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DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT
Author’s full name :
MOHD
NURHADI BIN MAD ZIN
________________________________________________
Date of birth
:
31
JULY 1989
________________________________________________
Title
:
MAXIMUM
POWER POINT TRACKING (MPPT) BATTERY CHARGER
________________________________________________
________________________________________________
FOR
SMALL WIND POWER SYSTEM
Academic Session :
2011/2012
________________________________________________
I declare that this thesis is classified as :
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(Contains confidential information under the Official Secret
Act 1972)*
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(Contains restricted information as specified by the
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6 JULY 2012
SIGNATURE OF SUPERVISOR
PN. NORZANAH ROSMIN
NAME OF SUPERVISOR
Date :
6 JULY 2012
If the thesis is CONFIDENTAL or RESTRICTED, please attach with the letter from
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sufficient in terms of scope and quality for the award of the Bachelor of
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Signature
:
Name of Supervisor
: Pn Norzanah Binti Rosmin
Date
: 6 July 2012
MAXIMUM POWER POINT TRACKING (MPPT) BATTERY
CHARGER FOR SMALL WIND POWER SYSTEM
MOHD NURHADI BIN MAD ZIN
A thesis submitted in fulfillment of the requirement for the award of degree of
Bachelor of Engineering (Electrical)
FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI
MALAYSIA
JULY 2012
ii
“I declare that this is my work as the product of my own effort with the exception of
excerpts cited from other works of which the sources were duly noted”
Signature
: ………………………….
Author’s Name
: MOHD NURHADI BIN MAD ZIN
Date
: 6 JULY 2012
iii
To family who encouraged, guided and inspired me throughout my journey of
education
iv
ACKNOWLEDGEMENT
Alhamdulillah, with Allah’s will. I am able to finish this project successfully.
Special thank to my supervisor, PN NORZANAH BINTI ROSMIN for the guidance
and enthusiasm given throughout the progress of this project. My appreciation
also goes to my coursemates whom are under the same supervision for the
guidance and encouragement that they gave to me led to the success of this project.
My appreciation also go to my family who has been so tolerant and
supports
me all these
years. Thanks for their encouragement,
love and
emotional supports that they gave to me.
Lastly, I extend my sincere appreciation to all people that have helped and
supported me in finishing my thesis.
v
ABSTRACT
It is important to ensure any system can be operated in the optimum
condition. The same concept goes to the small wind turbine system. The charging
process of the battery charger for the small wind turbine system must be able to track
the maximum power point in order to harvest the maximum output in the wide range
of wind speed variations. In this thesis, the issue regarding how this charging process
is managed was studied. The well management during battery charging process is
significantly required since the lack of charging management may affecting the
battery’s performance, particularly in terms of its lifetime and the degradation rate.
Hence, in this thesis, a battery charger with a satisfactory performance was proposed
and designed by using the concept of controlling the pulsating charging current.
Using this approach, it was found that the efficiency of the charging process is very
good since the resulting pulsating current that was produced, was working as
expected, whereas the overvoltage and the overcurrent condition was successfully
prevented in order to prolong the battery lifetime.
.
vi
ABSTRAK
Adalah keutamaan bagi sesuatu sistem untuk beroperasi dalam keadaaan
yang paling optimum. Begitu juga dengan sistem turbin angin. Proses mengecas
sesebuah bateri untuk kegunaan sistem turbin angin mestilah berkebolehan untuk
mengenal pasti titik kuasa maksimum agar kuasa maksimum dapat dihasilkan oleh
turbin, walaupun dalam lebar jalur perubahan angin yang luas. Di dalam tesis ini, isu
tentang pengurusan proses mengecas telah dikaji. Pengurusan yang baik semasa
proses mengecas sangat diperlukan kerana pengurusan yang tidak baik mampu
mempengaruhi prestasi dan kecekapan bateri, terutamanya dari segi jangka hayat dan
kadar degradasi bateri. Oleh itu, di dalam tesis ini, sebuah pengecas bateri yang
berkecekapan memuaskan telah dicadangkan dan direka dengan menggunakan
konsep pengawalan arus denyut pengecas. Sistem ini telah dibangunkan dengan
menggunakan perisian MATLAB/Simulink. Menggunakan pendekatan ini, didapati
bahawa kecekapan proses pengecasan adalah sangat baik disebabkan denyutan
isyarat arus yang diharapkan telah terhasil, sementara keadaaan lebihan voltan dan
lebihan arus juga telah berjaya dikekang demi memanjangkan jangka hayat bateri.
vii
TABLE OF CONTENT
CHAPTER TITLE
1
2
PAGE
DECLARATION OF THESIS
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENT
vii
LIST OF TABLES
x
LIST OF FIGURE
xi
LIST OF ABBREVIATIONS
xiii
INTRODUCTION
1
1.1 Background
1
1.2 Problem statement of project
3
1.3 Project’s objective
3
1.4 Scope of study
4
LITERATURE REVIEW
5
2.1 MPPT of permanent magnet
5
2.2 MPPT for based on suppport-vector regression
6
viii
3
2.3 MPPT based on limit cycle for a small wind turbine
7
2.4 Current control mode scheme for point load
8
2.5 Buck converter with zero and modified average current
10
2.6 Peak current control of buck converter
11
METHODOLOGY
13
3.1 Basic concept of wind energy system
13
3.2 Concept of MPPT
14
3.3 Buck converter operational circuit
16
3.3.1 Type of converter
16
3.3.2 Component in buck converter
17
3.3.2.1 Switch
17
3.3.2.2 Capacitor
18
3.3.2.3 Operating switching frequency
18
3.3.2.4 Inductor
19
3.3.2.5 Freewheeling diode
19
3.3.3 Feedback
20
3.3.4 Control scheme
20
3.3.5 Buck converter topology
21
3.4 Lead acid battery
24
3.4.1 Cell design
24
3.4.2 Charging process for the battery
25
3.4.3 Discharging process for the battery
26
3.4.4 Battery management issues
27
3.5 PID controller
3.5.1 Proportional term
27
29
ix
4
3.5.2 Integral term
30
3.5.3 Derivative term
31
3.5.4 Methods of tuning
33
3.5.5 Effect of increasing a parameter indipendently
34
3.6 Theory of calculation
34
3.7 Design specification
36
3.8 Battery charger circuit
37
3.8.1 Current control mode
39
3.8.2 Voltage control mode
40
RESULT AND DISCUSSION
42
4.1 Continuous current mode
43
4.2 Discontinuous current mode
46
4.3 Zero current mode
51
4.4 Characteristic of operation for voltage control and
current control
4.4.1 Voltage control characteristic
53
4.4.2 Current control characteristic
5
53
54
4.5 Result summary
54
CONCLUSION AND RECOMMENDATION
55
5.1 Conclusion
55
5.2 Recommendation
56
REFERENCES
57
x
LIST OF TABLE
PAGE
TABLE
TITLE
2.1
Parameter of unified three terminals
10
3.1
Relationship of converter
16
3.2
Method of tuning
33
3.3
Effect of increasing a parameter independently
34
3.4
Battery Charger Specification
36
4.1
Inductor current when battery is not fully charged
50
4.2
Inductor current when battery is fully charged
52
xi
LIST OF FIGURES
FIGURE
TITLE
PAGE
2.1
Block Diagram of PWM phase shift converter
6
2.2
Flow chart of wind speed estimation
7
2.3
MPPT using boost chopper
8
2.4
Current mode controlled buck converter
9
2.5
Peak current control system
12
3.1
Power converter application in wind energy system
14
3.2
Wind generator power vs generator speed
15
3.3
Rotor power versus rotor voltage
15
3.4
Basic control scheme
20
3.5
Pulse width modulator
21
3.6
Gate signal
22
3.7
Discontinuous inductor current
22
3.8
Continuous inductor current
22
3.9
Lead acid battery construction
24
3.10
Charging process
25
xii
3.11
Discharging process
26
3.12
PID block diagram
28
3.13
Proportional response
30
3.14
Integral response
31
3.15
Derivative response
32
3.16
Battery charger circuit
37
3.17
Controller for the battery charger
38
3.18
Measurement for the battery charger
39
3.19
Current controller
40
3.20
Voltage controller
41
4.1
Continuous current graph
44
4.2
Zoomed-in of continuous current graph
44
4.3
Input voltage 80 Volts for CCM operation
45
4.4
Input voltage 90 Volts for CCM operation
45
4.5
Discontinuous current graph
47
4.6
Zoomed-in of discontinuous current graph
47
4.7
Input voltage 50 Volts for DCM operation
48
4.8
Input voltage 60 Volts for DCM operation
49
4.9
Zero current graph
51
4.10
Inductor current when battery is fully charged
52
xiii
LIST OF ABBREVIATIONS
MPPT
Maximum power point tracking
PWM
Pulse width modulation
W/G
Wind generator
DC
Direct current
AC
Alternating current
MOSFET
Metal oxide semiconductor field effect transistor
IGBT
Insulated gate bipolar transistor
JFET
Junction gate field effect transistor
BJT
Bipolar junction transistor
CCM
Continuous current mode
DCM
Discontinuous current mode
ZCM
Zero current model
PID
Proportional-Integral-Derivative
PI
Proportional-Integral
PD
Proportional- Derivative
P
Proportional
I
Integral
Kp
Proportional constant
Ki
Integral constant
CHAPTER 1
INTRODUCTION
1.1
Background
Renewable energy sources derive their energy from on going natural
processes such as sunshine, wind, flowing water, biological processes and
geothermal heat flows. A general definition of renewable energy sources is that
renewable energy is captured from an energy resource that is replaced rapidly by a
natural process. The alternative energy had been developed to reduce the problem of
the fossil-fuel exhaustion and environmental issues. One of the feasible energy is a
wind-power system. For urban areas, wind power can be integrated with main grid.
In contrast with the rural areas with weak grid, the stand-alone wind power system
with the battery bank is essential
in order for providing stable and reliable
electricity.
Wind energy is one of the most promising alternative energy technologies of
the future [1]. Throughout recent years, the amount of energy produced by winddriven turbines has increased due to the significant breakthrough in the wind energy
system. It offers many advantages over the fossil based energy which explains why it
becomes fast growing energy source in the world. Since the wind energy is clean and
free, generating maximum energy from the wind is very important. It is due to the
2
appreciation of the pollution-free source energy and not wasting the available
chance. Even though wind energy has many environmental and supply advantages,
there are several disadvantages that limit the usability of wind power. The main
disadvantage to wind power is that it is unreliable because it does not blow at a
constant rate. This will cause the changes in maximum power available at a
particular moment.
Hence, in order to improve the battery’s lifetime for the small-sized stand
alone wind power system, a Maximum Power Point Tracking (MPPT) control
strategy equipped with the pulsating charging current is proposed. In this study, a
power electronic converter was considered where this converter has the ability to
provide pulsating charging current with MPPT control strategy. This technique has a
simple circuit diagram but provides high reliability [2].
So, the best option to realize the proposed idea is using buck converter with
pulse-width modulation (PWM) switching. The PWM will track the maximum
power point by control the output process using duty cycle or its switching
frequency. The converter will have three types of current modes known as
continuous current mode, discontinuous current mode and zero current mode. These
current modes will operate once at a time depend on the feedback input coming from
the main battery charger circuit.
Basically, the battery charger is not only for the maximum power point
tracking purpose but it also provides an overspeed protection for the battery in term
of overvoltage and overcurrent [1]. In addition, the pulsating current will increase the
charging efficiency and eventually increase the battery lifetime [1],[3],[4].
3
1.2
Problem Statement
For a small wind power system. It is difficult to draw maximum power from
the wind turbine and the degradation of the battery bank usually give significant
effect on the system reliability [1].
1.3
Project’s Objectives
This project has 3 objectives as follows:
a) To study the operation work of a small wind power system.
b) To study the operation work of buck-converter as a MPPT battery charger
c) To design and model the MPPT battery charger using MATlab Simulink.
4
1.4
Scope of Study
This project will only focus on the scope that given below:
a)
Small stand-alone wind power system with permanent magnet generator.
b)
Buck converter as maximum power point tracker.
c)
Lead acid battery as the battery bank
CHAPTER 2
LITERATURE REVIEW
For this project, there will be a number of literature reviews on the related
topic from various sources of information such as technical paper and report, books
and journal.
2.1
MPPT of permanent magnet [5],[6]
The maximum power point tracking is mostly applied on the permanent magnet
synchrounous generator (PMSG) which are commonly used in wind turbine system.
It using the suitable DC-DC converter to extract maximum power from the PMSG.
The PWM is manipulating the phase angle and output voltage in order to achieve the
desired output required.
6
Figure 2.1 below details the construction of the controller and how the phase
angle is manipulated. It shows that Proportional-Integrator (PI) has been used at the
early stage of the control process. The PI controller will adjust the value of the angle
so that the PWM modulator can generate a different duration of on-time.
Figure 2.1
2.2
Block diagram of PWM phase-shift controller
MPPT for based on support-vector regression(SVR) [7]
A different approach of MPPT function can be realized by using SVR. It was
based on the wind turbine power and rotational speed of the generator. Then, a
mathematical algorithm for the estimation was develop based on that two
information. The simulation was started as an offline training before the data is taken
from the on-line instantaneuos input.
The process of estimating the relation between wind and speed of the generator
can be clearly seen in figure 2.2. It shows the six main procedures in order to
generate the function that will be used in the controller. It was begin with the on-line
7
and off-line training. After that, both data from the on-line and off-line training will
be integrated together to find the output function.
Figure 2.2
2.3
Flow chart of wind-speed estimation
MPPT based on limit cycle for a small wind turbine [8],[9]
Another technique of MPPT for a small sized of wind turbine system can be
realized by using limit cycle information. This control scheme is a bit simple where
there is no information of the generator is needed such as instanteneous power,
torque and rotating speed. The technique is utilizing the inherent limit cycle
phenomena of the system itself
8
This 3 phase small scale power system was using the boost chopper in order to
provide MPPT feature to the system. The characteristic of the boost chopper is
analyzed so that optimum point of duty ratio can be determined. The duty ratio will
be set as a reference value to control the output power of the generator. This
approach enables the output to follow the set value although the generator rotational
speed is changing based on the variation of the wind speed. Figure 2.3 shows that the
boost chopper and the inverter operates together as a one control system to achieve
the MPPT function.
Figure 2.3
2.4
MPPT using boost chopper
Current control mode scheme for point of load [10]
Most of the current control mode is realized by using PWM controller. This
current mode is available with different type of technique which are the peak-current
control, valley-current control, constant on-time control and constant off-time
control. Each scheme has a unique characteristic which can be explained by the
unified small signal equivalent circuit model.
9
Figure 2.4 shows how the process of controlling the peak current work. The
compensator is acted as a feedback controller in this scheme. The control scheme is
based on the calculated parameter. So, we have to decide the maximum allowable
value for inductor current before proceed to the next stages.
Figure 2.4
Current mode controlled buck converter
Those unified parameters involved in the circuit were calculated by using the
formula that was provided in table 2.1 below.
10
Table 2.1
2.5
Parameter of unified three terminals
Buck converter with zero and modified average current [11],[12]
Most of the buck converter has been modified so that it can be operated with
high efficiency, small volume and weight, simple control circuit and low price. Each
method has a number of advantages and disadvantages depend on the selection. The
MCT semiconductor has a low conduction of voltage drop but it has longer turn-off
duration which can increase the losses. This problem can be overcome by
implementing the zero current to reduce the turn-off losses.
These method optimizing the battery charger circuit by implement a feedback
clamp circuit on the converter circuit. The current clamping method is based on the
feedback control and not depend on the saturation of the error amplifier. This
approach can produce pulsating charging current technique.
11
2.6
Peak current control of buck converter [13]
The inductor peak value plays an important part during the charging process. So,
its value and type of waveform need to carefully designed in order to prolong the
lifetime of the battery. Otherwise, the battery will degrade faster than expected.
Some aspect that can be took into consideration is the peak value of the inductor
current. It is important to ensure that this value never exceed the rated value of the
battery. Figure 2.5 shows one of the circuit that has been used to control the peak
value of the current inductor. The controller itself contained 7 main parts:
a)
Peak current detector
b)
Gate drive
c)
Logic control
d)
Comparator
e)
Zero-cross detector
f)
Start-up
12
Figure 2.5
Peak current control system
CHAPTER 3
METHODOLOGY
3.1 Basic Concept of Wind Energy System
Before selection of the type of the converter can be done, the wind turbine
system characteristic must be firstly determined. In this chapter, the basic principle
of the energy conversion system for the small stand-alone wind turbine system will
be explained in detail. By referring to figure 3.1, the application of the battery
charger in the wind turbine system can be undertood more easily.
Generally, the operation of the wind turbine system can be divided into 3
stages. The first stage is from kenitic energy to kinetic energy where wind energy is
captured and rotate the turbine’s blade.Then it converted from kinetic energy to the
electric energy. This is where the blades rotate the rotor, and
then inducing
electricity. The last stage is the storage system. In this stages, current that was
generated from the generator changes its form from the alternating current to the
direct current. The battery charger will store the produced direct current into the
battery bank.
14
Then it is important to have a high perfomance battery charger to ensure that
the generated current can be stored in the battery bank wisely. Besides that, the
battery bank type issue should also be considered to be studied since the common
battery bank that usually used in the industry, e.g. the lead-acid battery is quite
expensive.
3.2
Concept of MPPT
There are many methods to achieve MPPT function for wind energy system.
One of the method is using power electronic converter. For wind energy application,
the latest design of power electronic converter may offer a cheaper approach [1]. In
the carried out literature so far, there were a numbers of converter designs for wind
power application, though the objective was same.
Figure 3.1
Power converter application in wind energy system [1]
In order to obtain maximum output power from the turbine, we should find the
optimal value of speed for each wind speed. If the voltage is changing from V1 to
V2, the rotor speed should be changed from ω1 to ω2 for the best operation of the
wind-turbine system. It is shown in the figure 3.2.
15
Figure 3.2
Wind generator power vs wind generator speed [2]
Figure 3.3
Rotor power versus rotor voltage [1]
Figure 3.3 shows the relationship between the rotor power and the rotor
voltage. By comparing both graphs, we can conclude that wind speed, and the rotor
voltage is directly proportional. To be able to achieve a MPPT function, a suitable
range of
voltage must be determined. It is based on the local wind speed
information. The range is very useful since it determines the input voltage of the
battery charger. These are the point that need be tracked by the integrated controller
in order to achieve the MPPT functions.
16
3.3
Buck Converter Operational Circuit
3.3.1
Type of converter
The battery charger will be designed by using power electronic converter.
The main function of this converter is to convert voltage at a different voltage level.
It is vital to know the relationship of the converter before continue with the design
process.
Table 3.1
Relationship of converter
Type of converter
Rectifier
Inverter
Buck, Boost, Buck-Boost
Ac voltage controller
Input
AC
DC
DC
AC
Output
DC
AC
DC
AC
To design a battery charger, it should be noticed that the input and the output
current must be in direct value (d.c). And a condition must be fulfilled where the
input voltage must be greater than the output voltage. If we referred to Table 3.1, it
can be seen that, the suitable converter that fulfill the mentioned condition is the
buck, boost and buck-boost converter type. However, for battery charger application,
buck converter was considered to be used in this study since it is a step-down device
[14],[15],[16].
Due to the reason of buck converter has a few special characteristics that will
give good impact on the battery charger, this converter was used in this study. The
benefits of this converter is as listed below [17] :
17
a)
It is highly efficient
b)
It has a simple structure
c)
Voltage can be drop without any transformer needed
d)
Load current rate can be limited
e)
Load current can be protected by the inductor
3.3.2
Component in Buck Converter
Any basic switched power supply consists of five standard components:
•
pulse-width modulating controller
•
transistor switch (active switch)
•
inductor
•
capacitor
•
diode (passive switch)
3.3.2.1 Switch
Transistors that have been choosen as a switching power supplies must have
fast switching time and must be able to withstand the voltage spikes created by the
18
inductor. The input for the gate of the transistor is usually a Pulse Width Modulated
(PWM) signal. This signal will determine the ON and OFF time. T h e sizing of the
power switch is then determined based on the load and voltage stability.
The power switch (transistor) can either be a MOSFET, IGBT, JFET or a
BJT. However MOSFET is more suitable for high frequency application. Therefore,
MOSFETs have now replaced BJT in new designs for the operation at high
frequencies and low voltages. At high voltages MOSFET still has their limitations.
The intrinsic characteristics of the MOSFET produce a large on resistance which
increases excessively when the devices breakdown voltage is increases . Due to that,
MOSFET is restricted only to low voltage applications.
3.3.2.2 Capacitor
Output capacitance (across the load) is required to minimize the voltage
overshoot and ripple present at the output of a step-down converter. The capacitor
should be larged enough so that its voltage does not produce any noticeable change
during the time the switch is off. The maximum allowed output-voltage overshoot,
and ripples are usually specified during the design, in order to meet the ripple
specification for a step-down converter circuit.
3.3.2.3 Operating switching frequency
The operating frequency determines the performance of the switch. Switching
frequency selection is typically determined by the efficiency requirements. There is
now a growing trend in research work and new power supply designs in increasing
the size of switching frequencies. The higher is the switching frequency, the smaller
the physical size and component value. However, higher frequency will cause more
19
magnetic losses in the inductor or switching losses in the regulator circuit. Therefore,
higher frequency reduces MOSFET efficiency to an impractical level and also the
size of the output capacitor.
3.3.2.4 Inductor
The function of the inductor is to limit the current slew rate (limit the current
in rush) through the power switch when the circuit is ON. The current through the
inductor cannot change suddenly. When the current through an inductor tends to fall,
the inductor tends to maintain the current by acting as a source. This limits the
otherwise the high-peak current that would be limited by the switch resistance alone.
The key advantage is when the inductor is used to drop voltage, it stores energy. The
size of the inductor can be reduced by the implementation of high switching
frequency. A smaller inductor value enables a faster transient response and results in
a larger ripple current, which causes higher conduction losses in the switches.
3.3.2.5 Freewheeling Diode
Since the current in the inductor cannot change suddenly, a path must exist
for the inductor current when the switch is OFF (open). This path is provided by the
freewheeling diode.
The purpose of this diode is not to rectify, but to direct current flow in the
circuit and to ensure that there is always a path for the current to flow through the
inductor. Thus, the diode enables the converter to convert stored energy in the
inductor to the load. When the switch closes, the current rises linearly. When the
switch opens, the freewheeling diode causes a linear decrease in the current. At the
steady state, we have a saw tooth response with an average value of the current.
20
3.3.3
Feedback
Feedback and control circuitry can be carefully nested around buck converter
circuits to regulate the energy transfer and maintain a constant output within normal
operating conditions. Control by pulse-width modulation is necessary for regulating
the output. The transistor switch is the heart of the switched supply and it controls
the power supplied to the load.
3.3.4
Control Scheme
Sometimes we need to design a regulated output model which has an input
voltage under varying condition. This will be accomplished by employing a negative
feedback system to the circuit. The objectives of the feedback control are to ensure a
constant output value and to reduce the noise level. Parts of the control system
consist of error amplifier and pulse width modulator (PWM). It controls the circuit
by adjusting the duty ratio of the power switch to maintain the desired output value.
This control scheme in figure 3.4 also provide excellent noise immunity to the spike
sensed in the inductor current [17].
Figure 3.4
Basic control scheme
The PWM consist of sawtooth generator, reference signal and a comparator.
The comparator will compare the reference signal with the sawtooth waveform to
produce logic output either logic ‘0’(GND) or logic ‘1’(VDD) as shown in figure 3.5.
21
Figure 3.5
3.3.5
Pulse width modulator [18]
Buck converter topology
This study will focus on the latest design, as depicted in Figure 3.1. The
construction of this device is basically from the buck converter. However, it has been
modified based on the need and application. This modification will alter the result
and affect the calculation of the parameter. Circuit that has been modified must be
analyzed first. Then, the result must be related with the inductor current, as
illustrated in Figure 3.6, 3.7 and 3.8 before proceed further into the derivation
process. There are two states of operation for buck converter that is the ON state
and the OFF state[1].
22
Figure 3.6
Figure 3.7
Figure 3.8
Gate signal [1]
Discountinuous inductor current [1]
Continuous inductor current [1]
From figure 3.6, 3.7 and 3.8, the relations between the gate signal,
discontinuous current mode (DCM) and the continuous current mode (CCM) can be
explained. And as depicted in Figure 3.6, 3.7 and 3.8, the switching period of the
gate signal is represented by d 1 , d 2 and d 3 . For both operations of DCM and CCM,
the switch duration of ON state is represented by d 1 . For DCM operation, during ON
state, the inductor current will be increased from zero to the maximum value. Whilst,
for CCM, the inductor current will be increased from the designed minimum current
(I min ) to the maximum value during the ON state.For OFF state switching period, the
23
operation for DCM and CCM is different. For DCM, the OFF state is separated into
two parts, d 2 and d 3 . d2 represents the duration of the fall time whereas d3 represents
the zero current condition. Both d 2 and d 3 have a similar duration of the OFF
switching period. It’s meant that d2 will cause the inductor current to be decreased
from the maximum value to the zero value during the half of the total OFF state
switching duration. Then, for the next half duration, the switch d3 will be operated.
However, for CCM operation, the whole OFF state duration will be controlled by the
switch d2 where the inductor current will be decreased from the maximum value to
the Imin .
As briefly explained in the two previous paragraphs, the switching process of
the designed converter was controlled by the PWM. This PWM is able to generate
the pulse at a certain frequency (f ), and also be able to send the signal to the switch
gate.
There are many types of controller avalaible in the liturature and can be
integrated with PWM controller such as :
a)
PWM fuzzy logic controller [19]
b)
PWM PID controller [19]
However, PWM PID controller will be used in this project because it can
avoid from the steady state error which will produce a better result. Moreover, it can
control various systems or devices with little human interaction. Besides that, they
can be tuned using several methods as shown in table 3.2. Due to that, the user can
choose the most familiar method and have more flexibility during the tuning process.
The PID will control either sawtooth signal or the reference signal so that it can
control the process of the charging activity.
24
3.4
Lead Acid Battery
The energy generated by the wind turbine must be stored in the suitable storage
system. Based on the consideration of a few aspect, it is decided that lead acid
battery is still the common energy storage component for the stand-alone wind power
system [1]. Lead acid battery that have been used for the storage systems will be
connected to the proposed battery charger before the charging process can be
analyzed.
3.4.1 Cell Design
As shown in figure 3.9, lead acid battery contains lead dioxide (PbO2) used as
the positive plate and the sponge lead (Pb) used as the negative plate while sulphuric
acid used as the electrolyte to complete the internal circuit [17].
Figure 3.9
Lead acid battery construction
The chemical reaction during the discharge and recharge is normally written:
Discharge
PbO2 + Pb +2H2SO4
2PbSO4 + 2H2O
Charge
(1)
25
3.4.2
Charging Process for the battery [17]
The principle how charging process occurs in a battery is explained below:
a)
The connection of an electrical power source from the external circuit pushes
the electrons to move from positive to negative terminal.
b)
This will eventually increase the charge and the voltages at the electrodes.
c)
The chemical reactions is in the reverse direction where the electrical energy is
converted as chemical energy.
d)
As the battery is being charged, the lead sulfate covered on the surface of both
electrodes is removed and the electrolyte become stronger.
Figure 3.10
Charging process
26
3.4.3
Discharging Process for the battery [17]
Here, the principle work of the discharging process of a battery is explained:
a)
Connection of an electrical load at the external circuit allows electrons to flow
from negative to positive terminals.
b)
This will eventually reduces the charge and the voltages at the electrodes.
c)
This will create foward chemicals reactions that generating new electrons.
During this process, power is converted to electrical form to drive the external
electrical load.
d)
As the battery is discharged, both electrodes become covered with the lead
sulphate and the electrolyte become weaker.
Figure 3.11
Discharging process
27
3.4.4
Battery Management Issues
There are two main problems that were commonly occur along the battery operation:
a)
Over discharge leads to the process called sulfation which will ruins the battery.
The reaction will becomes non reversible when the quantity of the lead-sulfate
formations become too large
b)
Overcharging causes other undesirable reactions to occur:
i. Electrolysis of water and generation of hydrogen gas
ii. Electrolysis of other compounds in electrodes and electrolyte which
can generate harmful gases
iii. Deformation of cases of sealed battery
3.5
PID Controller Theory
A proportional-integral-derivative controller or widely known as PID is a
control loop feedback mechanism. A PID controller calculates an error values as the
difference between a measured variable and a reference value. The controller will try
to reduce the error by changing the control inputs [20].
Generally, PID controller will involve three separate parameters, denoted
as P, I, and D and these values can be interpreted in terms of time as shown in figure
3.12 below:
28
Figure 3.12
Defining
PID block diagram [20]
as the controller output:
(5)
Where:
Proportional gain
Integral gain
Derivative gain
Time or instantaneous time (the present)
Parameter interpretation in term of time
i.
P which based on the present errors,
ii.
I which depends on the information of the past errors,
iii.
D is depends on the prediction of the future errors.
29
The response of the controller can be described in terms of:
i.
Responsiveness of the controller to an error
ii.
Degree to which the controller overshoots the set point and
iii.
Degree of system oscillation.
Some applications may require using only one or two parameter only to
provide the suitable controller for the system. This can be achieved by adjusting
others parameters to zero. A PID controller will be called a PI, PD, P or I controller
in the absence of the respective control actions.
3.5.1
Proportional term
The proportional term creates an output value that is proportional to the
present error value. The proportional response can be changed by multiplying the
error by a proportional gain constant, K p [20].
The proportional term is given by:
(2)
Figure 3.13 shows that a huge change in proportional gain will results a huge
change in the output for a given change in the error. If the proportional gain is too
high, the system can become unstable. On the other hand, a small gain results in a
small output response to a large input error which making the controller less
sensitive.
30
Figure 3.13
3.5.2
Proportional response
Integral Term
The integral term is proportionally effecting to both magnitude of the error and
the duration of the error. The integral in a PID controller is the sum of the
instantaneous error over time and gives the accumulated offset that should have been
corrected previously. The accumulated error is then multiplied by the integral gain
constant, K i [20].
The integral term is given by:
(3)
31
The integral term accelerates the movement of the process towards the
reference value and eliminates the steady-state error that occurs with a pure
proportional controller as shown in figure 3.14. However, since the integral term is
responds to accumulate the errors from the past. It can cause the present value
to overshoot the reference value.
Figure 3.14
3.5.3
Integral response
Derivative term
The derivative of the process error is calculated by determining the slope of
the error over time and multiplying this rate of change by the derivative gain
constant, K d . The magnitude of the contribution of the derivative term to the overall
control action is denoted as the derivative gain constant, K d [20].
32
The derivative term is given by:
(4)
The derivative term reduce the rate of change of the controller output.
Derivative control is used to decrease the magnitude of the overshoot produced by
the integral component and increase the combined controller process stability as
shown in figure 3.15. However, the derivative term slows the transient response of
the controller. Besides that, the differentiation of the signal will amplify the noise.
This will significantly cause a problem by making the process become unstable if the
noise and the derivative gain are sufficiently large.
Figure 3.15
Derivative response
33
3.5.4
Methods of tuning
Table 3.2 shows that the PID controller can be tuned by using several method
[20]. The user can choose any method that depend the situations. In this project, the
designing process involved manual tuning where the parameter proportional, integral
and derivative is adjusted based on the information given from table 3.3 [20].
Table 3.2
Method of tuning
Method
Advantages
Manual
No math required.
Tuning
Ziegler–
Nichols
Disadvantages
Requires experienced
personnel.
Process upset, some
Proven Method.
trial-and-error, very
aggressive tuning.
Consistent tuning. Online or offline method.
Software
May include valve and sensor analysis.
Some cost and training
Tools
Allow simulation before downloading. Can
involved.
support Non-Steady State (NSS) Tuning.
34
3.5.5
Effect of increasing a parameter independently
It is important to understand the role of each parameter in order to make the
tuning process easier. Each parameter will affect the rise time, overshoot, settling
time and steady-state error differently as shown in table 3.3 [20].
Table 3.3
Parameter
3.6
Effect of increasing a parameter independently
Rise time
Overshoot
Settling time
Steady-state error
Decrease
Increase
Small change
Decrease
Decrease
Increase
Increase
Eliminate
Minor change
Decrease
Decrease
No effect in the error
Theory of calculation
From the topology’s point of view, the load is considered as a constant load.
So, the duty ratio, D obtained using standard expression is [21]:
𝐷=
𝑉𝑜
𝑉𝑠
(6)
35
Where 𝑉𝑜 is output voltage and 𝑉𝑠 is the supply voltage for buck converter. A
feedback loop is required to the output voltage, compare it to a reference. Hence, we
can set the duty ratio in order to obtain the desired output voltage. In the buck
converter, the average inductor current is same as the average load current, since the
average capacitor current at steady-state is zero [21].
𝐼𝐿 = 𝐼𝑅 =
𝑉𝑜
𝑅
(7)
Since the inductor current is changing, the maximum and minimum inductor currents
are [21]:
𝐼𝑚𝑎𝑥 = 𝐼𝐿 +
1
𝑅
∆𝑖𝐿
2
= 𝑉𝑜 � +
𝐼𝑚𝑖𝑛 = 𝐼𝐿 −
1
𝑅
1
𝑇
2𝐿𝑓
∆𝑖𝐿
2
= 𝑉𝑜 � −
where =
(1−𝐷)
(1−𝐷)
2𝐿𝑓
�
�
(8)
(9)
, is the switching frequency.
In the charger designing, the current flow must be in continuous current mode
(CCM). Thus, the minimum value of the current must be zero. Therefore, the
minimum inductance required for continuous current is [21]:
𝐿𝑚𝑖𝑛 =
(1 − 𝐷)𝑅
2𝑓
(10)
In the charger designing, the output voltage must be kept constant. However,
the output voltage cannot be kept constant. The ripple output voltage is obtained and
its expression is [21]:
∆𝑉𝑜
1−𝐷
=
𝑉𝑜
8𝐿𝐶𝑓 2
(11)
36
3.7
Design Specification
The battery charger specification was designed based on the parameter given
as shown in table 3.4. The input voltage was assumed to be varying from 30 to 100
Volts with rated value of 70 Volts. The leads acid full charge voltage is sets to 28
Volts. Then, the charging current is sets to 18 Amperes since the best charging
current is ¼ of the rated current value [1].
The value for switching frequency is depend on the designer. High switching
frequency will cause more losses but giving a smaller value of inductor used in the
battery charger. On the other hand, low switching frequency will have smaller losses
but causing a bulk inductor size need to be installed in the system. Huge size of
inductor making the battery charger less flexible and more costly.
Other parameter such as inductor, capacitor and duty cycle can be calculate
once the value of the swicthing frequency was fixed.
Table 3.4
Battery Charger Specification [1]
No Specification
Value
1
Wind turbine rated voltage, Vin rated
70 V
2
Full charge voltage, Vo
28 V
3
Charger input voltage, Vin
30 -100 V
4
Duty cycle, d1
0.4
5
Switching frequency, fsw
22 kHz
6
Inductor, L
23 µH
7
Capacitor, C
1.347mF
8
I rated
80 A
9
I charging
18 A
37
3.8
Battery Charger Circuit
The battery charger circuit shown in figure 3.16
was based on the
conventional buck converter except there is an additional diode included in the
circuit which is diode 2. This diode is controlled by switch 2 which the action is
based on the input voltage of the battery charger. The other component will operates
as explained in the of section 3.3.2
PID controller was used in the circuit as the control scheme system where any
changes of the feedback parameter will be analyzed by the PID controller to adjust
the value of either duty cycle or switching frequency, so that the output is following
the reference output value.
Figure 3.16
Battery charger circuit
38
Figure 3.17 shows the main controller that consist of two separate controllers
known as voltage control and current control. Logic block diagram is used to create
the condition to ensure that both controller are synchronize. Hence, the controller can
differentiate and decide correctly when it is necessary to make decision. Its consist of
three input and three output.
The feedback from the battery voltage will be compared with the voltage
reference of 28 Volts. This comparison will decide either the circuit need to be
operated using voltage control mode or current control mode
Figure 3.17
Controller for the battery charger
Figure 3.18 shows the measurement circuit for the simulation. The current and
voltage feedback data is linked with the simout and scope block diagram. So, both
39
feedback can be further analyzed in order to understanding the relation between the
input and the output.
The analyzed data can be presented in the graph style, as shown in section 4.0.
All the graphs were generated from the simout block diagram.
Figure 3.18
3.8.1
Measurement for the battery charger
Current Control Mode
The current control mode is a mode where feedback from the charging current is
taken before comparing with the reference value of 18 Amperes. As shown in figure
3.19, PID controller was used so that the switching frequency can be adjusted to the
suitable value. This action will eventually change and affecting the charging current
value. The duty cycles is remain fixed in this type of control.
40
Since the duty cycle is remained constant during this control mode, its meant
that the PWM switching cycle for switch 1 is also remain constant. Due to that, it
was well known as the contant on-time control [1].
Figure 3.19
3.8.2
Current controller
Voltage Control Mode
Figure 3.20 shows that the voltage control mode is a mode where feedback
from the battery voltage was taken before comparing with the reference value of 28
Volts which is the value of the fully charged voltage for the battery. The proportional
constant contributes more changes on the rise time of the signal. Since, its only
involved proportional controller, this mode is quite easy to control compare to the
current control mode because the controller just need to control one signal called rise
time [20].
41
The controller will control the duty cycle to ensure that the battery is not
overvoltage or overcharging once it is fully charged. Then, the duty cycle will
manipulate the switching time for the switch 1. This will provide some protection
scheme to the battery bank. Another part of the voltage control mode which is the
switching frequency is remains fixed.
Figure 3.20
Voltage controller
CHAPTER 4
RESULT AND DISCUSSION
Result is the most important part of this research because the only
thing that can be analyzed to support the theory will be based on the output data. The
results of the modelling work will be presented in this chapter. All results will be
presented in graph and table styles so that it can be easily understand and easy to be
analyzed. Generally, the result can be separated into three parts, which are the
continuous current mode, discontinuous current mode and zero current mode.
To assess the successness of the designed battery charger, it is significant to
ensure that the resulting pulsating current may operate in the normal behaviour. To
ensure that the proposed battery charger is working under its normal behaviour
situation, an observation must be paid on the shape of the resulting pulsating current;
continuous and discontinuous current mode. These two conditions are differentiated
in terms of its fixed boundary of input voltage value. First section will discuss about
the continuous current mode, whereas the discontinuous current mode will be
presented in the second section. Meanwhile, in the third section, the zero current
mode will be discussed, where this condition is also very significant to be discussed
since this current can show, the termination of the designed charging process.
Finally, a summary of the finding is given..
43
4.1
Continuous current mode (CCM)
Continuous current mode occurs when the battery is not fully charged and
the input voltage is exceeded 70 Volts. This condition shows that the input voltage is
over the rated voltage. During this situation, the charging current will exceed the
value of the best charging current which is 18 Amperes. However, the excess value
current will be always monitored by the controller to ensure that it will never exceed
the rated value which is 80 Amperes.
This continuous current will shift up from the zero current value making
the charging process continuously. This charging process is faster but it is not the
best option because the lifetime of the battery will be reduced. Moreover, the
efficiency for the charging process during this type of operation is not as good as the
discontinuous current mode (DCM) [1].
Figure 4.1 shows the steady-state response of the CCM charging process.
Meanwhile, figure 4.2 shows the zoom-in image of the current. The continuous
current mode contains two components:
a)
Current is increased when switch 1 is at ‘ON’state.
b)
Current is decreased when switch 1 is at ‘OFF’ state.
44
Figure 4.1
Figure 4.2
Continuous current graph
Zoomed-in of continuous current graph
The peak charging current value depends on the input voltage value.
Greater input voltage value will increasethe value for peak chargingcurrent. This is
proven by comparing figure 4.3and figure 4.4.Figure 4.3 represents the smaller
45
voltage (80V), whereby figure 4.4 represents the larger voltage (90V). The existence
of inductor in this circuit be able to limit the current slew rate and the shape of the
current as a sawtooth signal. If this inductor is taken out from the circuit, charging
current still be able to continuously pulsating but with a square shape signal.
Figure 4.3
Input voltage 80 Volts for CCM operation
Figure 4.4
Input voltage 90 Volts for CCM operation
46
4.2
Discontinuous current mode (DCM)
Discontinuous current mode occurs when the battery is not fully charged
and the input voltage is less than or equal to 70 Volts which is the rated value for this
design. It has the same component as the CCM except one extra component where
the current lies on zero value. That means, the electrons from the previous charging
process have a rest time to achieve the equillibrium state inside the lead acid battery.
Figure 4.5 shows the steady-state response of the DCM charging process.
Meanwhile, figure 4.6 shows the enlarge image of the current. This charging process
is a bit slower compared to the continuous current mode but it is better in terms of
efficiency because it contains 3 components:
a)
Current is increased when switch 1 is at ‘ON’state.
b)
Current is decreased when switch 1 is at ‘OFF’ state.
c)
Current is zero when switch 1 as at’OFF’ state.
47
Figure 4.5
Figure 4.6
Discontinuous current graph
Zoomed-in of discontinuous current graph
48
Zero current is actually the extention of the decreasing current but the
designed buck converter has limit current from decreasing further. So, instead of
decreasing, the current becomes zero for a certain time before enter the next cycle.
The similar characteristic as CCM, DCM also shows that higher input voltage will
cause higher peak charging current. It is shown in figure 4.7 that when the input
voltage is 50V the highest peak charging current is 34.63A. Meanwhile, in figure 4.8
shows that when the input voltage is 60V the highest charging current is 49.15 A. As
in the previous mode, the existence of inductor plays the same role regardless in
which modes the battery operates.
Figure 4.7
Input voltage 50 Volts for DCM operation
49
Figure 4.8
Input voltage 60 Volts for DCM operation
Table 4.1 shows that the value of inductor current is maintained around 18 ±
0.5 Amperes most of the time regardless of the charge percentage. This is
appoximately to the reference value that involve during the design process which is
18 Amperes. The uncertainty value can be reduced further by adjusting the value of
the capacitor used in the circuit using equation 11. The output voltage value is
depends on the quantity of the charge stored inside the battery. Thus, higher percent
of charge will result higher output voltage. The input voltage do not has significant
effect on the value of output voltage and inductor current. However, the input
voltage effecting on the type of the charging current, whether it is continuous current
mode or discontinuous current mode.
50
Table 4.1
Inductor current when battery is not fully charged
85% charged
70% charged
input
input
voltage
output
inductor
voltage
output
inductor
(V)
voltage (V)
current (A)
(V)
voltage (V)
current (A)
40
26.28
18.02
40
26.01
18.04
50
26.21
18.07
50
26.01
18.04
60
26.21
18.06
60
26.17
18.08
70
26.41
18.12
70
26.17
17.99
80
26.26
18.05
80
26.09
18.04
90
26.25
17.89
90
26.10
18.07
100
26.28
18.14
100
26.11
18.05
55% charged
40% charged
input
input
voltage
output
inductor
voltage
output
inductor
(V)
voltage (V)
current (A)
(V)
voltage (V)
current (A)
40
25.78
18.01
40
25.27
17.99
50
25.78
18.01
50
25.31
18.09
60
25.84
18.01
60
25.20
18.07
70
25.72
18.10
70
25.41
18.08
80
25.76
18.10
80
25.24
17.94
90
25.82
17.86
90
25.24
18.06
100
25.76
18.00
100
25.27
17.91
51
4.3
Zero Current Mode (ZCM)
The figure 4.9 shows that the charging process is terminated once the
battery is fully charged. This is one of the protection scheme provided by the battery
charger to the battery bank called overcharging or overvoltage protection.
However, there is still current flow to the battery with small value which
is approximately to zero. This can be observed from the figure 4.9 where the
operating current is about 0.1522 Amp. The reason is because the purpose of this
current is just to maintain the battery voltage and to overcome self discharging
problem. This small current flow also known as trickle charging current.
Figure 4.9
Zero current graph
52
In ZCM, the similar characteristic applied where the value of current flow
into the battery is directly proportional with the value of input voltage as shown in
table 4.2 and figure 4.10.
Table 4.2
Inductor current when battery is fully charged
100% charged
input voltage
output voltage
inductor current
30
28.05
1.92E-05
40
28.05
1.23E-04
50
28.05
2.22E-04
60
28.05
3.19E-04
70
28.05
4.18E-04
80
28.05
5.18E-04
90
28.05
6.22E-04
100
28.05
7.18E-04
Inductor current (A) vs Input voltage (V)
during 100% charged
8.00E-04
7.00E-04
6.00E-04
5.00E-04
4.00E-04
3.00E-04
2.00E-04
1.00E-04
0.00E+00
30
Figure 4.10
40
50
60
70
80
90
100
Inductor current when battery is fully charged
53
4.4
Characteristic of operation for Voltage control and Current control
The operation of different modes involve a different characteristic of operation.
This operation can be differentiated by looking at the duty cycle, switching
frequency, switch 2, and the battery voltage.
4.4.1
Voltage control characteristic
•
Control duty cycle
•
Fix switching frequency
•
Fix switch 2 – ‘ON’state
•
Battery voltage greater than 28 Volts
4.4.2
Current control characteristic
•
Fix duty cycle
•
Control switching frequency
•
Control switch 2
 ‘ON’ state when input voltage is greater than 70 Volts
 ‘OFF’ state when input voltage is equal or less than 70 Volts
54
•
4.5
Battery voltage equal or less than 28 Volts
Result summary
The boundary and the pulsating current was successfully designed based on the
CCM’s result and DCM’s result. These two modes can maintain their average
charging current, closed to the best value of charging current which is 18 A. Then,
the protection scheme worked well during the charging process, as proven by the
ZCM’s result.
CHAPTER 5
CONCLUSION AND RECOMMENDATION
5.1
Conclusion
The principle and the characteristic of the battery charger was clearly
understood. It was compatible to be merged with the stand-alone wind energy system
based on the characteristic of the chosen wind turbine’s generator speed, generator
power and rotor voltage. To analyze the system reliability, Matlab/Simulink was
used to develop the proposed system.
The PID tuning process in this project was done manually with trial-error
method. The PID controller for the current control mode was adjusted until the
desired output is successfully achieves. The same goes to the voltage control mode
but only P controller is needed because this mode is more simple compared with the
current control mode.The simulation of the software proved the theoretical concepts
for this project.
From this study, it was found that the designed battery charger was able to
produce the best charging current of 18 Amperes, which is approximately ¼ of the
56
rated current value during the charging process even though the input voltage level
is different, as shown in table 4.1. Besides that, the designed battery charger also
enable the overcharging and the overvoltagecan be avoided clearly shown in figure
4.9. In the same time, the designed battery charger provides another feature which it
can avoid the self-discharging process by providing a small current flow which is
less than 72 mA. This was called as a trickle charging current, as shown in table 4.2.
This current value is just enough to block the current flow out from the battery.
5.2
Recommendation
It is recommended to develop hardware based on this simulation to ensure
whether the simulation is practically success or not. If it is practically success, the
hardware can be used to analyze the impact of the battery charger to the wind turbine
system. There are some aspects that can be further study such as :
a)
Efficiency of the battery charger
b)
Charging rate of the battery charger
c)
Lifetime of the battery charger
57
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