“an Adaptive Control Scheme For Flexible Power Point

A Project Report On “AN ADAPTIVE CONTROL SCHEME FOR FLEXIBLE POWER POINT TRACKING IN PHOTOVOLTAIC SYSTEM” Submitted in partial fulfillment of the requirement for the award of the degree of BACHELOR OF TECHNOLOGY In ELECTRICAL AND ELECTRONICS ENGINEERING Submitted By G.MADHAVI LATHA (16JD1A0222) K.RUPASRI (16JD1A0246) G.PAVANI (16JD1A0219) K.RAMYA RL (16JD1A0239) M.N.V.D.BHAVANI (16JD1A0250) Under the guidance of Mr.D.J.K.KISHORE,M.Tech Assistant professor Department of EEE

ELECTRICAL AND ELECTRONICS ENGINEERING ELURU COLLEGE OF ENGINEERING & TECHNOLOGY (Affiliated to Jawaharlal Nehru Technological University, Kakinada)Duggirala (V),Pedavagi (M),ELURU-534004,W.G.Dist,ANDHRA PRADESH. (2016-2020)

ELURU COLLEGE OF ENGINEERING & TECHNOLOGY (Affiliated to JawaharlalNehru Technological University, Kakinada, A.P) Duggirala, Eluru, W.G.Dist. Department of Electrical & Electronic Engineering

CERTIFICATE This is certified that the project entitled, “AN ADAPTIVE CONTROL SCHEME FOR FLEXIBLE POWER POINT TRACKING IN PHOTOVOLTAIC SYSTEM”” is being submitted for the partial fulfillment of the requirements for the award of the Degree in Bachelor of Technology from Jawaharlal Nehru Technological University, Kakinada, bonafide work by G.MADHAVILATHA(16JD1A0222),K.RUPASRI(16JD1A0246),G.PAVANI (16JD1A0219),K.RAMYA.RL(16JD1A0239),M.N.V.D.BHAVANI(16JD1A0250) Under the guidance and it has been found suitable for acceptance according to the requirements of the university. This work is not submitted to any other institution or organization for the award of any degree. Project guide

Head of the Department

Mr. D.J.K.KISHORE, M.Tech

Mrs.G.RADHIKA,M.Tech,MISTE(PhD)

Assistant Professor

Associate Professor

Department of EEE

Department of EEE

External Examiner

ACKNOWLEDGEMENT It gives my immense pleasure to express my gratitude to everyone who helped me in the successful completion of my project titled “AN ADAPTIVE CONTROL SCHEME FOR FLEXIBLE POWER POINT TRACKING IN PHOTOVOLTAIC SYSTEM”. I express my sincere thanks to Project guide Mr.D.J.KKishore, Assistant Professor, Electrical and Electronics Engineering, Eluru College of Engineering And Technology, Duggirala, Eluru. For his extraneous support in providing the requirements and his valuable suggestions and co-operation in making this project a Success. I express my sincere thanks to Mrs. G. RADHIKA, Associate professor and Head of the Department of Electrical and Electronics Engineering. EluruCollege of Engineering & Technology, Duggirala, and Eluru for her extraneous support in providing the requirements and his valuable suggestions and co-operation in making this project a success. I wish to thank our respected Principal Dr. P. BALA KRISHNA PRASAD, Eluru College of Engineering & Technology, Duggirala, and Eluru forproviding such a great opportunity of completion of the project. I wish to express our sincere gratitude to the management for providing facilities in the completion of the project successfully. Finally, last but not least I express our heart full gratitude for our lecturers, library staff, friends and my family members for supporting me and helping me in every aspect of my project. SUBMITTED BY G.MADHAVI LATHA (16JD1A0222) K.RUPASRI (16JD1A0246) G.PAVANI (16JD1A0219) K.RAMYA RL (16JD1A0239) M.N.V.D.BHAVANI (16JD1A0250)

ii i

CONTENTS LIST OF FIGERS

vi

LIST OF GRAPHS

vii

ABBREVATIONS

viii

ABSTRACT

ix

CHAPTER No

CHAPTER NAME

CHAPTER 1

PAGE No

INTRODUCTION

1.1 The need for renewable energy

10

1.2 Brief history of photovoltaic cells

11

1.3 Outline CHAPTER 2

17

MODELLING AND CHARACTERISTICS

2.1 Terminology of PV cell

19

2.2 Photovoltaic technology

23

2.3 Solar cell

23

2.3.1 Working of solar cell

24

2.3.2 PV Module

25

2.3.3 PV Array 2.4 Mathematical modeling of PV array

26 27

2.5 Simulink model

30

2.5.1 I-V and P-V Curves for different

31

Irradiance values for a fixed temperature 2.5.2 I-V and P-V Curves for different temperature values for a fixed irradiance

32

iv CHAPTER 3

MAXIMUM POWER POINT TRACKING 3.1 Need for maximum power point tracking

34

3.2 Implementation of MPPT using DC- Converter

35

3.3 Different MPPT techniques

38

3.3.1 Applications of different MPPT techniques

39

3.3.2 Incremental conductance (INC) method MPPT 39 CHAPTER 4

CHAPTER 5

ADAPTIVE FLEXIBLE POWER POINT TRACKING 4.1 Flexible power point tracking

43

4.2 Perturb & Observe method

46

4.3 P&O Algorithm 47 ADAPTIVE CONSTANT POWER GENERATION ALGORITHM 5.1 Operational model evaluation calculation

49

5.2 Adaptive voltage step calculation algorithm

53

5.2.1 Drawbacks and solution

5.3 Voltage reference calculation algorithm CHAPTER 6

54

57

MATLAB SIMULATION CIRCUITS & RESULTS MPPT

CHAPTER 7

6.1 Simulink

58

6.2 Simulation results

59

6.2.1 Right side of

59

6.2.2 Left side of MPPT

65

CONCLUSION

71

CHAPTER 8

REFERENCE

72 v

LIST OF FIGURES

FIG No

FIG NAME

Fig 2.1

Photo-voltaic effect on solar cell

Fig 2.2

Structure of PV module with 36 cells connected in

PAGE No 24 26

Series Fig 2.3

Structure of PV array

27

Fig 2.4

Electrical equivalent circuit of PV cell

28

Fig 2.5

Simulink model of PV array

30

Fig 4.1.1

Circuit configuration

45

Fig 4.2.1

Perturb and Observe method algorithm

47

Fig 5.1.1

Operational mode evaluation algorithm

50

Fig 5.3.1

Voltage reference calculation algorithm

56

Fig 6.1

Simulink model of MPPT

58

vi

LIST OF GRAPHS GRAPH No Graph 2.11

GRAPH NAME I-V Curves of PV array for different irradiance

PAGE No 31

valves 32 Graph 2.12

P-V Curves of PV array for different irradiance values 32

Graph 2.13

I-V Curves of PV array for different values of temperature

Graph 2.14

P-V Curves of PV array for different values of

32

temperature Graph 3.2

Range of Rin for BUCK converter

37

Graph 3.3

Range of Rin for BOOST converter

37

Graph 3.4

Range of Rin for BUCK-BOOST converter

38

Graph 4.2

I-V Curve of PV array

46

Graph 5.12

The different operational modes of the PV system

52

Voltage step during transients

55

Voltage step calculation at steady-state

57

Graph 5.2.1 Graph 5.32

LIST OF TABLES TABLE No Table 2.1

TABLE NAMEPAGE No Parameters of PV array

31

vii

ABBREVATIONS PV

=

Photo Voltaic

DC

=

Direct Current

AC

=

Alternating Current

CGP

=

Constant power generation

MPP

=

Maximum power point

MPPT

=

Maximum power point tracking

FPPT

=

Flexible power point tracking

GCPVPP

=

Grid connected photo voltaic power plant

viii

ABSTRACT One of the major concerns associated with the increasing penetration of grid connected photovoltaic (PV) power plants is the operational challenges (e.g., overloading and overvoltage), imposed due to the variability of PV power generation. A flexible power point tracking (FPPT), which can limit the PV output power to a specific value, has thus been defined in grid-connection regulations to tackle some of the integration challenging issues. However, the conventional FPPT algorithm based on the perturb and observe method in which we have an algorithm called constant power generation algorithm in that we follow three steps to they are voltage reference calculation, step calculation, operation mode evaluation although they suffers from slow dynamics in between adaptive voltage step calculation rectifies it. In this, an adaptive FPPT algorithm is thus proposed, which features fast dynamics under rapidly changing environmental conditions (e.g., due to passing clouds), while maintaining low power oscillations in steady-state. The proposed algorithm employs an extra measured sampling at each perturbation to observe the change in the operating condition (e.g., solar irradiance). Afterwards, the voltage-step is adaptively calculated following the observed condition (e.g., transient or steady-state) in a way to improve the tracking performance in terms of fast dynamics and high accuracy under various operational conditions.We can find maximum point at which we can track more power and that can be limited to a specific value.

ix

INTRODUCTION CHAPTER 1 1.1 The need for renewable energy Energy plays an important role in the development of a country. Developing countries like India have greater energy intensity and therefore greater energy dependence when compared to the developed countries. The development of renewable energy has been an increasingly critical topic in the 21 st century with the growing problem of global warming and other environmental issues. With greater research, alternative renewable sources such as geothermal, wind, water and solar energy have become increasingly important for electric power generation. Energy has been an important component to meet day to day need of human being. The degree of civilization is measured by the energy utilization for human advancement needs. The world’s fossil fuel supply natural gas, petroleum, coal will be depleted in few hundred years. The rate of energy consumption has been increasing as a result supply has been depleting in turn results in inflation and energy shortage. Since any country will be developed over a period of time, the growth of Gross Domestic Product (GDP) occurs along with an increase in population. With this consequence, demand for housing, transportation, consumer goods and services increases. So growth of GDP is almost parallel to that of energy consumption. Per capita energy consumption of a country decides the standard of living of a country. In developing countries, the energy sector is of critical importance because of ever increasing energy needs and the huge investments to meet them. Thus the interest is being focused to develop and utilize renewable and nonconventional energy sources which have been largely ignored so far due to unhampered and cheap accessibility of oil. As a result, extensive R and D efforts are going on 11

toward the development of these resources and related technologies. Consequently, the knowledge related to development in these technologies needs frequent updating. An advantage of renewable and nonconventional energy sources lies in the fact i.e. •

The demand of energy is increasing due to rapid industrialization and population growth

and

enhance the conventional sources of energy will not be sufficient to meet the growing demand. •

Conventional sources (except hydro) are non-renewable and are bound to finish up one day.

•

Conventional sources (fossil fuel, nuclear) also caused pollution thereby their use degrades

the

environment. •

Large hydro resources influence wild life, cause deforestation and pose various social problems.

1.2 Brief history of photovoltaic cells In 1839, a French physicist, A.E. Becquerel recognized the photoelectric effect for the first time. He noticed the flow of current in an electrode due to the chemical reaction when it was exposed to light. The similar effect was observed by W.G. Adams and R.E. Day in solids after many years. In 1878 they presented a paper on photovoltaic effect. The first solar cell was developed using Selenium on a thin layer of gold by Charles Fritts. In the twentieth century many remarkable works had been made in the field of PV technology and is equipped with some prominent Nobel Prize winning works [1]. In the beginning The German scientist Max Planck engulfed in the job of trying to explain the nature of light emitted by hot bodies. According to his assumptions the energy is made up of several discrete layers. This helped Albert Einstein to articulate his theory of light, which he explained that light was made up of small particles called photons. And they vary in their energies based on their color. The Ultraviolet rays have the photons with more energy which causes sunburns etc.., which are invisible to human eye. The red photons have half the energy of the blue photons. 12

Finally the most important scientific solar cells are Infrared photons which are invisible have further less energy. The Einstein’s proposal leads to the development of Quantum mechanics which led to the Elwin Schrodinger’s wave equation in 1926. Wilson solved the equation for solid materials in 1930. This led to the breakthrough in differentiating the metals and insulators as good and bad conductors of electricity and also the properties exhibited by semiconductors. Semiconductors are almost like insulators needed energy to bring electrons to motion but in a considerable amount. The energy present in the red photons is enough for an archetypical semiconductor to liberate its electrons. Russel Ohl was the first one to discover the silicon solar cell. When he shone a flash of light on a silicon rod he observed large electrical voltage. Later he observed due to the presence of impurities the densely formed electrons is formed which is negatively high as n-type and due to deficiency of electrons there formed the protons led to positively high as p-type. In 1949 William Shockley designed the first practical transistors based on the above theory and worked on different properties shown by p-n junctions. After this great accomplishment the solar cells was developed in 1954 with a conversion efficiency of 6%. The direct conversion of light into electricity and photo-voltaic cells is considered as revolutionary in the process of power generation. Since the Photovoltaic cells were produced in 1950’s, till 1960’s the usage of photovoltaic cells is limited to provide power to geo stationary satellites. Later in 1970’s the advancement in the technology and improvement in performance and quality of PV cells resulted in the decrease in the manufacturing cost and unfolded various opportunities in various terrestrial applications. PV cells played key role in powering telecommunication equipments, signals, and other lower power needs like battery charging etc. In 1980’s PV cells also became famous in consumer electronic devices like calculators, radios, watches etc. Significant efforts also began to develop PV power systems for residential and commercial uses, both for stand-alone, remote power as well as for utility-connected applications.

13

India has launched the Jawaharlal Nehru Solar Mission (JNNSM) in 200910 with the grand target of installing 20 GW of solar power, solar photovoltaic (PV) as well as solar thermal, in the country by the year 2022 which was later increased to 100 GW in 2015 Union budget of India. The JNNSM promotes solar PV system installations by providing incentives both at grid connected PV system and off-grid PV system levels. The total commissioned status of grid connected solar projects capacity of India based on the survey of Ministry of New Renewable Energy (MNRE) till 29-05-2015 is 3883.57 MW. In a developing country like India there is immense demand of electricity. There is a huge shortage of electricity. Since Solar Photovoltaic technology converts sunlight into electricity directly it is of great use as there are no other additional conversion procedures. The solar PV modules are fixed in the areas where there is need for electrical energy provided sufficient amount of sunlight is incident on it. The photovoltaic cell converts sun light into electricity directly without any other additional energy conversion step. It consists of a very thick n-type crystal covered by a thin n-type layer. When it is exposed to sun light it produces a voltage of 0.5-0.7 volt and a current of 30mA/cm2. The output of the solar cell is DC and for consumer applications it must be converted to AC by means of inverters. The series and parallel connected solar cells can be called as solar module. Depending up on the consumer requirements modules are connected in series and parallel and it can be called as an array. The sources which are used traditionally are voltage sources, the I-V characteristic of voltage source is constant with respect to current axis and power delivered is based on the load demand. But PV cell is a current source with an antiparallel diode whose magnitude depends up on the solar isolation. Because of this nature I-V characteristic of the PV cell is nonlinear and at one particular point of I-V curve only it gives maximum power output. Prof. S. Banerjee Dept. of electrical engineering IIT khargpur in his great lecture has explained the basic concepts of photo voltaic systems, its working principle, exact and approximate equivalent circuits and corresponding mathematical relations. 14

Kun Ding, XinGaoBian, HaiHao Liu, and Tao Peng analysed that the PV characteristics are nonlinear and very much dependent on weather and can be noticed that PV output voltage greatly governed by temperature while PV output current has approximate linear relationship with solar irradiances [2]. The cost of the PV cell is high and the conversion efficiency (i.e. light energy to electrical energy) of the cell is very less in the order of 6-16% based up on the technology used in the manufacturing process. So PV systems are utilized always at their maximum power output conditions. Manually it is so difficult to operate the panel at maximum power output condition because the power output of the solar module depends up on the atmospheric conditions and the load resistance. With the help of maximum power tracking algorithm and power electronic converters it is possible to operate the PV panels at maximum power output conditions. Earlier these are implemented in the stand alone systems. K.H. Hussein, I Muta T. Hoshino and M. Osakada explained the detailed principle of perturb and observe (P&O) and incremental conductance method (INC) MPPT techniques for standalone system under rapidly changing atmospheric conditionsand also compared the efficiencies of these MPPT techniques. Based on his experimental work he concluded that INC method is more efficient than the P&O method [3]. TrishanEsram, and Patrick L. Chapman compared about 20 MPPT techniques based up on the different aspects i.e. is PV array dependent, is true MPPT, about periodic tuning, convergence speed, Implementation complexity and sensed parameters. Also gives the applications for different MPPT techniques [4]. The standalone system along with maximum power point tracking (MPPT) needs battery storage. Under lightly loaded conditions the battery is charged. Otherwise MPPT is not preferable for standalone systems. Even though MPPT is applied for lightly loaded condition without battery charging, the load may fail due to over voltage. Battery is an additional cost. It requires maintenance and the conversion efficiency of around 50% only. The grid connected PV systems along with MPPT gave the solution for the above problem. It does not require battery storage. Traditionally with the two stages of power conversion is needed 15

to feed the PV power to grid. First stage is DC-DC conversion stage where MPPT is implemented and the second one is DC-AC inversion stage. The limitations of the two stage converted grid connected system is high cost, less reliable, more losses and more complexity of the circuit because of the two stages of power conversion. To overcome the limitations in the two stage grid connected systems, single stage grid connected PV systems are developed. In single stage grid connected systems there is no DC-DC conversion stage. Here MPPT is applied directly at the DC-AC inversion stage with some modifications in the traditional MPPT algorithm. Tsai-Fu Wu, Chih-Hao Chang, Li-Chiun Lin, and Chia-Ling Kuo compared the different power loss factors of single stage and two stage grid connected PV systems. He concluded that from a view point of efficiency, cost, and system size, a single-stage grid connected PV system is feasible in dcdistribution and grid-connected applications if the operating voltage range is properly selected [5]. The power output of the PV panel is DC. With the use of inverter along with DC link capacitor DC power is converted to AC and is feed to grid or load. In grid connected system inverter switches are necessarily controlled such that the maximum power available at the PV panel delivered to grid or load. The control of the switches depends up on the reference signal given by MPPT algorithm. Here the inverter is capable to deliver reactive power also. But it will not deliver the reactive power even though the load demands. Since the source impedance of the grid is very less nearly to zero, and the source impedance of PV system is finite. So that the reactive power drawn from the grid only. The gating signals are given in such a way that the inverter delivers reactive power along with active power. During the night time also the inverter along with DC-link capacitor are capable to deliver reactive power. Several control techniques were proposed in the literature for active and reactive power control like synchronous frame control, unit vector control, Instantaneous reactive power control theory (IRP theory or PQ theory) and so on.

16

Wu Libo, Zhao Zhengming, and Liu Jianzheng introduced modified MPPT technique for a single stage grid connected photovoltaic systems and also explained the reactive power compensation [6]. Georgios A. Tsengenes and Georgios A. Adamidis investigated the behaviour of a three phase gridconnected photovoltaic system to control active and reactive power for a linear balanced load [7]. Georgios A. Tsengenes, Georgios A. Adamidis describes the active and reactive power control of distributed generated systems by using different control techniques like space vector pulse width modulation and P-Q theory [8]. After introducing power electronics in the late 1960s, nonlinear loads that consume non-sinusoidal current have increased significantly. In some cases, they represent a very high percentage of the total loads. Today, it is common to find a house without linear loads such as conventional incandescent lamps. In most cases, these lamps have been replaced by electronically controlled fluorescent lamps. In industrial applications, an induction motor that can be considered as a linear load in a steady state is now set with a rectifier and inverter for the purpose of attaining adjustable-speed control. The induction motor together with its drive is no more a linear load. The proliferation of non-linear loads has impelled interest in research on new power theories, thus leading to the instantaneous reactive power (IRP)Theory. This theory can be used for the design of power electronics devices, particularly those anticipated for reactive-power compensation. Various issues related to harmonic pollution in power systems have been explored and discussed for a long time. These are listed as follows •

Voltage waveform distortion

•

Overheating of transformers

•

Overheating of capacitors

•

Voltage flicker

• Interference with communication systems Previously, these issues were isolated and few. These days, with the expanded number of nonlinear loads present in the electric grid, they are much more common. On the other hand, the need for highly 17

efficient and reliable systems has forced researchers to find solutions to these problems. In many cases, harmonic pollution cannot be tolerated. IRP theory is first used in the application of shunt active power filter. The main functionalities of shunt active power filter are •

Elimination of real power oscillations

•

Improvement in power factor

•

Elimination of current harmonics

•

Provision of

harmonic damping. Hirofumi Akagi, Yoshihira Kanazawa and Akira Nabae introduced new instantaneous reactive power compensator comprising switching devices is proposed which requires practically no energy storage components . Now in the proposed work besides the applications of shunt active power filter, IPR theory is used for active and reactive power control of a single stage grid connected PV system along with MPPT. 1.3 Outline •

Chapter 2 gives an introduction to the photo voltaic technology, working principle, modelling of the

PV cell including the frequently used terms and simulation results of I-V and P-V curves. •

Chapter 3 gives the need of maximum power point tracking(MPPT), implementation of MPPT by using DC-DC converters, different methods, applications, detailed working principle of Incremental conductance (INC), modified incremental conductance methods and simulation results for INC

MPPT by using buck converter.

18

•

The basic knowledge on adaptive flexible power point tracking ,in that FPPT is briefly explained in this we adapt perturb and observe method in which includes algorithm .The algorithm is present in chapter 4.

•

Chapter 5 describes the main objective of the proposed constant power generation algorithm, in this three calculation algorithms are ge step calculation algorithm, voltage reference calculation algorithm

•

Chapter 6 summarizes the simulation results for different case studies explained in chapter 5.

•

Chapter 7 gives the conclusions of the present work and also scope for the future extension.

CHAPTER 2 PV MODELLING AND CHARACTERISTICS 19

In order to know the behavior of a PV cell, computer simulations are necessary. To achieve this PV cell has to be modeled. Modelling needs the knowledge of the operation of PV cell. For modelling it is necessary to know the terminology behind the PV cell.

2.1 Terminology in PV cell The following parameters determine the effectiveness of sun light to electricity conversion.

Short circuit current (Isc) It is the maximum current a solar cell can produce. The current drawn by the cell when the terminals are connected together is the short circuit. In other way it is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). The value of short circuit current depends on the cell technology, cell area and radiation falling on the cell. Iph = Isc

(2.1)

Where Iph is the photon current generated from the PV cell.

Open circuit voltage (Voc) It is the maximum voltage a solar cell can produce. When light hits a solar cell it develops a voltage analogous to the e.m.f. of a battery in a circuit. The voltage developed when the terminals are isolated (infinite load resistance) is called the open circuit voltage. It depends upon the cell operating temperature and cell technology.

Efficiency (η) The efficiency of a solar cell is defined as the maximum output power (P m) to the input power (P in). The efficiency of a cell is given in terms of percentage (%). Means what percentage of input power is converted to electric power. 20

The cell efficiency is written as (2.2) Where Vm and Im are the voltage and currents corresponding to maximum power point, FF is the fill factor and Pin is the input power. Normally efficiency of a PV module is specified at Standard Test Condition (STC), which is corresponding to input power density of 1000 W/m 2 and 250C cell temperature in PV module. The efficiency of solar cell varies from one technology to other technology and from one manufacturer to other manufacturer. The name of the technology comes from the material used in making solar cells. The efficiencies of common PV technologies are 1. Mono crystalline silicon cells (14-17)% 2. Poly crystalline silicon cells (14-16)% 3. Amorphous silicon cells (6-9)%

Fill factor (FF) Fill factor is the ratio of the areas covered by V mpp-Impp rectangle with the area covered by VocIscrectangle. It indicates the square-ness of I-V curve. Higher the FF indicates, better is the cell. The FF of a cell is given in terms of percentage (%).

FF

(2.4)

21

Where, VmppandImpp are the voltage and current corresponding to maximum power output.

Series resistance (Rs) It is the resistance offered by the metal contacts and the semiconductor body resistance. For an ideal PV cell whose value is equal to zero. The main impact of series resistance is to reduce the fill factor, although excessively high values may also reduce the short-circuit current. Series resistance does not affect the solar cell at open-circuit voltage. Shunt resistance (Rsh) Considerable power losses caused by the presence of a shunt resistance, R SH, are typically due to manufacturing defects, rather than poor PV cell design. Low shunt resistance causes power losses in solar cells by providing an alternate current path for the light-generated current. Such a distraction reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell.

Band gap energy (Eg) A band gap also called an energy gap is an energy range where no electron states can exist. The energy gap between the valence band and conduction band of an atom is called band gap at which the electron has to cross to move freely and conduct.

22

Irradiance or Insolation (G) Irradiance is an instantaneous quantity describing the flux of solar radiation incident on a surface (kW/m2). The density of power radiation from the sun at the outer atmosphere is 1.373 kW/m 2, but only a peak density of 1 kW/m2 is the final incident sunlight on earth’s surface.

Reverse saturation current (IS) It is defined as the part of the reverse current in a diode caused by diffusion of minority carriers from the neutral regions to the depletion region. This current is almost independent of the reverse voltage.

Maximum power point (MPP) It is the operating point at which the power dissipated in the resistive load is maximum. A solar cell may operate over a wide range of voltages and currents. By increasing the resistive load in the cell from zero to infinitely high values one can determine the maximum power point. Pm=Vmpp*Impp

(2.5)

Where Vmpp and Imppare the voltage and current corresponding to maximum power point

23

2.2 Photovoltaic Technology Photovoltaics technology and research is related to the devices which uses semi conductors that directly convert sunlight into electricity .Photovoltaic effect involves the creation of voltage in a material upon exposure to electromagnetic radiation. In 1839 Edmund Becquerel French physicist found that certain materials would bring into being small amounts of electric current when uncovered to light. Albert Einstein characterized the photoelectric effect and nature of light on which photovoltaic technology is established, for which he won a Nobel prize in physics. The first photovoltaic module was made by Bell Laboratories in 1954. It was advertised as a solar battery and just a interest as it was too expensive to gain universal use. In the 1960s, the space industry began to make the first use of the technology to provide power for spacecraft. Through the space programs, the technology advanced, its reliability was fixed, and the cost began to reduce. The solar cell is the building block in the fabrication of Photovoltaic modules. These photovoltaic modules are the main component in the solar power generation systems. A clear understanding of the sciences of the solar cells, the fabrication, the principle of operation and their electrical behavior and the efficiency are essential to understand and work with the technological aspects of the solar photovoltaics.

2.3 Solar cell Solar cell is a semiconducting device made up of silicon. It directly converts sunlight into electricity by photovoltaic effect. Hence it can also be called as a photovoltaic cell. The amount of electricity generated by the solar cell is mainly depends up on the intensity of light, call area and the angle of incidence of the light falling on the cell. The output current of the cell is mainly depends on the cell area. The electricity generated by the solar cell is directly proportional to the intensity of sunlight and the area of solar cell. 24

2.3.1 Working of solar cell When the sun light is falling on the surface of a solar cell, photons are absorbed by the semiconducting material. If the photon energy is bigger than the band gap energy of semi conducting material electron hole pairs are generated. The generated holes and electrons are collected by positive and negative terminals respectively. These terminals are also known as anode and cathode. Electrical potential is generated at the terminals due to separation of charge carriers. The potential difference between the terminals is nothing but

voltage. This voltage is used to drive the current in the external circuit. The most common material used in PV cell manufacturing is mono-crystalline or poly-crystalline silicon. Although other materials and techniques have been developed, silicon is used in more than the 80% of the production. Silicon is popular because it’s abundant availability in the Earth’s crust, in the form of non-toxic silicon dioxide. The third type is amorphous silicon, but it is less used because its efficiency is worse than the previous.

Fig. 2.1 Photo-Voltaic effect in a Solar Cell Other new solar cells are made of Copper indium gallium (di) selenide(CIGS) or cadmium telluride(CdTe)[1]. Much research and development(R&D) effort is being made to develop new materials, but there are no commercial substitutes to the above types of solar cells. Each cell is typically made of square or rectangular wafers of dimensions measuring about 10cm x10cm x0.3mm [2]. In the dark 25

the PV cell's behaviour is similar to that of a diode and the well known Shockley-diode equation can be used to model its behavior i.e. efficiency, which is the percentage of solar radiation that is transformed into electricity. It is measured under Standard Test Conditions(STC), irradiance of 1000 W/m², air mass (A.M) coefficient as 1.5, and a cell junction temperature of 25°C. The higher efficiency, the smaller surface is needed for a given power. This is important because in some applications the space is limited and other costs and parameters of the installation depend on the installed PV surface.

2.3.2 PV Module A single solar cell can produce a voltage of 0.5-0.7 volts and a current of 30mA/cm 2. In order to meet the power specifications these cells need to be connected in either series or in parallel. If the cells are connected in series their voltages get added up but at constant current level and if cell are connected in parallel their currents get added up at a constant voltage level. An environmentally protected assembly of solar cells designed to generate required electric power at particular current and voltage ratings is called as a solar module or panel. Most of the cells are usually connected into a series string of cells typically 36 or 72 as shown in the Fig. 2.2 to achieve the desired output voltage of 12 volts dc at wattage of 100 to 120 watts. The modules are made in a wide range of sizes depending on the applications.

26

Fig. 2.2 Structure of a PV module with 36 cells connected in series

2.3.3 Array For increasing the voltage rating of the system, PV modules are connected in series. A single row of series connected PV cells can be called as PV module string. To increase the current rating of the system, PV module strings or PV modules are connected in parallel. Such a series and parallel combination of modules is referred as PV array. Fig. 2.3, which has 4 parallel connections of 4 module strings connected in series. The voltages for n modules in series are given as 𝑉𝑠𝑒𝑟𝑖𝑒𝑠 = ∑𝑛𝑗=1 𝑉𝑗 = 𝑉1 + 𝑉2 + ………. + 𝑉𝑛 for I > 0

𝑉𝑠𝑒𝑟𝑖𝑒𝑠𝑜𝑐=∑𝑛𝑗=1 𝑉𝑗= 𝑉𝑜𝑐1 + 𝑉𝑜𝑐2 + ………. + 𝑉𝑜𝑐𝑛 for I = 0

(2.6)

(2.7)

The voltage and current for m modules in parallel is given

𝐼𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑙 = ∑𝑛𝑗=1 𝐼𝑗 = 𝐼1 + 𝐼2 + ………. + 𝐼𝑛

(2.8)

𝑉𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑙 = 𝑉1 = 𝑉2 = …….. = 𝑉𝑛 (2.9) For an array to perform well the modules must not be shaded. If shaded they act as a load resulting in heat that may cause damage. Bypass diodes are used to avoid damage although they result in further increase in

27

cost. Integration of bypass diodes in some large modules during manufacturing is not uncommon and reduces the extra wiring required.

Fig. 2.3 Structure of a PV array 2.4 Mathematical modeling of PV array For modeling the PV array first it is necessary to model the PV cell [2]. Group of series connected cells to form a PV module. Depending up on the voltage and power requirements the group of modules are connected in series and parallel and it can be called as PV array. Ideal electrical equivalent circuit consists of a current source in parallel with anti-parallel diode. But in practical some losses are present in the PV cell. To represent those losses series and shunt resistances (Rs&Rsh) are included in the circuit. The value of these two resistances can be obtained from measurement or by using curve fitting methods based on I-V characteristics of the cell. For a typical, high quality, one square inch silicon cell R s is 0.05 to 0.10 ohms and Rsh is 200 to 300 ohms. Fig. 2.9 represents electrical the equivalent circuit of a practical PV cell.

Fig. 2.4 Electrical equivalent circuit of PV cell ref 28

From the above circuit output current (Icell) of the cell is given by (2.10) Where, Iph is the photon current whose magnitude depends upon solar irradiance (G) and Temperature (T). ID and Ish are the diode and shunt branch currents respectively. Where,

(2.11)

(2.12)

G= solar irradiance (W/m2)

T= operating temperature (oC)

Gref= reference value of irradiance (Gref=1000W/m2) Tref= reference value of temperature (Tref=25oC) Isc,ref= short circuit current of a cell referred to Tref and Gref CIsc= solar cell short circuit temperature coefficient Io= is reverse saturation current of diode (A); Vcell=output voltage of a cell Icell=output current of a cell η=diode ideality factor (1-3) K=is Boltzmann constant,

1.38x10 J / K 29

Rs=series resistance Rsh=shunt resistance Let us assume that if the operating temperature is equal to the reference temperature the term is equal to zero and the remaining equation can be written as I ph G. so the photon current of the cell is directly

proportional to the solar insolation G.

Under short circuited condition the current through the short circuited path is Isc=Iph-ID. and it can be written as IscIphG. so from the relation short circuited current of the solar cell is directly proportional to the photon current, which in turn depends up on the insolation value G.

By

substituting the equations (2.12) and (2.13) in (2.10) and rearranging the terms results

(2.14)

Iph=

(2.15)

open circuit condition Icell=0 and Vcell =Voc. The term Voc/Rsh is negligible because the value of R sh value is high. om the equations open circuit voltage lowery depends up on photon current Iphor the insolation value G. so the open circuit voltage Voc will not change much even the change in insolation. The output current I for connecting Ns number of cells in series and Np number of cells in parallel The current I represents the output current of a PV module is further extended to group of modules which is known as PV array. An illuminated PV cell converts only a small fraction (approximately less than 20%) of irradiance 30

into electrical energy. The balance is converted into heat resulting heating of the cell. As a result, the cell can be expected to operate above the ambient temperature. Keeping insolation level as constant if the temperature is increased, there is a marginal increase in the cell current but a marked reduction in the cell voltage. An increase in temperature causes reduction in the band gap. This in turn causes some increase in photo- generation rate and thus a marginal increase in current. However the reverse saturation current increases rapidly with temperature. Due to this cell, the cell voltage decreases by approximately 2.2mV/0C rise in its operating temperature depending on the resistivity of the silicon used. Higher the silicon resistivity more marked is the temperature effect. Also the fill factor decreases slightly with the temperature.

2.5 Simulink model

Fig. 2.5 Simulink model of PV array The simulink model of PV array for getting I-V and P-V curves for different0 values of irradiance and different values of temperatures is shown in Description

Parameter

No. of series connected modules per string

22

No. of strings connected in parallel

2

Number of cells per module

36

Open circuit voltage of a complete array at STC

868V

31

Short circuit current of a complete array at STC

6.9A

Maximum power output at STC

3830W

Table 2.1 Parameters of a PV array Note: STC means Standard Test Condition (G=1000W/m2 and T=25oC)

2.5.1 I-V and P-V curves for different irradiance values for a fixed temperature Fig.2.11 & Fig.2.12 represents the I-V and P-V curves of PV array for different values of irradiance values from the curves it is evident that with increase in solar irradiance there is a significant increment in the short circuit current and the small decrement in the open circuit voltage, as a result maximum power is increased.

Fig. 2.11 I-V curves of a PV array for different irradiance values

32

Fig. 2.12 P-V curves of a PV array for different irradiance values

2.5.2 I-V and P-V curves for different temperature values for a fixed irradiance

Fig. 2.13 I-V curves of a PV array for different values of temperature

Fig. 2.14 P-V curves of a PV array for different values of temperature From the figures, 2.13 & 2.14, it is evident that with increase in temperature significant decrement in voltage and the minute increment in current, so the power output is decreased. In this chapter operation of the PV cell, modeling and characteristics for different load resistance values and different atmospheric conditions are presented. Different types of PV systems and its advantages and disadvantages are discussed. Simulink results for I-V and P-V curves are presented for different insolation values and for different temperature values.

33

CHAPTER 3 MAXIMUM POWER POINT TRACKING In the last chapter I-V and P-V curves are plotted for different insolation values and different temperature values. The observation made from the P-V curves is at one particular load resistance only the PV panel gives maximum power output and whose value is different for different insolation and temperature values. Our intension is to operate the panel always at maximum power output condition. With the help of maximum power point tracking techniques maximum power will track from the PV panel. This chapter deals the concepts of maximum power point tracking of the PV systems. 34

3.1 Need for Maximum Power Point Tracking (MPPT) Solar energy system uses a photovoltaic (PV) solar panel or PV cell to convert the incident sunlight to electrical energy. The cost of the PV array is high and the conversion efficiency of light to electricity is very less. so it is necessary to extract as much energy as possible from such a system to make a PV module useful. A PV module is used efficiently only when it is made to operate at its optimal operating point. The amount of power that can be extracted from the array is depended by the operating voltage of that array. At any moment the operating point of a PV array depends on v varying insolation levels, the load of the system and temperature. The atmospheric conditions and load variables are changing constantly making it very difficult to extract all of the convert energy available from panels without a controlled system. With the use of maximum power point tracking algorithms along with power electronic converters maximum power is extracted from the array.The concept of MPPT is simple; to automatically vary PV array’s load conditions so that it can produce maximum output power. PV cell has non linear current–voltage characteristics as shown in the Fig. 3.1. The power delivered by an array increases as the current drawn increases. Any additional current drawn from the array will result in the rapid drop off the cell voltages thereby reducing the array output power. MPP is the kneeoftheI-VorP-Vcurve, thus the aim of MPPT’s subsystem is to find where this point is and to regulate current accordingly.

3.2 Implementation of MPPT using DC-DC converter A PV panel is operated at its MPP normally with the help of a DC/DC converter. This converter transfers maximum power from the PV panel to the load. According to Maximum

35

Power Transfer theorem, the power output of a circuit is maximum when the Thevenin impedance of the circuit (source impedance) matches with the load impedance. Here the panel resistance is fixed for a fixed insolation and temperature, if change in insolation or temperature results the resistance of the panel changes. For matching the load resistance to panel resistance dc to dc converter is placed in between PV panel and load. The main control objective of the MPPT controller is to adjust the duty ratio of the converter such that the resistance seen from the input terminals of the converter is equal the PV panel impedance. Here converter along with the actual load acts as a load for the PV panel. If the load resistance is less than the PV panel resistance buck converter is preferable because the input resistance of the converter is given by R in=Ro/D2, which is always more than the load resistance Ro. Where Rinis the load resistance seen from the input terminals, R o is the load resistance and the D is the duty ratio of the converter whose value is in between 0 to 1 which is decided by MPPT algorithm. Boost converter is preferred when the load resistance is greater than the PV panel resistance. The load resistance of the converter seen from the input terminals of the boost converter is given by Rin=Ro(1-D2), for any value of D between 0 to 1 which is less than the load resistance Ro. In general load resistance is not in our hand it may be lesser than or greater than the panel resistance. For this type of loads buck-boost converter is used. The load resistance of the converter seen from the input terminals of the buck-boost converter is given by R in=Ro((1D2)/D2), whose value is greater than Ro for a values of D between 0 to 0.707 and for above 0.707 it value is less than Ro. The panel resistance is also changes with respect to atmospheric conditions, it may be 36

less than the load resistance or greater than the load resistance. Changes in the load resistance or panel resistance, buck-boost converter is more suitable for matching the load resistance to a panel resistance. The range of Rinvalues for different converters is given in the following Figures. This also implies the range of load that the PV panel can deliver maximum power.

Fig. 3.2 Range of Rin for buck converter

Fig. 3.3 Range of Rin for boost converter 37

Fig. 3.4 R ange of Rin for buck-boost converter

3.3 Different MPPT techniques There are different techniques used to track the maximum power point. Few of the most popular techniques are 1)

Perturb and Observe method (hill climbing method) (P&O)

2)

Incremental Conductance method (INC)

3)

Fractional short circuit current (FSCC)

4)

Fractional open circuit voltage (FOCV)

5)

Neural networks

6)

Fuzzy logic control

7)

Ripple correlation control

8)

Current sweep control

9) DC link drop droop control 10) Load current or voltage maximization 38

11) dP/dV or dP/dIfeed back control. 12) One cycle control (OCC).

3.3.1 Applications of different MPPT techniques Different MPPT techniques will suit for different applications [4]. For example in space satellites and orbital stations its performance and reliability is the main considerations rather than the costs and complexity of the MPP tracker. The tracker should be able to continuously track the true MPP in minimum amount of time and should not require periodic tuning. In this case, hill climbing/P&O, Incremental Conductance, and RCC are suitable. Solar vehicles would mostly require fast convergence to the MPP. Fuzzy logic control, neural network, and RCC are good choice in this case. The goal when using PV arrays in residential areas is to minimize the payback time and to do so, it is essential to constantly and quickly track the MPP. Therefore, the two-stage Incremental Conductance and the current sweep methods are suitable. Since a residential system might also include an inverter, the OCC MPPT can also be used. For street lighting PV systems are used only for charging up batteries during the day. They do not essentially need fixed constraints; easy and cheap implementation might be more important, making fractional VOCorISCfeasible.

Here incremental conductance method and modified

incremental conductance methods are used along with detailed flowchart.

3.3.2Incremental Conductance (INC) method MPPT The object of maximum power point tracking is to adjust the actual operating voltage of array according to the voltage corresponding to maximum power [3]. The basic idea of INC based MPPT is, at the maximum power operating pointthe derivative of the power with respect to the voltage is equal to zero. From Fig.3.1 note that to the left of the MPP the power is increasing with the voltage, i.e. 39

dP/dV> 0, and it is decreasing to the right of the MPP, i.e. dP/dV< 0. This can be rewritten in the following equations

dP/dV= 0 at the MPP

dP/dV<0 to the right of the MPP

(3.1)

dP/dV>0 to the left of the MPP

(3.2)

(3.3)

These equations can be written in terms of the array current and voltage using dP/dV= d(IV)/dV = I + V dI/dV At MPP

(3.4) (3.5)

left of MPP

(3.6)

At right of MPP

(3.7)

Hence, the PV array terminal voltage can be adjusted relative to the MPP voltage by measuring the incremental and instantaneous array conductance’s (dI/dVandI/V, respectively) and making use of equations (3.5-3.7). Fig. 4.5 represents the complete operation of the Incremental Conductance algorithm. In this algorithm the incremental changes are represented as the difference between present values I(k),V(k) and the corresponding values stored at the end of the preceding cycle, I(k-1) and V(k-1) i.e. dI=I(k)-I(k-1) and dV= V(k) – V(k-1). In the algorithm, mainly the search is carried out by comparing dI/dVagainst- I / V. The array terminal voltage will be shifted towards MPP voltage by adjusting the control reference signal D based on this search. At the MPP, dI/dV= - I/V, no control action is needed, therefore the adjustment stage will be by passed and the algorithm will update the stored parameters at the end of the cycle as usual. Two other checks are included in the algorithm to detect whether a control action is required when the array was operating at the MPP in the preceding cycle (dV=0); in this case the change in the atmospheric conditions is detected using (dI≠0). Now the control signal D, adjustment will depend on whether dIis positive or 40

negative, as shown in the flow chart. When the above Incremental Conductancealgorithm was tested and it is observed that the condition dP/dV= 0 (or dI/dV= -I/V) seldom occurred because of the approximation made in the calculation of dIanddV. However, this condition can be detected by allowing a small marginal error (E) in the above comparisons, i.e. dP/dV= ±E and the value of E depends on the required sensitivity of Maximum power point tracking. Traditionally MPPT technique is applied at the dc-dc converter stage by adjusting the duty ratio of converter in such a way that output voltage of the converter corresponding to maximum power output. But in a single stage grid connected system the tracking is done at the inverter itself by adjusting the angle of inverter voltage with respected to grid voltage based on the value of inverter output power. The output current, voltage, and power f the PV panels in the grid-connected system are defined as V, I, and P, respectively. According to INC method, when the P-V panels operate at the MPP, the 𝑑𝑃/𝑑𝑉 value must be equal to zero. Under MPP conditions, equation is true, the inverter output power PREF should not be changed since the PV panels are operated already at the MPP. If the equation is less than zero means the PV panels are operated in the voltage source region, and the reference output power of the inverter PREF should be increased to move toward the MPP. The PV panels are operated in the current-source region, and PREF should be decreased quickly to avoid a voltage collapse and to move toward the MPP simultaneously. The modified MPPT control objective is similar to the traditional MPPT method. Here instead of adjusting the duty ratio of converter for regulating the output voltage of the array according to the voltage corresponding to maximum power, the output power of the inverter is adjusted. In the tracking process, if the working voltage of PV modules is greater than the MPP 41

voltage, the MPPT controller will increase the power output of the grid-connected inverter to bring it down; If the supply voltage of PV modules is less than the MPP voltage, the MPPT controller will decrease the power output of the grid-connected inverter to drive it up. Fig. 3.6 represents the modified incremental conductance flow chart, where PREF is the active power supplied by inverter and δP is the step change in power.

CHAPTER 4 ADAPTIVE FLEXIBLE POWER POINT TRACKING 42

The control objective of the FPPT algorithm is to regulate the output power of the PV system to be constant at a certain set-point. Conventionally, the P&O-based FPPT algorithm, which intentionally perturbs the PV voltage away from the MPP to reduce the output power.

4.1 Flexible Power Point Tracking In the past, the focus of most research studies in the literature was the maximum power point tracking (MPPT) from PV strings to increase the overall power conversion efficiency and energy utilization . In addition to the conventional MPPT algorithms, like perturb & observe (P&O) and incremental conductance several advanced algorithms like model-predictive , particle swarm optimization method and dual-Kalman filter method are also introduced to extract the maximum power from PV strings. Furthermore, the operation of PV strings under partial shading conditions is considered in .With the introduction of FPPT requirements, several FPPT algorithms have also been introduced for different configurations of GCPVPPs. There are mainly two categories of methods to achieve the FPPT operation: i)Modifying

the dc-dc converter controller in two-stage ordc-ac inverter controller (e.g., Proportional

Integral - PI controller) in single-stage GCPVPPs. The fundamentals of the FPPT are introduced in the focus on stability issues. A voltage reference calculation method is also introduced in, based on the P&O algorithm to calculate the voltage reference related to the required active power. However, moving the ii) operation point to the right-side of the maximum power point (MPP) reduces the robustness of these algorithms, as the operation point may go beyond the open-circuit voltage of the PV panel under fast irradiance reductions. These algorithms apply multi-mode operations to regulate the output power of the PV panels. Clearly, the controller initialization during the operational mode transitions is required and thus slow dynamics are observed.

43

iii)Adjusting

the voltage reference of PV strings per the re-quired power reference according to the

powervoltage (PV) characteristics of the PV panels, Such approaches do not require any modifications in the dc-dc or dc-ac converter controllers. Since the second category of FPPT algorithms do not require any changes in the controllers and can achieve fast dynamics, they are chosen in this study for the generation of constant power from GCPVPPs. These algorithms perform well during constant environmental conditions (e.g., irradiance and temperature). However, the power and voltage characteristic of the PV arrays can vary considerably due to environmental changes. Thus, the previous solutions can encounter issues in the calculation of the voltage reference under rapid irradiance changes. Several studies are available in the literature to enhance the operation of MPPT algorithms during rapid environmental changes. In that case, the performance of FPPT algorithms can be highly affected by environmental condition changes, especially when the operating point is far away from the MPP, because: •

MPPT operating range is narrow around the MPP, while the FPPT operating range covers the entire region of the P-V curve. Therefore, it is more challenging to adapt the control parameters according to the environmental conditions.

•

The impact of environmental changes on the PV power during the FPPT operation could be more pronounced compared to the MPPT operation, because the change of the voltage during FPPT has greater impact on the power compared to the MPPT operation.

44

4.1.1 Circuit Configuration And Overall Control Structure of GCPVPPs

The output power in the photovoltaic modules depends on solar radiation and temperature of the solar cells. 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 therefore, to maximize the efficiency of the renewable energy system, it is necessary to track the maximum power point of the PV array. 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. The proposed scheme consists of a solar panel, a zeta dcdc converter, and MPPT controller.

4.2 Perturb and Observe Method: The most commonly used MPPT algorithm is P&O method. This algorithm uses simple feedback arrangement and little measured parameters. In this approach, the module voltage is periodically given a 45

perturbation and the corresponding output power is compared with that at the previous perturbing cycle . In this algorithm a slight perturbation is introduce to the system. This perturbation causes the power of the solar module various. If the power increases due to the perturbation then the perturbation is continued in the same direction. After the peak power is reached the power at the MPP is zero and next instant decreases and hence after that the perturbation reverses as shown in Figure.

Fig 4.2 I-V curve of PV cell When the stable condition is arrived the algorithm oscillates around the peak power point. In order to maintain the power variation small the perturbation size is remain very small. The technique is advanced in such a style that it sets a reference voltage of the module corresponding to the peak voltage of the module. A PI controller then acts to transfer the operating point of the module to that particular voltage level. It is observed some power loss due to this perturbation also the fails to track the maximum power under fast changing atmospheric conditions. But remain this technique is very popular and simple.

46

4.3 Perturb and Observe Algorithm

Perturb and Observe (P&O) and constant duty cycle techniques are used, because these require less hardware complexity and low-cost implementation .In this method only voltage is sensed, so it is easy to implement. In this method power output of system is checked by varying the supplied voltage. If on increasing the voltage, power is also increases then further ‘∆𝑣’ is increased otherwise start decreasing the ‘δ'. Similarly, while decreasing voltage if power increases the duty cycle is decreased. These steps continue 47

till maximum power point is reached. The corresponding voltage at which MPP is reached is known as reference point (Vref). The entire process P&O algorithm is shown in Fig 4.2.1

CHAPTER 5 ADAPTIVE CONSTANT POWER GENERATION ALGORITHM

48

Under a constant or slow changing solar irradiance condition, the change in the PV power is mainly induced by the perturbation of the CPG algorithm. Thus, the P&O CPG algorithm can accurately regulate the PV power according to the set-point.

5.1 Operation Mode Evaluation Algorithm There are two main operational modes as depicted in Fig. (a). A power threshold dpthis defined to distinguish between the two operation modes as:

in which the error dp is defined as:

dp= ppv(k) − pref, where ppv(k) is the instantaneous PV power at the urrent calculation-step k. In steady-state, the error in is close to zero, while during transients it can be relatively large, due to the change in the solar irradiance condition. The implementation of the comparison in can result in a wrong selection of operation mode in the condition that the PV system operates at the MPP. As illustrated in Fig. this condition can happen under two circumstances: •

The controller is set to extract the maximum power from the PV system, instead of operating at FPPT. In this case, the controller sets the power reference to a value larger than the nominal maximum PV power, as depicted in Fig.

49

•

Due to partial shading or other reasons, the maximum available PV power (pmpp) is smaller than the constant power reference during the FPPT operation. In this case, the operation mode is also similar to

Fig 5.1.1 Operation Mode Evaluation Algorithm The proposed voltage reference calculation algorithm is able to calculate the MPP voltage during the above conditions. In order to achieve similar or smaller power oscillations compared to the conventional MPPT algorithms, it should be ensured that these conditions are classified as steady-state. It is known that the slope of the PV panels P-V curve (dp/dv) at MPP is close to zero. Accordingly, the absolute value of dp/dv is compared to a threshold (Thr) to identify whether the current operation point is close to the MPP. If the operation point is not close to the MPP (|dp/dv|>Thr), the PV system is in transient mode. It should be noted that if the current operation point is close to the MPP, two different conditions can happen: •

The power reference is larger than pmpp, as illustrated in Fig. This operation condition should be classified as steady-state. In this operation mode, dp is positive, as calculated.

•

The power reference can be smaller than pmppat the current calculation time-step. However, due to the step decrease of pref, the operation point is still at the MPP, as demonstrated in Fig. This operation condition results in dp <0 and should be classified as transient to achieve fast dynamics. 50

In order to differentiate the two conditions, the sign of dp is determined in the proposed algorithm, as it is shown in Fig. After the detection of the operation mode, the parameter α is defined as:

The proposed voltage reference calculation algorithm is able to calculate the MPP voltage during the above conditions. In order to achieve similar or smaller power oscillations compared to the conventional MPPT algorithms. When the operation mode evaluation algorithm is implemented, it is ensured that all the operation conditions are classified correctly

.The main advantage of this algorithm is to properly classify the operation at the MPP. It guarantees that the MPPT operation is classified as steady-state, which results in smaller power oscillations compared to the conventional

51

Fig 5.12 Thedifferent operation modes of the PV system in constant power generation: (a) Operation at steady-state, (b) operation at MPP under steadystate, while prefis larger than the maximum available PV power, and (c) operation at MPP under transient, while prefis smaller than pmpp.

52

5.2 Adaptive Voltage-Step Calculation Algorithm The selection of voltage-step (Vstep) is critical in the design of the FPPT algorithm. A large value of Vstepresults in fast dynamics during transients, while it generates large power oscillations in steady-state. On the other hand, with small values, relatively small power oscillations in steady state can be achieved. However, such a choice results in slow dynamics. Thus, an adaptive voltage-step calculation algorithm is introduced in the following to improve both the dynamic and steady-state performances. One objective of the proposed FPPT algorithm is to provide similar MPPT performance compared to conventional MPPT algorithms. In this regard, a fixed voltage-step, which is the optimal voltage-step for the MPPT operation, can be applied in the FPPT algorithm as

Vstep= Vstep-b, in which Vstep-b is the optimal voltage-step for the MPPT operation, which can be designed by following . When the fixed voltage-step Vstep-b is adopted for the FPPT algorithm, the dynamics of the system under rapidly changing environments become slow as aforementioned. Note that the change of the voltage in an FPPT operation vp-ref for a specific constant power reference is larger than that of the voltage changes at MPP vmppunder similar environmental condition variations. This is due to the fact that the MPPT operating range is concentrated around the MPP; where the slope of the P-V curve is close to zero. Accordingly, a larger voltage-step should be applied during transients to improve the dynamics as Vstep= Vstep-tr is the selected voltage-step for transient operations

Vstep-tr the selected voltage-step for transient operations larger than the optimal voltage-step Vstep-b. During transients, α = 0 and Vstep= Vstep-tr, which results in faster dynamics, while in steady-state with α = 1, relatively low power oscillations can be achieved.. 53

5.2.1Drawbacks and its solution •

The FPPT operation in the right-side of the MPP with relatively small power references results in large power oscillations, even considering Vstep-b as the voltage-step, because the slope of the P-V curve (dp/dv) is large. This means smaller voltage-step values should be applied for operation points with larger dp/dv values to maintain low power oscillations.

•

The dynamic transients can lead to large power deviations from the power reference (power errors). Using small voltage-step values increases the response time, as depicted in Fig. (a). On the other hand, by applying large voltagestep values during transients, the operation point may go beyond the steadystate region, in which large power oscillations are observed, as depicted in Fig. (b). In this case, the operation point oscillates beyond the steady-state region. To solve these drawbacks, an adaptive voltage-step calculation algorithm is proposed as Vstep

(10)

in which α is determined by the operation mode evaluation algorithm in the previous subsection, while k1 and k2 are scaling factors. During the transient operation, α = 0, which gives Vstep= k2 ×dp ×Vstep-b. In this method, the value of Vstep depends on the error between the instantaneous power and its reference value. During transients with large errors, the voltage-step becomes large, which reduces the response time. When the PV power becomes closer to its reference value, the voltage step becomes smaller, as illustrated in Fig. (c).

54

Fig 5.2.1 Voltage step calculation during Transients at small voltage –step,constant,constant voltage step and adaptive voltage step . Insteady-state, α=1, whichresults in of |dp|/|dv| is close to zero at the MPP, while it increases to relatively large values inVstep= (1 − k1|dp|/|dv|) × Vstep-b. The P-V curve of the PV panels and the curve of | dp|/|dv| are illustrated in Figs. (a) and (b). The value in which α is determined by the operation mode evaluation algorithm in the previous subsection, while k1 and k2 are scaling factors.

55

During the transient operation, α = 0, which gives Vstep= k2 ×dp ×Vstep-b. In this method, the value of Vstepdepends on the error between the instantaneous power and its reference value. During transients with large errors, the voltage-step becomes large, which reduces the response time. When the PV power becomes closer to its reference value, the voltagestep becomes smaller, as illustrated in Fig. (c). In steady-state,α=1, whichresults in Vstep= (1 − k1|dp|/|dv|) × Vstep-b. The P-V curve of the PV panels and the curve of |dp|/|dv| are illustrated in Figs. (a) and (b). The value of |dp|/|dv| is close to zero at the MPP, while it increases to relatively large values in the right-side of the MPP. The voltage-step values in the proposed algorithm are plotted in Fig. (c). It is seen in Fig. (c) that Vstepis equal to Vstep-b at the MPP, while it is reduced to a minimum value (Vstep-min) in the right-side of the MPP. Additionally, the voltage-step Vstepremains close to a constant value in the left-side of the MPP due to the linear behavior of the P-V curve in this region. Further observations in Fig. (c) confirm that with the proposed algorithm, the voltage-step is adaptively modified according to the operation point of the PV panels. Therefore, the voltage oscillations can remain small in steady-state for all operation point 5.3

Voltage Reference Calculation Algorithm

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5.3.1 Voltage Reference Calculation The voltage reference calculation algorithm for the proposed FPPT operation scheme is illustrated in Fig. 4. If the instantaneous power of the PV system is smaller than the power reference (dp <0), a conventional P&O is applied to move the operation point towards the MPP to increase the power. If the instantaneous power is larger than the power reference, based on the intended operation region (i.e., right- or leftside of the MPP) the voltage reference increases or decreases, respectively. The details of the voltage reference calculation algorithm for FPPT operation can be found.

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Fig 5.32 The proposed voltage-step calculation algorithm in steadystate: (a) The P-V curve of the PV panels, (b) ppvover vpvderivation, and (c) calculated voltage-step

CHAPTER 6 MATLAB SIMULATION CIRCUITS AND RESULTS

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6.2 SIMULATION RESULTS 6.2.1 Right side of MPPT (CASE-1) Power1KWMode1

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Voltage1KWMode1

Power 2KW Mode 1

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Voltage 2KW Mode 1

Power 2KW Mode 2

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Voltage 2KWMode 2

Power 1KWMode 2 62

Voltage 1KW Mode 2

Power1KWMode3 63

Voltage 1KW Mode 3

Power 2KW Mode 3 64

Voltage 2KW Mode 3

6.2.2 Left-side of MPPT(case-2) Power 1KW Mode1 65

Voltage 1KW Mode 1

Power 2KW Mode1

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Voltage 2KW Mode1

Power 1KW Mode 2 67

Voltage 1KW Mode

Power 2KW Mode 2

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Voltage 2KW Mode2

Power 1KW Mode 3 69

Voltage 1KW Mode 3

Power 2KW Mode 3 70

Voltage 1KW Mode3

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7 .CONCLUSION An adaptive flexible power point tracking (FPPT) algorithm for calculating the voltage reference of PV panels, which regulates the output power to a certain power reference, has been introduced. The main target of the proposed algorithm is to tackle the power system challenges (i.e., overvoltage), which may occur due to the increasing growth of the installation of GCPVPPs Also, it has been shown that if the target power reference is larger than the maximum available power of the PV string, the proposed algorithm operates at the maximum power point, with performance comparable to conventional MPPT algorithms. The flexibility of the proposed adaptive FPPT algorithm has been demonstrated by calculating tracking error The results based on the tracking error under t he test conditions can be determined .In this we have two cases, In those cases(FPPT operation with the movement of the operation point to the right-side of MPP,FPPT operation with the movement of operation point to left-side of MPP) the 𝑃𝑟𝑒𝑓 s are taken as 2KW and 1KW with respective to the three methods they are, Fixed voltage step, Conditional voltage step and Adaptive voltage step. We have the results, in case1 𝑃𝑟𝑒𝑓 = 2𝐾𝑊 the error % obtained as 4.7%, 4.2% and 3.2% respectively in the three methods. While in the same case when 𝑃𝑟𝑒𝑓 = 1𝐾𝑊 the error% obtained as 23.4%, 20.7% and 18.2% with respective the three methods. Whereas in case 2 while 𝑃𝑟𝑒𝑓 = 2𝐾𝑊 the error% are obtained as 20.3%, 12.5% and 6.4% with respective the three methods. In the same case while 𝑃𝑟𝑒𝑓 = 1𝐾𝑊 the error% are obtained as 45.8 %, 24.8% and 14.4% with respective to the three methods. The flexibility of the proposed adaptive FPPT algorithm has been demonstrated by calculating tracking error. It is also seen that when power reference is increased then the error will be decreased which results in the constant power generation. The power can also limited to a certain point in all operational conditions (Eg.cloudy situations) The results demonstrated the applicability and effectiveness of the proposed FPPT algorithm as an additional function for existing MPPT algorithms in GCPVPPs.

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8. REFERENCES

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