UNIVERSIDAD DE LOS ANDES

EV Battery Charger: Design and RT

Test on a Battery Emulation Platform

by

Eng. Maria Paula Puentes Castilla

A thesis submitted in partial fulfillment for the

degree of Master in Electrical Engineering

in the

Faculty of Engineering

Department of Electrical and Electronic Engineering

January 2014

Authorship Declaration

I, Maria Paula Puentes Castilla, declare that this thesis titled, ‘EV Battery Charger:

Design and RT Test on a Battery Emulation Platform’ and the work presented in it are

my own. I confirm that:

This work was done wholly or mainly while in candidature for a research degree

at this University.

Where any part of this thesis has previously been submitted for a degree or any

other qualification at this University or any other institution, this has been clearly

stated.

Where I have consulted the published work of others, this is always clearly at-

tributed.

Where I have quoted from the work of others, the source is always given. With

the exception of such quotations, this thesis is entirely my own work.

I have acknowledged all main sources of help.

Where the thesis is based on work done by myself jointly with others, I have made

clear exactly what was done by others and what I have contributed myself.

Signed: Maria Paula Puentes Castilla

Date: January 2014

i

“We are what we repeatedly do, Excellence then, is not an act but a Habit.”

Aristotle

UNIVERSIDAD DE LOS ANDES

Abstract

Faculty of Engineering

Department of Electrical and Electronic Engineering

Master in Electrical Engineering

by Eng. Maria Paula Puentes Castilla

This thesis document presents the complete design process of an Electric Vehicle (EV)

Charger, beginning with the study of the topologies suitable for the purpose until the test

of the full converter. Tests were carried out on an RT platform, which included a battery

emulator. The first design step was the selection of the best suited topology for the

charger, intended to be economically implemented for a small electric vehicle; beginning

with a State-of-the-Art revision, whose theoretical results were a reduced number of

topologies pre-selected. These where later evaluated through Multisim-NI Simulation,

verifying the full charger. The second design step was focused on the development

of a Battery emulation platform; beginning with the evaluation of the different model

approaches evaluation, to finally obtain per cell and battery package model tested on

RT with a Power Amplifier. The third and final design step, was to test the complete

charger. The model for the converter was implemented on LabView for the final complete

RT tests of the charger at On-Line RT simulation on CRIO9082.

Acknowledgements

I would like to thank Professor Gustavo Ramos for his advice and supervsion over the

course of the last years at the University. For his strong belief in looking for goals we

cannot yet see clear from the present. I would also like to thank Miguel Hernndez for

his incredible dedication in helping others, he is undoubtedly the best Engineer I know.

Finally to my parents, and friends who have always support my dreams, even if they

seem unreacheable.

iv

Contents

i

Abstract iii

Acknowledgements iv

List of Figures vi

List of Tables vii

1 Introduction 1

2 STATE-OF-THE-ART REVISION 3

2.1 AC/DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 DC/DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Converter Topology Design 6

3.1 Converter Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Control 11

4.1 AC/DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2 DC/DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 Battery Emulator 14

6 Tests and Results Analysis 17

6.1 Offline Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

6.2 Online Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7 Conclusions 23

A Simulation Interface 25

Bibliography 27

v

List of Figures

1.1 Smart Grids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Bidirectional Battery Charging Scheme . . . . . . . . . . . . . . . . . . . 2

3.1 Full Bridge: a) Rectifier, b) Inverter . . . . . . . . . . . . . . . . . . . . . 7

3.2 Three-Level Clamped Diode: a) Rectifier, b) Inverter . . . . . . . . . . . . 8

3.3 Dual Active Bridge Variation . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4 Half Bridge Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.5 Definitive Bi-Directional EV Charger Topology . . . . . . . . . . . . . . . 10

4.1 SVPWM Sectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 Control Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.1 Battery Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.2 Nyquist Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.3 Battery Cell Voltage Profile . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.4 Circuit Model Refinement . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.5 Battery Emulation Platform . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.6 Battery Emulation Test: a) Voltage, b) SOC, c) Current . . . . . . . . . . 16

6.1 Charger Simulink Result: Vbatt and Vcharger . . . . . . . . . . . . . . . 18

6.2 Charger Performance: Efficiency . . . . . . . . . . . . . . . . . . . . . . . 18

6.3 Charger Performance: Displacement Power Factor . . . . . . . . . . . . . 19

6.4 Charger Performance: THDi . . . . . . . . . . . . . . . . . . . . . . . . . 19

6.5 Charger Performance: Vdclink Ripple . . . . . . . . . . . . . . . . . . . . 19

6.6 Charger Performance: Vbatt Ripple . . . . . . . . . . . . . . . . . . . . . 20

6.7 Charger Performance: Ibatt Ripple . . . . . . . . . . . . . . . . . . . . . . 20

6.8 Charger Performance: THDv . . . . . . . . . . . . . . . . . . . . . . . . . 20

6.9 Charger LabView Results: a) Vdclink, b) SOC, c) Vbatt . . . . . . . . . . 21

6.10 Charger LabView Results with Separate Models . . . . . . . . . . . . . . . 21

6.11 Parallel Functioning CRIO 9082 . . . . . . . . . . . . . . . . . . . . . . . 21

6.12 Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6.13 Test results: Left SOC, Right: Voltage . . . . . . . . . . . . . . . . . . . . 22

A.1 Simulink Code Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

A.2 LabView Simulation Interface (SIT) Manager . . . . . . . . . . . . . . . . 25

A.3 LabView Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

A.4 LabView VI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

A.5 LabView Front Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

vi

List of Tables

2.1 AC/DC CONVERTERS COMPARISON [1][8][10][13][11][12] . . . . . . . 4

2.2 DC/DC CONVERTERS COMPARISON [1][8][14][16][15][17][12][18][19][20] 5

3.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.1 Battery Cell Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.1 Charger Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

vii

Chapter 1

Introduction

Electrical vehicles (EV) are an interesting alternative to internal combustion ones,

mainly because of their reduced greenhouse gases emission levels, but they show others

advantages such as non-dependency on fossil fuels and relative low electrical energy cost.

Small EV, as those for residential users, have battery packs ranging around 16kWh (Mit-

subishi i-MiEV) to 24kWh (Nissan Leaf). These types of users are expected to recharge

their vehicles during the night. With a level 2 charger, a full charge would take 6-8

hours [1][2][3][4][5][6], and this level of charger is the one expected to be the prefered

one. Electrical Vehicles are just another link in the Smart grid Fig.1.2.

Yet not everything about EVs is positive. EV’s and the its charger are a nonlinear load

which might rapidly grow and become a threat to the distribution system. This is why

the idea of bidirectional chargers has grow. This type of enery flow, allows not only to

reduce the damages caused to the grid by the charging, but also to provide support to

the grid (harmonic filtering, load balancing, and reactive power compensation, among

others). The flow of energy might be from the grid to the vehicle in the standard Grid

Figure 1.1: Smart Grids

1

2

Figure 1.2: Bidirectional Battery Charging Scheme

to Vehicle mode, and might also be from the vehicle to the grid, in what is known as

Vehicle to Grid (V2G) mode [3][4][7][6].

EV Chargers may have one or two stages. The one selected for this research has two

stages: first an AC/DC converter and second a DC/DC converter; both must support

bidirectional power flow. The AC/DC converter which is the interface with the grid

has mainly two tasks: provide low harmonic distortion for both voltage and current

(in and out) and unitary power factor. The DC/DC must take care of the battery

through the control the charge current, assure the proper voltage level is being delivered

to the battery at every moment and minimizing the ripple for both current and voltage.

General battery charging scheme is presented in Fig.1.2 [8][5][9].

Chapter 2

STATE-OF-THE-ART

REVISION

EV battery chargers vary mainly in terms of it power level. For small electric vehicles,

the level 2 of charging is preferred. This allows a 6 hours full recharge for a 16kWh EV,

from a single phase 240V plug. It is the expected to be the chosen residential level for

charging at night [6].

2.1 AC/DC

This converter is the input seen by the grid; its correct behavior will determine the

impacts or relives on the grid. The major topologies are presented in Table 1. Full

Bridge and Multilevel are the pre-selected ones for the AC/DC stage. Simulation will

determine which is better suited to satisfy the required characteristics: allow a unitary

power factor charging mode (Grid to Vehicle), not current neither voltage distortion

seen at the mains whether the flow is from or into the grid and to deliver to the DC/DC

stage a higher power quality; while maintaining a good relation in semiconductors (size

and number), passive elements size, number of control signals, total losses, stresses on

the semiconductors, and high efficiency at all loads (since Battery is a light load 2/3 of

the charging time) [1][8][10][11][12][6].

2.2 DC/DC

This converter must guarantee a secure charge and discharge, protecting battery life;

voltage must always be between accepted levels, and charge current must not threat

3

4

Table 2.1: AC/DC CONVERTERS COMPARISON [1][8][10][13][11][12]

AC/DC Advantages Disadvantages N Transistors

Split-Phase No VcBalance L and C too Large 6

Three-Leg Problems, Low I Heat Dissipation

ripple, Small in Filter issues

Half- Simpler Design, Fewer Highly Distorted 2

Bridge Components Current. High

Stresses on Devices

Full Smaller Components Reduced Stresses on 4

Bridge than HB. Smaller Devices. Highly

Passive Elements Distorted Current

Multilevel Reduced Harmonic Increased 6

(Three- Content. Small in components

Level) Filter. Low EMI. Number

Smaller Components Complicated

with Reduced Stresses Design

High PF, Reduced

Losses

Boost Simpler Design High Current 5

PFC Fewer Ripple. Increased

Components Losses. L and C too

large

Interleaved Reduced Current Heat Dissipation 6

Boost Ripple. Low EMI Issues. L and C too

large

Phase High Efficiency at High Stresses on 6

Shifted light Load. Reduced Components. Low

Semi Harmonic Content Efficiency at full

Bridgeless High PF Load

Boost

Bridgeless High PF. Reduced Increased 8

Interleaved Harmonic Content Components

Boost Good Efficiency at all Number

loads Levels Complicated

Control. L too large

High Stress

Components

battery integrity. For security reasons isolation must be taken into account for the

charger, and due to nature of this converter is the better suited place for this isolation.

The principal DC/DC converters are presented on Table 2. Dual Active Bridge Variation

presented in [14], and Half Bridge Variation Presented in [15], are the preselected DC/DC

converters topologies. Simulation will determine which is better suited to protect the

battery in both charge and discharge mode, and preserving good relation among passive

elements size, semiconductors devices (size and number), control signals and total losses.

5

Table 2.2: DC/DC CONVERTERS COMPARISON[1][8][14][16][15][17][12][18][19][20]

DC/DC Advantages Disadvantages N Transistors

Buck- Reduced Components Only Allows Buck 2

Boost 2 Number Mode to Charge and

Quadrants Boost to Discharge

Floating Ground

Dual Very Good Isolation Increased 8

Active Allows High Components

Bridge Switching Frequency Number. Low

Reduced Losses Efficiency at light

load. High

Component Stresses

Dual Fewer Components Increased Number 4

Active Reduced Components of Passive Elements

Bridge Stresses

Variation[14]

Half Fewer Components Large Output 4

Bridge Reduced Components Filter

Variation[15] Stresses

Chapter 3

Converter Topology Design

Overall the bidirectional converter is expected to have a high efficiency at all loads levels

and high power density. While output filter in Fig. 1 might not be needed, input filter

helps to maintain high power quality (reducing the harmonic content caused by the

PWM). An increased number of transistors results not only in a more complex control

but also in higher conduction losses. Smaller semiconductors have reduced stresses.

Snubber circuits are recommended in order to control overvoltages without affecting the

efficiency. The series combination of transistors reduces the voltage stresses, while the

parallel combination reduces the current stresses and result in lower switching losses

[10][14][2][13][11][16][17][18][5][19][4].

3.1 Converter Simulation

In order to determine the better suited topologies for the battery charger, simulations

were carried out using NI Multisim software. All four preselected topologies (Three-Level

Clamped Diode and Full Bridge for the AC/DC converter, Variation to Dual Active

Bridge and Variation to Half Bridge for the DC/DC converter) were tested without

considering the control strategy; as a result from this stage the Three-Level Clamped

Diode and the Dual Active Bridge Variation[14] were chosen for the battery charger that

will be constructed.

As for losses, both conduction and commutation were simply estimated respectively as

follows

Pco = VF IF +RDSI2D (3.1)

Psw = DRDSI2D +

1

2VDSIDfs (3.2)

6

7

Figure 3.1: Full Bridge: a) Rectifier, b) Inverter

Even though Three-Level Clamped Diode have 2 more transistors than Full Bridge,

its required voltage capacity is half of the later thus making the topologies compara-

ble in terms of losses. As it will be shown the Three-Level topology advantages fully

compensate its additional semiconductors.

In Fig.3.1 both power flow directions for the Full Bridge converter are shown. When

operating in charge mode, the AC/DC converter is used as a rectifier, in this previous

selection stage in which no special attention is being paid to control scheme passive

rectification was used, i.e. all transistor are left open and diodes perform the task.

Simple PWM technique (setting fs = 48kHz and inductance was calculated to assure

a current ripple of 5%) was used in the inverter mode when battery is being discharged

so that +Vdc and −Vdc are seen at the the RL filter that smooths the signal until the

desired sinusoidal shape is achieved. Special attention was put on Harmonic Distortion,

for Full bridge Total Harmonic Distortion for Voltage was 2.1% in the Battery-to-Grid

mode, while in the charge mode the voltage ripple was 5% which is in the acceptable

limit.

In order to make a fair comparison trough the simulation results same filters and PWM

technique were used for the Three-Level Clamped Diode Bidirectional AC/DC converter

8

Figure 3.2: Three-Level Clamped Diode: a) Rectifier, b) Inverter

as shown in Fig.3.2. For passive rectification voltage ripple was 2.4%, while as an inverter

Total Harmonic Distortion for Voltage was 1.18%. In both power flow directions Three-

Level Clamped Diode converter proved to have a better performance, although it requires

2 more transistors its rating is reduced as voltage stress is Vin/2, making it the better

suited topology for the AC/DC stage. Special attention will be put on the design of the

control technique to enhance Bi-Directional charger capabilities.

Both DC-DC converters considered are variations of Dual Active Bridge whose behavior

can be described as follows. The power delivered is function of the angle difference

among the gate signals for the two bridges and the Voltage relation is dependent upon

the duty cycle as

P =1

2π

∫ 2π

0Vin(ωt)iin(ωt)d(ωt)

=Vi

2nφ

4πLlk(1 −D)(4πD − φ

(1 −D))

=VinVoutωNLlk

(φ− φ2

π) (3.3)

9

Figure 3.3: Dual Active Bridge Variation

Figure 3.4: Half Bridge Variation

Vsw =Vin

(1 −D)(3.4)

Modified Dual Active Bridge is shown in Fig.3.3. High frequency isolation transformer

is crucial to protect the battery. Stresses for voltage and current are dependent upon the

operation point (i.e. the duty cycle and angle). Considering the battery life preservation

the main task for the DC/DC converter. For this purpose the converter must guarantee

minimum ripple for both current and voltage. Considering a perfect DC voltage input,

voltage ripple was 0.36% and current ripple 0.4%. This topology is well suited for the

purposes of the Bi-Directional Battery Charger, and it is the selected one.

The main advantage for the Half Bridge topology is the absence of snubber due to

active switch which reduces the voltage stresses. As shown in Fig.3.4 this converter

is composed by two half bridges. One is voltage-fed and the other is current-fed. Its

behavior resembles the one presented above. Ripple for voltage was 0.1% and for current

was 0.65%; again assuming an ideal DC input voltage. Even though its ripple is lower

in voltage, the di/dt stress is too high, making it unpractical for both the battery and

the MOSFETs.

Three-Level Clamped Diode as the AC/DC and Modified Dual Active Bridge [14] for

the DC/DC stage were chosen due to its capabilities to compose the definitive topology

as shown in Table 3. A total of 10 MOSFETs, 5 Capacitors, 2 Diodes, 2 Inductances

and 1 high frequency isolation transformer compose the bi-directional EV charger as

shown in Fig.3.5.

10

Table 3.1: Simulation Results

AC/DC THDV ∆Vdc

Full Bridge 2.1% 5% NO

Three Level 1.18% 2.4% YES

Clmaped Diode

DC/DC ∆Vout ∆Iout

Dual Active 0.36% 0.4% YES

Bridge Variation[14]

Half Bridge 0.1% 0.65% NO

Variation[15]

AC/DC + DC/DC THDV ∆Vout ∆Iout

TLCD + DAV 3.65% 0.48% 0.67%

Figure 3.5: Definitive Bi-Directional EV Charger Topology

Chapter 4

Control

Control must guarantee that the energy sent into grid posses acceptable power quality

levels, as well as battery life must be preserved taking care of voltage and current ripples

and controlling the level of the charge current.

4.1 AC/DC

For the AC/DC stage SVPWM single phase was implemented. Transformation to the

dq coordinate frame was used to accommodate the three phase SVPWM to the present

case.

IL = Idsin(Ωot) − Iqcos(Ωot) (4.1)

V = Vdsin(Ωot) − Vqcos(Ωot) (4.2)

The purposes of this control are: unity displacement power factor, voltage balancing

of the capacitors (lowering harmonic distortion), and DC link voltage regulation. The

system in terms of it dq components can be expressed as follows

dIddt

= −ΩoIq + VauxL − Vd

L (4.3)

11

12

Figure 4.1: SVPWM Sectors

dIqdt

=ΩotId − VqL (4.4)

C

2

VDClinkdt

=VdId + VqIq2VDClink

− VDClinkRload

(4.5)

From these, the voltage of the DC link is controlled from the direct component of

input current, while the displacement power factor is controlled through the quadrature

component. Considering the switching options, there are 9 feasible instantaneous voltage

vectors. The reference voltage for the modulation is

Vref =√V 2d + V 2

q (4.6)

σ=arctan(Vq/Vd) (4.7)

φ=Ωot− 0.5π − σ (4.8)

The reference voltage is guided through the five sectors as shown in Fig.4.1. The forehand

modulation purpose is to balance the charge in the capacitors. In order to do this, three

voltage vector are used in each switching period.

13

Figure 4.2: Control Scheme

4.2 DC/DC

On the other hand for the DC/DC converter the control is focused in soft switching.

This allows to reduce ripple in both voltage and current while minimizing the losses.

Since voltage relation between the DC link and the output to the battery is related to

the duty cycle. Then the second control loop which is the one for the angle, which is the

one that takes the soft switching action. The transistors from each half bridge switch

complementary, and each transistor has a range for the angle, in which the commutation

can be done under zero voltage [21]. The limit for the ZVS operation is

D ≤ 1

(1 − 0.5π)(4.9)

Since the angle determines the power delivered, ZVS operation may be not possible

to achieve under light current requirements from the battery. The control reads the

battery voltage, and compares it with the DC link voltage. The angle is controlling the

current, while maintaining the ZVS if it is possible. The other two parameters to take

into consideration during the control design, are the transformer leakage inductance and

the interlock delay time. The final control scheme is shown in Fig.4.2.

Chapter 5

Battery Emulator

There are several types of battery models. Empirical models use equations with pa-

rameters tuned to match experimental data. The Electrochemical models, describe the

discharge mechanism, despite being the most accurate model type, its application on

Real Time simulation is limited due to slow computation times. Abstract models, use

pure mathematical methods to describe the battery behavior. The electrical circuit

model is the most widely used, there are several variations of it, looking for a better

performance.

The model for a lithium-ion 3.7V cell used is shown in Fig.5.1, parameters are presented

on Table 4. It is simple but accurate, coulomb counting is used to estimate the Sate

of Charge. Parameters for the circuits where chosen to obtain the desired performance

[22].

SOC = SOC0 −∫

icQcdt (5.1)

Battery cell may be characterized through its Nyquist diagram. The frequency perfor-

mance represents the three main characteristics: Ohmic Losses (low frequency range),

Diffusion Losses (low-medium frequency), and Charge Transfer (High frequency), this is

Figure 5.1: Battery Model

14

15

Table 5.1: Battery Cell Characteristics

V 3.6 V

I 20 Ah

Vco 4.2 V

Charge Mode CC/CV

Ohmic Losses R 1.13 mΩ

Charge Transfer R 0.42 mΩ

C 8.8 F

Diffusion R 1.88 mΩ

C 13375 F

Figure 5.2: Nyquist Diagram

shown in Fig.5.2. While the voltage profile of the cell is shown in Fig.5.3. Circuit refine-

ment is shown in Fig.5.4. The charge process is divided in two stages, first a continuous

current (CC) is used, followed by continuous voltage (CV) control.

Using 21 series cells, a battery package of 77.7V and 20Ah was used to built a battery

emulation platform. Battery model was implemented on FPGA CRIO 9082, and passed

through a Power Amplifier (Fig.5.5), which just for a validation test, was used to feed

an electronic load. An example is shown in Fig.5.6, where a 400Ohm electronic Load

was used and current consumption was up to 3Ah.

16

Figure 5.3: Battery Cell Voltage Profile

Figure 5.4: Circuit Model Refinement

Figure 5.5: Battery Emulation Platform

Figure 5.6: Battery Emulation Test: a) Voltage, b) SOC, c) Current

Table 6.1: Charger Performance

Efficiency 0.85 − 0.97

DPF 0.97

THDi 0.01%

THDv 1.46%

Vdclink: Ripple 0.45%

Vbatt: Ripple 0.002%

Ibatt: Ripple 1.03%

Chapter 6

Tests and Results Analysis

In order to prove converter design a series of off-line and on-line simulation were carried

out. Debugging the control parameters as well as veryfing the battery model functioning.

6.1 Offline Simulation

Full converter simulation was carried out both on Simulink and LabView. In Simulink

full charger including the control scheme and battery model aforementioned are together

in one model. In Fig.6.1, it can be seen how the battery is charged. On the other hand,

charger performance is shown from Fig.6.2 to Fig.6.8, satisfying the requirements. Table

5 Presents the main characteristics of charger performance.

On the other hand when implementing the model in LabView, for purposes of the

consequent RT simulation, models are implemented separately. There is a model for

each physical stage (i.e. AC/DC converter, DC/DC converter, Battery Package and

Control). Furthermore, LabView panel control allows the user to change the initial

SOC as well as the battery capacity for each simulation. Fig.6.9 shows the result of the

17

18

Figure 6.1: Charger Simulink Result: Vbatt and Vcharger

Figure 6.2: Charger Performance: Efficiency

19

Figure 6.3: Charger Performance: Displacement Power Factor

Figure 6.4: Charger Performance: THDi

Figure 6.5: Charger Performance: Vdclink Ripple

20

Figure 6.6: Charger Performance: Vbatt Ripple

Figure 6.7: Charger Performance: Ibatt Ripple

Figure 6.8: Charger Performance: THDv

21

Figure 6.9: Charger LabView Results: a) Vdclink, b) SOC, c) Vbatt

Figure 6.10: Charger LabView Results with Separate Models

Figure 6.11: Parallel Functioning CRIO 9082

simulation for an SOCo = 0.1, using one model for the full charger as in the Simulink

case. While in Fig.6.10 all the models are separated, and there were not appreciable

changes in the results.

6.2 Online Simulation

As mentioned above, separate models were implemented in order to reproduce reality.

The Labview models implemented on CRIO 9082 run in parallel. The board modules

are interconnected, sending and receiving the required signals through physical cables,

as shown in Fig.6.11.

Setup for the test is shown in Fig.6.12, and the satisfactory results obtained through an

oscilloscope are shown Fig.6.13. Finally all design stages carried out correctly let to the

verification test, with the performance expected.

22

Figure 6.12: Test Setup

Figure 6.13: Test results: Left SOC, Right: Voltage

Chapter 7

Conclusions

After an extensive state-of-the art revision on chargers and converters for EV purposes,

four topologies were chosen to be tested through a simulation stage with NI software

Multisim in order to select the best suited topology for a bi-directional charger for an

small Electric Vehicle. Two-Stage Bi-Directional charger is composed by a AC/DC stage

and a DC/DC converter. AC/DC converter main task is to preserve power quality in

the grid when energy is being sent into it from the battery, as well as minimizing the

DC link voltage ripple. With the DC link voltage as its input the DC/DC converter

must control the charge current and minimizing the ripple both for voltage and current

to preserve the battery pack life.

For the first stage Full Bridge converter and Three-Level Clamped Diode were compared

with the later providing both the lower total harmonic distortion for voltage, and the

lower DC link voltage ripple making it the best option. On the other side two variations

for the Dual Active Bridge presented in [14] and [15] were tested, while both proved

to satisfy the ripple restriction the later di/dt stress was too high, reason why it was

discarded.

A Control scheme for the use of the converter as a charger (G2V). The control includes

a first stage made of a SVPWM single phase for the AC/DC converter, for balancing

capacitors voltage and obtaining unity displacement power factor. While a second stage

is a Soft Switching ZVS PWM for the DC/DC converter, which is used in order to reduce

both current and voltage ripple, as well as reducing losses. Control was tested through

offline Simulink simulations, where the PI’s parameter were tuned.

A Battery Emulator Platform was build using a circuit model for a 3.6V and 20A Lithium

Ion-Cell. The Model was implemented in Simulink, in order to import it to LabView

and be mounted on NI CRIO 9082. Battery Emulator Platform output is voltage, and

23

24

its input is the current consumption of the load. It was tested with a Power Amplifier

and Electronic Load, returning satisfactory results.

Finally RT simulation was carried out using separate models for both converters, control

and battery package. Models were implemented on NI CRIO 9082, where they run in

parallel, and communicate with physical signals. Tests proved the expected performance

of the battery charger and battery emulator platform.

Appendix A

Simulation Interface

Simulink Circuit and Control model can be implemented on LabView through the Sim-

ulation Interface Toolkit.

First in the simulink model, simulation parameters, the code is generated to obatian a

dll file, Fig.A.1.

Later on LabView SIT tools, this dll model is selected, and FPGA configured, Fig.A.2.

The LabView project uses an image of the model, so code is quite simple, Fig.A.3.

While the driver is just configuring what was chosen on the control panel to control and

show, Fig.A.4.

As can be seen in the control panel, there are two control parameters: Initial SOC, and

Battery Capacity (this means the cell capacity, of the series package), Fig.A.5.

Figure A.1: Simulink Code Generation

Figure A.2: LabView Simulation Interface (SIT) Manager

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26

Figure A.3: LabView Project

Figure A.4: LabView VI

Figure A.5: LabView Front Panel

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