Nghiên cứu thiết kế sử dụng hệ thống năng lượng mặt trời

5 The maximum power point The MPP During the day time the solar panel capture sun irradiations and convert them into electricity due to the photovoltaic phenomenon.. Figure 1.1: Equival

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

PAULINO VICTORINO RODRIGUES MUEBE

STUDY AND DESIGN OF A SOLAR PV SYSTEM

MASTER OF SCIENCE

HANOI 2016

MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

-

PAULINO VICTORINO RODRIGUES MUEBE

STUDY AND DESIGN OF A SOLAR PV SYSTEM

CONTROL AND AUTOMATION

MASTER THESIS IN SCIENCE

SCIENTIFIC SUPERVISOR:

ASSOC PROF TA CAO MINH, PhD

HANOI 2016

i

Contents

Declaration iii

List of Figures iv

List of Tables vii

List of Acronyms viii

Acknowledgement ix

Dedication x

ABSTRACT xi

1 CHAPTER I - INTRODUCTION 2

1.1 Generalities 2

1.2 Renewable energy 3

1.2.1 The solar panel 4

1.2.2 Typical configuration of a PV system 10

1.2.3 Main applications 11

1.2.4 Configuration of a 150W PV stand-alone system 11

Conclusions of Chapter 1 11

2 CHAPTER II - SYSTEM SIZING 13

Conclusions of Chapter 2 16

3 CHAPTER III - SYSTEM CONTROL 18

3.1 Charge controller 18

3.1.1 Buck converter modelling 19

3.1.2 Charger modelling 21

3.2 Inverter Control 27

3.2.2 Current mode control modelling 35

ii

3.2.3 PWM (Pulse Width Modulation) Technique 40

3.3 DC/AC Converter (Inverter) 40

3.3.1 Three level Carrier PWM (single pole) 41

3.3.2 Working principle of the inverter 43

3.3.3 Output Filter equations 47

3.3.4 Mathematic modelling 47

3.3.5 Power circuit calculation 50

3.3.6 Determination of Modulation index 51

Conclusions of Chapter 3 52

4 CHAPTER IV: SIMULATION RESULTS 54

4.1 Charge controller 54

4.2 DC-DC step up converter 59

4.3 The DC/AC Converter 60

Conclusions of Chapter 4 63

Conclusions and future scope of work 64

Bibliography 65

Apendix 68

iv

List of Figures

Figure 1.1 Equivalent circuit of a solar panel 5

Figure 1.2 Block diagram of a modern PV system 6

Figure 1.3: Buck converter schematic [5] 6

Figure 1.4: MPPT P&O method graph 7

Figure 1.5: P&O algorithm flowchart [6] 8

Figure 1.6: Graph power versus Voltage in INC algorithm [7] 9

Figure 1.7: INC algorithm flowchart 9

Figure 1.8 : Typical configuration of a PV system [8] 10

Figure 1.9: Basic diagram of a stand-alone PV [8] 10

Figure 1.10: Basic diagram of a grid-tied system [8] 10

Figure 2.1: PV system project 13

Figure 3.1: Single solar array and single battery arrangement [10] 18

Figure 3.2: Operation of Buck converter at the MPPT [11] 19

Figure 3.3: Buck converter circuit [11] 20

Figure 3.4: Equivalent buck circuit in ON state [11] 20

Figure 3.5: Equivalent Buck converter in the OFF state [11] 21

Figure 3.6: Control structure of a constant power (CP) mode [10] 22

Figure 3.7: Power flow diagram of a CP mode 22

Figure 3.8: Control structure of a constant voltage (CV) mode [10] 23

Figure 3.9: Power flow diagram of a CV mode 23

Figure 3.10:Single battery dual-mode control structure 24

Figure 3.11: Averaged system model 24

Figure 3.12: MPPT control scheme [13] 26

Figure 3.13: Arduino of solar charge controller [14] 26

Figure 3.14: Inversion scheme 27

Figure 3.15: Push pull converter 28

Figure 3.16: Current wave forms of the push pull circuit 29

Figure 3.17: Proposed inverter overall structure circuit 30

v

Figure 3.18: Schematic of proposed inverter 30

Figure 3.19: Output wave form 31

Figure 3.20: Simplified circuit of block1 converter 31

Figure 3.21: Simplification of Fig 3.21 by transformer elimination 31

Figure 3.22: Switching network circuit 32

Figure 3.23: Simplified network with parasitic parts 32

Figure 3.24: Large signal model of switch network 33

Figure 3.25: Averaged dc and small signal model 33

Figure 3.26: Small signal model of the converter 34

Figure 3.27: Current mode controlled push pull converter 36

Figure 3.28: Control block diagram of the converter 36

Figure 3.29: Proposed voltage and current controller [18] 37

Figure 3.30 Matlab graphs for the inverter circuit 39

Figure 3.31 PWM graphs 40

Figure 3.32: Structure of the full bridge inverter 41

Figure 3.33: Output impedance of single phase inverter 41

Figure 3.34: Details of the three levels modulation applied to the single phase inverter [20] 43

Figure 3.35: Simplification of the single phase inverter 44

Figure 3.36: First stage of operation of the inverter 44

Figure 3.37: Second stage of operation of the inverter 45

Figure 3.38: Third stage of operation of the inverter 45

Figure 3.39: Fourth stage of operation of the inverter 46

Figure 3.40: Vab voltage and switching commands 46

Figure 3.41: Closed loop control diagram 47

Figure 3.42: Closed loop block diagram of the system 48

Figure 3.43: Vab during the positive half cycle 48

Figure 3.44: Sinusoidal PWM 51

Figure 4.1: First Stage of battery charging (MPPT) 55

vi

Figure 4.2: MPPT charge controller graphs 56

Figure 4.3: Charge controller without MPPT 56

Figure 4.4: Connection in parallel of two solar panels in PSIM 57

Figure 4.5: First stage of charging 58

Figure 4.6: Second stage of charging 58

Figure 4.7: Third stage of charging 59

Figure 4.8: DC-DC push pull circuit 60

Figure 4.9: Voltage and current graphs of the push pull converter 60

Figure 4.10: Power circuit 61

Figure 4.11 carrier PWM circuit 61

Figure 4.12: The inverter circuit 62

Figure 4.14 Voltage at A-B terminals of the inverter 63

Figure 4.15: RMS output Inverter current (in blue) and voltage (in red) graphs 63

vii

List of Tables

Table 1: Loads vs watt hours/day consumption 11

Table 2: Total energy demand/day 13

Table 3: Inverter specifications 28

Table 4: Output filter specifications 51

Table 5: Specification for the inverter project 61

viii

List of Acronyms

DC - Direct current

AC - Alternative current

PWM - Pulse Width Modulation

MPP - Maximum power point

MPPT- Maximum power point tracker

Cin/Ci - input capacitor

Cout/Co - output capacitor

Lf - filter inductor

Vin - input voltage

Vout - output voltage

D - duty cycle

Fsw - switching frequency

Fc - cutoff frequency

Voc- open circuit voltage

Isc - short circuit current

At the last to all my family and friends (Mozambicans and internationals) for the moments we had

Paulino Muebe

x

Dedication

I would like to dedicate my thesis to my family, my parents, brothers and sister for the support in this period of studies abroad, for all the time they wanted me near and I couldn’t, due to my researches and work, for their patience and support

The solar PV energy is considered clean, easy to implement and it became one alternative to generate electricity, mainly in countries where the peak sun/hour per day is greater than 3 There are many countries developing these new sources, Germany is the leader with an annual production of around 24.700MW with an average peak sun hours/day of 4.3h, which is quite similar to that of Vietnam and less than that of Mozambique (for example) which is 7h The second is Italy with 12.700MW annual production

The conversion from the sun irradiation into electricity is done by the photovoltaic phenomenon These systems can operate in three modes: Stand alone, grid connected or hybrid (when it operates with other sources of energies)

This study presents the modeling of a stand-alone system, with the intend to check how it can power an AC load, as well as study the storage of energy in batteries for use of the energy when there’s no sun irradiation The simulation was done in PSIM and all the calculations are presented in the report

Were done some variations in irradiance and temperature values to check the behavior of the system; the results are presented in graphs for easy understanding

Key words: Solar energy, photovoltaic energy, environment, renewable energy,

simulations

1 CHAPTER I INTRODUCTION

2

1 CHAPTER I - INTRODUCTION

1.1 Generalities

“Electricity is a modern necessity of life.” Said Franklin D Roosevelt at a rural

trade fair in 1938 [1] The use of electricity has becoming very common in our lives

due to many factors such as industrialization, agriculture, use of electronic devices, electric cars, etc This fact made that many new sources of energy were developed and highly studied in the world In this new energy types we focus on Renewable energy (photovoltaic, wind, biomass, geothermal power and heat ), each one has its advantages and disadvantages.This Thesis will focus on Photovoltaic energy (PV energy)

One of the most important rights to the humans is the education In order to guarantee this, the governments in many countries opened more schools, prepared qualified teachers, bought new books, encouraged the use of electrical appliances such as computers and smartphones, open virtual laboratories and libraries, etc

In developing countries in Africa, such as Mozambique, there some programs for adult literacy which must be done during the evenings, because they work during the day time

Around 18% of the world population is illiterate; in this number about 64% are women In sub-Saharan Africa, the population of illiterate people is around 38%, where 61% are female Between youth (15-24 years old), the illiterate taxes are 12%

of the world population; in sub-Saharan 23% can’t read not even write, where 59%

are female [2]

At the other hand the electricity access in Africa is deficit, as an example: “The average annual power consumption per capita in Africa is just over 500 kWh compared with 13,500 kWh in the USA This is mainly used in business, industry &

government” [3] This fact makes quite impossible the study during the evenings,

the access to new technologies (laptops, smartphones) the access of information (TV, radio, newspaper)

3

Paradoxically, Africa is one of the best continents for solar irradiation and the suns power exceeds the needs by many magnitudes With the cost reductions in photovoltaic cells, this is no longer a matter of cost/benefit but a matter of education and deployment With all the advantages that can be seen in the PV energy, this thesis aims to show how can be implemented a stand-alone photovoltaic system in a small house to help children study or do their homework during the evening, using new technologies and also to guarantee the information access to the families The thesis brings an overview of how a stand-alone PV system works, and also has a simple project of the design and simulation of a 150W PV stand-alone system The simulations were made in the software PSIM and the results are shown

The thesis is divided in four chapters: Chapter one – is the introduction of the work, where is possible to find an overview of the PV system in general, the motivations for the study and are defined the goals of the work; Chapter two – it’s possible to find the design of the system all calculations are shown; Chapter three –

is the main chapter of the work, here is possible to find the control aspects of each component of the system as well as the modeling of the circuits; Chapter four – shows the results and the discussions, after this we have the conclusions and the references are presented

1.2 Renewable energy

Renewable energies are a range of all kinds of energy obtained from resources which are naturally replenished on a human timescale, such as sunlight, wind, rain,

tides, waves and geothermal heat [4]

Solar Energy is radiant light and heat from the sun harnessed using a range of ever-evolving technologies such as solar heating, photovoltaics, solar thermal energy, etc

The PV energy is obtained from the photovoltaic phenomenon, i.e it uses a photovoltaic cell that converts the sun irradiation into electricity This kind of energy is considered clean (not emissions during operation), easy to implement and there are no fuel costs

4

1.2.1 The solar panel

For best understanding, we start from the solar cell: the photovoltaic cells are devices formed by semi-conductive material, which can transform the light energy, coming from the sun or another light source, into electric energy The most used material for making the cell is silicon (4 electrons) When combined with elements

of five electrons, for example, such as phosphorus, it can cause an excess of one electron that will stand alone, without connection, this electron will be weakly connected in the original atom, and if this atom has a small quantity of sun-light irradiance, the stand-alone electron can move inside the atom This orientated movement we call electrical energy The silicon is the second most abundant element in the world So that is possible to make different cell materials such as: Single-crystal silicon, polycrystalline silicon cells (also known as multicrystal silicon), amorphous silicon cells (also known as thin film silicon)

Only one silicon cell can produce between 3A to 5A and a 0,7V The association

of cells in series or parallel is called module and of modules is called solar panel, an array is the association of solar panels The intent of these associations is to have big values of current and voltage

There are also cells made by the combination GaAs (gallium arsenic), for high efficiency cells used essentially in space technology

Just to increase the voltage or the current of the cells we can connect them in series or parallel Generally in series a module can contain about 30 or 32 cells that

we call self-regulated modules If 36 cells are possible to have more voltage so that are most used in photovoltaic industries, they can have 16.7V in source, but when exposed to high temperatures it can decrease There is another kind of modules, composed by 44 cells, and this module can generate 20,3V output voltage In some cases are used reflectors to increase the signal efficiency, although many panels are not designed to support high temperatures resulting in use of this method Remember that the higher temperature, the less output power

5

The maximum power point (The MPP)

During the day time the solar panel capture sun irradiations and convert them into electricity due to the photovoltaic phenomenon It’s known that the sun shine varies according to the day time, it means, there are periods of time that is possible

to capture more sun irradiations than others This fact will, of course, influence the power provided by the solar module to a certain circuit The temperature is also an important parameter to a solar module; the more heat the less power emitted by the solar module The material used to build the solar cell, the resistance of the load and the shadow-shading, are another factors that can compromise the efficiency of the solar panel as well as the charging of a battery To overcome these problems was developed a strategy of tracking the maximum power point - The MPPT

Figure 1.1: Equivalent circuit of a solar panel

If the impedance of the Load is equal to the impedance of the source (solar panel), then is derived the maximum power to the circuit So we would need a block called impedance matching between the solar panel and the load The control circuit must read the impedance of the Load and match it with the impedance of the source

in order to reach the MPP – Maximum Power Point

6

Figure 1.2: Block diagram of a modern PV system

The block “impedance matching” can be a circuit DC/DC converter It is generally used a buck converter (step-down) but can also be used a Boost converter (step-up), a Buck-Boost converter or any other The most important thing is to know the proposal of the circuit

Figure 1.3: Buck converter schematic [5]

MPPT Algorithms

There are several algorithms to track the point of maximum power We will talk about two roles of categories of MPPT techniques: Indirect (fixed voltage method; Fractional Open circuit voltage method) and Direct (Perturb and Observe method; Incremental Conductance method)

Fixed Voltage method

This method is used adjusting the voltage levels based only in seasonal information The base is to know the MPP in winter (for example) and the MPP in summer Then fix the voltage between these two voltage points already known This will guarantee that the system works in an approximate MPP during the year The

7

method is not exact because of this point and also, does not care with the changes in weather or irradiation during the day

Fractional Open Circuit

(Given in the datasheet of the solar panel)

– Voltage at the maximum power point

– Open circuit voltage

The Vmpp can be easily found by the expression above, the Voc is the Open circuit voltage which is given in the datasheet of the solar panel Once known the

Vmpp can be known the MPP

Under operating mode, can either change the temperature or the irradiance

values, this will change the Voc, thus to measure the new Voc the controller must

suddenly disconnect the load This results in loss of production of PV power as this

measurement of Voc has to be done more frequently So to avoid this problem, can

be used a pilot cell which is a PV cell that totally matches the others PV cells which

constitute the module This will guarantee that the Voc can be measured without

disconnecting the PV module The method is not totally exact because of estimation

of K

Perturb and Observe method (the most popular)

Figure 1.4: MPPT P&O method graph

Figure 1.5: P&O algorithm flowchart [6]

Incremental conductance method (INC/ IC)

The INC method overcomes the disadvantage of the P&O method to track the peak power under fast varying atmosphere The INC can determine when the MPPT

has reached the MPP and stop perturbing the operating point The relationship dI/dV and –I/V derived from the fact that dP/dV is negative when the O.P is to the right of

the MPP and positive when it is to the left of the MPP helps us to calculate which direction the O.P must be perturbed until we meet the MPP The advantage of this

9

algorithm is that it can determine when the O.P has reached the MPP, while the P&O method oscillates around it The INC method can also, tracks rapidly incising and decreasing irradiance conditions with higher accuracy than P&O

Figure 1.6: Graph power versus Voltage in INC algorithm [7]

The graph shows that the slope of the P-V array power curve is zero at The MPP, increasing on the left of the MPP and decreasing on the Right side of the MPP The basic equations of this method are as follows

Decrease the Operating Voltage

Increase the Operating Voltage

Decrease the Operating Voltage

Return

Yes

Yes Yes

Yes

No No

Figure 1.7: INC algorithm flowchart

10

1.2.2 Typical configuration of a PV system

Figure 1.8: Typical configuration of a PV system [8]

Depending on the kind of loads, the system can be simple or sophisticated For water pumping, for example, just an array and the water pump, thus the system can have two classifications: stand-alone systems (off-grid systems)

Figure 1.9: Basic diagram of a stand-alone PV [8]

Either Grid connected systems

Figure 1.10: Basic diagram of a grid-tied system [8]

11

1.2.3 Main applications

It can be used as power supply to communities in remote areas, water pumping,

in agriculture for irrigation, power supply to telecommunication systems in remote areas, public illumination of roads, parks, squares, etc

1.2.4 Configuration of a 150W PV stand-alone system

The total power of the system is 150 W, it will be composed by two solar panel 80W each, a battery bank, charge controller and an inverter The loads will be as shown in the table:

Appliance Quantity Watts Hours ON/day Watt hours/day

Table 1: Loads vs watt hours/day consumption

Note that, for better efficiency the laptop and the cell phones will be charged when the TV is off

Conclusions of Chapter 1

In this chapter were showed the overview of the energies, were discussed the solar panel structure as well as the solar cell atoms components We showed the internal structure of a solar panel and a solar cell Were presented the Maximum power point most used algorithms and were discussed how to implement the Perturb and Observe as well as the Incremental conductance algorithm Then were discussed the typical configuration of a photovoltaic system, showed some applications and finally the configuration of the system that will be studied, in this part we discussed why and where to implement the studied system

12 CHAPTER II SYSTEM SIZING

13

2 CHAPTER II - SYSTEM SIZING

This system will be tested in Hanoi, Vietnam Hanoi is located at Latitude: 21°01′28″ N, Longitude: 105°50′28″ E

“Vietnam is considered a nation with high solar potential, especially in the central and southern parts of the country The average sunshine at 150kcal/m2 in Vietnam is between 2,000 - 5,000 hours, which leads to a theoretical potential of

43.9 billion TOE (To Quoc Tru, 2010; Trinh Quang Dung, 2010)” [9]

(B) adjust factor 0.85

(C) adjust Wattage [A/B]

(D) Hours/day usage

(E) Energy/day [CxD]

Compact

Table 2: Total energy demand/day

b) Total energy required

14

To avoid under sizing is important to begin by dividing the total average energy demand per day by the efficiencies of the system components to obtain the daily energy requirement from the solar array:

Is the minimum peak sun hours/day in the worst month of sun irradiation;

In Hanoi is January where 2.44 is the average The total current needed for the system will be:

Where is the system DC voltage

The total number of solar panel depends on the product of solar panels in series and solar panels in parallel

Sizing of the battery bank

First must determine the amount of rough energy storage required by the multiplication of the total power demand and the number of autonomy days .

Then, for safety, calculate the maximum energy considering the maximum allowable level of discharge (MDOD)

(11)

; With is the DC voltage of the system (14)

Sizing of the controller

(16)

Where I is the current of the voltage regulator and is the safety factor Now

is possible to choose the controller that matches the requirements for the system (must be greater than 12.125 amps)

After this point, it’s possible to determine the number of controllers needed for the system

(17) One controller needed

Sizing the inverter

The inverter needed must be able to handle the at 220V AC

Sizing the system wiring

It’s advised to select the correct size and type of wire to enhance the performance and reliability of the system Thus the wires for the system will be 1.5 mm2

c) Sizing the solar array

The specification of the selected PV panel is:

- Maximum power ratings STC (Pmax): 80W

- Open circuit Voltage (Voc): 21.92 V

16

- Short Circuit Current (Isc): 4.85 A

- Maximum power voltage (Vmp): 17.64V

- Maximum power current (Imp): 4.54A

With this specifications will be needed two solar panels in parallel to match de requirements of the project

Conclusions of Chapter 2

The sizing of a PV system is an important stage of the PV project In this point was defined exactly what is pretended to do, what are the appliances that the system must handle, their power demands, the number of hours usage, how many days of non-sun shine Then it was sized the battery bank, the voltage controller and the inverter The PV array was sized at the last

Some points to consider are the location in the world map where the system will

be implemented as well as the climate conditions during the year

It was seen that to perform this system its necesary to have 3 modules of 80W each, due to our limitations in laboratory we can work with two (worst case of operation) The battery bank can be composed by 2 batteries, a controller of 12 A and an Inverter

17 CHAPTER III SYSTEM CONTROL

18

3 CHAPTER III - SYSTEM CONTROL

3.1 Charge controller

In off-grid installation, the photovoltaic source is usually connected to battery via

a power processing interface (DC-DC converter) When the micro-grid’s power consuming abilities are limited, power limiting operation (in which the amount of harvested photo-energy is lower than that available) may occur The mentioned DC-

DC converter is often referred to as “charger” since its operation is usually independent on whether the storage element is connected to other devices or not The main function of a charge controller in a stand-alone PV system is to protect the battery from overcharge by the panels and over-discharge by the loads Any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial costs), or any characteristics that would allow excessive battery overcharging or over-discharging requires a charge controller and/or low-voltage load disconnect Lack of a controller may result in shortened battery lifetime and decreased load availability Consequently, modern battery chargers must be capable of dual mode operation: maximum power tracking (as long as the storage device is capable of absorbing the whole generated power) and power limiting (once the absorbing capabilities of storage device are lower than

generated power) [10]

The figure 3.1 shows the typical configuration of one solar panel connected to a charge to a battery by the use of a dc-dc converter arrangement

Figure 3.1: Single solar array and single battery arrangement [10]

As discussed in chapter 2, multiple cells can be connected in series to form a SA (solar array) with sufficient rated power As a result, the nominal operating array

19

voltage is usually higher than the portable electronics’ equipment battery voltage, necessiting the use of Buck converters as power electronics interfaces Moreover, since the current control (either peak or average) has become a very popular method due to the desirable features it provides, such a high attenuation of input voltage disturbances, nearly first-order control dynamics of the voltage loop and inherent current limiting, most of the modern PWM control IC’s include current-mode-

controller circuitry [10]

Figure 3.2: Operation of Buck converter at the MPPT [11]

The Fig 3.2 shows a schematic of a solar panel connected to the Load by a Buck converter interface The converter works at the MPPT, the picture also shows that the current and voltage are controlled by the MPPT block; finally, the block sends a signal to the Buck converter

3.1.1 Buck converter modelling

The dc-dc buck converter, also known as step-down converter, is a type of circuit that converts higher DC input voltage to lower DC output voltage The basic topology is shown in the figure 3.3, it consists of a controlled switch S (usually a MOSFET due to high frequency and low voltage application), an uncontrolled

switch diode (D), an inductor L, a capacitance C and a load resistance R [11]

20

Figure 3.3: Buck converter circuit [11]

In the continuous conduction mode (CCM), the buck converter could be in two different states: ON state and OFF state In the ON state, power MOSFET S is ON

and diode D is OFF The corresponding circuit is shown in Figure 3.4, where R S is

the on-resistance of the MOSFET, R L is the resistance of inductor, i S is the current

flows through the MOSFET, and i L is the current through inductor L The system

equation is expressed as:

Figure 3.4: Equivalent buck circuit in ON state [11]

In the OFF state, MOSFET S is off, diode D is on The corresponding circuit is shown in Figure 3.5 In this state, since the current flowing in the inductor could not change instantly, the current flows to the diode The current in the inductor will decrease and cause the voltage across the conductor in reverse polarity The system equation is expressed as:

Figure 3.5: Equivalent Buck converter in the OFF state [11]

In the steady state condition, the average inductor voltage is the summation of Equations (19) and (22), and yields:

Where d is the period of the ON state The average capacitor current is the

summation of Equations (21) and (24), and yields:

(26)

Thus the average input current i s is given as

22

MPPT creates the reference value of the input voltage, as shown in figure 3.6 and 3.7 Moreover, since battery terminal voltage may be considered constant during the MPPT algorithm convergence period, MPP tracking implies maximizing the current injected into the battery, i.e., a single-sensor MPPT algorithm should be utilized

[10]

The power, generated by SA is transferred to the battery side taking into account

the converter efficiency η as PB= η.PPV The battery charging current is given as

Where and are battery equivalent series resistence and internal voltage, respectively, depending on battery temperature, age and state of charge

Figure 3.6: Control structure of a constant power (CP) mode [10]

Figure 3.7: Power flow diagram of a CP mode b) Constant Voltage (CV)

The system switches to the second operating mode when the battery terminal

voltage reaches its rated value In this mode, the converter output voltage vo is

23

regulated to the voltage of employed battery, as shown in 3.8 The charging current

is determined by the battery as:

Figure 3.8: Control structure of a constant voltage (CV) mode [10]

Figure 3.9: Power flow diagram of a CV mode

Consequently, the intersection of P-V curve with constant power PI, determines the solar array operating point In addition, output voltage controller again creates reference command to the current controller Therefore, the same current controller may be utilized for both operating modes, as shown in figure 3.10, where the reference current fed to the current controller is determined by operation mode

selector [10]

24

Figure 3.10: Single battery dual-mode control structure 3.1.2.1 System Analysis

Figure 3.11: Averaged system model

The switching cycle averaged model of the system is shown in 3.11 is governed

by the following set of equations (continuous conduction mode assumed)

(36) Output Inductor

26

the previous cycle On contrary, if the perturbation voltage is the opposite of the power, then the direction or slope of perturbation voltage is the opposite from the

previous cycle [12] The figure 3.12 shows the schematic of implementation of

MPPT to control the charging of a battery

Figure 3.12: MPPT control scheme [13]

The block MPPT is the one who has the algorithm for gating the circuit The left side graphs are related to the solar panel I-V and P-V characteristics while the right

side graphs are referred to duty cycle, Icell (current of the panel) and output power

Likewise, the system must contain a microchip programmable by a P&O algorithm following the scheme in figure 3.13

Figure 3.13: Arduino of solar charge controller [14]

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3.2 Inverter Control

Inverters convert lower voltage DC electricity into a higher voltage AC In the process of converting DC to AC, they use energy They are typically about 90 percent efficient Thus when planning large systems, inverter loses must be included

in the calculations There are two ways that inverters can be used in off-grid PV systems:

a) In some systems, a small inverter is used to power a single appliance

b) In other systems, a larger inverter is used to power whole circuits

Note that some inverters can be coupled to a charger

This project aims to convert 12V DC into 220VAC by using a topology called push pull converter to boost from 12V DC into 400V DC then from 400V DC to 220VAC The figure 3.14 shows the scheme of the inverter proposed by the researcher

Figure 3.14: Inversion scheme

The DC-DC section is a critical part of the converter design In fact, the need for high overall efficiency (close to 90%) together with the specifications for continuous power rating, low input voltage range leading to high input current, and the need for high switching frequency to minimize weight and size of passive

components, makes it a quite challenging design [15]

Due to the constraints given by the specifications in Table 3, few topologies are suitable to meet the efficiency target Since the input voltage of the DC-AC converter must be around 400V, it is not feasible to use non-isolated DC-DC converters Moreover, the output power rating prevents the use of single switch topologies such as the flyback and the forward Among the remaining isolated