M , S

M , S
MODELING, SIMULATION AND IMPLEMENTATION OF
LOW POWER PHOTOVOLTAIC ENERGY CONVERSION
SYSTEM
R. Sudharshan Kaarthik
Nayan Kumar Dalei
R. Vigneshwaran
Rabinarayan Das
Department of Electrical Engineering,
National Institute of Technology Rourkela,
Rourkela – 769008, India.
Modeling, Simulation and Implementation of
Low Power Photovoltaic Energy Conversion
System
Project Report Submitted in partial fulfillment of the requirements for the
degree of
Bachelor of Technology
in
Electrical Engineering
by
R. Sudharshan Kaarthik (10602028)
Nayan Kumar Dalei (10602033)
R. Vigneshwaran (10602066)
Rabinarayan Das (10502002)
National Institute of Technology Rourkela,
Rourkela – 769008, India.
May 2010
2
Department
of
Electrical
Engineering
National Institute of Technology Rourkela,
Rourkela – 769008, India. www.nitrkl.ac.in
B.Chitti Babu
Assistant Professor
May 13, 2010
CERTIFICATE
This is to certify that the project entitled Modeling, Simulation and
Implementation of Low Power Photovoltaic Energy Conversion System submitted by
Mr. R. Sudharshan Kaarthik (Roll No. 1602028 ), Mr. Nayan Kumar Dalei (Roll. No.
10602033), Mr. R. Vigneshwaran (10602066) and Mr. Rabinarayan Das (Roll No.
10502002) in partial fulfillment of the requirements for the award of Bachelor of
Technology Degree in Electrical Engineering at NIT Rourkela is an authentic work
carried out by them under my supervision and guidance.
B. Chitti Babu
3
ACKNOWLEDGEMENT
We would like to thank NIT Rourkela for giving us the opportunity to use
their resources and work in such a challenging environment. First and foremost we
take this opportunity to express our deepest sense of gratitude to our guide
Prof. B. Chitti Babu for his able guidance during our project work. This project would
not have been possible without his help and the valuable time that he has given us
amidst his busy schedule. We would also like to extend our gratitude to our friends
and senior students of this department who have always encouraged and supported
in doing our work. Last but not the least we would like to thank all the staff
members of Department of Electrical Engineering who have been very cooperative
with us.
R. Sudharshan Kaarthik
Nayan Kumar Dalei
R. Vigneshwaran
Rabinarayan Das
4
TABLE OF CONTENTS
Certificate .......................................................................................................................................................................................... 3
Acknowledgement ........................................................................................................................................................................ 4
List of figures ................................................................................................................................................................................... 7
Abstract .............................................................................................................................................................................................. 9
Chapter 1 ....................................................................................................................................................................................... 10
Introduction ................................................................................................................................................................................... 10
Motivation ............................................................................................................................................................ 10
Work Summary .................................................................................................................................................. 10
Report Organization ........................................................................................................................................ 11
Chapter 2 ....................................................................................................................................................................................... 12
Renewable Energy ...................................................................................................................................................................... 12
Renewable energy scenario in India ......................................................................................................... 12
Solar Energy in India ....................................................................................................................................... 14
Chapter 3 ....................................................................................................................................................................................... 15
PV Cell and Modeling ................................................................................................................................................................. 15
I-V Characteristics of a Photovoltaic Module ........................................................................................ 15
Temperature and Irradiation Correction factors ................................................................................ 19
Chapter 4 ....................................................................................................................................................................................... 21
Maximum Power Point Tracking .......................................................................................................................................... 21
Converter Choice for MPPT .......................................................................................................................... 22
Algorithm for finding Maximum Power Point ...................................................................................... 24
Chapter 5 ....................................................................................................................................................................................... 26
Buck Converter ............................................................................................................................................................................. 26
Inductor and Capacitor Design.................................................................................................................... 28
Control of Buck Converter............................................................................................................................. 28
Analysis of Buck Converter ........................................................................................................................... 29
5
Chapter 6 ....................................................................................................................................................................................... 31
Circuit and Practical Implementation ................................................................................................................................ 31
Description of Power Supply ....................................................................................................................... 32
Power Circuit ...................................................................................................................................................... 33
PV Module ....................................................................................................................................................... 33
Mosfet ............................................................................................................................................................... 34
Inductor ........................................................................................................................................................... 34
Diode ................................................................................................................................................................. 34
Capacitor.......................................................................................................................................................... 34
Control Circuit .................................................................................................................................................... 35
Op-Amps .......................................................................................................................................................... 35
Ramp Generator 8038 ................................................................................................................................ 37
LM311 ............................................................................................................................................................... 37
Gate Pulse to Driver .................................................................................................................................... 38
Driver Circuit ................................................................................................................................................. 38
Charging Circuit ................................................................................................................................................. 38
Chapter 7 ....................................................................................................................................................................................... 40
Results and Discussion.............................................................................................................................................................. 40
Simulation Results ............................................................................................................................................ 40
PV Simulink Model Block .......................................................................................................................... 41
Internal Components Of PV Simulink Model .................................................................................... 41
Output Waveforms of buck converter and Control Circuit.............................................................. 44
Output Waveforms of Photovoltaic Array .............................................................................................. 48
Hardware of the circuit implemented ...................................................................................................... 51
Experimental Results / Waveforms from CRO ..................................................................................... 52
Chapter 8 ....................................................................................................................................................................................... 57
Conclusion and Future Work ................................................................................................................................................. 57
References .................................................................................................................................................................................... 58
Appendix ........................................................................................................................................................................................ 60
6
LIST OF FIGURES
Fig. 1 - Complete Photovoltaic Energy Conversion System
Fig. 2 - Percentage of installed Power capacity in India
Fig. 3 - Renewable energy generation in India
Fig. 4 - PVA Characteristics V (Y axis) Vs I (X axis)
Fig. 5 – Voltage Vs Current characteristics of PVA with Variation of Insolation
Fig. 6 – Characteristics of PVA incorporating effect of Temperature
Fig. 7 - Simplified equivalent circuit of PV Array
Fig. 8 – Operating point of PVA
Fig. 9 – Converter acting as a Maximum Power Point Tracker
Fig. 10 – Region of operation of Buck Converter
Fig. 11 - MPPT Implementation on a PV array
Fig. 12 – Buck Converter Topology
Fig. 13 – Input Voltage as a function of Switch position
Fig. 14 – Inductor Current
Fig. 15 – Control scheme for Buck Converter
Fig. 16 – The complete circuit of the Buck Converter
Fig. 17 – Charging Circuit
Fig. 18 – Op-Amp used as a differential Amplifier
Fig. 19 – LM 324 used as an Adder
Fig. 20 – LM 741 as an inverting amplifier
Fig. 21 - Complete simulation model of the photovoltaic energy conversion system
Fig. 22 - Complete PV Cell Simulink model block
Fig. 23 - Block diagram of the PV sub-module that gives out cell current and cell voltage
Fig. 24 - Block diagram of PV sub-module that determines correction factors for current
Fig. 25 - Block diagram of PV sub-module that measures PV cell output voltage
7
Fig. 26 - Block diagram of the internal sub-modules of PWM generator
Fig. 27 - Response of the error voltage signal from the comparator
Fig. 28 - Response of the ramp voltage generated
Fig. 29 - Generated gate pulse from the PWM controller
Fig. 30 - Current response of the inductor
Fig. 31 - Response of the voltage across the inductor
Fig. 32 - Response of the Sensed output current
Fig. 33 - Response of the sensed reference current
Fig. 34 - Response of voltage across MOSFET IRF9530NS
Fig. 35 - Response of the output voltage from the photovoltaic array
Fig. 36 - Response of the output current of the photovoltaic array
Fig. 37 - Trajectory curve of the operating point in the plot between output current Vs output voltage
Fig. 38 - Trajectory curve of the operating point in the plot between output power Vs output voltage
Fig. 39 - Photograph of the PCB designed
Fig. 40 - The complete experimental setup
Fig. 41 - Reference voltage (blue) and Output voltage (yellow) in 2.5ns resolution
Fig. 42 - Reference voltage (blue) and Output voltage (yellow) in 250ns resolution
Fig. 43 - Inductor voltage
Fig. 44 - Generated Ramp Signal
Fig. 45 - Error Signal (blue) and Ramp Signal (yellow)
Fig. 46 - Comparator’s output (blue) and Ramp signal (yellow)
Fig. 47 - Comparator output (yellow) and Gate signal (blue)
Fig. 48 - Gate signal with respect to ground
Fig. 49 - Gate signal with respect to Source
8
ABSTRACT
Remote areas in India are still not connected to the power grid. But they have
mobile network connectivity. The people face problems in charging their cell
phones. They are forced to travel a long distances to get access to electrical outlets.
This project focuses on providing a Photovoltaic System which could charge a cell
phone battery. The developed system provides a solution to this problem. The
system comprises of PV array, Maximum Power Point Tracker, Buck Converter and
Charging Circuit. The system is modeled and simulated in Matlab-Simulink
Environment. Hardware for the system is also implemented. We find proper
synchronism between the results.
9
CHAPTER 1
INTRODUCTION
The use of new efficient photovoltaic solar cells (PVSCs) has emerged as an alternative
measure of renewable green power, energy conservation and demand-side management. Owing to
their high initial cost, PVSCs have not yet been fully an attractive alternative for electricity users who
are able to buy cheaper electrical power from the utility grid. However, they can be used extensively
for water pumping and air conditioning in remote and isolated areas, where utility power is not
available or is too expensive to transport [1].
MOTIVATION
In India there are about 300 clear sunny days in a year and solar energy is available in most
parts of the country, including the rural areas. But still we have miles to cover before solar power is
effectively utilized to replace the fossil fuels and become a cheap and effective solution for domestic
and commercial applications. With the growing demand for renewable sources of energy, the
manufacturing of solar cells and photovoltaic arrays has advanced dramatically in recent years. Its
efficient usage has led to increasing role of photovoltaic technology as scalable and robust means of
harnessing renewable energy.
WORK SUMMARY
A photovoltaic energy conversion system for converting solar power into useable DC at 5V to
15V for charging batteries of low power devices like mobile phones has been proposed and
implemented. The energy obtained from the photovoltaic module is unregulated. But for charging
Lithium ion batteries, we require approximately 4.5V steady DC supply. The 18V unregulated DC
obtained from the PV module is stepped down to 9V by DC-DC buck converter. Fig.1 shows the
complete structure of PV energy conversion system, which comprises PV array with DC-DC buck
converter.
10
The inductor design of the buck converter circuit is discussed in detail, which is a significant
part in designing of the converter. For efficient usage of photovoltaic energy conversion system, it is
essential to design a maximum power point tracking (MPPT) system. The concept of MPPT is to
automatically vary a PV array's operating point so as to get maximum power [2]. This is necessary
because the PV cell has a very low conversion efficiency and to reduce the cost of the overall system.
The power delivered by array increases to maximum as the current drawn rises and after a particular
value, the voltage falls suddenly making the power drop to zero. A boost converter is not preferred
here because it cannot track maximum power point at low radiation levels, as this point is located in
the non-operating region.
Fig. 1 - Complete Photovoltaic Energy Conversion System
REPORT ORGANIZATION
Renewable energy utilization in Indian scenario is presented in Chapter 2 along with the
prospects of the photovoltaic energy. Characteristics of Photovoltaic cells are discussed in Chapter 3.
Chapter 4 is about Maximum Power Point tracking and choice of MPP tracker. Chapter 5 elaborates
on Buck converter operation. Chapter 6 discusses the practical implementation of the circuit. Chapter
7 is dedicated to results and discussion of the work done. Finally, Chapter 8 describes the concluding
remarks and future work.
Simulation of the whole system has been carried out using Matlab-Simulink environment via
the graphical user interface. The hardware implementation of the system is also made and we find
proper correlation between the two.
11
CHAPTER 2
RENEWABLE ENERGY
In recent years, there is a substantial increase of energy consumption in India. This fast rate
of energy consumption is influenced by the population growth and economic development in India.
In the last four decades the commercial energy consumption in India has grown by about 700
percent. This has lead to the per capita consumption in India to be in region of 400 KWH per annum.
Driven by the rise in population, ever expanding economy and an ultimate quest for improved quality
of life, energy usage in India is expected to grow in an exponential rate.
Compared to the other developing countries the per capita energy consumption in India is
still very low even though there is an overall increase in energy demand every year. Today, India is
one of the potential competitors for the effective usage of renewable energy. India is the world’s
largest producer of wind power after Denmark, Germany, Spain and the USA. India has a significant
potential for generation of power from renewable energy sources - Small hydro power, wind energy,
bio-mass and solar energy.
RENEWABLE ENERGY SCENARIO IN INDIA
Renewable Energy in India is a sector that is still undeveloped. India was probably the first
country in the world to set up a separate ministry of non-conventional energy resources in early
1980s. However the results have been very mixed and in recent years it has lagged far behind other
developed nations in using renewable energy (RE). RE contribution to energy sector is less than 1%
of India's total energy needs.
India is one of the largest and fastest growing economies in the world with an expansive
populace of above 1.1 billion people. There is a very high demand for energy, which is currently
satisfied mainly by coal, foreign oil and petroleum, which apart from being a non-renewable, and
12
therefore non-permanent solution to the energy crisis, it is also detrimental to the environment. The
price of crude oil has risen sharply over the last few years, and there are no signs of a change in this
trend [3]. Thus, it is imperative that India obtains energy security without affecting the booming
economy, which would mean that alternative energy sources must be developed. This would mean
that the country must switch from the non-renewable energy (crude oil and coal) to renewable
energy. Figure 2 gives an account of installed power capacity of various power generation systems.
Natural gas
Nuclear
Diesel
Renewables
Large Hydro
Coal
Fig. 2 - Percentage of installed Power capacity in India
India is determined to become one of the world’s leading clean energy producers. The
Government of India has already made several provisions, and established many agencies that will
help it achieve its goal. Renewable Energy, excluding large hydro projects already accounts for 9% of
the total installed energy capacity, equivalent to 12,610 MW. In combination with large hydro, the
capacity is more than 34%, i.e. 48,643MW, in a total installed capacity of 1,44,980 MW. Refer figure 3.
Biomass
(non
bagasse), 46
Bio
Power, 542
Solar
Power, 2
Small
Waste to Hydro, 2013
Energy, 43
Biomass
Cogeneratio
Gassifier, 86
n, 635
Energy
Recovery
from
waste, 20
Wind
Power, 7230
Fig. 3 - Renewable energy generation in India
13
SOLAR ENERGY IN INDIA
Solar power, a clean renewable resource with zero emission, has got tremendous potential of
energy which can be harnessed using a variety of devices. With recent developments, solar energy
systems are easily available for industrial and domestic use with the added advantage of minimum
maintenance. Solar energy could be made financially viable with government tax incentives and
rebates. An exclusive solar generation system of capacity of 250KWh per month would cost around
Rs. 5 lakhs, with present pricing and taxes (2010). Most of the developed countries are switching
over to solar energy as one of the prime renewable energy source. The current architectural designs
make provision for photovoltaic cells and necessary circuitry while making building plans.
India is a country near the equator – which means that given its geographical location, it is
subject to a large amount of solar irradiation throughout the year. India is also, according to area, the
seventh largest country in the world. Combining the two points together, it is not difficult to gauge
that solar energy in India is a vast and plentiful resource. Much of the country does not have access to
electrical grid; one of the first applications of solar power has been for water pumping; to begin
replacing India's four to five million diesel powered water pumps, each consuming about
3.5 kilowatts, and off-grid lighting. Some large projects have been proposed, and a 35,000 km² area of
the Thar Desert has been set aside for solar power projects, sufficient to generate 700 to 2,100 Giga
Watts.
About 7.7 lakhs solar lanterns, 5.1 lakhs solar home lighting systems, 82,500 solar street
lighting systems, 7,247 solar water pumping systems, stand-alone and grid connected solar
photovoltaic (SPV) power plants of about 10 MW peak aggregate capacity, about 3.12 million square
meter solar water heater collector area and 6.57 lakhs solar cookers have been distributed/installed
in the country, as on 30.11.2009, under the solar energy programs. The present cost of electricity
generation from solar thermal and solar photovoltaic energy systems is Rs. 13.45 and Rs. 18.44 per
unit, respectively as fixed by Central Electricity Regulatory Commission.
14
CHAPTER 3
PV CELL AND MODELING
I-V CHARACTERISTICS OF A PHOTOVOLTAIC MODULE
The performance characteristics of a photovoltaic module depend on its basic materials,
manufacturing technology and operating conditions.
Voltage Vs Current of PVA
20
15
10
5
0
0
0.5
1
1.5
Fig. 4 - PVA Characteristics V (Y axis) Vs I (X axis)
Three points in these curves are of particular interest:
1.
Short circuit point, where the voltage over the module is zero and the current is at its
maximum (short circuit current Isc).
2.
Maximum power point or MPP, where the product of current and voltage has its maximum
(defined by Impp.Vmpp).
3.
Open circuit point, where the current is zero and the voltage has its maximum (open circuit
voltage Voc).
15
The measurements taken for obtaining an I-V curve is done by controlling the load current.
At open circuit, when no load current is generated, a first characteristic value can be measured: the
open circuit voltage Voc. Increasing the load fed by the photovoltaic module leads to a decreasing
voltage V with an increasing current I. In other words, by increasing the load current from zero to its
maximum value, the operating point moves from the open circuit voltage at zero current to the short
circuit current Isc at zero voltage. The series of all measured pairs (V, I) yields the characteristic I-V
curve of the module.
From the characteristic curve of the module, it is clear that the open circuit voltage of the
photovoltaic module, the point of intersection of the curve with the horizontal axis, varies little with
solar radiation changes. It is inversely proportional to temperature, i.e., a rise in temperature
produces a decrease in voltage. Short circuit current, the point of intersection of the curve with the
vertical axis, is directly proportional to solar radiation and is relatively steady with temperature
variations. Actually, the photovoltaic module acts like a constant current source for most parts of its
I-V curve [4].
Fig 5. – Voltage Vs Current characteristics of PVA with Variation of Insolation
As demonstrated in Fig. 5, an increase in solar radiation causes the output current to
increase and the horizontal part of the curve moves upward. An increase in cell temperature causes
the voltage to move leftward, while decreasing temperature produces the opposite effect. Thus, the
I-V curves display how a photovoltaic module responds to all possible loads under different solar
radiation and cell temperature conditions.
16
Fig. 6 – Characteristics of PVA incorporating effect of Temperature
The operating point of a photovoltaic module will move by varying solar radiation, cell
temperature, and load values. For a given solar radiation and operating temperature, the output
power depends on the value of the load current. As the load increases, the operating point moves
along the curve towards the right. So, only one load value produces a maximum power. The
maximum power points the line which is positioned at the knees of the I-V curves, has a nearly
constant output voltage at varying solar radiation conditions. When the temperature varies, the
maximum power points are generated in such a manner that the output current stays approximately
constant.
The fill factor (FF) of a photovoltaic generator is defined as the ratio of output power at MPP
to the power computed by multiplying Voc by Isc. It determines the shape of the photovoltaic
generator characteristics. The factors which affect the fill factor are the series and shunt resistances
of the photovoltaic generator. A good fill factor is between 0.6-0.8 [5]. As the photovoltaic generator
degrades with age, its series resistance tends to increase resulting in a lower fill factor.
During the design of a PVA powered system, a simulation must be performed for system
analysis and parameter settings. Therefore an efficient and user-friendly simulation model of the
PVAs is always needed. The PVA model proposed in [6] is a circuit based model to be used with
Simulink. The proposed model was simulated with various types of loads for performance checking.
17
PV arrays are built up with combined series/parallel combinations of PV solar cells, which
are usually represented by a simplified equivalent circuit model such as the one given in Fig. 7 or by
eq. (3.1)
𝑉𝑐 =
𝐴𝑘 𝑇𝑐
𝑒
𝑙𝑛
𝐼𝑝 𝑕 +𝐼0 −𝐼𝑐
− 𝑅𝑠 𝐼𝑐
𝐼0
(3.1)
The PV cell output voltage is a function of the photocurrent which is mainly determined by
the load current and the solar radiation level during the operation where the symbols are defined as
follows:
e
k
Ic
: electron charge ( 1.602 × 10−19 ℃ )
: Boltzmann constant ( 1.38 × 10−23 J/K )
: Cell output current, A
Iph : Photocurrent - a function of irradiation level and junction temperature
I0
: Reverse saturation current of diode (0.0002 A)
Rs
: Series resistance of cell (0.001 Ω)
Tc
: Reference cell operating temperature (20 °C)
Vc
: Cell output voltage, V
Rs
Io
IPh
IC
VC
D
Fig. 7 - Simplified equivalent circuit of PV Array
Both k and Tc should have the same temperature unit, i.e. ℃ or Kelvin. The curve fitting
factor A is used to adjust the I-V characteristics of the cell obtained from eq. (3.1) to the actual
characteristics obtained by testing. The equation also gives the voltage of a single solar cell which is
then multiplied by the number of the cells connected in series to calculate the full array voltage. Since
the array current is the sum of the currents flowing through the cells in parallel branches, the cell
18
current IC is obtained by dividing the array current by the number of cells connected in parallel
before being used in eq. (3.1), which is only valid for a certain operating temperature Tc with its
corresponding solar irradiation level Sc. If the temperature and solar irradiation level changes, the
voltage and current output of the PV array will follow this change. Hence, the effects of the changes in
temperature and solar irradiation levels should be included in the final PV array model.
TEMPERATURE AND IRRADIATION CORRECTION FACTORS
For a known temperature and a known solar irradiation level, a model is obtained and then
this model is modified to handle different cases of temperatures and irradiation levels. Eq. (3.1) is the
benchmark model for the known operating temperature Tc and known solar irradiation level Sc as
given in the specification. When the ambient temperature and irradiation level change, the cell
operating temperature also changes, resulting in a new output voltage and a new photocurrent value.
The solar cell operating temperature varies as a function of solar irradiation level and ambient
temperature. The ambient temperature Ta affects the cell output voltage and cell photocurrent. These
effects are represented in the model by the temperature coefficients CTV and CTI for cell output
voltage and cell photocurrent, respectively, as
𝐶𝑇𝑉 = 1 + 𝛽𝑇 (𝑇𝑎 − 𝑇𝑥 )
𝐶𝑇𝐼 = 1 +
𝛾𝑇
𝑆𝐶
(𝑇𝑎 − 𝑇𝑥 )
(3.2)
(3.3)
where, βT = 0.004 and γT = 0.06 for the cell used and Ta=20˚C is the ambient temperature during the
cell testing. This is used to obtain the modified model of the cell for another ambient temperature Tx.
Even if the ambient temperature does not change significantly during the daytime, the solar
irradiation level changes depending on the sunlight and clouds.
A change in solar irradiation level causes a change in the cell photocurrent and operating
temperature, which in turn affects the cell output voltage. If the solar irradiation level increases from
Sx1 to Sx2, the cell operating temperature and the photocurrent will increase from Tx1 to Tx2 and from
Iphl to Iph2, respectively. Thus the change in the operating temperature and the photocurrent due to
variation in the solar irradiation level can be expressed with the help of two constants, CSV and CSI,
which are the correction factors for changes in cell output voltage VC and photocurrent Iph,
respectively.
19
𝐶𝑆𝑉 = 1 + 𝛽𝑇 𝛼𝑆 (𝑆𝑥 − 𝑆𝑐 )
𝐶𝑆𝐼 =
1
𝑆𝐶
(𝑆𝑥 − 𝑆𝑐 )
(3.4)
(3.5)
where, Sc is the benchmark reference solar irradiation level during the cell testing. Sx is the new level
of the solar irradiation. The temperature change ΔTC occurs due to the change in the solar irradiation
level which is obtained using
∆𝑇𝑐 = 𝛼𝑆 (𝑆𝑥 − 𝑆𝑐 )
(3.6)
The constant αS represents the slope of the change in the cell operating temperature due to a
change in the solar irradiation level and is equal to 0.2 for the PVA used. Using correction factors CTV,
CTI, CSV and CSI, the new values of the cell output voltage VCx and photocurrent Iphx are obtained for the
new temperature Tx and solar irradiation Sx.
𝑉𝐶𝑋 = 𝐶𝑆𝑉 𝐶𝑇𝑉 𝑉𝐶
(3.7)
𝐼𝑝𝑕𝑥 = 𝐶𝑆𝐼 𝐶𝑇𝐼 𝐼𝑝𝑕
(3.8)
VC and Iph are the benchmark reference cell output voltage and reference cell photocurrent,
respectively. The Current versus voltage graph and the power versus voltage graphs for the
simulated model and the actual solar panel are attached in the discussion section.
20
CHAPTER 4
MAXIMUM POWER POINT TRACKING
The power output from the solar panel is a function of insolation level and temperature. But
for a given operating condition, we have a curve which gives the voltage level maintained by the
panel for a particular value of current. This plot is known as the characteristics of the cell. From the
characteristics plot, we will be able to derive the power output with respect to the output current.
From [7] we adopt the method to find the current which has to be extracted so as to fix the operating
point of the cell at its maximum power.
The operating point of any source sink mechanism is the intersection point of load line with
the source characteristic plot [8]. What we attempt here to do is change the load angle theta (𝜃) to
intersect the characteristics at maximum power point. The principle is described below.
Fig. 8 – Operating point of PVA
Photovoltaic modules have a very low conversion efficiency of around 15% for the
manufactured ones. Besides, due to the temperature, radiation and load variations, this efficiency can
be highly reduced. In fact, the efficiency of any semiconductor device drops steeply with the
temperature. In order to ensure that the photovoltaic modules always act supplying the maximum
power as possible and dictated by ambient operating conditions, a specific circuit known as
Maximum Power Point Tracker (MPPT) is employed.
21
In most common applications, the MPPT is a DC-DC converter controlled through a strategy
that allows imposing the photovoltaic module operation point on the Maximum Power Point (MPP)
or close to it. On the literature, many studies describing techniques to improve MPP algorithms were
published [9], [10], permitting more velocity and precision of tracking. On the other hand, there is no
a theory to guide the designer to choose, among the DC-DC converters family, the best one to operate
as MPPT, thus, in most cases, the designers are tempted to use the simplest DC-DC converters –
namely buck converter or boost converter.
CONVERTER CHOICE FOR MPPT
Is
IL
+
POWER
SOURCE
+
DC/DC
Vs
-
VL
RESISTIVE
LOAD
-
Fig. 9 – Converter acting as a Maximum Power Point Tracker
The load voltage can be obtained in terms of load current by the relation
𝑉𝑙𝑜𝑎𝑑 = 𝐼𝑙𝑜𝑎𝑑 × 𝑅𝑙𝑜𝑎𝑑
(4.1)
where Vload is the load voltage, Iload is the load current and Rload is the load resistance. We shall assume
the operation of a buck converter and proceed in the analysis.
𝑉𝑙𝑜𝑎𝑑 = 𝐷 × 𝑉𝑚𝑜𝑑𝑢𝑙𝑒
(4.2)
The average input power to the DC-DC converter equals the average output power thereby, we get a
relation
𝐼𝑙𝑜𝑎𝑑 =
𝐼𝑚𝑜𝑑𝑢𝑙𝑒
𝐷
(4.3)
Combining the above two equations, we write
𝑉𝑚𝑜𝑑𝑢𝑙𝑒
𝐼𝑚𝑜𝑑𝑢𝑙𝑒
=
𝑅𝑙𝑜𝑎𝑑
𝐷2
22
(4.4)
D here represents the voltage conversion ratio of the buck converter. So, when seen from the
source side, the effective resistance will be
𝑅𝑙𝑜𝑎𝑑
𝐷2
. This is a function of D, which we can control to fix
the operating point near the MPP. We have to note that the range of D is zero to one.
𝐷 𝜖 [0,1]
(4.5)
The load line is a straight line through the origin. So we can express the angle of inclination as
𝜃 (𝐷, 𝑅𝑙𝑜𝑎𝑑 ) = tan−1
𝐷2
𝑅𝑙𝑜𝑎𝑑
(4.6)
Hence the range of angle is also fixed from zero to arctan(1/R)
𝜃 𝐷, 𝑅𝑙𝑜𝑎𝑑 𝜖 0, tan−1
1
𝑅𝑙𝑜𝑎𝑑
(4.7)
Fig. 10 – Region of operation of Buck Converter
The above results allow an important verification. When a Buck converter is applied as
MPPT, the maximum power point will be tracked just if it is localized into the operation region. In
any other case, the load and generation curves intersection will determinate the operation point. Still,
it is important to notice that the load connected to the module imposes the upper angle limit, thus,
when this load is changed, the system can operate at non-operational region, i.e., out of the MPP.
Applying the same procedure to the others DC-DC converters, similar results are obtained.
Depending on the static transfer characteristic of each DC-DC converter, the resistance Rload( D,R ) is
characterized by a different equation. Table-1 summarizes these equations for DC-DC Buck, Boost,
Buck-Boost, Cuk, Sepic and Zeta converters.
23
Table 1 - Conversion Factor and Range of Operation of Converters
DC-DC Converter
Buck Converter
Boost Converter
Buck-Boost Converter
/ Cuk converter
𝑹𝒆 (𝑫, 𝑹𝒍𝒐𝒂𝒅 )
Range 𝜽
𝑅𝑙𝑜𝑎𝑑
𝐷2
0° ≤ 𝜃 ≤ tan−1
(1 − 𝐷)2 𝑅𝑙𝑜𝑎𝑑
1−𝐷
𝐷
tan−1
2
𝑅𝑙𝑜𝑎𝑑
1
𝑅𝑙𝑜𝑎𝑑
1
𝑅𝑙𝑜𝑎𝑑
≤ 𝜃 ≤ 90°
0° ≤ 𝜃 ≤ 90°
As the MPP is always requested, and this point can be found in any position on I-V curve,
depending on temperature and radiation levels, the natural DC-DC converters to be applied as MPP
Trackers are Buck-Boost, Cuk, Sepic or Zeta, because they have no non-operational region. However,
as these converters are more complex and are more expensive than Buck or Boost, usually the
designers choose the last two. In order to check the Buck and Boost limitations, these two converters
must be studied more deeply. In sequence, some simulations results of the buck converter will also
be presented.
ALGORITHM FOR FINDING MAXIMUM POWER POINT
The MPPT controller is a power electronic DC/DC chopper or DC/AC inverter system
inserted between the PV array and its electric load to achieve the optimum characteristic matching,
so that PV array is able to deliver maximum available power which is also necessary to maximize the
photovoltaic energy utilization. PV cell has a single operating point where the values of the current
and voltage of the cell result in a maximum power output. These values correspond to a particular
resistance, which is equal to V/I as specified by Ohm's Law. Also the PV cell has an exponential
relationship between current and voltage, so the maximum power point (MPP) occurs at the knee of
the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI).
Additional current drawn from the array results in a rapid drop of cell voltage, thereby
reducing the array power output. The aim of this MPPT sub-system is to determine just where that
point is, and to regulate current accordingly and thus to allow the converter circuit to extract the
maximum power available from a cell. The control methodology presented in this paper will adopt an
approach in which designing of the power converter is done by using the relationship existing
24
between the short-circuit current Isc and the MPP current Im. By simulating with various sample data
for Isc and Im it is ascertained that the ratio of Im to Isc remains constant at 0.9. One such control
scheme is shown in the figure 8.
iL
IO
L
+
G1
Mm
PV
ARRAY
vL
+
-
iC
Load
G2
D
RS
VO
C
0.1
0.1
REFERENCE
CURRENT
CONTROL
DRIVER
0.9
AMPLIFIER
SENSED
OUTPUT
CURRENT
AMPLIFIER
Fig. 11 - MPPT Implementation on a PV array
Determining the MPP for a specific insolation condition and operating the converter for this
condition is the critical part in the design of PV conversion system. Initially the short circuit current
Isc is measured and then the actual load current adjusted in such a way it is equal to a desired fraction
value of 0.9Isc.
25
CHAPTER 5
BUCK CONVERTER
A buck converter falls in to the category of switch-mode DC-DC converters. These switchmode DC-DC converters convert one DC voltage level to another level by temporarily storing the
input energy and then releasing that energy to the output at a different voltage level. The preferred
storage element can be either a magnetic field storage component (inductors) or electric field storage
components (capacitors). This conversion methodology has greater power efficiency (often 75 to 98
percent) than linear voltage regulation (which dissipates unwanted power as heat). A buck-converter
produces a lower average output voltage than the DC input voltage Vdc. Regulated DC power supplies
and DC motor speed control are the main applications.
iL
id
iO
L
+
+ vL
-
Vd
R
C
Fig. 12 – Buck Converter Topology
Figure 12 shows the topology of the Buck converter.
𝑉𝑜 =
1 𝑇𝑠
𝑉
𝑇𝑠 0 𝑜
𝑡 𝑑𝑡 =
1
𝑇𝑠
𝑡 𝑜𝑛
0
𝑉𝑑 𝑑𝑡 +
𝑇𝑠
𝑡 𝑜𝑛
0 𝑑𝑡 =
𝑡 𝑜𝑛
𝑇𝑠
𝑉𝑑 = 𝐷𝑉𝑑
(5.1)
When an ideal condition is assumed i.e. an ideal switch, a constant input voltage Vdc and a
pure resistive load, then the instantaneous voltage waveform is shown in figure 13 as a function of
the switch of position. Generally the average output voltage is expressed in terms of the switch duty
ratio.
26
Voi
Vd
Vo
t on
t
t off
Ts = 1 / f s
Fig. 13 – Input Voltage as a function of Switch position
It is noted that the diode enters the reverse biased mode during the interval when the
switch is ON and the input provides energy to the load as well as to the inductor. During the interval
when the switch is OFF, the diode carries the inductor current flowing in the circuit and transfers
some of stored energy of the inductor to the load. Under the ideal conditions the filter capacitor at
the output is assumed to be very large. This is the common consideration in applications requiring a
constant or nearly constant instantaneous output voltage 𝑣𝑜 𝑡 ≅ 𝑉𝑜 . Figure 14 shows the average
inductor current in the buck-converter which is equal to the average output current 𝐼𝑜 , the main
reason behind this being the average capacitor current in the steady-state is zero.
I CLIM
I PEAK
I DC= I O
I / 2
ITROUGH
I / 2
Continuous Mode
(I O = MAX LOAD)
I
Discontinuous Mode
( CRICTICAL BOUNDARY )
DT
(1-D)T
Fig. 14 – Inductor Current
27
INDUCTOR AND CAPACITOR DESIGN
Inductor voltage current relation is given by 𝐿
𝑑𝑖
𝑑𝑡
. During time interval 𝑇𝑠 , the change in the
inductor current ∆𝑖𝐿 and voltage across the inductor is 𝑉𝑑 − 𝑉𝑜 . Hence, we have
∆𝑖𝐿 =
𝑉𝑑 −𝑉𝑜 𝐷𝑇𝑠
(5.2)
𝐿
𝐼𝐿 = 𝐼𝑜 =
𝑉𝑜
(5.3)
𝑅
which yields a current ripple of
∆𝑖 𝐿
𝐼𝐿
=
1−𝐷 𝑅𝑇𝑆
(5.4)
𝐿
The charge carrying capacity of the capacitor must be (refer fig. 14)
∆𝑞 =
∆𝑉𝑜 =
1 ∆𝑖 𝐿 𝑇𝑆
=
2 2
2
∆𝑞
∆𝑖 𝐿 𝑇𝑠
=
𝐶
8𝐶
∆𝑖 𝐿 𝑇𝑆
(5.5)
8
=
𝑉𝑜 (1−𝐷)𝑇𝑆2
8𝐿𝐶
(5.6)
So, the value of ripple voltage is given by
∆𝑉𝑜
𝑉𝑜
=
(1−𝐷)𝑇𝑆2
8𝐿𝐶
(5.7)
By fixing the average load current, source voltage, average load voltage, voltage and current
ripple, the critical minimum values of the inductor and capacitor can be found out using eq. (5.3) and
eq. (5.7). In all our analysis, we assume that the converter operates in the continuous current
conduction mode. The inductor designed has EI core with 80 turns.
CONTROL OF BUCK CONVERTER
In response to the changes in the output load and the input line voltages there is always a
specified tolerance band (e.g. ± 1% around its nominal value) within which the output voltages of
the DC power supplies are regulated. A negative-feedback control system is used to accomplish this
as shown in figure 15. Here the output voltage 𝑣𝑜 of the converter is compared with a reference value
𝑉𝑜,𝑟𝑒𝑓 . The output from the comparator is fed to an error amplifier [11].
28
Zf
Vd
Zi
-
V o, ref
Vc
PWM
CONTROLLER
d
+
POWER CIRCUIT
WITH THE
OUTPUT FILTER
Vo
Fig. 15 – Control scheme for Buck Converter
The control voltage 𝑣𝑐 is the output from the error amplifier which is used to adjust the duty
ratio D of the switches in the converter. With the arrangement of a ramp generator circuit, ramp
signal of required frequency is generated. These ramp signals are compared with the control voltage
𝑣𝑐 to generate the gate pulses - wherever the control voltage Ve found to be greater than the
generated ramp signal a gate pulse is generated.
A proportional controller has been used to accomplish the above mentioned task. It is a
linear type feedback control system. The controller output in this proportional control algorithm is
proportional to the error signal, which is the difference between the process variable and the set
point. In other words, the output of the proportional controller is the product of the proportional
gain and the error signal obtained.
ANALYSIS OF BUCK CONVERTER
By analyzing the on-state and off state topologies (fig. 12), we can write the state space
model for the system as follows.
ON State:
d
dt
0
𝑖𝐿
= 1
V𝐶
𝐶
𝑉𝑂 = 0
−1
𝐿
−1
𝑅𝐶
1
𝑖𝐿
+ 𝐿
𝑉𝐶
0
0
𝑉𝑔
−1
𝑖𝑙𝑜𝑎𝑑
𝐶
𝑉𝑔
𝑖
1 𝑉𝐿 + 0 0
𝑖𝑙𝑜𝑎𝑑
𝐶
29
(5.8)
(5.9)
OFF State:
d
dt
0
iL
= 1
vc
𝐶
𝑉𝑂 = 0
−1
𝐿
−1
𝑅𝐶
0
𝑖𝐿
+ 0
𝑉𝐶
0
𝑉𝑔
𝐶
𝑖𝑙𝑜𝑎𝑑
−1
𝑉𝑔
𝑖
1 𝑉𝐿 + 0 0
𝑖𝑙𝑜𝑎𝑑
𝐶
(5.10)
(5.11)
By averaging the two models, we get
𝑥 𝑡 = 𝑑 𝑡 𝐴1 + 𝑑 𝑡 − 1 𝐴2 𝑥 𝑡 + 𝑑 𝑡 𝐵1 + 1 − 𝑑 𝑡 𝐵2 𝑢(𝑡)
(5.12)
Linearization of this model is done by incorporating a perturbation to the duty cycle, the
state vector and the input voltage. The equation now becomes
(𝑥 + 𝑋) =
𝐷 + 𝑑 𝐴1 + 1 − 𝐷 + 𝑑 𝐴2 𝑋 + 𝑥 + [ 𝐷 + 𝑑 𝐵1 + 1 − 𝐷 + 𝑑 𝐵2 ](𝑈 + 𝑢) (5.13)
Transfering the above equation to Laplace domain, we have
𝑥 𝑠 = 𝑠𝐼 − 𝐴𝑜
−1
𝐵0 𝑢 𝑠 + 𝑠𝐼 − 𝐴𝑜
−1
𝐸𝑑 (𝑠)
(5.14)
where
𝐴𝑜 = 𝐷𝐴1 + (1 − 𝐷)𝐴2
(5.15)
𝐵0 = 𝐷𝐵1 + 1 − 𝐷 𝐵2
(5.16)
𝐸 = 𝐴1 − 𝐴2 𝑋 + 𝐵1 − 𝐵2 𝑈
(5.17)
From eq. 5.14 we have the transfer function Gvd given by
𝐺𝑣𝑑 𝑠 =
𝑅𝑉𝑔
(5.18)
𝑠 2 𝐿𝐶𝑅+𝑠𝐿+𝑅
The remaining parts have straight forward transfer functions like Kp for the proportional
controller, and
1
𝑉𝑟𝑎𝑚𝑝
for the comparator. Now the system can be analyzed by the classical control
theory.
30
9V
9V
9V
Port
1000F
1000F
1
2
IC7912
2
1
3
1
2
IC7805
IC7812
3
3
+5V
-
2KΩ
31
50 k 
220k
LM
3
1
1
22K
pot
Fig. 16 – The complete circuit of the Buck Converter
+12V
PV
ARRAY
+
IN4001
10 k
8
0
3
8
100k
LM
3
1
1
J1
7474
47 pF
190 k 
190 k
7
6 Q’
5 Q
1
With heat sink
IRF9530
D
1000F
3
CLK
14
2
IN4001
13mH
J2
0.1
330 k
pot
22 k
330 k
B
CHARGING
CIRCUIT
A
22 k
330 k
-12V
GND
GND
-12V
+12V
LM
7
4
1
22 k
LM
3
2
4
3M 
33k
33k
33k
33k
Ref
pot
22 k
GND
-12V
+12V
CHAPTER 6
CIRCUIT AND PRACTICAL IMPLEMENTATION
1KΩ
OUT J9
1K
1 .2 K 
J8
1.8K
B
1 ,1W
1 ,1W
J3
J2
-IN
470 
470 
A
+IN
Green
LED
Red
LED
BC548
IN4001
10 
470 
0 .1 F
0 .1 F
1
J1
CTC880
33 
1 ,1W
470 
J7
470 
OUT+
2
Vo
A
3 V1
LM317TB(with
heat sink)
Fig. 17 – Charging Circuit
DESCRIPTION OF POWER SUPPLY
The power supply circuit constitutes the battery, capacitors and voltage regulator ICs
required for the voltage regulation. Three 9V batteries are connected in series to give positive and
negative terminals for ICs. By using two capacitors of 1000𝜇𝐹 connected in series with a center
tapped ground terminal the supply voltage from the three batteries, which is 27V is divided into two
13.5V. There terminal voltage across the positive terminal of the supply along the upper 1000𝜇𝐹 to
the center tapped ground terminal is 13.5V. And the between the negative terminal of the supply
along the lower 1000𝜇𝐹 capacitor to the center tapped ground terminal produces a voltage of -13.5V.
These two supply voltages are the base voltages.
32
Now with these available supply voltages other required supply voltages are derived with
use of different ICs. Starting with the IC 7812, which is basically a linear voltage regulator. The supply
voltage of +13.5V is connected to the PIN1 and the PIN2 is grounded. Output from this IC is from
PIN3, it produces a +12V. This +12V is connected to the PIN1 of IC 7805 with its PIN2 grounded
through the center tapped ground terminal used in the capacitor arrangement. Output of this IC is
taken from the PIN3 which produces +5V. This +5V supply is mainly used to supply the flip-flop.
Now in order to derive a -12V supply from the base supply voltage IC 7912 is used. The
negative supply terminal voltage i.e. -13.5V is fed to the PIN1 of the IC 7912. And the PIN2 is
grounded. The output of this IC 7912 is taken from the PIN3. It produces -12V supply voltage which
is mainly used for supplying ICs like LM 741 and LM 324.
POWER CIRCUIT
The power circuit consists of the photovoltaic module, the buck – converter, the filter unit
and the load.
PV MODULE
It is basically a packaged interconnected assembly of photovoltaic cells. The photovoltaic
module consists of the 36 solar cells arranged in series-parallel pattern to produce16.5V. These
solar cells are manufactured using mono crystalline silicon materials. The use of silicon in
manufacturing the solar cells achieves its highest efficient usage mainly by incorporating the
mono-crystalline cells for photovoltaic modules. In manufacturing these photovoltaic modules the
encapsulation of the cells are done by using the UV stabilized polymer (EVA) and the protective
back cover using Tedler-Polyester-Tedler [12]. The usage of the high transmission toughened
glass superstrate and anodized aluminum frame for mounting the panel are mainly targeted for
increasing the service life of these photovoltaic modules. The maximum power rating of this
photovoltaic module is 24W. And the current rating is 1.82A. Under nominal operating cell
temperature the rated irradiance level is 800W per sq.m. With this whole arrangement within the
internal structure is as described above and this photovoltaic module weighs 4.2kg.
33
MOSFET
The buck-converter consists of a power electronic switch, an inductor, a diode and a
capacitor. The power electronic switch chosen is the MOSFET IRF9530NS. It provides the highest
power capability with lowest possible on-state resistance, so it is extremely efficient and reliable.
With the low internal connection resistance it can dissipate up to 2W. The MOSFET is used with a
heat sink.
INDUCTOR
The inductor used has inductance value of 13mH. The inductance value chosen also reflects
the static power losses that can occur in this buck-converter circuit. The static power losses
constitute the 𝐼2 𝑅 (conduction) losses in the wires or PCB traces as well as the inductor. The
efficiency depends on the conduction losses and the switching losses. But the conduction losses
depend on the load and the inductor winding resistance. So the overall efficiency dependent factors
include the chosen inductance value. It has a maximum operating frequency of 1𝑀𝐻𝑧.
DIODE
The diode used is IN4007. These diodes can operate under low forward voltage condition. It
has a high current capability with low leakage current and high surge capability. Its operating voltage
range is 50 to 100V with an operating current of 1A. The main use of the diode in the buck-converter
circuit is to provide a current carrying path during switch OFF mode.
CAPACITOR
The capacitor used has a value of 1000𝜇𝐹. These capacitors used are electrolytic capacitors.
With this chosen value of capacitance (i.e. large enough) the terminals are maintained at constant
voltage during the commutation cycle. It turn the average value of the current flowing is zero. This
output capacitor has enough capacitance to supply power to the load (a simple resistance) without
any noticeable variation in its voltage.
The operation of the buck converter is already explained in the Chapter 5.
34
CONTROL CIRCUIT
The control scheme used is the voltage mode control [13]. Initially a reference voltage signal
is generated to create reference voltage which is necessary for the control technique adopted. A
voltage divider arrangement is used for generating the reference voltage signal. The potential divider
creates the reference voltage which is connected in between +12V and ground. A 22K pot is used for
this purpose.
This reference voltage signal generated is compared with the output voltage from the buckconverter. For accomplishing this task IC LM324 is used. It is a single supply quad Op-Amp IC [14].
This quad amplifier can operate at supply voltages ranging from 3V to32V.
OP-AMPS
As shown in the figure 18 the reference voltage signal is connected to the PIN-12 of LM 324
with a 33K resistance in series. PIN-12 is the non-inverting terminal. The output voltage from the
buck-converter is connected to the PIN-13, which is the inverting terminal. A 33K resistance is
connected in series with the output voltage fed to the LM324. The output for this Op-Amp is taken
from the PIN-14. This output voltage is the required error voltage signal, which is connected in series
with a 33K resistance to provide a negative-feedback to this Op-Amp. This Op-Amp acts as a
differential amplifier.
33K
Voutput
33K
PIN-13
Ref from
22k ohms
POT
PIN-12
33K
+
PIN-8
Fig. 18 – Op-Amp used as a differential Amplifier
𝑉𝑜𝑢𝑡 = 𝐴𝑑 𝑉𝑖𝑛+ − 𝑉𝑖𝑛− + 𝐴𝐶 (𝑉𝑖𝑛+ + 𝑉𝑖𝑛− )
35
(6.1)
Refer to figure 19. The error voltage signal generated is fed to PIN-9, which the inverting
terminal of another Op-Amp in the same chip of IC LM324. A 33KΩ resistance is connected in series
with this input error voltage signal. And the PIN-10 is connected to ground as shown in the Figure.
The output for this Op-Amp is taken from the PIN-8, which is connected to a 3MΩ resistor as a
negative-feedback to the inverting terminal of this Op-Amp. This Op-Amp acts as an inverting
amplifier.
330K
PIN-8
330K
PIN-2
PIN-6 of
LM741
330K PIN-3
+
TO PIN-2 OF
IC LM311
PIN-1
Fig. 19 – LM 324 used as an Adder
The output signal obtained from the PIN-8 is connected to the PIN-2 through a 330KΩ series
resistance. PIN-2 is the inverting terminal of another OP-Amp of LM324 (fig. 19). PIN-3, which is the
non-inverting terminal of this Op-Amp, is connected to ground terminal. The output of this Op-Amp is
taken from the PIN-1, which is connected to a 330KΩ resistance to act as a negative-feedback to this
Op-Amp. This Op-Amp can act as an adder.
PIN-2 of LM741 (which is the inverting terminal ) is connected to a 22KΩ resistor is which
connected to the +12V through a series connected POT of 22KΩ .Simultaneously this point i.e. the
terminal from 22KΩ pot connected to the +12V supply. The PIN-3 is the non-inverting terminal of
this Op-Amp, this is connected to the ground terminal. The PIN-4 is connected to negative supply of 12V. Refer figure 20.
36
22K
22K
+12V through
22kilo ohms
POT
PIN-2
PIN-3
+
TO PIN-2 OF
IC LM324
PIN-6
Fig. 20 – LM 741 as an inverting amplifier
The output of this Op-Amp is taken from PIN-6 as shown in the figure; this terminal is
connected to the PIN-2 through a 22KΩ resistance in series which acts a negative-feedback to this
OP-Amp. This output at PIN-6 is connected to the PIN-2 of the IC LM324. This IC provides a positive
or a negative offset. This offset can be added or subtracted respectively by connecting it to PIN-2 of IC
LM324.
RAMP GENERATOR 8038
IC 8038 is the ramp generator. This waveform generator is a monolithic integrated circuit
capable of producing high accuracy triangular, saw-tooth and pulse waveforms with minimum of
external components [15]. In this PIN-4 and PIN-5 are connected to the +12V supply voltage through
220KΩ and 50KΩ series resistor respectively. These are resistors connected at the terminals PIN-4
and PIN-5 are the resistances which decide the rise time and fall time of the ramp signal. The square
wave generated by 8038 is tapped at PIN-9 and fed to the clock PIN-3 of IC 7474 (D-Flip Flop).
LM311
LM311 ICs can operate over a wider range of supply voltages i.e. from the standard ±15V opamp supplies to single 5V supply for IC logic [16]. In these ICs both the inputs and the outputs can be
isolated from the system ground and the output can drive loads referred to ground, positive supply
or the negative supply.
37
The output ramp signal generated at PIN-3 of IC 8038 is connected to PIN-3 of IC LM311.
PIN-3 of LM311 is the inverting terminal. The output error voltage obtained from the PIN-1 of IC
LM324 is connected to the PIN-2 of IC LM311. This voltage comparator compares the error voltage
signal and the generated ramp signal to produce the firing signals. The output for this IC is taken at
the PIN-7.
GATE PULSE TO DRIVER
The output signal obtained from PIN-7 of IC LM311 is connected to PIN-2 and PIN-1 of IC
7474. PIN-7 is connected to the ground terminal. The square wave pulse generated at PIN-9 of IC
8038 is connected to the PIN-3, which is the clock input to this D –flip flop. The main use of this IC is
to provide the same output previously present it till the next clock pulse is provided to the PIN-3.
This is used to supply the driver chip of the MOSFET IRF9530NS. The output of this IC is taken at
PIN-5.
DRIVER CIRCUIT
The output signal obtained from PIN-5 of IC 7474 is connected to the PIN-3 (which is the
inverting terminal) of IC LM311 (which acts as the driver chip of MOSFET IRF9530NS). PIN-1 is
connected to the negative supply terminal of the Photovoltaic module through a 2KΩ series
resistance. PIN-4 is connected to the ground terminal. +12V supply voltage is connected to PIN-2
(which is the non-inverting terminal) through a 22KΩ POT. PIN-8 is connected to the +12V supply
voltage terminal. The output for this IC is taken at PIN-7 i.e. the gate pulses are generated at this PIN.
This output signal is connected to the gate of MOSFET IRF9530NS.
CHARGING CIRCUIT
The charging circuit is connected across the output load terminal of the buck-converter
circuit. The green LED indicates the supply voltage. A 470Ω resistance is connected in series to
protect the LED. Another set consisting of a red LED and a 470Ω resistor connected in series with a
transistor BC548 are connected in parallel to the previous arrangement as shown in the figure 17.
38
The positive supply terminal is directly connected to the PIN-3 of IC LM317TB.This IC is used
as a voltage regulator. This IC can supply more than 1.5A of load current with an adjustable output
voltage range of 1.2 to 37V. The variable resistor must be set to give the requires output voltage
(approx. 4.5V for charging one cell). PIN-1 of the IC LM317TB is connected the power transistor
CTC880. And the load to be charged is connected across the terminals marked as OUT+ and OUT- as
shown in the figure.
There are some jumpers available in this charging circuit to facilitate better usage of this
circuit. Jumpers J7, J8 and J9 are connected to charge one, two and three cells respectively. Jumpers
J1, J2 and J3 decide the charging rate of the batteries. But among all these options generally the
jumper j1 is preferred. This type of charging is called the trickle charging. This takes a long duration
for charging, but this type of charging is good for the service life of batteries.
When the supply is ON and the battery is charging, transistor CTC880 switches ON the red
LED. This will indicate the charging mode of the circuit. So during the charging time both the green
and the red LEDs will glow. The red light is off when the battery is fully charged.
39
CHAPTER 7
RESULTS AND DISCUSSION
SIMULATION RESULTS
The figure below shows the complete block diagram of the circuit. This includes the PV
module block, the power circuit and the control circuit. The modeling and the simulation of the whole
system has been done in MATLAB – SIMULINK environment.
Fig. 21 - Complete simulation model of the photovoltaic energy conversion system
40
PV SIMULINK MODEL BLOCK
Fig-22 shows the complete PV cell Simulink model block. The various input parameters
include photocurrent, reference cell operating temperature, cell output current and solar insolation
level. Scope1 will show the response of the cell output voltage.
Fig. 22 - Complete PV Cell Simulink model block
INTERNAL COMPONENTS OF PV SIMULINK MODEL
Fig-23 shows the block diagram of the photovoltaic sub-module used to measure the cell
output current and voltage. The radiation and the temperature effects are taken into consideration,
which is done using a sub-module shown in the below figure. The input parameters chosen to solve
the equation-1 (see eq. 3.1) are cell output current, photocurrent at changed condition and the cell
operating temperature as shown in fig-23.
41
Fig. 23 - Block diagram of the PV sub-module that gives out cell current and cell voltage
Figure 24 shows the block diagram of the photovoltaic sub-module used in determining the
correction factors for change in cell current and cell voltage. The input parameters chosen includes
the temperature coefficients for cell output voltage and cell photocurrent and the solar insolation
level coefficients for cell output voltage and cell current.
Fig. 24 - Block diagram of PV sub-module that determines correction factors for current
42
Figure 25 shows the block diagram of the photovoltaic sub-module used in calculating the
PV cell output voltage. The input parameters chosen are cell output current, cell photocurrent and
the operating temperature of the cell. Some of the input constants are temperature conversion
constant, Boltzmann constant, ideality factor and electron charge.
Fig. 25 - Block diagram of PV sub-module that measures PV cell output voltage
Figure 26 shows the block diagram of internal sub-module blocks that constitutes the PWM
generator. The proportional gain controller is chosen in this case. Some of the input parameters
chosen are reference current and sensed current. Initially these two currents are passed through a
subtracter and then multiplied by a gain K(=1000000).
43
Fig. 26 - Block diagram of the internal sub-modules of PWM generator
OUTPUT WAVEFORMS OF BUCK CONVERTER AND CONTROL CIRCUIT
Fig-27 shows the error voltage signal obtained from the PIN-1 of 𝐼𝐶 𝐿𝑀324. Initially the
error signal is high and gradually reduces to a lower value.
Fig. 27 - Response of the error voltage signal from the comparator
Fig-28 shows the generated ramp voltage signal. This ramp signal is taken from PIN-3 of
𝐼𝐶 8038. The frequency of the generated ramp signal can be calculated using the following
expression.
𝑓=
1
𝑡 1 +𝑡 2
= 𝑅𝐴 𝐶
0.66
44
1
𝑅𝐵
2𝑅 𝐴 −𝑅 𝐵
1+
(7.1)
Fig. 28 - Response of the ramp voltage generated
Fig-29 shows the gate pulse generated by comparing the sensed current and the reference
current. The thicker band indicates the response during the switching condition.
Fig. 29 - Generated gate pulse from the PWM controller
Fig – 30 shows the inductor current waveform. With higher value of inductance the ripple
current ratio reduces and with a lower RMS current in the output capacitor can be observed.
45
Fig. 30 - Current response of the inductor
Fig - 31 shows the voltage across of the inductor. Initially the voltage remains at zero value.
Within a short duration it reaches around 5.2V. Then it gradually decreases under the switched off
condition. The thicker band indicates the fluctuations of the voltage across the inductor under
switching conditions.
Fig. 31 - Response of the voltage across the inductor
Fig-32 shows the sensed output current, which is sensed at the output of the buck converter
circuit. A 0.1Ohm resistor is used for sensing this output current.
46
Fig. 32 - Response of the Sensed output current
Fig-33 shows the reference current sensed from the power circuit. This reference current is
sensed only once in a duration. Therefore, only one single pulse in the beginning is shown in the
above figure.
Fig. 33 - Response of the sensed reference current
Fig-34 shows the voltage across the MOSFET IRF9530NS. Initially the curve starts with an
offset value of 1V. Then with a steep increase it reaches 5.4V. This occurs under the switched off
condition. Under switch ON condition MOSFET acts as a short, so the voltage reaches the zero value
as shown in the figure. With the practical case there is always an on-state voltage drop, which is a
negligible value. The thicker band indicates the fluctuation of the voltage across the MOSFET during
the switching condition.
47
Fig. 34 - Response of voltage across MOSFET IRF9530NS
OUTPUT WAVEFORMS OF PHOTOVOLTAIC ARRAY
Fig-35 shows the output voltage across the supply terminals of the photovoltaic array.
Initially the voltage is low, suddenly the value increases to a value around 6.2V. The gradual dip of
the curve indicates the voltage variations that occur in a practical circuit. The thicker band indicates
the fluctuation of the output voltage under the switching condition. When the current decreases to
zero the output voltage is equal to the open circuit voltage and vice versa.
Fig. 35 - Response of the output voltage from the photovoltaic array
48
Fig-36 shows the output current of the photovoltaic array. Initially the short-circuit current
is measured, which is around 5A as shown. Instantly, the current decreases to zero. With the MPPT
control technique the current increases gradually to 0.9 times the short-circuit current value. When
the current reaches 4.5A it becomes a constant. The thicker band indicates the output current
fluctuation during the switching condition.
Fig. 36 - Response of the output current of the photovoltaic array
Fig-37 shows the trajectory of the operating point when the reference current is measured.
Initially the curve starts with the value equal to the zero current. With the steep increase in the
output current value, at short-circuit current value the output voltage is a small finite voltage value
from the PV array. Then with adopted MPPT control scheme the current gradually increases to the
MPP current.
49
Fig. 37 - Trajectory curve of the operating point in the plot between output current Vs
output voltage
Fig-38 shows the trajectory of the operating point when the reference current is calculated.
Initially the curve starts at the zero power output value. Then the power increases to nearly 10W,
where the initial short-circuit current is measured. With the adopted MPPT control technique the
output power increases gradually to the MPPT power value.
Fig. 38 - Trajectory curve of the operating point in the plot between output power Vs output
voltage
50
HARDWARE OF THE CIRCUIT IMPLEMENTED
Fig. 39 - Photograph of the PCB designed
Fig. 40 - The complete experimental setup
51
EXPERIMENTAL RESULTS / WAVEFORMS FROM CRO
The outputs obtained are direct results from the CRO.
Fig. 41 - Reference voltage (blue) and Output voltage (yellow) in 2.5ns resolution
Fig. 42 - Reference voltage (blue) and Output voltage (yellow) in 250ns resolution
52
Fig. 43 - Inductor voltage
Fig. 44 - Generated Ramp Signal
53
Fig. 45 - Error Signal (blue) and Ramp Signal (yellow)
Fig. 46 - Comparator’s output (blue) and Ramp signal (yellow)
54
Fig. 47 - Comparator output (yellow) and Gate signal (blue)
Fig. 48 - Gate signal with respect to ground
55
Fig. 49 - Gate signal with respect to Source
56
CHAPTER 8
CONCLUSION AND FUTURE WORK
From the observations made above, we conclude that the system developed is capable of
extracting maximum power from the photovoltaic module at the same time providing a regulated DC
supply. The results obtained from experiment are in synchronism with the theoretical results.
The ambient temperature of the system is assumed not to change for a reasonably long time
(about 5 minutes). But practically, his may not be the case. The insolation may change in two to three
minutes. In such cases, we need to derive the reference voltage from the short circuit current of the
PV panel. The value obtained can be latched as the reference voltage and MPP can be obtained
automatically without any manual intervention.
57
REFERENCES
[1]
I.H. Atlas and A.M. Sharaf, “A Photovoltaic Array Simulation for Matlab-Simulink GUI
Environment”, Proce. of IEEE International Conference on Clean Electrical Power, ICCEP 2007,
Capri, Italy.
[2]
Roberto F. Coelho, Filipe Concer, Denizar C. Martins, “A Study of Basic DC-DC Converters
Applied in Maximum Power Point Tracking”, Proceedings of IEEE 2009 Conference, ISBN :
978-1-4244-3370-4, pp. 673-677.
[3]
Resources from Ministry of New and Renewable Energy, Annual Report 2009,
http://mnre.gov.in/
[4]
Abou El-Maaty Metwally Metwally Aly Abd El-Aal, “Modeling and Simulation of a
Photovoltaic Fuel Cell Hybrid System”, Dissertation for Dr.-Ing, University of Kassel,
Germany, 15 April 2005.
[5]
Christian DUMBS, “Development of analysis tools for Photovoltaic hybrid systems”, PhD
Thesis, Ecoledes Mines de Paris 1999.
[6]
J.C. Amphlett, R.M. Baumert, R.F. Mann, B.A. Peppley and P.R. Roberge. “Performance
Modelling of the Ballard Mark IV Solid Polymer Electrolyte Fuel Cell: II Emperical model
Development”, Journal of the Electrochemical Society, Vol 142, No. 1, 1995, pp. 9-15.
[7]
T. Esram, P.L. Chapman, “Comparison of Photovoltaic Array Maximum Power Point Tracking
Techniques”, IEEE Transactions on Energy Conversion, Vol. 22, N 2, pp. 439-449, June 2007.
[8]
Pandey, N. Dasgupta, A.K. Mukherjee, “A Single-Sensor MPPT Solution”, IEEE Transactions on
Power Electronics, Vol 22, N 2, pp-698-700, July, 2007.
[9]
S. Yuvarajan, Dachun Yu, Shanguang Xu, “A novel power converter for photovoltaic
applications”, Journal of Power Sources, Elsevier Science, Vol. 135, pp. 327-331, 2004.
[10]
D.L. King, J.H. Dudley, W.E. Boyson, “PVSIM: A simulation program for photovoltaic cells,
modules and arrays”, Proceedings of the 25th IEEE Photovoltaic Specialists Conference, May
1996.
[11]
Mohan, Uderland and Riobbins, “Power Electronics converters, applications and design”,
Wiley India, 4th edition 2006.
58
[12]
D.L. King, et al., “Field experience with a new performance characterization procedure for
photovoltaic arrays”, Proceedings of the Second World Conference on Photovoltaic Solar
Energy Conversion, July 1998.
[13]
Somnath Maity, Tapas K. Bhattacharya, Soumitro Banerjee, “Experimental Study of Chaos
and Bifurcation in the Buck Converter”, National Conference on Nonlinear Systems &
Dynamics”, IIT Kharagpur, February 24-26, 2005.
Datasheets
[14]
http://pdf1.alldatasheet.com/datasheet-pdf/view/11666/ONSEMI/LM324.html
[15]
http://www.intersil.com/data/fn/fn2864.pdf
[16]
http://www.national.com/mpf/LM/LM311.html
59
APPENDIX
List and cost of components used are given below. All components were acquired from the
open market. The prices given are approximate.
S. No.
Item name
Quantity
Price in Rs.
1
Diode µr160
1
20
2
P MOSFET IFR9530
1
60
3
Capacitor 47µf
2
10
4
Resistor 0.01Ω
2
14
5
IC 8038
1
450
6
IC 7474
1
18
7
IC LM 324
1
30
8
Capacitor 1000pf
2
16
9
Variable resistor 22K
2
6
10
Pot 47K
2
30
11
IC 7805
1
20
12
IC 7812
1
15
13
Resistance total
One set
40
14
Transistor BC548
1
4
15
Transistor D880
2
60
16
Jumper switch
3
18
60
17
Led
15
30
18
IC LM 317
2
40
19
Heat sink
1
7
Total
888
Some components are not included as their price varies a lot with the place of purchase and
its value is very less (e.g. connecting wires etc.). The PV array contributes almost 50% of the total
cost. Again its exact price depends on the manufacturer. PV module used for this project is
manufactured by CEL India. Module model number is PM 648 bearing a serial of 59377.
61
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