SIMULATION DESIGN OF VARIABLE DC SOURCE TO CONSTANT AC
BY USING PWM
NUR FARAHANA BINTI RAMLI
A thesis submitted in fulfillment of
the requirements for the award of the degree of
Bachelor of Engineering (Electrical)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
APRIL 2010
Dedicated to my beloved father and mother
Ramli bin Serom and Ruziah binti Hussin
Brother
Muhammad Adib Firas bin Ramli
And
My all friends in SEE programme
For their encouragement
ACKNOWLEDGEMENT
In the name of Allah and It is deepest sense gratitude of the Almighty that gives
me strength and ability to complete this final project report. Special thank to my beloved
family, especially my mother and father which always pray for my success.
I am very grateful to everyone that gives me inspiration to complete my final
year project. Especially thanks to my respectful supervisor, Encik Alias bin Mohd. Yusof
for the time, ideas, supports, and advices throughout this project. Without his support,
this final project report may have not been completed.
Finally to all my friends who helped me directly or indirectly to complete my
project. Thank for all your times, ideas, advices and guidance through my project.
Finally, I can see myself through real working environment which I can use the
experience and also poses a strong communication and interpersonal skills towards
excellent future.
ABSTRACT
The purpose of this project is to design a circuit in a way to get the constant AC
sine wave from variable DC input. The variable DC input will be regulated by using DCDC converter. Next the DC wave will be converted to high purity of AC sine wave by
using the PWM inverter. The output voltage magnitude and frequency will be controlled
by the PWM controller to sinks pure DC pulse wave. The simulation will be presented by
MATLAB Simulink. The proposed control scheme is verified through the simulation in
various conditions.
ABSTRAK
Tujuan dari projek ini adalah untuk merancang sebuah litar untuk mendapatkan
gelombang sinus AC konstan dari masukan DC yang mempunyai magnitud dan frekuensi
yang berbeza-beza. Masukkan DC yang berbeza magnitude dan frekuensi akan
ditetapkan dengan menggunakan pengubah DC-DC. Kemudian gelombang DC akan
diubah ke gelombang sinus AC yang rendah kadar karmoniknya dengan menggunakan
pengubah DC-AC PWM. Magnitud dan frekuensi keluaran akan dikawal oleh pengawal
PWM untuk menghasilkan gelombang AC dengan nilai yang ditetapkan. Simulasi ini
akan menggunakan MATLAB Simulink. Skema kawalan yang dicadangkan disahkan
melalui simulasi pada berbagai keadaan.
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
DECLARATION
Ii
DEDICATION
Iii
ACKNOWLEDGMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xi
LIST OF SYMBOLS
Xiv
LIST OF APPENDICES
xv
1 INTRODUCTION
1
1.1
Objective
1
1.2
Scope of the Project
2
1.3
Methodology
2
1.4
Thesis Organization
3
2 LITERATURE REVIEW
4
2.1
Pulse Width Modulation (PWM)
4
2.2
PWM Inverter
7
2.3
DC-DC Converter
9
2.4
Buck Converter
10
14
3 METHODOLOGY
3.1
Project Implementation
14
3.2
Components
15
3.2.1
Ideal Switch
15
3.2.2
Universal Bridge
15
3.2.3
Three-Phase VI Measurement
17
3.2.4
Three-Phase Parallel R Load
18
3.2.5
Discrete PWM Generator
19
3.2.6
Scope
20
Run Simulation
22
3.3
4 RESULT AND DISCUSSION
4.1
4.2
4.3
24
Simulation Design of PWM Inverter
24
4.1.1
Specifications
25
4.1.2
Simulation
25
4.1.4
Results and Discussions
26
Simulation Design of Buck Controller
35
4.2.1
Specifications
35
4.2.2
Simulation
36
4.2.3
Results and Discussions
36
PWM Inverter with DC-DC Buck Controller
41
4.3.1
Simulation
41
4.3.2
Results and Discussions
42
4.3.2.1 Output waveform when DC input =
300 V
42
4.3.2.2 Output waveform when DC input =
350 V
43
4.3.2.3 Output waveform when DC input =
400 V
45
4.3.2.4 Output waveform when DC input =
500 V
46
5 CONCLUSION AND RECOMMENDATIONS
48
5.1
Conclusion
48
5.2
Recommendations
48
REFFERENCES
50
APPENDIX A
52
APPENDIX B
55
LIST OF TABLES
TABLE
TITLE
PAGE
4.1 Specifications of PWM inverter.
25
4.2 Specifications of Buck controller.
35
LIST OF FIGURES
FIGURE
TITLE
PAGE
2.1 A pulse wave, showing the definitions of ymin, ymax, and D
5
2.2 A simple method to generate the PWM pulse train
7
2.3 General block diagram of DC-DC converter
10
2.4 Buck converter circuit diagram
10
2.5 The two configurations of a buck converter
11
2.6 Naming conventions
12
3.1 Ideal Switch
15
3.2 Universal bridge
15
3.3 Universal Bridge dialog box
16
3.4 Three phase VI measurement
17
3.5 Three phase VI measurement dialog box.
18
3.6 Three phase parallel R load
18
3.7 Discrete PWM Generator
19
3.8 Discrete PWM generator dialog box
20
3.9 Scope block
21
3.10 Parameters of scope block
22
3.11 Save the scope data in data history.
23
4.1 Simulation circuit of PWM inverter
26
4.2 Inverter voltage before filter
27
4.3 Load voltage before filter
27
4.4 Phase voltage before filter
27
4.5 Phase current before filter
28
4.6 Load voltage THD before filter
28
4.7 Phase voltage THD before filter
29
4.8 Phase current THD before filter
30
4.9 Inverter voltage after filter
30
4.10 Load voltage after filter
31
4.11 Phase voltage after filter
31
4.12 Phase current after filter
31
4.13 Load voltage THD after filter
32
4.14 Phase voltage THD after filter
33
4.15 Phase current THD after filter
34
4.16 Simulation circuit of DC-DC buck converter
36
4.17 DC input = 300 V, DC output = 239.3 V
37
4.18 Buck controller output voltage when Vinput = 300V
37
4.19 DC input = 350 V, DC output = 239.7 V
38
4.20 Buck controller output voltage when Vinput = 350V
38
4.21 DC input = 400 V, DC output = 239.9 V
39
4.22 Buck controller output voltage when Vinput = 400V
39
4.23 DC input = 500 V, DC output = 240.1 V
40
4.24 Buck converter output when Vinput = 500 V
40
4.25 The simulation circuit of PWM inverter with the
42
controller
4.26 Inverter output voltage before filter
43
4.27 Load voltage
43
4.28 Phase current
43
4.29 Phase voltage
43
4.30 Inverter output voltage before filter
43
4.31 Load voltage
44
4.32 Phase current
44
4.33 Phase voltage
44
4.34 Inverter output voltage before filter
45
4.35 Load voltage
45
4.36 Phase current
45
4.37 Phase voltage
47
4.38 Inverter output voltage before filter
47
4.39 Load voltage
47
4.40 Phase current
48
4.41 Phase voltage
48
LIST OF SYMBOLS
-
Duty cycle
T
-
Period
m
-
Modulation index
mf
-
Modulation frequency
VLL
-
Line-to-line Voltage
Imin
-
Minimum current
IL
-
Minimum inductance
R
-
Resistance
L
-
Inductance
Lmin
-
Minimum inductance
f
-
Frequency
Vo
-
Output voltage
C
-
Capacitance
LIST OF APPENDICES
NO. APPENDIX
TITLE
PAGE
APPENDIX A
Simulation circuits
52
APPENDIX B
Diagrams of Output Waveforms of Different Input
55
Values
CHAPTER 1
INTRODUCTION
Today‟s enhancement of technology makes it possible to use the power
electronic devices in the power system. The power electronic devices development
growth rapidly as nowadays it has been widely used to control the power system. In
recent years, the tendency to operate the converter with pulse width modulated (PWM)
to improve the input and output of the performance of the converter is increases. This
project is concerned with the variable-high-speed of wind that will produce variable
magnitude and frequency of voltages. The studies of the influence of pulse width
modulation and its controller before the energy injected to the grid, and analyzing the
stability for the power electronic converters will be conducted. Computer simulations
will be obtained by using MATLAB Simulink software and conclusions will be drawn.
1.1
OBJECTIVES
The objective of this project is to design and develop a simulation circuit in a
way to get the constant AC sine wave from variable DC waveforms input. The
objectives of this project including:
2
To design and simulate DC-DC converter as a controller for the PWM inverter to
regulate the high magnitude variable DC to the desired value of constant DC
before it is injected to PWM inverter.
To design and simulate the Pulse Width Modulation (PWM) inverter to convert
the DC constant signal to the desired constant magnitude and frequency AC
signal.
1.2
SCOPE OF PROJECT
The scope of this project is the design and simulation study of circuit involving
the DC-DC converter as the controller for the PWM inverter and DC-AC PWM inverter
itself. First, a study and design of the inverter to convert the DC signal into AC signal by
using Pulse Width Modulation (PWM) technique. Next, a study and design of the DCDC converter as the controller to regulate the variable DC input to get the desired value
of DC signal output that will the input of the PWM inverter.
1.3
METHODOLOGY
This project will be started with the literature review on the theory of the power
electronics converter to understand the fundamental concepts. Besides that, the literature
review will help to understand the structure and operation of the converters devices.
Next is the crucial part, where the work of designing the simulation circuit and run the
simulation circuit with the controller that had been designed by using the MATLAB
Simulink software.
PWM inverter will be converted the DC signal to AC sinusoidal waveform. The
desired output of the PWM inverter is 240 V AC, 50 Hz. The input of the PWM inverter
3
will be 240 V DC. This input will be obtained from the DC-DC buck controller output,
where this buck controller will control the variable DC inputs to get the constant DC
output to be injected to PWM inverter. The analysis will be conducted after getting the
simulation results and the results will be discussed in the later chapter. This project will
be summarized after the discussions and the work will be continued by doing the report
writing.
1.3
THESIS ORGANIZATION
This paper is organized as follows:
Chapter 1 - Introduction on this project. The highlights of the objectives, methodology,
and scope of work will be stated in this chapter.
Chapter 2 – This literature review chapter will be discussing in details about the
converters and PWM in terms of its function and the control scheme that will be used in
this project.
Chapter 3 – This chapter will be discussing the implementation of the pulse width
modulation (PWM) on MATLAB Simulink software. This will include the brief
explanation of the development of the simulation blocks that has been used.
Chapter 4 – The simulation results and analysis on performance will be stated in this
chapter. The simulation is conducted under various input voltage to see whether the
output obtained achieve the objectives or not.
Chapter 5 – This chapter will conclude the work that has been carried out in this project
that is related to the operation and the performance. The discussion on further research
possible in the future will be included too.
4
CHAPTER 2
LITERATURE REVIEW
This chapter will be reviewing some of the previous paper that shows how to
design and analysis the operation of the pulse width modulation (PWM) inverter and
DC-DC buck converter as its controller. The most important requirement during the
operation of the electric power system is the reliability which is to get the pure
sinusoidal waveform with the desired magnitude output value of the PWM inverter. The
principal of a PWM inverter and the DC-DC buck controller will be explained. The
proposed circuit design are modeled and simulated by using MATLAB Simulink
software. Results will be referred on the output waveform which is AC sinusoidal signal
with the desired magnitude value.
2.1
PULSE WIDTH MODULATION (PWM)
Pulse width modulation is a modulation technique for generating variable width
pulses to represent the amplitude of an input analog signal or wave. Pulse width
modulation is used to reduce the total power delivered to a load without resulting in loss,
which normally occurs when a power source is limited by a resistive element [1]. The
average power delivered is directly proportional to the modulation duty cycle according
5
to its principle. It is possible to smooth out the pulse train using passive electronic filters
and recover an average analog wave form if the modulation rate is high.
High frequency pulse width modulation power control systems can be realized
using semiconductor switches. The discrete ON or OFF state of the modulation itself can
be used to control the switches and controlling the voltage or current across the load.
The major advantage with these types of switches is that the voltage drop across it
during conducting and non-conducting states is ideally zero. Pulse width modulation is
widely used in voltage regulators. It works by switching the voltage to the load with the
appropriate duty cycle, thus the output will maintain a voltage at the desired level [2].
Figure 2.1
A pulse wave, showing the definitions of ymin, ymax, and D
Pulse-width modulation uses a rectangular pulse wave whose pulse width is
modulated resulting in the variation of the average value of the waveform. If we
consider a pulse waveform f(t) with a low value ymin, a high value ymax and a duty cycle
D, the average value of the waveform is given by:
(2.1)
As f(t) is a pulse wave, its value is ymax for 0 < t < D.T and ymin for D.T < t < T. The
above expression then becomes [2]:
6
(2.2)
This latter expression can be fairly simplified in many cases where ymin=0 as
ŷ=D.ymax. From this, it is obvious that the average value of the signal, ŷ is directly
dependent on the duty cycle D.
The simplest way to generate a PWM signal is the interceptive method, which
requires only a saw tooth or a triangle waveform which is easily generated using a
simple oscillator and a comparator. By referring to the Figure 2.2 below, when the value
of the reference signal (the sine wave) is more than the modulation waveform (the
triangular wave), the PWM signal (pulses) is in the high state, otherwise it is in the low
state.
7
Figure 2.2
A simple method to generate the PWM pulse train corresponding to a
given signal is the intersective PWM: the signal (sinewave) is compared with a sawtooth
waveform [2]. When the latter is less than the former, the PWM signal (pulse) is in high
state (1). Otherwise it is in the low state (0).
2.2
PWM INVERTER
The three phase inverter based on Pulse Width Modulation (PWM) will be used
to convert the signal from DC to AC to be used by the load. In wind power generation
system, the grid-connected inverter is an important section for energy conversion and
transmission, of which the performance has a direct influence on the entire wind power
generation system [3].
Modern, high performance PWM converter provides unity power factor and low
harmonic distortion of current. It has substantial influence for power quality, because
non-sinusoidal currents delivered to the grid, can introduce an additional non-sinusoidal
voltage drop across the line impedance and as a consequence, increase the grid voltage
distortion, which is supplied to many other loads or could affect other generators [4].
8
Various control strategies have been proposed in recent works on this PWM inverter.
Some research has examined the closed-loop regulation of PWM inverters to achieve
good dynamic response and most of them have focused on transient response
improvement through instantaneous feedback control [5]. By using a sinusoid of the
desired frequency as the control voltage for a PWM circuit, it is possible to produce a
high-power waveform whose average voltage varies sinusoidally.
In the past there has been intensive research on this topic and there is much
literature on it. All the PWM schemes may be evaluated under a certain switching
frequency, fs and the reference signal frequency ration, and the input and output voltage
ratio, which is also named as the modulation index, m. The definition of the modulation
index, m is given by:
m=
(2.3)
where VLL is the peak value of the line-to-line voltage, Vd
out
is the converter output
voltage, Vcont is the peak amplitude of the control signal, Vtri is the peak amplitude of the
triangular signal which kept constant in all applications. The frequency modulation ratio
mf is defined as:
(2.4)
where fs and f1 is the switching and modulation frequencies respectively. It is always
desirable to minimize the distortion of the output voltage and current. It may change
with the modulation index in a over-modulation region (> 1 ma). In the linear region
(≥1.0 ma), the fundamental frequency component in the output voltage varies linearly
with the amplitude modulation ratio, ma. The line-to-line rms voltage at the fundamental
frequency can be written as:
9
(2.5)
The power loss in the load due to the harmonic frequencies may not be as high
in the over-modulation region as the presence of additional sideband harmonics would
suggest. The output power from the wind turbine generator model fed to the permanent
magnet synchronous generator then to a three phase diode bridge rectifier. So, to get
constant voltage at the terminals of PWM, DC-DC buck converter has been used in the
simulation.
The controller is used to achieve the optimal operating by keeping the dc
voltage constant of the buck converter to utilize completely the available wind energy.
The proposed controller has a stable operation for different high speed of wind. The
electrical utility line currents have a very low THD. The system has higher efficiency
and reliability.
2.3
DC-DC CONVERTER
In electronic engineering, a DC-DC converter is an electronic circuit which
converts a source of direct current (DC) from one voltage level to another. It is a class of
power converter. DC to DC converters are important in portable electronic devices such
as cellular phones and laptop computers, which are supplied with power from batteries
primarily. Such electronic devices often contain several sub-circuits, each with its own
voltage level requirement different from that supplied by the battery or an external
supply (sometimes higher or lower than the supply voltage, and possibly even negative
voltage).
10
Figure 2.3
2.4
General block diagram of DC-DC converter
BUCK CONVERTER
Figure 2.4
Buck converter circuit diagram
The basic structure of a buck converter is shown in Figure 2.4 above. A buck
converter produces an average output voltage Vload less than the DC input voltage Vsupply.
By varying the duty ratio;
D=
(2.6)
of the switch, Vload can be controlled, where:
11
ton:
switch on duration
toff:
switch off duration
Ts:
switching time period
L:
inductance
C:
capacitance
The idea output voltage vLoad of the buck converter without considering the
conduction losses is:
vLoad = vSupply
(2.7)
During „switching on‟ the voltage drop decreases whereas the current rises,
causing high losses. Contrarily, during „switching off‟ the losses are causes by a rising
voltage drop and a decrease of the current.
Figure 2.5
The two configurations of a buck converter: On state, when the switch is
closed, and Off-state, when the switch is open.
The conduction and switching losses are considered in the ideal switching
converter models. Converter losses of the ideal switching model are affected by forward
state-on resistance and the forward threshold voltage of the transistor and the diode,
respectively.
12
Figure 2.6
Naming conventions of the components, voltages and current of the buck
converter
Most buck converters are designed for continuous current operation. The choice
of switching frequency and inductance to give continuous current is given by;
Imin = 0 =
(2.8)
and the output ripple is described by;
(2.9)
As the switching frequency increases, the minimum size of the inductor to
produce continuous current and the minimum size of the capacitor to limit output ripple
both decrease. Therefore, high switching frequencies are desirable to reduce the size of
both the inductor and the capacitor;
Lmin =
(2.10)
Imax = IL +
(2.11)
Imin = IL (2.12)
13
The trade-off for high switching frequencies is increased the power loss in the
switches. Increased power loss for the switches decreases the converter‟s efficiency and
the larger heat sink required for the transistor switch offsets the reduction in size of the
inductor and capacitor.
The inductor wire must be rated at the rms current, and the core should not
saturate for peak inductor current by equation;
IL,rms =
(2.13)
The capacitor must be selected to limit the output ripple to the design
specifications, to withstand peak output voltage, and to carry the required rms current
where the equation is;
C=
(2.14)
14
CHAPTER 3
METHODOLOGY
MATLAB is a high-level technical computing language and interactive
environment for algorithm deviation, data visualization, data analysis, and numerical
computation [6]. MATLAB can also be used as computing platform for solving circuit
equations and developing/testing algorithms [7]. All the algorithms can be analyzed and
modeled by using the conjunction with the available toolboxes such as the signal
processing toolbox, the filter design toolbox and the control toolbox. The behavior of
power electronic system prior to prototyping can be studied using MATLAB. In
MATLAB, Simulink is the one of the simulation tools that provide libraries of pre-built
blocks called block sets. Simulink is also a graphical block diagram hierarchical
modeling tool. It provides signal source like signal generators to simulate the models
and sinks, such as oscilloscopes and spectrum analyzers, and also to visualize the output
of results. This chapter will explain about the methodology of designing the simulation
circuit of buck converter, PWM inverter and controller by using MATLAB Simulink.
3.1
PROJECT IMPLEMENTATION
The implementation of the circuit design in MATLAB Simulink in this project
is basically by using the block sets provided in the Simulink library.
15
3.2
COMPONENTS
Several components of the block sets should be completely used to design the
simulation circuits. This project will be design for the buck converter, PWM inverter,
and the controller. It is important to understand the component characteristic and setting
to build a functional circuit.
3.2.1
Ideal Switch
Figure 3.1
Ideal Switch
The switch is simulated as a resistor Ron in series with a switch controlled by a
logical gate signal g. Ron used in this project is 0.001Ω.
3.2.2
Universal bridge
Figure 3.2
Universal bridge
16
The universal bridge block implements a universal three-phase power converter
that consists of up to six power switches connected in a bridge configuration. The types
of power switch and converter configuration are selectable from the dialog box as figure
below:
Figure 3.3
Universal Bridge dialog box.
The output of universal bridge (inverter) block is a sinusoidal waveform. The
block is connected to a low pass filter consisting of an inductor, L, and a capacitor C.
The low pass filter is use to filter the higher frequency harmonic in order to generate a
purely sinusoidal output voltage.
17
3.2.3
Three-Phase VI Measurement
Figure 3.4
Three phase VI measurement.
The three-phase VI measurement block is used to measure three-phase voltages
and currents in a circuit. When connected in series with three-phase elements, it returns
the three phase-to-ground or phase-to-phase voltages and the three line currents.
18
Figure 3.5
3.2.4
Three phase VI measurement dialog box.
Three-Phase Parallel R Load
Figure 3.6
Three phase parallel R load
The three-phase parallel R load block implements a three-phase balanced load
as a parallel of R elements. At the specified frequency, the load exhibits constant
impedance. The active and reactive powers absorbed by the load are proportional to the
square of the applied voltage.
19
3.2.5
Discrete PWM Generator
Figure 3.7
Discrete PWM Generator
By selecting „Internal generation‟, the modulation index m, frequency, and
phase of the output voltage can be controlled from the internal parameters; m, f, and
phase. Otherwise, external signal are used for pulse generation. The width of the input
vector must be 1 for single phase bridges (1 arm or 2 arm) and 3 for 3-phase bridges
(single or double bridge).
20
Figure 3.8
3.2.6
Discrete PWM generator dialog box.
Scope
Figure 3.9
Scope block
This scope block will contain the determined the waveform of any output such
as voltage or current output waveform. The number of axes and the time range can be set
up by clicking at the „parameters‟ button as shown in Figure 3.12. User can analyze the
waveform in the Fast Fourier Transform (FFT) analysis to determine the frequency
21
spectrum or the total harmonic distortion (THD) of the waveform. This can be done by
saving the Scope Data in the Data History tab as shown in Figure 3.13.
Figure 3.10
Parameters of scope block.
22
Figure 3.11
Save the scope data in data history.
Other blocks used in this simulation are voltage measurement block, current
measurement block, and FFT (Fast Fourier Transform) using Powergui block.
3.3
Run Simulation
After complete all the circuit design, setting and requirements, the user need to
run the circuit in order to get the result. Before run the simulation, user need to set the
simulation time. To run the simulation, simply click on the „start simulation‟ button in
the main toolbar as shown in Figure 3.12 below:
23
Click here to „start
simulation‟
Figure 3.12
The location of „start simulation‟ button in the tool bar.
When this button is pressed, MATLAB will initialize the circuit before starting
the simulation. The user will see a message window pop up on the screen and display
messages if there is any error in the circuit and user need to do the correction.
Throughout the run time, the work will be compiling by the MATLAB. The result or
output just can be getting if there is no error in the setting of the circuit in the drawing or
all connection is connected. The user also able to pause and zoomed the graph. To see
the graph of results, user needs to double-click on the scope blocks. Once simulation is
completed, user may open the Powergui and select 'FFT Analysis' to display the
frequency spectrum of signals saved in the data history. To observe the frequency
spectrum, users have to click on the „Display‟ and observe the frequency spectrum.
24
CHAPTER 4
RESULTS AND DISCUSSIONS
This chapter presents the verification of each operation stage of the simulation
circuit with respect to various conditions. The results obtained from the simulation work
are then analyzed and discussed. The inverter circuit has been simulated with pulse
width modulation method and DC-DC buck controller is the proposed design technique
to get the constant PWM input; 240 V AC, 50 Hz. Simulation results show through the
current and voltage output waveform.
4.1 SIMULATION DESIGN OF PWM INVERTER
This part will show the simulation design of the PWM inverter and the results of
simulation. The results will also be discussed in this part.
25
4.1.1
Specifications
Carrier frequency, fc
2 kHz
Sample time, Ts
5×10-6 s
Modulation index, m
0.71429
Output frequency, f
50 Hz
Low pass filter, LC
L = 10-3 H
C = 5×10-3 F
Table 4.1: Specifications of PWM Inverter
4.1.2
Simulation
Figure below will show the simulation circuit of inverter circuit with pulse
width modulation by using MATLAB Simulink. The input voltage that has been used is
240 V before connected to the DC-DC buck controller. After the PWM inverter is
connected to the DC-DC buck controller, the output voltages are observed when the
variable input of the DC-DC buck controller has been used.
26
Figure 4.1
4.1.3
Simulation circuit of PWM inverter.
Results and Discussions
Result below shows the output waveform of the inverter that use the pulse width
modulation (PWM) technique. This PWM inverter is using the 240 V DC as the input in
order to get the output of 240 V AC, 50 Hz. The output waveforms are observed before
filter, after filter, and their total harmonic distortion (THD) before and after filter.
27
Figure 4.2
Inverter voltage before filter
Figure 4.3
Load voltage before filter
Figure 4.4
Phase voltage before filter
28
Figure 4.5
Figure 4.6
Phase current before filter
Load voltage THD before filter
29
Figure 4.7
Phase voltage THD before filter
30
Figure 4.8
Figure 4.9
Phase current THD before filter
Inverter voltage after filter
31
Figure 4.10
Load voltage after filter
Figure 4.11
Phase voltage after filter
Figure 4.12
Phase current after filter
32
Figure 4.13
Load voltage THD after filter
33
Figure 4.14
Phase voltage THD after filter
34
Figure 4.15
Phase current THD after filter
The results of the PWM inverter simulation shows that the output voltage is
purely sinusoidal as expected. The load voltage is 90 V, with frequency of 50 Hz. The
PWM inverter output with the filtering has decreased the THD of the load voltage from
THD = 102.89% to THD = 30.36%. The total harmonic of the phase voltage and phase
currents also decreased. The THD for both phase voltage and phase current before filter
is 103.13% and the THD after filter is 31.20%. By using this PWM inverter, it is shown
that the DC signal has been converted to AC signal with low total harmonic distortion.
35
4.2
SIMULATION DESIGN OF DC-DC BUCK CONTROLLER
This part will show the simulation design of DC-DC buck controller and the
results. The results of the simulation will be discussed at the end of this part.
4.2.1
Specifications
10 kHz
Switching
frequency, fs
Input voltage,
300 V, 350 V, 400 V, 500 V
Vs
240 V
Output
voltage, Vo
Ron=0.001 Ω
Diode
Rs=500 Ω
Cs=250×10-9 F
Inductor, L
3.125×10-5 H
Capacitor, C
1.6×10-3 F
Voltage ripple
≤0.5%
Table 4.2
Specifications of buck converter.
36
4.2.2
Simulation
Figure 4.16 below shows the simulation blocks developed using MATLAB
Simulink. Variable input has been used as the analogy of the variable wind speed. The
input voltage is assumed to be higher than 240 V. So, the variable inputs that had been
used in this simulation are 300 V, 350 V, 400 V, and 500 V.
Figure 4.16
4.2.3
Simulation circuit of DC-DC buck converter
Results and Discussions
The input of the DC-DC buck converter has been varied. The input that has
been used are 300 V, 350 V, 400 V, and 500 V. The following result is taken from the
simulation in the MATLAB Simulink. The results show that the all output voltage of the
DC-DC buck converter is almost constant to the value of 240 V.
37
Figure 4.17
Figure 4.18
DC input = 300 V, DC output = 239.3 V
Buck controller output voltage when Vinput = 300V
38
Figure 4.19
Figure 4.20
DC input = 350 V, DC output = 239.7 V
Buck controller output voltage when Vinput = 350V
39
Figure 4.21
Figure 4.22
DC input = 400 V, DC output = 239.9 V
Buck controller output voltage when Vinput = 400V
40
Figure 4.23
Figure 4.24
DC input = 500 V, DC output = 240.1 V
Buck converter output when Vinput = 500 V
The results of the simulation of the buck converter shows that this controller is
able to produce almost constant output DC voltage from variable DC input voltage. The
output voltage has been compared to the reference and feeded back to the switch of the
41
buck controller. The output voltages of the buck converter are not exactly at 240 V, but
all results shown that the output voltage is approximately to the value of 240 V.
4.3 PWM INVERTER WITH DC-DC BUCK CONTROLLER
This part will show the simulation of the PWM inverter with the controller. The
results of this simulation will be shown by the graphs and will be discussed at the end of
this part.
4.3.1
Simulation
Figure 4.25 shows the simulation circuit of the PWM inverter with the DC-DC buck
converter controller. This buck controller has been used to regulate the variable DC
input to constant DC to be injected to PWM inverter.
Figure 4.25
The simulation circuit of PWM inverter with the controller.
42
4.3.2
Results and Discussion
Figure below show the results of the PWM inverter circuit with four variable inputs. The
inputs that had been used are 300 V, 350 V, 400 V, and 500 V. The result shows that all
output waveform are approximately at 240 V as expected.
4.3.2.1 Output waveform when DC input = 300 V
Figure 4.26
Inverter output voltage before filter.
Figure 4.27
Load voltage
43
Figure 4.28
Phase current
Figure 4.29
Phase voltage
4.3.2.2 Output waveform when DC input = 350 V
Figure 4.30
Inverter output voltage before filter.
44
Figure 4.31
Load voltage
Figure 4.32
Phase current.
Figure 4.33
Phase voltage
45
4.3.2.3 Output waveform when DC input = 400 V
Figure 4.34
Inverter output voltage before filter
Figure 4.35
Load voltage
Figure 4.36
Phase current
46
Figure 4.37
Phase voltage.
4.3.1.4 Output waveform when DC input = 500 V
Figure 4.38
Inverter output voltage before filter
Figure 4.39
Load voltage
47
Figure 4.40
Phase current
Figure 4.41
Phase voltage
Variable voltage source of 300 V, 350 V, 400 V, and 500 V feeds through a
DC-DC buck converter. The output of the buck converter is in range of 239-240 V that
is approximately to 240 V. This results show that this buck converter can be the
controller to produce constant value of DC output from variable DC input voltage.
The DC-DC buck converter control operates by using the comparator. When the
comparator feedback voltage exceeds the internal reference voltage, the comparator
output turns off the main power switch. Then, when the feedback voltage drops below
the reference, the power switch turns on and the cycle repeats itself to get the desired
output results.
48
CHAPTER 5
CONCLUSION AND RECOMMENDATIONS
5.1
CONCLUSION
In this project, the simulation circuit design of DC-DC buck converter has been
proposed as the controller for the PWM inverter. This buck controller design procedure
has been suggested by using MATLAB Simulink software. By using this controller, the
variable DC inputs can be converter to constant DC output to be the input for the PWM
inverter. Then PWM inverter circuit is then converting the DC input to AC output with
voltage magnitude of 240 V AC as expected. This simulation has successfully been
carried out by using MATLAB Simulink software.
5.1
RECOMMENDATIONS
There are few recommendations for future research:
This project has been using the dc-dc buck converter because assuming that the
wind speed is larger than the expected speed. So in the future this project can be
improve by using buck-boost converter by assuming the wind speed variations
are higher and lower than the expected speed.
49
The work on the PWM inverter is minimal as it covers only the simulation aspect
in verifying its operation. So, for the future research, it is recommended to
implement this project for the hardware as well as the software implementation.
The future research also can be developed by using the more proper of the
controller technique to control the PWM inverter output to get the best output
results.
50
REFERENCES
[1]
–
Techbits.com.
Tech
Community,
Pulse
Width
Modulation,
http://www.topbits.com/pulse-width-modulation.html
[2]
–
Wikipedia,
the
free
encyclopedia,
Pulse-width
modulation,
en.wikipedia.org/wiki/Pulse-width_modulation
[3] – Yong Yang, Yi Ruan, Huan Qing Shen, Yan-yan Tang, and Yin Yang, “Gridconnected Inverter for Wind Power Generation System,” Shanghai University Press, vol.
13, Number 1/ February 2009, pp. 51-56.
[4] – Mariusz Malinowski and Steffen Bernet, “Simple Control Scheme of Three-Level
PWM Converter Connecting Wind Turbine with Grid,” Warsaw University of
Technology, Institute of Control (Poland) & Industrial Electronics &Technical
University of Berlin, Institute of Energy and Automation Technology (Germany).
[5] – H. J. Cha, S. S. Kim, M. G. Kang, and Y.H. Chung, “Real-time Digital Control of
PWM Inverter with PI Compensator for Uninterruptible Power Supply,” in IEEE
IECON Conf. Rec, vol. 2, 1990, pp. 1124-1128.
[6] - http://www.mathworks.com/access/helpdesk/help/helpdesk.html
[7] – Mohamad Khaizul bin Zakaria, “Dead beat Controller for PWM Inverter,” Faculty
of Electrical Engineering, UTM, 2004/2005.
[8] – Takashi Nabeshima, Terukazu Sato, Shinichi Yoshida, Shin Chiba, Kenichi Onda,
“Analysis and Design Considerations of a Buck Converter with a Hysteretic PWM
51
Controller”, Oita University & Renesas Technology Corp, Japan, 2004 35th Annual
IEEE Power Electronics Specialists Conference, Aachen, Germany, 2004.
52
APPENDIX A
Simulation Circuits
Figure A1
Simulation circuit of PWM Inverter
53
Figure A2
Simulation circuit of DC-DC buck controller
54
Figure A3
Simulation circuit of PWM inverter with buck controller
55
APPENDIX B
Diagrams of Output Waveforms of Different Input Values
DC Input = 300 V
Figure B1
Inverter output voltage before filter.
Figure B2
Load voltage
56
Figure B3
Phase current
Figure B4
Phase voltage
DC Input = 350 V
Figure B5
Inverter output voltage before filter.
57
Figure B6
Load voltage
Figure B7
Phase current.
Figure B8
Phase voltage
58
DC Input = 400 V
Figure B9
Inverter output voltage before filter
Figure B10
Load voltage
Figure B11
Phase current
59
Figure B12
Phase voltage.
DC Input = 500 V
Figure B13
Inverter output voltage before filter
Figure B14
Load voltage
60
Figure B15
Phase current
Figure B16
Phase voltage