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UNIVERSITI PUTRA MALAYSIA
DC-TO-AC INVERTER DESIGN FOR PHOTOVOLTAIC
SYSTEM
MOHAMED MOHAMED EDARDAR
FK 1999 20
DC-TO-AC INVERTER DESIGN FOR PHOTOVOLTAJC SYSTEM
By
MOHAMED MOHAMED EDARDAR
Thesis Submitted in Partial Fulfilment of the Requirements for the Degree of M aster of Science in the Faculty of
Engineering Universiti Putra Malaysia
March 1999
To my younger brother Mahmud, who takes the responsibility of the family while I am studying to get the Master of Science Degree
ACKNOWLEDGEMENTS
1 would like to thank my Lord, Allah the most gracious and merciful who gives me
the ability to finish this project sllccessfully.
The author gratefully acknowledges the guidance, advice, support and encouragement
he received from his supervisor, Dr Norman Mariun, who keeps advising and
commenting throughout this project until it turns to a real success.
Thanks for Dr. Rahaman Wagiran and Dr lshac Aris for their comments and valuable
suggestions during the practical work and thesis writing
Great appreciation is expressed to Mr. Yasin M and the other technicians of Electrical
and Electronic Engineering Department for their technical support and valuable
suggestions.
Appreciation also to the Faculty of Engineering for providing the facilities and the
components required for undertaking this project.
The author would like to thank his family and his friends for the encouragement and
support without which is impossible for the success of this project.
111
TABLE OF CONTENTS
ACKNOWLEDGEM ENTS LIST O F FIGURES LIST OF PLATES LIST OF SYMBOLS AND ABBREVIATIONS ABSTRACT ABSTRAK
CHAPTER
I INTRODUCTION Prologue Importance of the Project Objectives of the Project ThesIs Layout
II LITERA TURE REVIEW Photovoltalc Histoncal Background
Why PhotovoItalc') PhotovoItaic Applications Solar Radiation
Solar Spectrum Effects of Earth's Atmosphere Geometncal Effects Weather Effects Solal Enelgy Collector Configuration Radiation on InclIned Surface
Solar Cell PrinCiple of Opel atlon Solar Cell Technology Solar Cell Junction Types and Materials
DC-to-AC Inverter Power Semiconductor Devices
Types of Static Switches Types of S witchIng Control
Power MOSFET Pnnclple of Operation Safe Operatmg Area SWitchIng Chal actenstlcs Heat S ink SwitchIng Aid Circuits ( snubber circuits) Gate Dnve Circuits
Inverter Types Current Source Inverter Voltage SOUl ce 1 nverter
IV
Page
III Vll
Vlll IX
X X11
1 1 2 3 4
6 6 7
8 9
1 1 1 1 1 2 1 3 1 3 14 1 5 1 6 18 20 20 22 23 7" _J
24 25 26 27
28 29 3 1 32 32 33
Invertel Cll cult Topolugle'> 3-1 The Half bndge C onfiguranon 34 The Full blldge C onfiglll atlon 3b
Contl Ol of the AC Output Voltdge 1S Pul<;e WIdth J\;Iuduldtlon 39 Stepped-'>ll1e Wave InveIter 42
Shapmg of Output Voltage (SlI1usOlddl PWM) 44 Harm011lcs Ftltel ll1g De!:>lgn and MagnetIc ConSIderation 44
I I I MATERIALS AND METHODS 46 Photovoltalc Sy!:>tems 46 Modules and Arrays 48
Flat Plate An ays 49 Concentratll1g An ays 50
Power CondltlO11lng and Control 50 Storage 5 1 Cost AnalysIs 52
Module Cost 53 Balance-of-System Costs 53 System Cost 54
System SIZll1g 55 SIZIng ConSIder dtlon 56 ProcedUl e of How to SIze a PhotovoltaIC System 58
RelIabIlIty 59 Invel tel CIrcuit AnalysIs 60 The InverSIon Cll CUlt ( the Power ClrclIlt) 60
The Load 6 1 The DC SOUl ce 62 Heat SlI1k De'>lgn 63
The Gatll1g CIrCUIt 64 SlI1gle PWM CIrCUit AnalysIs 64 MultIple PWM ClrcUlt AnalysIs 65
Clock DeSign 68 Dnve Cll cult DeSign 70 ProtectIon CIrCUIt 73
PI mted Clrclllt BOdl d 74 Invelter CirCUit Testll1g 77
ClrcLllt PI ep,u dtlon 77 Testmg Procedure 78
I V RES U L TS AND DISCUSSION 79 The Complete 111vel ter Circuit 79 The Conti 01 ClrcUlt (Low Power SectIOn) 80 The Power Circuit (High Power SectIOn) 82
The TI anstOi mer Connected to the Inverter 82 The Trdl1!:>tOl mel not Connected to the Inverter 85
A filtelll1g C ,lpaCitol Connected to the Transformer 85 Summery 89
v
V CONCLUSION 90 The Invel1er 90 RecommendatIons 92
REFERENCES 93
APPENDIX
A Power MOSFET Data Sheets 96
B MGD IR2 1 1 0 Data Sheets 1 0 1
VITA 1 1 3
VI
LIST OF FIG URES
Figlll"e Page
1 The zel1lth angle 1 3 2 Collector configuratIon 1 -l 3 Solar cell structul e 1 6 4 a- Equivalent circuit of the solar cell 1 7
b- J-V c,haractenstics of the solar cell 1 7 5 Maximum power pomt 1 8 6 a- Output characteristics of power MOSFET 26
b- Safe operatmg area (SOA) of power MOSFET 26 7 MOSFET switchmg charactenstics 27 8 di/dt and dv/dt protection circuit 3 0 9 Current source inverter 3 3 1 0 Voltage source inverter 34 1 1 Half-bridge inverter configuration 3 5 1 2 Full-bridge inverter configuration 3 7 1 3 Single pulse width modulation generating signals 39 14 Multiple pulse width modulation generating signals 4 1 1 5 Stepped sine waveform 42 1 6 Output-filters 45 1 7 Block diagram of general photovoltaic system 47 1 8 Parallel and series module configuration 48 1 9 Wiring diagram of a typical air conditioner 6 1 20 Single PWM control circuit schematic diagram 64 2 1 Timing diagram of single PWM 65 22 Multiplt, PWM control circUit schematic diagram 67 23 Timing diagram of Multiple PWM 68 24 IC555 timer circlllt 69 25 Single-phase bndge mverter and gating signals 70 26 The amplifying Cll cuit 7 1 27 IR2 1 1 0 MOSFET gate dl ive connected to bootstrap
Components 72 28 Schematic diagram of complete control circuit layout 75 29 The control circuit layout (the artwork) 76 3 0 Inverter circuit prepared for testing 77 3 1 The gating signals waveform 8 1 32 The inverter signal wavefOl m without transformer 83 33 The signal waveform with the snubber circuit 84 3 4 The 680-ohm resistive load connected t o the transformer 86 3 5 The 3 80-ohm resistive load connected to the transformer 87 36 The effect of a filtering capacitor to the waveform shape 88
VII
LIST OF PLATES
Plate
The constmcted power and control circuit
viii
Page
79
LIST OF SYMBOLS AND ABBREVlATONS
ARC Anti-reflection Coating BJT Bipolar Junction Transistor BOS Balance of System C Capacitor CJP Cold-Junction Processing I> I>rain f Frequency FF Fill-Form G Gate I Insolation I Current IGBT Isolated Gate Bipolar Transistor L Inductor M Modulation index MGI> MOSFET Gate I>rive MOSFET Metal Oxide Semiconductor Field Effect Transistor NSDB Non-Sun-Drain-Period PCB Printed Circuit Board PV Photovoltaic PWM PU/$e Width Modulation R Resistor SN Switch number N, where N = 1,2,3, or 4 SOA Safe Operating Area SOG-Si Solar Grade Silicon T Temperature TTL Transistor-Transistor Logic Wp Peak Watt e Thermal resistance "( Time constant 6 Pulse width
11 Efficiency
IX
Abstract of thesis presented to the Senate of Universiti Putra Malaysia in partial fulfilment of the requirements for the degree of Master of Science
DC-TO-AC INVERTER DESIGN FOR PHOTOVOLTAIC SYSTEM
By
MOHAMED MOHAMED EDARDAR
Mal"ch 1999
Chairma n: Norman M�lI"iun, Ph.D.
Faculty: Engineering
With the recurrent oil crises and the new environmental boundary conditions
for energy production and use, the Photovoltaic, PV, system development can be a
very promising strategic solution, which realises together with the Photovoltaic
energy cost effectiveness goal, an actual clean, renewable, and alternative energy
option. However, the continuos decline in Photovoltaic module prices in the
international market gives more attention to design the supplementary equipment with
low cost and high reliability, such as DC-to-AC inverter
For that reason, this project has the objective of designing and implementing a
prototype DC-to-AC invel1er which used to power small houses or huts in the rural
areas . The input of this inverter is four batteries of twelve volt, which would be
charged by a photovoltaic solar array. The design of the solar array and its controller
is not within the aim of this project. However, a theoretical study is given for the
photo voltaic system in the first chapters for a purpose of illustration.
x
The DC-to-AC invel1er is designed as a single phase bridge switched mode
circuit, using power MOSFET switches and electronic components that are common
and with low cost . For the sake of simplicity, a single pulse width modulation is
applied which gives a reasonable efficiency with rejection of the third harmonic and its
multiple. The detailed analysis of the design and experimental construction of the
inverter are discussed in this report.
xi
Abstrak tesis yang dikemllkakan kepada Senat Universiti Putra Malaysia Sebagai memenllhi keperillan llntuk ijazah Master Sains
MEREKA BENTUK PEN UKAR UNTUK APLIKASI S ISTEM PHOTOVOLTAIK
Oleh
MOHAMED MOHAMED EDARDAR
Mac 1999
Pengerusi : Norman Mari nn, Ph.D.
Fakulti : Kejuruteraan
Berdasarkan permasalahan minyak kini dan terbatasnya pengeluaran tenaga
dan kesannya kepada alam sekitar, maka penggunaan fotovoltaik (PV), dibangunkan
untuk memberikan sllatll penyelesaian yang strategik . Kita sedia maklum bahawa
tenaga fotovoltaik memberi altenatif kepada tenaga yang sedia ada sekarang.
Ditambah pula dengan penurunan harga pada modul fotovoltaik di pasaran
antarabangsa, telah memberikan tumpuan untuk mencipta peralatan tambahan, pad a
kos yang rendah, sebagai contoh, penukar DC kepada AC.
Tujuan projek ini adalah untuk mencipta dan melaksanakan prototaip penukar
DC kepada AC yang boleh digllna llntuk memberi bekalan kuasa kepada rumah kecil
di luar bandar. Input llntuk penllkar ini menggunakan 4 bateri ( 12 volt), yang boleh
dicas semula oleh sllsllnan fotovoltaik suria. Pembentukan susunan sel suria dan
kawalannya, bukanlah matlamat kepada projek ini. Walaubagimanapun kaj ian secara
teori mengenai sistem fotovoltaik diberikan pada bab pertama bertujuan memberikan
gambaran menyeluruh.
XII
Penukar DC kepada AC direkabentuk sebagai litar jejambat mod SUIS.
menggunakan suis MOSFET dan komponen elektronik yang biasa dan murah Supaya
mudah, modulasi lebar denyllt tllnggal digllnakan yang boleh memberikan nilai
kecekapan munasabah serta menghapus harmonik ketiga dan seterusnya analisa
mengenai rekabentuk dan pembinaan ujikaj i dibincangkan secara lanjut di dalam
laporan ini .
xiii
CHAPTER I
INTRODUCTION
Prologue
This thesis contains two parts. The first part is a study of a photovoltaic (PV)
system and related topics such as the solar radiation and the solar cel l . The
photovoltaic effect is the direct conversion of light energy into electricity by solar
cel ls . Electricity is a high quality form of energy, which is suitabl e for almost every
type of energy-consuming activity. It uses range from such highly technical tasks as
powering computer or electron beam welders to domestic tasks such as water heating
or powering the kitchen toaster. An example of how to design a small photovoltaic
system for a home application is given with a description of the subsystems and the
cost analysis.
The second part of the project, the main part, is to design and construct a DC
to-AC inverter, which i s one of the main components of the photovoltaic system for
AC appl ications. More attention has been given to this part, since the author has done
a complete design and the construction of the inverter. The inverter is the circuit
which convert the DC, or the direct current, of the solar cell s to AC, or the
alternating current which is suitable for home applications such as lamps, fans,
refrigerators, radios and so on.
Importance of the Project
There are several reasons that attract the researchers to photovoltaic systems
and subsystems design. These reasons have been gathered and presented by the author
in this thesis in Chapter II . It is more important in developing countries where the
majority of the population lives in dispersed communities in rural areas. The provision
of an electricity supply to these areas are difficult and costly; extension of the mains
grid over difficult terrain is generally not economic for small power loads and the use
of diesel generator sets relies on availability of fuel supplies and maintenance skills .
The only limitation of using the photovoltaics as a source of energy is the cost . As a
result emphasis is put in most scientific research and development institutions to
reduce the cost to the minimum. The goal of reducing the cost of the photovoltaic
system to be equivalent to the other sources of the energy will put it in the first
options to be chosen because of its other advantages presented in Chapter II.
Research works are being carried out to reduce the cost in the field of solar
cel ls as well as the other parts of the system such as batteries, inverters, system
control lers and so on. In this project a DC-to-AC inverter has been designed and
constructed for a photovoltaic application. This inverter has the same working
principles as the other power-electronic devices and circuits. But three important
factors should be given more emphasis in the designing an inverter for photovoltaic
application as fol lows:
1- S implicity: Most of photovoltaic applications are applied in communication
systems or the rural areas with a low population where the connecting to the
utility grid is difficult. In these areas it is also difficult to find people who have
high skills of maintenance. Consequently, a system with a high reliability is
3
important in these applications To improve the total reliability of the subsystems,
the inverter circuits should be simple to reduce the probability of circuit failure A
simple circuit gives a chance for a person with a moderate skil l to maintain the
circuit when fault occurs.
2- High efficiency: The solar cell efficiency in the markets now is in the range of 1 0%
to 1 5%. As a result most of the solar energy converted to electrical energy is
required to be used at the load, and to minimise the losses in the other parts of the
system.
3- Low cost: This is the most important factor to be achieved while keeping the
other factors within an acceptable range. Reducing the cost of the invener will
share in reducing the total cost of the system.
Objectives of the Project
In this project the author has the aim to master the knowledge in two fields,
which usually is faced by the electrical and electronic engineers in their jobs. The first
is in designing the system, which gives the author the ability to design and implement
electrical system in different applications, once he exposed to one of these systems.
The design of electrical system will give the electrical engineer the chance to deal with
other factors which is not an electric in its nature such as the geographical and
weather effects, the system location and the people. The photovoltaic system has
been chosen for such a study because of its importance, which the author presented in
his thesis.
The second, is the circuit design. This will give the author more chance to deal
with requirements of designing and constructing the electronic and power circuits
Testing the circuit also allow the author to know more about the problems of circuit
design and comparison is made between the theoretical values and the measured ones.
The DC-to-AC inverter gives more chance to deal with low currents and voltage of
the control circuit that mainly contains an electronic digital and analogue devices. On
the other hand the power circuit deals with higher currents and voltages, which
requires more safety precautions for the circuits and the operators.
Thesis Layout
The thesis is organised into five chapters. Chapter I gives an introduction to
the author's work and his objective of designing and constructing the DC-to-AC
inverter for a photovoltaic application .
Chapter II contains a literature review of both the photovoItaic systems and
the DC-to-AC inverter. A general description of a photovoItaic system and related
subjects such as the solar radiation and solar cells' operation and construction is
given. This knowledge is basic for a system designer. More emphasis is given in this
chapter for the study of DC-to-AC inverters, since the author has designed and built
the inverter in the laboratory. Inverter theory, inverter types and topologies, the
power semiconductor devices, and the required protection circuits are described and
analysed.
5
In Chapter III, a design for a photovoltaic sys��� "''''��ystem UNIvEaSlTI PUTRAv� sizing are given. This is followed by the main design. the inverter. The voltage and
current values of the circuit are calculated. Then the components required to
construct the circuit are chosen.
Chapter IV contains the results of the tested circuit at different points and with
different loads. Oscilloscope waveforms of the circuit output are presented. A
discussion of the results is also included in this chapter.
In Chapter V, a general conclusion about the research work has been
presented and recommendations for future work and the areas of possible
improvements has been discussed.
CHAPTER II
LITERATURE REVIEW
Photovoltaic Historical Background
Photovoltaics have been technological ly driven since the discovery of the
sil icon cell . In the first few years following the invention of the solar cell , it was
l imited to the space appl ication because of its high cost, hundreds of US dollars,
which is relatively small in relation to the total project cost. Following 1 973 oil
crisis, i nterest in photovoltaics as a terrestrial source of power increased. Many
countries insisted photovoltaic research, development and demonstration programs.
In 1 974, the goal was to reduce the cost by a factor of a hundred, to i mprove
the working lifetime by a factor of ten times and the efficiency by a factor of two.
S ince then, modules cost has decreased from USD70/peak watt (Wp) to about
USD2IWp for amorphous sil icon modules and about USD4IWp for wafer sil icon
modules. Lifetime increased from months to years, more than twenty years.
Efficiency goal of 1 0% was easily met by using si l icon wafers of 1 3- 14%.
Amorphous sil icon modules have yet to exceed 1 0% (Hil l , 1 989).
6
7
Why Photovoltaics?
PY modules provide an independent, reliable electrical power source at rhe
point of use making it particularly suited to remote or inaccessible locations. PY
systems are technically and economically viable. Their principal advantages are:
1 - PV systems have no fuel requirements:
In remote areas diesel or kerosene are erratic and often very expensive. The recurrent
costs of operating and maintaining PV systems are small .
2- PY systems are modular:
A solar array is composed of individual PY modules so each system can be sized to
meet particular demand.
3 - PY system can be used to improve quality of life:
For example, the provision of lighting in a rural school allows evening education or
community activities. Refrigeration at a health centre improves effectiveness of
immunization programmes.
4- PV systems are highly reliable:
The reliability of PY modules is significantly higher than of diesel generators and wind
generators.
5- PY systems are easy to maintain:
Operating and the routine maintenace requirements are simple. r
6- PV modules have a long life :
There is a little degradation in performance over 1 5 years.
7 - PV systems provide national economic benefits:
Reliance on imported fuels such as a coal and oil is reduced.
8- PV systems are environmentally harmless:
There is no harmful pollution through the use of a PY system.
9- PV systems are economically viable:
On a life cycle cost basis and taking into consideration the higher reliability of PV,
many small applications can be more economically powered by PY than with diesel
systems or other small power supplies (Derrick et al . , 1 99 1 ) .
Photovoltaics Applications
The photovoltaics industry is growing rapidly throughout the world. The
previous markets in professional systems for telecommunications, cathodic protection
etc. are still growing, whilst the applications in developing countries have passed
beyond the demonstration stage into an acceptance of photovoitaic systems as a
reliable engineering solution to specific problems. In recent years, the most rapidly
growing market segment has been in consumer products, but this is likely to be
overtaken by the grid-photovoltaics, in research, in production, and in applications has
developed such momentum that its future is assured as a major technology.
Photovoltaics have been used in space since the first years of solar cell
applications, but they found the widest range of applications in terrestrial applications.
At present there are three market categories for photo voltaic systems as the following
(Partain, 1 995) :
1- Consumer market: there are many applications under this category. Examples of
them are: small electronic devices, garden light, battery charger, security lighting,
ventilation systems, and water pumping.
2- Industrial market: they are also called professional systems; Examples of this
category are: telecommunication links, remote sensing, cathodic protection,
navigation lights, military equipment, irrigation pumping, and refrigeration services
3 - Social benefits market: usually the governments support the applications of this
category. Examples of this category are: lighting and power supplies for remote areas,
battery charging stations, pumping for drinking water, lighting in rural clinics, blood
storage refrigerators, remote weather measuring, emergency telephones, and road
lighting.
Solar Radiation
The solar efficiency of a photovoltaic system depends critically on the spectral
distribution of the radiation coming to us from the sun. The study of the solar
insolation is complex and involved one that is a field all by itself. Here a brief survey
of information is presented about solar insolation and the way in which this
information can be interpreted (Faherenbruch and Bube, 1 983) .
Solar insolation: the radiation when integrated over one-hour periods. The symbol I
is commonly used.
Irradiance: the radiation flux on a unit area of a spherical surface.
Air-mass: the bath length through the atmosphere is conveniently described in terms
of equivalent air-mass Ammr .
Solar constant : the rate at which energy is received on a unit surface perpendicular to
the sun direction in free space at the earth's mean distance from the sun.
One-sun: is used to define 1 000 watt/m2 .
Bright sunshine: hours when sunshine is bright enough to cast a shadow.
The sun has a diameter of 865 ,000 miles, and radiates energy at rate of some
3. 8x 1020 Mwatts. The energy output from the sun is considered being constant
(Neville, 1 995) . The distance between sun and earth varies from 92 to 95 million
miles. This variation implies that the light energy reaching the earth in June (when the
earth is at its maximum distance from the sun) is approximately 94% of the light
energy reaching the earth in December. The energy density available to a collector
positioned just outside the earth's sphere, which is known by a solar constant, IS
measured variously as lying between 1 .3 53 and 1 .418 Kwatt/m2 .
Once the sunlight has reached the earth's atmosphere a number of additional
effects play a part. These effects result from the weather, and photon absorption by
water vapour, ozone, and other atmospheric constituents. They have the general
overall effect of reducing energy density in sunlight at the earth' s surface.
Atmospheric effects are at minimum on a dry, cloudless day with the sun that directs
overhead. Under these conditions (known collectively an air-mass one, AMd the
power flux in sunlight at earth's surface is 1 .07 Kwatt/m2 at a potential energy of
approximately 1 2 hours a day of sunlight. Air-mass zero AMO condition corresponds
to a solar flux density of 1 .3 53 kwatt/m2 and a potential of 24 hours of sunlight each
day.
Two other factors influencing the availability of a solar energy are geometric
in nature. The first, the earth rotates on its axis with a period of approximately 24
hours . Hence, sunlight is available for only an average of 1 2 hours a day. Second, the
earth' s axis of rotation is tilted approximately 23 . 5° to the normal of its plane of
revolution about the sun. Together, these two effects act to produce a shift in number
II
of hours of daylight and a geometrical situation in which the sun is almost never
directly overhead (Neville, 1 995).
Solar Spectrum
The sun is a complex radiator whose spectrum can be approximated by a 6050
oK black body. In outer space 98% of total energy radiated by the sun lies between
(0 .25 and 3 .0) xl0-6 m wavelength.
A significant fraction of solar spectrum irradiance lies at wavelengths greater
than I. 1 5x 1 0-6 m. this region of the solar spectrum is often termed the far infrared, as
opposed to the near infrared, which lies between 0 .7 and 1 . 1 5 x l 0-6 m. This far
infrared has been demonstrated is not available to solar cells as energy source. This is
a major limitation on the efficiency of energy produced by photovoltaic devices.
Effects of Earth's Atmosphere
Both intensity and spectral distribution of radiation arnvmg at the earth's
surface depend on the composition of the atmosphere, as well as the path length of
the radiation through the atmosphere. The most important parameters of the
atmosphere are: 1 ) water content, 2) the turbidly coefficient, B, expressing the effect
of haze and related scattering, 3 ) the cloudiness of the sky, and 4) the effect of the
ground reflection (Faherenbruch and Bube, 1 983) .
