controller gains and sluggish response to sudden changes [8]. As a solution to overcome the above mentioned shortcomings, this paper presents a new bidirectional ac-dcac converter based charging system using Fuzzy Logic Control (FLC). Compared to the traditional full bridge converters for high frequency generation, the proposed topology has the advantage of simple circuit with reduced number of switches and diode. Thus, it has half the component count for the same rating. In addition, a simple bidirectional dc/dc interface is used with ICBCS to step up or down the voltage depending on the battery ratings. Furthermore, the application of FLC, in inductive charging system is found to be flexible enough to handle batteries of different ratings such as terminal voltage Ampere-hour (Ah) and State of Charge (SOC) levels. Finally, a charging system model is designed and simulated for a 200kW system at a transmitting frequency of 18 kHz. The proposed ICBCS model and its associated controllers are simulated to verify its power transfer capability during G2V and V2G process. The remainder of the paper is organized as follows, an overview of ICBCS system is given in section II, Section III and IV explains its modeling and controllers descriptions. Simulated results and conclusions are given in section V.

II.

BI-DIRECTIONAL INDUCTIVELY COUPLED BATTERY CHARGING SYSTEM (ICBCS)

A. Structure of bi-directional ICBCS systems Fig.1 shows, the basic block diagram of the proposed bidirectional ICBCS. It has two units on either side of coreless coils. The primary side has the charging station and the secondary side has the EV battery system. At the primary side, a reduced bidirectional converter representing charging station is used for high frequency generation with sinusoidal grid current as input waveform. At the secondary side of charging system, a bidirectional 1- converter and a buckboost converter with battery pack is connected. The dc/dc interface is used to step or step down the voltage depending on the battery ratings.

stopped in the charging system, electric power is transferred from the charging station to the EV battery system through this coils. It has an inverter circuit with a dc/dc interface. III.

MODELLING OF BIDIRECTIONAL CHARGING SYSTEM

A. Contactless coil design 1.

Self and mutual inductance calculation Although spiral circular geometry is the one having better couple [9], the geometry used here is square or rectangular with planar coil distribution. They show better tolerance to misalignment, which is one of the important characteristics for EV applications. The calculation of L1 and L2 for rectangular winding and planar coil distribution can be approximated using Neumann’s formula (1). The rectangular winding with N 1 and  N 2   turns and their equivalent radius

“ r ” is given by L 

Fig.1. Basic block representation of inductive charging system

o N 4

2

  r1

r2

d ld l  r

(1 )

                                                      

Due to large air gap between windings of contactless system the leakage inductances are large as compared to conventional transformer. Therefore, compensation (Cp, Cs) is required in primary and secondary to enhance the power transfer capability. B.

Charging station An EV charging station, also called as electric recharging point is a Power Electronic Unit (PEU) present off the vehicle. It has a primary winding of coreless coil attached with it. The main purpose of this PEU is to generate a high frequency current on the coreless coil from the 50Hz main supply. These power converters convert three phase 50Hz ac to single phase high frequency ac. Fig.2. shows, the system description of charging station associated with EV battery system.

 

Fig.3. Parameters of the rectangular coils for any dimension and any relative position between them [9]

The equation (1), applied to the system shown in Fig.3, gives 

  r i ( ai  ai  bi ri (bi  ai  bi   2 2  2( ai  bi  ai  bi )  0.25( ai  bi ) 

o 2  L Ni   

ai .ln

2ai bi

2

2

 bi .ln

2ai bi

2

2

(2)

Where ri is the equivalent radius of the winding, defined as ri 

Fig.2. Configuration of inductively coupled battery charging system

C. EV battery system    EV Battery system is present on the vehicle. The secondary winding of the coreless coil with its converters are attached to the underside of the EV’s.  Once the EV’s are

N iSi

(3)



Winding resistance Ri is given by the following expression: Ri   C u N i

2 ( a i  bi ) Si

(4)

Where in (2)-(4), i=1 should be used for the primary winding and i=2 for the secondary winding.

Fig.4. shows the induced and reflected voltages, which are specified in terms of mutual inductance “ M ”, the operational frequency “ o ” and I p , I s , V p , Vs are the primary and secondary current and voltages. The mutual inductance is related to the magnetic coupling coefficient, k and is given by M L p Ls

k 

(5)

2.

Electrical circuit parameter calculation Fig.3. shows the mutual inductance coupling model of a series compensated contactless coil.

 

Where n 

Np Ns

is the turn’s ratio of the transformer.

Therefore, the required compensation, Ccomp of the series capacitor is given by C c om p 

1

 o 2 L leak

(12 )

B. dc/dc interface design There are several topologies for bidirectional buck-boost converter; a simple dc-dc converter is used in ICBCS model. Fig.5. shows the selected topology for buck-boost converter. This circuit has the ability to provide an output voltage higher or lower than the input voltage and also it can transfer power in both directions. It is selected due to its simple structure, well-known dynamic behavior and possibility of pulse by pulse current limiting and instantaneous shut down. The output voltage is controlled by adjusting the duty ratio D of the switches.

Fig.4. Mutual inductance coupling model

The reflected impedance “ Z r ” from the secondary to the primary is found by dividing the reflected voltage by the primary current resulting in  o2M 2 Z

r



Z

(6)

s

Where Z s , is the impedance of the secondary network and depends on the selected compensation topology. Therefore, the power transferred from the primary to the secondary is given by P  (Re Z r ) I p 2

(7)

Where the operator “ Re ” represents the real component of corresponding variable. The current flowing through the secondary winding is given by j o M I p Is 

(8)

Zs

The voltages across the primary and secondary windings are therefore, given by V p  jo L p I p  jo MI s

(9)

Vs  jo MI p  jo Ls I s

(10)

3.

Compensation level The primary compensation is necessary to minimize the VA rating of the supply. The secondary compensation is to boost the power transfer capability, which is normally low due to loose coupling. The series resonant circuit shown in Fig.4 is formed by compensation capacitors C p and Cs , and

leakage inductances Lp and Ls . The approximate resonant frequency of the resonant circuit is given by o 

1

C ( L p  n L s ).( C p  2s ) n 2

(11)

Fig.5. Bidirectional buck-boost converter topology

D

ton t  on ton  toff T

(13)

The values of filter inductance “ L ” and capacitance “ Cin , Cout t” can be derived from the basic equation and is given by

L

C

Vs D f I Ia D

(14)

(15)

f V

Where, Vs represents the maximum input voltage, f switching frequency,  I is the peak-to-peak ripple current, ,  V is peak-to-peak ripple voltage and maximum current I a . IV.

CONTROLLER DESCRIPTIONS

A. G2V controller Fig.6 shows, the simplified control block diagram of G2V operation. In order to generate a high frequency ac current in the primary winding of the contactless coil, a high frequency generating circuit is employed in the charging station converter.

obtain, the required regulation of the battery, the energy stored in the converter is sensed i.e., inductor current is sensed as input. The system is normalized to p.u system and the error is calculated from iL and iLref . The result obtained is given as input to the controller. Based on the error, the FLC P provides a signal proportional to the converter duty-cycle, which is then applied to a standard PWM modulator and pulses are produced. The output reference is usually available as an external input depending on the operating point of the battery. Fig.6. G2V control block diagram

This is achieved by sensing the primary side current ( I p ) of the contactless coil in conjunction with a pulse generating circuit at a frequency of 18 kHz. Apart from this, an FLC based P controller is used for regulating the charging current of the battery based on the reference current ( iLref ). This is achieved by sensing the inductor current ( iL ). The iLref indeed is produced based on the demand of the consumer (i.e., Power requirement) and the grid command. The entire charging current controller is normalized in per unit  p.u  values. B. V2G Controller Fig.7 illustrates a control block diagram during V2G mode. In contrast to G2V controllers, the power flows in the reverse direction i.e., from battery to the utility grid. This operation is achieved by the conducting switches in the charging system. The buck boost converter in EV battery system maintains a voltage of 440V at the terminals of contactless coil using FLC P, irrespective of the battery ratings.

Fig.8. Block diagram of FLC-P based control scheme for controlling the charging current in loosely coupled system 

Seven fuzzy subset NH (Negative High), NM (Negative Medium), NL (Negative Low), Z (Zero), PL (Positive Low), PM (Positive Medium), PH (Positive High) have been chosen for the input variable iL and for the output VVL (Very Very Low), VL (Very Low), L (Low), M (Medium), H (High), VH (Very High), VVH (Very Very High) are chosen in order to obtain the required control action. Triangular membership functions are used as it is simpler and easier to implement. Fuzzy control rules are obtained from the analysis of the system behavior. Fig.9 shown below has the rules and membership function of the proposed controller.

Fig.7. V2G control block diagram

The contactless winding in the charging station converter, maintains the voltage at its dc terminal equal to the peak voltage of the grid to achieve a synchronized operation. Because of the importance of the quality of the converter voltage for grid synchronization, a Sinusoidal pulse Width Modulation (SPWM) is used on both side inverters at a frequency of 18 kHz. C. Fuzzy based Current Controller Fig.8. shows the control structure of FLC P i.e., fuzzy based charging and discharging current controller. Here, to

 

Fig.9. FLC a) Input b) Output c) Rule Base

     

 

 The rules are derived based on the general knowledge about the charging system depending on the operating conditions of the battery. Suitable control rules are introduced to make iL follow the iLref . V.

RESULTS AND DISCUSSION

The proposed ICBCS system explained in section II-IV is designed and simulated to deliver 200kW power at 18 kHz frequency. The system parameters are shown in Table I. The simulated results are shown from Fig 10.  a    j  . TABLE I CIRCUIT PARAMETERS OF THE PROPOSED BCPT SYSTEM Parameter

Parameter

L

Value 0.0275H

Cin

Value 0.02666F

Cout

0.02666F

Lp

109H

Ls

50.7H

M

11.88H

Cp

0.7121F

Cs

1.5329F

Fig. 10  a    c  , explains the power transfer between battery and the grid. The simulated results shows, the power is regulated based on the reference power in both directions during G2V and V2G mode. Fig. 10  a  Shows battery supplies power to the grid during discharging operation. In Fig. 10.  b    c  , Grid receives power from the EV battery during V2G operation. Thus the performance curves of the system, shows its bidirectional power transferring capability.

Fig.10 (c): Grid supplies 35kW power to EV battery during G2V

The proposed fuzzy controller explained using Fig.8, 9 is examined in the ICBCS model by simulation for the battery specifications shown in table II. The entire charging system with FLC P is evaluated subjected to various types of batteries with different voltages, ampere-hour, initial SOC’s and C-ratings based on the controller explained in section IV. Fig. 10  d    f  , shows the regulated currents in G2V and V2G mode operations. TABLE II SPECIFICATIONS OF BATTERY Voltage(V)

(Ah)

300 250 350 400

40 50 80 60

SOC (%) 40 30 80 90

Fig.10 (a): EV battery supplies -160kW power to grid during V2G Fig.10 (d): Discharging current of 300V, 40Ah, 4.5C during V2G

Fig.10 (b): Grid supplies 120kW Power to EV battery during G2V

Fig.10 (e): Charging current of 200V, 60Ah at 4C during G2V

Fig.10 (f): Charging current for 350V, 80Ah at 5C during G2V

Fig. 10.  g   ( j ) , shows the rest of the curves obtained by

Fig.10 (j): CL power delivering 100kW power to the grid during V2G

simulation in ICBCS system. In Fig. 10  g  and  h  , the charge

VII.

rate and discharge rate characteristics of different battery ratings are shown for 5s. In Fig. 10  i  and  j  , Contactless

CONCLUSION

In this study, a novel bidirectional ICBCS system has (CL) primary and secondary power is shown while been described. A mathematical model has been shown for transferring the power for different kW ranges. the design of contactless coils and converters used in the system. The entire charging system has been modeled and designed for a 200kW system. The charging and discharging currents of the battery are regulated by the proposed simple fuzzy controller. The entire ICBCS system with its controllers is simulated with different nominal voltages, ampere-hour and SOC levels of the battery. The results obtained by simulation show the effectiveness and feasibility of the proposed system with its controller to handle EV’s batteries of different ratings. REFERENCES Fig .10(g): Charge rate characteristics of 300v, 40Ah, 40% SOC

[1] [2] [3]

[4] Fig .10(h): Discharge rate characteristics for200V

[5]

[6] [7] [8]

Fig.10(i): CL power receiving 200kW power during charging mode

[9]

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