Chapter 1INTRODUCTION

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1.1 IntroductionPhotovoltaic (PV) power supplied to the utility grid is gaining more and more attention nowadays. However, depending on the characteristics of the PV panels, the total output voltage from the PV panels varies greatly due to variation of temperature, irradiation conditions, shading and clouding effects. When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically. In view of continuity & stability, a stable energy source and an energy source that can be dispatched at the request are desired. As a result, energy storage such as batteries and fuel cells for solar PV systems has drawn significant attention and the demand of energy storage for solar PV systems has been dramatically increased, since, with energy storage, a solar PV system becomes a stable energy source and it can be dispatched at the request, which results in improving the performance and the value of solar PV systems.There are different options for integrating energy storage into a Utility-scale solar PV system. Every integration solution has its advantages and disadvantages. Different grouping solutions can be compared with regard to the number of power stages, efficiency, storage system flexibility, control complexity, etc.Under the technical backgrounds a single-stage inverter cum chopper (SSICC) for PV-Battery system is presented in the project. The main objective of the project is to design a single-stage inverter cum chopper instead of multistage inverter cum chopper. This proposed single-stage inverter cum chopper (SSICC) performs different operations such as DC-AC & DC-DC in order to interconnect PV to Grid(dc to ac), PV to battery (dc to dc), battery to grid (dc to ac), and battery/PV to grid (dc to ac) for solar PV systems with energy storage. 1.2 Literature surveyLiterature survey has been carried out on information regarding single stage inverter cum chopper (SSICC) for PV- Battery system [1][9].The system described in [7] proposes a hybrid CAES system, where compression and expansion modes are operated under a Maximum Efficiency Point Tracking (MEPT) strategy.The system described in [3][7] proposes a two-stage high-frequency power conversion in cascaded configuration with dc link in the middle. The system described in [8][11] uses a line-commuted inverter along with an isolated dcdc stage. Also, many no isolated single-stage boost or buckboost derived inverter topologies have been developed [2][5]. The system described in [6], [7] overcomes the major drawbacks or limitation of input-voltage range and/or requirement of two input sources. The system described in [8] replaces the isolated Dcdc with non isolated or transformer less dcdc [8].The system described in [9] uses the transformer less dcdc stage will be more reliable and cost effective.1.3 Project overviewThe proposed single-stage inverter cum chopper (SSICC) for PV-Battery System is implemented by a small modification to conventional three phase inverter. The proposed Single-Stage inverter cum chopper (SSICC) minimizes the number of conversion stages, reduces the losses thereby improving efficiency and reducing cost, weight, and volume when compared to multi stage power converter for PV- battery system. Simulation & experimental results are presented to verify the validities of the proposed single-stage inverter cum chopper (SSICC) for PV-Battery System.1.4 Organization of the thesisThis thesis is comprised of eight chapters. Chapter 1 describes the introduction to the project, literature survey & overview of the project. Chapter 2 introduces block diagram of the Single-Stage inverter cum chopper (SSICC) circuit for PV-Battery System. Chapter 3 describes different modes of operation, system benefits compared to state of art methods & System control schemes of the proposed system. Chapter 4 describes necessary design considerations and modifications to the conventional three-phase PV converter & circuit components. Chapter 5 describes the modulation topologies for three phase voltage source inverter. Chapter 6 describes about maximum power point tracking. Chapter 7 verifies the SSICC & performance characteristics through simulation .Finally chapter 8 summarizes and concludes the project.1.5 SummaryIn this chapter introduction, literature survey, organization of the project is discussed and in the next chapter introduction to the proposed Single-Stage inverter cum chopper (SSICC) circuit, comparison with state of art methods, different operating modes and system benefits PV-Battery System are explained clearly.

Chapter 2BLOCK DIAGRAM

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2.1 IntroductionIn chapter1 introduction about the project has been discussed. This chapter explains about block diagram of the proposed Single-Stage inverter cum chopper circuit (SSICC).2.2 Block diagram

FIG.2.1 BLOCK DIAGRAM OF SSICCThe above diagram shows the block diagram of the SSICC .The block diagram consists of following blocks.2.2.1 Solar cell array2.2.2 Blocking diode2.2.3 Converter2.2.4 Battery Storage2.2.1 Solar cell arraySolar array is the one which converts the solar radiation in to useful DC electrical power.

2.2.1(A) Electricity from solar energy Electricity is directly generated by utilizing solar energy by the photo voltaic process. When photons from the sun are absorbed in a semi-conductor, they create free electrons with higher energies than the electrons which provide the bonding in the base crystal. Once these free electrons are created, there must be an electric field to induce these higher energy electrons to flow out of the semiconductor to do useful work. The electric field in most solar cells is provided by a junction of materials which have different electrical properties. 2.2.1(B) Solar electric power generationThe direct conversion of solar energy into electrical energy by means of the photovoltaic effect, that is, the conversion of light (or other electromagnetic radiation) into electricity. The photo-voltaic effect is defined as the generation of an electromotive force as a result of the absorption of ionizing radiation. Energy conversion devices which are used to convert sunlight to electricity by the use of the photovoltaic effect are called solar cells. A single converter cell or, more generally, a photovoltaic cell, and combination of such cells designed to increase the electric power output is called a solar module or solar array. Photovoltaic cells are made of semiconductors that generate electricity when they absorb light. As photons are received, free electrical charges are generated that can be collected on contacts applied to the surface of the semiconductors. 2.2.1(c) Solar cell principlesThe photo-voltaic effect can be observed in nature in a variety of materials, but the materials that have shown the best performance in sunlight are the semi-conductors as started above. When photons from the sun are absorbed in a semiconductor, they create free electrons with higher energies than the electrons which provide the bonding in the base crystal. Once these electrons are created, there must be an electric field to induce these higher energy electrons to flow out of the semi-conductor to do useful work. The electric field in most solar cells is provided by a junction of materials which have different electrical properties. To obtain a useful power output from photon interaction in a semi-conductor three processes are required.The photons have to be absorbed in the active part of the material and result in electrons being excited to a higher energy potential.1. The electrons-hole charge carrier created by the absorption must be physically separated and moved to the edge of the cell.1. The charge carriers must be removed from the cell and delivered to a useful load before their loose extra potential. For completing the above processes, a solar cell consists of:1. Semi-conductor in which electron hole pairs are created by absorption of incident solar radiation.1. Region containing a drift field for charge separation, and1. Charge collecting front and back electrodes. The photo-voltaic effect can be described easily for p-n junction in a semi-conductor. In an intrinsic semi-conductor such as silicon, each one of the four valence electrons of the material atom is tide in a chemical bond, and there are no free electrons at absolute zero. If a piece of such a material is doped on one side by a five valence electron material, such as arsenic or phosphorus, there will be an excess electrons in that side, becomes an n-type semi-conductor. The excess electrons will be practically free to move in the semi-conductor lattice. When the other side of the same piece is doped by a three valence electron material, such as boron, there will be deficiency of electrons leading to a p-type semi-conductor. This deficiency is expressed in terms of excess of holes free to move in the lattice. Such a piece of semi-conductor with one side of the p-type and the other of the n-type is called a p-n junction. In this junction after the photons are absorbed, the free electrons of the n-side will tend to flow to the p-side, and the holes of the p-side will tend to flow to the n-region to compensate for their respective deficiencies. This diffusion will create an electric field EF from the n-region to the p-region. This field will increase until it reaches equilibrium for Ve, the sum of the diffusion potentials for holes and electrons. If electrical contacts are made with the two semi-conductor materials and the contacts are connected through an external electrical conductor, the free electrons will flow from the n-type material through the conductor to the p-type material. Here the free electrons will enter the holes and become bound electrons thus, both free electrons and holes will be removed. The flow of electrons through the external conductor constitutes an electric current which will continue as long as more free electrons and holes are being formed by the solar radiation. This is the basis of photo-voltaic conversion, that is, the conversion of solar energy into electrical energy. The combination of n-type and p-type semi-conductors thus constitutes a photo-voltaic (PV) cell or solar cell. All such cells generate direct current which can be converted into alternating current if desired.

FIG.2.2: SCHEMATIC VIEW OF A TYPICAL SOLAR CELL

The most normal configuration for a solar cell to make a p-n junction semi conductor is as shown schematically in fig.2.2 The junction of the p type and n type materials provides an inherent electric field which separates the charge created by the absorption of sunlight. This p-n junction is usually obtained by putting a p- type base material into a diffusion furnace containing a gaseous n-type dopant such as phosphorus and allowing the n-dopant to diffuse into the surface about 0.2m. The junction is thus formed slightly below the planer surface of the cell and the light impinges perpendicular to the junction. The positive and negative charges created by the absorption of photons are thus encouraged to drift to the front and back of the solar cell. The back is completely covered by a metallic contact to remove the charges to the electric load. The collection of charges from the front of the cell is aided by a fine grid of narrow metallic fingers. The surface coverage of the conducting collectors is typically about 5% in order to allow as much light as possible to reach active junction area. An antireflective coating is applied on the top of the cell. The p-n junction provides an electrical field that sweeps the electrons in one direction and the positive holes in the other. If the junction is in thermodynamic equilibrium, then the Fermi energy must be uniform throughout. Since the Fermi level is near the top of the gap of an n-doped material and near the bottom of the p-doped side, an electric field must exist at the junction providing the charge separation function of the cell. Important characteristics of the Fermi level is that, in thermodynamic equilibrium, it is always continuous across the contact between the two materials. 2.2.1(D) Conversion efficiency and power outputA solar cell usually uses a p-n junction its physical configuration is shown schematically in fig.2.3.

FIG.2.3: THE EQUIVALENT CIRCUIT OF A SOLAR CELLWhereIc = cell output currentVc = cell output voltageIph = light generated currentIo = reverse saturation currentRs = series resistance of the cellCurrent and voltage relationship is given by 1WhereIoSaturation current also called the dark current and is applied when a large negative voltage is applied across the diode.V Voltage across junction.e Electronic chargek Boltzmanns constantT Absolute temperatureWhen light impinges on the junction, electron hole pairs are created at a constant rate providing an electrical flow across the junction. The net current is thus the difference between the normal diode current and light generated current IL. The internal series resistance Rs is mostly due to the high sheet resistance of the diffused layer which is in series with the junction. The light generated current acts as a constant current source supplying the current to either the junction or a useful load depending on the junction characteristic and the value of the external load resistance. The net current I is given by 2The internal voltage drop in a cell can usually be minimized, and for ideal cell Rs may be assumed equal to zero i.e. Rs=0. With these the corresponding I-V plot is given in figure. Open circuit voltage Voc for the ideal cell is then given by Since IL>>Io, the 1 in the equation can be neglected. Then open circuit voltage

In practice the open circuit voltage of the cell decreases with increasing temperature.The maximum power that can be derived from the device is given by Pmax=Vmp.Imp 5

FIG.2.4: A TYPICAL I-V PLOT FOR IDEAL SOLAR CELLWhere Vmp and Imp are the voltage and current at maximum power point as shown in figure respectively. It can be seen that the maximum efficiency for the cell is obtained by dividing Vmp Imp by the total power density of the sunlight Psun.

Thus = [Where Eg= Forbidden energy gap]The fill factor (FF) for a solar cell is defined as the ratio of two areas shown. FF= 7Solar cell designers, strive to increase the fill factor values, to minimize the internal losses. Maximum power can be defined in terms of Voc and IL is given by Pmax= IL VOC 8A typical value of the fill factor for a good silicon cell is about 0.8.The voltage factor (eVoc/Eg) is determined by the basic properties of the materials in the cell and typically about 0.5 for a silicon cell.2.2.2 Blocking diode A Blocking Diode which lets the array generated power flow only toward the battery or grid. Without a blocking diode the battery would discharge back through the solar array during times of no insulation.2.2.3 ConverterConverter is the one which converts the power from AC to DC or DC to DC or DC to DC. In order to integrate the solar arrays with the batteries there is a requirement for chopper which converts fixed DC to variable DC. Also to Integrate solar arrays or batteries to grid there is a need for DC to AC inverter .Integration of solar arrays, batteries & grid requires a number of conversion stages which increases the losses, cost & further reduces the efficiency. To overcome the above defects a SSICC emerges in this project.

2.2.3(A)Single-stage inverter cum chopper (SSICC)

FIG.2.5 SCHEMATIC OF SSICCThe schematic of the SSICC is presented in Fig. 2.5 the Single-Stage inverter cum chopper (SSICC) is basically a modified conventional three-phase voltage source inverter system. These modifications allow the SSICC to utilize the conventional three-phase voltage source inverter system to operate as both chopper & inverter for the purpose of charging the battery. The SSICC consists of a three-phase voltage source converter and its associated components such as additional cables and mechanical switches, as shown in Fig. 2. Optional inductors are included if the ac filter inductance is not enough for the charging purpose.2.2.3(B) Inverter

A power inverter, or inverter, is an electrical power converter that changes direct current (DC) to alternating current (AC).The input voltage, output voltage, and frequency are dependent on design.Static inverters do not use moving parts in the conversion process. Some applications for inverters include converting high-voltage direct current electric utility line power to AC, and deriving AC from DC power sources such as batteries. The main objective of static power converters is to produce an ac output waveform from a dc power supply. These are the types of waveforms required in adjustable speed drives (ASDS), uninterruptable power supplies (UPS), static VAR compensators, active filters, flexible ac transmission systems (FACTS), & voltage compensators, which are only a few applications. For sinusoidal ac outputs, the magnitude, frequency, and phase should be controllable. 2.2.3(C) Three phase voltage source inverters Single-phase VSIs cover low-range power applications and three-phase VSIs cover the medium- to high-power applications. The main purpose of these topologies is to provide a three-phase voltage source, where the amplitude, phase, and frequency of the voltages should always be controllable. The standard three-phase VSI topology is shown in Fig. 2.6 and the eight valid switch states are given in Table 2.1. As in single-phase VSIs, the switches of any leg of the inverter (S1 and S4, S3 and S6, or S5 and S2) cannot be switched on simultaneously because this would result in a short circuit across the dc link voltage supply. Similarly, in order to avoid undefined states in the VSI, and thus undefined ac output line voltages, the switches of any leg of the inverter cannot be switched off simultaneously as this will result in voltages that will depend upon the respective line current polarity. Of the eight valid states, two of them (7 and 8 in Table 2.1) produce zero ac line voltages. In this case, the ac line currents freewheel through either the upper or lower components. The remaining states (1 to 6 in Table 2.1) produce non-zero ac output voltages. In order to generate a given voltage waveform, the inverter moves from one state to another. Thus the resulting ac output line voltages consist of discrete values of voltages that are Vi , 0, and -Vi for the topology shown in Fig2.4 the selection of the states in order to generate the given waveform is done by the modulating technique that should ensure the use of only the valid states.

FIG. 2.6: THREE-PHASE VSI TOPOLOGYTABLE 2.1: VALID SWITCH STATES FOR A THREE-PHASE VSI

2.2.4. Battery storageAn electric battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. Each battery consists of a negative electrode material, a positive electrode material, an electrolyte that allows ions to move between the electrodes, and terminals that allow current to flow out of the battery to perform work. Primary (single-use or "disposable") batteries are used once and discarded; the electrode materials are irreversibly changed during discharge. Common examples are the alkaline battery used for flashlights and a multitude of portable devices. Secondary (rechargeable batteries) can be discharged and recharged multiple times; the original composition of the electrodes can be restored by reverse current. Examples include the lead-acid batteries used in vehicles and lithium ion batteries used for portable electronics.2.2.4(A) Principle of operation

FIG. 2.7: BATTERY CONSTRUCTIONIn this example the two half-cells are linked by a salt bridge separator that permits the transfer of ions, but not water molecules. Batteries convert chemical energy directly to electrical energy. A battery consists of some number of voltaic cells. Each cell consists of two half-cells connected in series by a conductive electrolyte containing anions and cat ions. One half-cell includes electrolyte and the negative electrode, the electrode to which anions (negatively charged ions) migrate; the other half-cell includes electrolyte and the positive electrode to which cat ions (positively charged ions) migrate. Redox reactions power the battery. Cat ions are reduced (electrons are added) at the cathode during charging, while anions are oxidized (electrons are removed) at the anode during discharge. The electrodes do not touch each other, but are electrically connected by the electrolyte. Some cells use different electrolytes for each half-cell. A separator allows ions to flow between half-cells, but prevents mixing of the electrolytes.Each half-cell has an electromotive force (or emf), determined by its ability to drive electric current from the interior to the exterior of the cell. The net emf of the cell is the difference between the emfs of its half-cells. Thus, if the electrodes have emfs and, then the net emf is; in other words, the net emf is the difference between the reduction potentials of the half-reactions.The electrical driving force or across the terminals of a cell is known as the terminal voltage (difference) and is measured in volts. The terminal voltage of a cell that is neither charging nor discharging is called the open-circuit voltage and equals the emf of the cell. Because of internal resistance, the terminal voltage of a cell that is discharging is smaller in magnitude than the open-circuit voltage and the terminal voltage of a cell that is charging exceeds the open-circuit voltage. An ideal cell has negligible internal resistance, so it would maintain a constant terminal voltage of until exhausted, then dropping to zero. If such a cell maintained 1.5 volts and stored a charge of one coulomb then on complete discharge it would perform 1.5 joules of work. In actual cells, the internal resistance increases under discharge and the open circuit voltage also decreases under discharge. If the voltage and resistance are plotted against time, the resulting graphs typically are a curve; the shape of the curve varies according to the chemistry and internal arrangement employed.The voltage developed across a cell's terminals depends on the energy release of the chemical reactions of its electrodes and electrolyte. Alkaline and zinccarbon cells have different chemistries, but approximately the same emf of 1.5 volts; likewise NiCd and NiMH cells have different chemistries, but approximately the same emf of 1.2 volts. The high electrochemical potential changes in the reactions of lithium compounds give lithium cells emfs of 3 volts or more. 2.2.4(B) Classification of batteries Primary batteries irreversibly transform chemical energy to electrical energy. When the supply of reactants is exhausted, energy cannot be readily restored to the battery. Secondary batteries can be recharged; that is, they can have their chemical reactions reversed by supplying electrical energy to the cell, approximately restoring their original composition.2.2.4(C) Primary batteriesPrimary batteries, or primary cells, can produce current immediately on assembly. These are most commonly used in portable devices that have low current drain, are used only intermittently, or are used well away from an alternative power source, such as in alarm and communication circuits where other electric power is only intermittently available. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms. Battery manufacturers recommend against attempt to recharge primary cells.In general, these have higher energy densities than rechargeable batteries, but disposable batteries do not fare well under high-drain applications with loads under 75 ohms (75 ).Common types of disposable batteries include zinccarbon batteries and alkaline batteries. 2.2.4 (D) Secondary batteriesSecondary batteries, also known as secondary cells, or rechargeable batteries, must be charged before first use; they are usually assembled with active materials in the discharged state. Rechargeable batteries are (re)charged by applying electric current, which reverses the chemical reactions that occur during discharge/use. Devices to supply the appropriate current are called chargers.The oldest form of rechargeable battery is the leadacid battery. This technology contains liquid electrolyte in an unsealed container, requiring that the battery be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas it produces during overcharging. The leadacid battery is relatively heavy for the amount of electrical energy it can supply. Its low manufacturing cost and its high surge current levels make it common where its capacity (over approximately 10 Ah) is more important than weight and handling issues. A common application is the modern car battery, which can, in general, deliver a peak current of 450 amperes.The sealed valve regulated leadacid battery (VRLA battery) is popular in the automotive industry as a replacement for the leadacid wet cell. The VRLA battery uses an immobilized sulfuric acid electrolyte, reducing the chance of leakage and extending shelf life. VRLA batteries immobilize the electrolyte. The two types are: Gel batteries (or "gel cell") use a semi-solid electrolyte. Absorbed Glass Mat (AGM) batteries absorb the electrolyte in special fiberglass matting.Other portable rechargeable batteries include several sealed "dry cell" types, which are useful in applications such as mobile phones and laptop computers. Cells of this type (in order of increasing power density and cost) include nickelcadmium (NiCd), nickelzinc (NiZn), nickel metal hydride (NiMH), and lithium-ion (Li-ion) cells. Li-ion has by far the highest share of the dry cell rechargeable market. NiMH has replaced NiCd in most applications due to its higher capacity, but NiCd remains in use in power tools, two-way radios, and medical equipment.Recent developments include batteries with embedded electronics such as USBCELL, which allows charging an AA battery through a USB connector,[and smart battery packs with state-of-charge monitors and battery protection circuits that prevent damage on over-discharge. Low self-discharge (LSD) allows secondary cells to be charged prior to shipping.

2.2.4.3 Battery chemistryTABLE 2.2: PRIMARY BATTERY CHEMISTRIES:Sl.No.ChemistryNominal CellVoltageSpecific Energy [MJ/kg]Elaboration

1Zinccarbon1.50.13Inexpensive.

2Zincchloride1.5Also known as "heavy duty", inexpensive.

3Alkaline(zincmanganese dioxide)1.50.4-0.59Moderate energy density.Good for high and low drain uses.

4Nickel oxy hydroxide(Zincmanganese dioxide/nickel oxy hydroxide)1.7Moderate energy density.Good for high drain uses

5Lithium(lithiumcopper oxide)LiCuO1.7No longer manufactured.Replaced by silver oxide (IEC-type "SR") batteries.

6Lithium(lithiumiron disulfide)LiFeS21.5Expensive.Used in 'plus' or 'extra' batteries.

7Lithium(lithiummanganese dioxide)LiMnO23.00.83-1.01Expensive.Only used in high-drain devices or for long shelf life due to very low rate of self discharge.'Lithium' alone usually refers to this type of chemistry.

8Mercury oxide1.35High drain and constant voltage.Banned in most countries because of health concerns.

9Zincair1.351.651.59Mostly used in hearing aids.

10Silver-oxide (silverzinc)1.550.47Very expensive.Only used commercially in 'button' cells.

TABLE2.3: RECHARGEABLE BATTERY CHEMISTRIES:Sl.No.ChemistryCellVoltageSpecific Energy[MJ/kg]Comments

2NiCd1.20.14Inexpensive.High/low drain, moderate energy density.Can withstand very high discharge rates with virtually no loss of capacity.Moderate rate of self discharge.Environmental hazard due to Cadmium use now virtually prohibited in Europe.

3Leadacid2.10.14Moderately expensive.Moderate energy density.Moderate rate of self discharge.Higher discharge rates result in considerable loss of capacity.Environmental hazard due to Lead.Common use Automobile batteries

4NiMH1.20.36Inexpensive.Performs better than alkaline batteries in higher drain devices.Traditional chemistry has high energy density, but also a high rate of self-discharge.Newer chemistry has low self-discharge rate, but also a ~25% lower energy density.Used in some cars.

5NiZn1.60.36Moderately inexpensive.High drain device suitable.Low self-discharge rate.Voltage closer to alkaline primary cells than other secondary cells.No toxic components.Newly introduced to the market (2009). Has not yet established a track record.Limited size availability.

6AgZn1.861.50.46Smaller volume than equivalent Li-ion.Extremely expensive due to silver.Very high energy density.Very high drain capable.For many years considered obsolete due to high silver prices.Cell suffers from oxidation if unused.Reactions are not fully understood.Terminal voltage very stable but suddenly drops to 1.5 volts at 70-80% charge (believed to be due to presence of both argentous and argentic oxide in positive plate one is consumed first).Has been used in lieu of primary battery (moon buggy).Is being developed once again as a replacement for Li-ion.

7Lithium ion3.60.46Very expensive.Very high energy density.Not usually available in "common" battery sizes.Very common in laptop computers, moderate to high-end digital cameras, camcorders and cell phones.Very low rate of self discharge.Terminal voltage unstable (varies from 4.2 to 3.0 volts during discharge).Volatile: Chance of explosion if short circuited, allowed to overheat, or not manufactured with rigorous quality standards.

2.2.4.3(A) Li-ion batteryA lithium-ion battery (sometimes Li-ion battery or LIB) is a member of a family of rechargeable battery types in which lithium ions move from the anode to the cathode during discharge and back when charging. Li-ion batteries use an intercalated lithium compound as the electrode material, compared to the metallic lithium used in non-rechargeable battery. Lithium-ion batteries are common in consumer electronics. They are one of the most popular types of rechargeable battery for portable electronics, with one of the best energy densities, no memory effect (note, however, that new studies have shown signs of memory effect in lithium-ion batteries), and only a slow loss of charge when not in use. Beyond consumer electronics, LIBs are also growing in popularity for military, electric vehicle and aerospace applications. For example, Lithium-ion batteries are becoming a common replacement for the lead acid batteries that have been used historically for golf carts and utility vehicles. Instead of heavy lead plates and acid electrolyte, the trend is to use a lightweight lithium/carbon anode and lithium iron phosphate cathode. Lithium-ion batteries can provide the same voltage as lead-acid batteries, so no modification to the vehicle's drive system is required. Chemistry, performance, cost and safety characteristics vary across LIB types. Handheld electronics mostly use LIBs based on lithium cobalt oxide (LiCoO2), which offers high energy density, but presents safety risks, especially when damaged. Lithium iron phosphate (LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC) offer lower energy density, but longer lives and inherent safety. Such batteries are widely used for electric tools, medical equipment and other roles. NMC in particular is a leading contender for automotive applications. Lithium nickel cobalt aluminum oxide (NCA) and lithium titanate (LTO) are specialty designs aimed at particular niche roles.Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard since they contain, unlike other rechargeable batteries, a flammable electrolyte and are also kept pressurized. This makes the standards of these batteries high, and it consists of many safety features. There have been many reported accidents as well as recalls done by some companies.2.2.4.3(B) Lithium-ion batteries constructionThe three primary functional components of a lithium-ion battery are the anode, cathode and electrolyte. Generally, the anode of a conventional lithium-ion cell is made from carbon. The cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent. The electrochemical roles of the electrodes reverse between anode and cathode, depending on the direction of current flow through the cell. The most commercially popular anode is graphite. The cathode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide).The electrolyte is typically a mixture of organic carbonates such as ethylene carbonate or diethyl carbonate containing complexes of lithium ions. Depending on materials choices, the voltage, energy density, life and safety of a lithium-ion battery can change dramatically. Recently, novel architectures using nanotechnology have been employed to improve performance. Pure lithium is highly reactive. It reacts vigorously with water to form lithium hydroxide and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes moisture from the battery pack. Lithium ion batteries are more expensive than NiCd batteries but operate over a wider temperature range with higher energy densities. They require a protective circuit to limit peak voltage. For notebooks or laptops, lithium-ion cells are supplied as part of a battery pack with temperature sensors, voltage converter/regulator circuit, voltage tap, battery charge state monitor and the main connector. These components monitor the state of charge and current in and out of each cell, capacities of each individual cell (drastic change can lead to reverse polarities which are dangerous), temperature of each cell and minimize the risk of short circuits.2.2.4.3(C) ElectrochemistryThe three participants in the electrochemical reactions in a lithium-ion battery are the anode, cathode and electrolyte.Both electrodes allow lithium ions to migrate towards and away from them. During insertion (or intercalation) ions move into the electrode. During the reverse process, extraction, ions move back out. When a lithium-based cell is discharging, the positive ion is extracted from the negative electrode (usually graphite) and inserted into the positive electrode (lithium containing compound). When the cell is charging, the reverse occurs.Useful work is extracted when electrons flow through a closed external circuit. The following equations show one example of the chemistry, in units of moles, making it possible to use coefficient.The positive electrode half-reaction is: .(1)The negative electrode half-reaction is:..(2)The overall reaction has its limits. Over discharge supersaturates lithium cobalt oxide, leading to the production of lithium oxide, possibly by the following irreversible reaction: Li++ e-+ LiCoO2 Li2 + CoO .(3) Overcharge up to 5.2 volts leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray diffraction: LiCoO2= Li++ CoO2+e ..(4)In a lithium-ion battery the lithium ions are transported to and from the cathode or anode by oxidizing the transition metal, cobalt (Co), in LixCoO2 from Co3+ to Co4+ during charge, and reduced from Co4+ to Co3+ during discharge.The cell's energy is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941 or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kg. This is a bit more than the heat of combustion of gasoline, but does not consider the other materials that go into a lithium battery and that make lithium batteries many times heavier per unit of energy.2.2.4.3(D) Charge and dischargeDuring discharge lithium ions Li+ carry the current from the negative to the positive electrode, through the non-aqueous electrolyte and separator diaphragm.During charging an external electrical power source (the charging circuit) applies an over-voltage (a higher voltage but of the same polarity) than that produced by the battery, forcing the current to pass in the reverse direction. The lithium ions then migrate from the positive to the negative electrode where they become embedded in the porous electrode material in a process known as intercalation.2.2.4.3(E) Charging procedureThe charging procedures for single Li-ion cells and complete Li-ion batteries are slightly different. A single Li-ion cell is charged in two stages:1. CC2. CV A Li-ion battery (a set of Li-ion cells in series) is charged in three stages:1. CC2. Balance (not required once a battery is balanced)3. CVCC: Apply charging current to the battery, until the voltage limit per cell is reached.Balance: Reduce the charging current (or cycle the charging on and off to reduce the average current) while the state of charge of individual cells is brought to the same level by a balancing circuit, until the battery is balanced. Some fast chargers skip this stage. Some chargers accomplish the balance by charging each cell independently.CV: Apply a voltage equal to the maximum cell voltage times the number of cells in series to the battery, as the current gradually declines asymptotically towards 0, until the current is below a set threshold of about 3% of initial constant charge current.2.3 SummaryIn this chapter block diagram of the proposed Single-Stage inverter cum chopper circuit (SSICC) are discussed and in the next chapter different modes of operation, system benefits compared to state of art methods & System control schemes of the proposed system.

CHAPTER 3MODES OF OPERATION & CONTROL OF SSICC

3.1 Introduction In chapter 2 block diagram of the proposed Single-Stage inverter cum chopper circuit (SSICC) are discussed. This chapter explains about different modes of operation, system benefits compared to state of art methods & System control schemes of the proposed system.3.2 Operating modes of SSICC system The five possible operation modes for the proposed SSICC system are as follows. 3.2.1 Mode 1PV to grid3.2.2 Mode 2PV to battery3.2.3 Mode 3PV/battery to grid3.2.4 Mode 4battery to grid

3.2.1 Mode 1PV to grid (s1 & s6 switches remain open) In Mode 1, the PV is directly connected to the grid through a dc/ac operation of the converter with possibility of maximum power point tracking (MPPT) control .In this mode S1 and S6 switches remains open keeping all the remaining switches in closed position.

FIG 3.1 OPERATING MODE 1 OF THE PROPOSED SYSTEM

3.2.2 Mode 2PV to battery (s5 & s3 switches remain open)In Mode 2, the battery is charged from PV panels through the dc/dc operation of the converter .In this mode S5 and S3 switches remains open keeping all the remaining switches in closed position. In this mode, the MPPT function is performed therefore; maximum power is generated from PV.

FIG 3.2 OPERATING MODE 2 OF THE PROPOSED SYSTEM3.2.3 Mode 3pv/battery to grid (switch s6 switch remains open)In Mode 3 both the PV and battery provide the power to the grid. In this mode S6 switch remains open keeping all the remaining switches in closed position .In this mode MPPT control is not possible.

FIG 3.3 OPERATING MODE 3 OF THE PROPOSED SYSTEM

3.2.4 Mode 4battery to grid (switch s4 & s6 switches remain open)In Mode 4 both the battery provide the power to the grid Mode 4 represents an operation mode that the energy stored in the battery is delivered to the grid. In this mode S4 & S6 switches remains open keeping all the remaining switches in closed position.

FIG 3.4 OPERATING MODE 4 OF THE PROPOSED SYSTEM

3.3 Advantages of single-stage inverter cum chopper (SSICC) conceptThe single stage inverter cum chopper (SSICC) concept provides significant benefits to system planning of utility-scale solar PV power plants. The current state-of-the-art technology is to integrate the energy storage into the ac side of the solar PV system. An example of commercial energy storage solutions is the ABB distributed energy storage (DES) solution that is a complete package up to 4 MW, which is connected to the grids directly and, with its communication capabilities, can be utilized as a mean for peak shifting in solar PV power plants. This concept allows not only the system owners to possess an expandable asset that helps them to plan and operate the power plant accordingly but also manufacturers to offer a cost-competitive decentralized PV energy storage solution with the single stage converter. Fig.3.5 shows examples of the PV energy storage solutions with the single stage converter and the current state-of-the-art technology. However, different system controls can be proposed based on the requested power from the grid operator specifically, a large solar PV power plant using the single stage converter s can be controlled more effectively and its power can be dispatched more economically because of the flexibility of operation. Developing a detailed operation Preq and available generated power form the plant Pgen .These two values being results of an optimization problem (such as unit commitment methods) serve as variables to control the solar PV power plant accordingly.

FIG.3.5. UTILITY-SCALE PV-ENERGY STORAGE SYSTEMS WITH THE SINGLE STAGE INVERTER CUM CHOPPER (SSICC) AND THE CURRENT STATE-OF-THE-ART SOLUTION

3.4 System control schemes of single stage inverter cum chopper (SSICC)In response to the request of the grid operator, different system control schemes can be realized with the single stage inverter cum chopper (SSICC) based solar PV power plant as follows:3.4.1 System control 1 for P generated > P demand3.4.2 System control 2 for P generated < P demand3.4.3 System control 3 for P generated = P demand

3.4.1 System control 1 for p generated > p demand:System control 1 will be activated when generated power is greater than the demand. In this control the system operates in mode1 & mode2 such that the PV cells supply the grid & excess amount of power be stored in the batteries.

FIG 3.6 FOR SYSTEM CONTROL 13.4.2 System control 2 for p generated < p demand: System control 2 will be activated when generated power is lesser than the demand. In this control the system operates in mode 3 such that the PV cells & batteries supply the grid.

FIG 3.7FOR SYSTEM CONTROL 2

3.4.3 System control 3 for p generated = p demand;System control 3 will be activated when generated power is equal to the demand. In this control the system operates in mode1 such that the PV cells supply the grid.

FIG 3.8 FOR SYSTEM CONTROL 33.4.4 System control 4 for p generated = 0:

FIG 3.9 FOR SYSTEM CONTROL 43.5 SummaryDifferent modes of operation, system benefits compared to state of art methods & System control schemes of the proposed system are discussed and in the next chapter circuit diagram of the proposed system, necessary design considerations and modifications to the conventional three-phase PV converter & circuit components are discussed.

Chapter 4CIRCUIT DIAGRAM AND DESIGN CONSIDERATIONS TO THREE PHASE INVERTER

4.1 Introduction It explains about circuit diagram of the proposed system, necessary design considerations and modifications to the conventional three-phase PV converter & circuit components.4.2 Circuit diagram4.2.1 Circuit diagram in dc/ac operation

FIG. 4.1.OVERALL CIRCUIT DIAGRAM OF SSICC IN DC-AC MODE. 4.2.1(A) Circuit description for dc-ac operationThe DC-AC operation of the SSICC is utilized for delivering power from PV to grid, battery to grid, PV and battery to grid, and grid to battery. The SSICC performs the MPPT algorithm to deliver maximum power from the PV to the grid. Like the conventional PV inverter control, the SSICC is implemented in the synchronous reference frame. The synchronous reference frame proportional-integral current control is employed. In a reference frame rotating synchronously with the fundamental excitation, the fundamental excitation signals are transformed in to dc signals. As a result, the current regulator forming the innermost loop of the control system is able to regulate ac cur-rents over a wide frequency range with high bandwidth and zero steady-state error. For the pulse width modulation (PWM) scheme, the conventional space vector PWM scheme is utilized. The above Fig. presents the overall control block diagram of the single stage converter in the DC-AC operation. For the dc/ac operation with the battery, the proposed system control should be coordinated with the battery management system (BMS), which is not shown in Fig.4.1. 4.2.2 Circuit diagram in dc/dc operation

FIG.4.2 OVERALL CIRCUIT DIAGRAM OF SSICC DC-DC MODE. 4.2.2(A) Circuit description for dc-dc operationThe DC-DC operation of the single stage converter is also utilized for delivering the maximum power from the PV to the battery. The single stage converter in the dc/dc operation is a boost converter that controls the cur-rent flowing into the battery. In this research, Li-ion battery has been selected for the PV-battery systems. Li-ion batteries require a constant current, constant voltage type of charging algorithm. In other words, a Li-ion battery should be charged at a set current level until it reaches its final voltage. At the final voltage, the charging process should switch over to the constant voltage mode, and provide the current necessary to hold the battery at this final voltage. Thus, the DC-DC converter performing charging process must be capable of providing stable control for maintaining either current or voltage at a constant value, depending on the state of the battery. Typically, a few percent capacity losses happen by not performing constant volt-age charging. However, it is not uncommon only to use constant current charging to simplify the charging control and process. The latter has been used to charge the battery. Therefore, from the control point of view, it is just sufficient to control only the inductor current. Like the DC-AC operation, the single stage converter performs the MPPT algorithm to deliver maximum power from the PV to the battery in the DC-DC operation. Fig. above shows the overall control block diagram of the single stage converter in the DC-DC operation. In this mode, the single stage converter control should be coordinated with the BMS, which is not shown in Fig.4.2.

4.3 Design considerations & modifications to conventional inverterOne of the most important requirements of the project is that a new power conversion solution for PV-battery systems must have minimal complexity and modifications to the conventional three-phase solar PV converter system. Therefore, it is necessary to investigate how a three-phase DC-AC converter operates as a DC-DC converter and what modifications should be made. It is common to use a LCL filter for a high-power three-phase PV converter and the single stage converter in the DC-DC operation is expected to use the inductors already available in the LCL filter. There are basically two types of inductors, coupled three-phase inductor and three single-phase inductors that can be utilized in the single stage converter circuit. TABLE 4.1 INDUCTANCE VALUE OF A COUPLED THREE-PHASE INDUCTOR IN THE DC/DC OPERATION

Using all three phases of the coupled three-phase inductor in the DC-DC operation causes a significant drop in the inductance value due to inductor core saturation. Table I presents an example of inductance value of a coupled three-phase inductor for the DC-DC operation, which shows significant drop in the inductance value. The reduction in inductance value requires inserting additional inductors for the DC-DC operation which has been marked as optional in Fig. 2. To avoid extra inductors, only one phase can perform the DC-DC operation. However, when only one phase, for instance phase B, is utilized for the DC-DC operation with only either upper or lower three insulated-gate bipolar transistors (IGBTs) are turned OFF as complementary switching, the circulating current occurs in phases A and C through filter capacitors, the coupled inductor, and switches, resulting in significantly high current ripple in phase B current, as shown in Fig.4.3. To prevent the circulating current in the dc/dc operation, the following two solutions are proposed;

1) All unused upper and lower IGBTs must be turned OFF2) The coupled inductor is replaced by three single-phase Inductors.

While the first solution with a coupled inductor is straight-forward, using three single-phase inductors makes it possible to use all three phase legs for the DC-DC operation. There are two methods to utilize all three phase legs for the DC-DC operation:

1) Synchronous operation2) Interleaving operation.

In the first solution, all three phase legs can operate synchronously with their own current control. In this case, the battery can be charged with a higher current compared to the case with one-phase DC-DC operation. This leads to a faster charging time due to higher charging current capability. However, each phase operates with higher current ripples. Higher ripple current flowing into the battery and capacitor can have negative effects on the lifetime of the battery and capacitors.To overcome the aforementioned problem associated with the synchronous operation, phases B and C can be shifted by applying a phase offset. For the interleaving operation using three phase legs, phases B and C are shifted by 120 and 240, respectively. The inductor current control in interleaving operation re requires a different inductor current sampling scheme, as shown in Fig. 9. How-ever, for three-phase interleaving, a modified sampling scheme is required to measure the average currents for all three phases. Therefore, the sampling points for phases B and C must be shifted by 120 and 240, respectively [see Fig.4.4], which may imply that computation required inductor current control for each phase should be done asynchronously. Using the interleaving operation reduces the ripples on the charging current flowing into the battery. Therefore, the filter capacitance value can be reduced significantly.

FIG.4.3. CIRCULATING CURRENT PATH IF ONE PHASE IS USED FOR THE DC/DC OPERATION OF SSICC WITH A COUPLED THREE-PHASE INDUCTOR.

FIG.4.4. INDUCTOR CURRENT SAMPLING SCHEMES IN THE INTERLEAVING OPERATION.(A) TWO-PHASE INTERLEAVING (B) THREE-PHASE INTERLEAVING. 4.4 Description of components used in SSICCThe circuit diagram of single-stage-converter consists of different components & description of those is as fallows. The conventional grid-tie PV inverter is connected to the grid and delivers the power from the PV to the grid. Therefore, the conventional grid-tie PV inverter requires grid synchronization and power factor control functions. The single stage converter consists of six IGBTs and diodes that have the rating of 1.2 kV and 100 Ap eak There is a pre charging circuit that limits an inrush current flowing into the capacitors of the three-phase inverter, when the dc power supply is initially connected to the three-phase inverter. The filter capacitors are used to reduce voltage and current ripples for the batteries. There is the voltage balancing circuit that limits an inrush current flowing into the filter capacitors of the batteries, when the battery system including the battery filter capacitors is initially connected to the inverter. There are three relays used for battery charging in the dc/dc operation. The relay rating is determined by the battery charging current requirement. As mentioned earlier, a passive load is used in single stage converter verification. A passive load has a maximum power of 3 kW under the air-cooled condition. At the top of the picture is the single stage converter consisting of six IGBTs, six diodes, filter inductors, capacitors, relays, and wires. At the bottom of the picture is the energy storage device, the K2 Li-ion battery. The specification of the K2 battery is described in Table II. The K2 battery has its own BMS. The master controls four slaves who have nine battery cells assigned. The BMS measures the state of the battery cell voltages, temperatures, and the current flowing into or out of the battery. It also determines the battery operating status such as normal, warning, and error in which status BMS uses the relays to protect the battery system and prevent any damage. The battery system includes a pre charging circuit to limit an inrush current flowing from the batteries into the capacitors that can be connected to the battery in parallel for a filtering purpose. The proposed system control algorithms are implemented with MATLAB/Simulink.

TABLE 4.2 LITHIUM-ION K2 BATTERY PARAMETERS

4.5 Summary In this chapter circuit diagram of the proposed system, necessary design considerations and modifications to the conventional three-phase PV converter & circuit components in brief & next chapter explains about modulation topologies of three phase voltage source inverter.

CHAPTER 5MODULATION TOPOLOGIES

5.1 IntroductionThis chapter explains about the modulation topologies for three phase voltage source inverter.5.2 Pulse width modulation in invertersOutput voltage from an inverter can also be adjusted by exercising a control within the inverter itself. The most efficient method of doing this is by pulse-width modulation control used within an inverter. In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as Pulse-Width Modulation (PWM) Control. The advantages possessed by PWM techniques are as under: (i) The output voltage control with this method can be obtained without any additional components. (ii) With the method, lower order harmonics can be eliminated or minimized along with its output voltage control .As higher order harmonics can be filtered easily, the filtering requirements are minimized. Pulse-width modulation (PWM) is a technique where the duty ratio of a pulsating waveform is controlled by another input waveform. The intersections between the reference voltage waveform and the carrier waveform give the opening and closing times of the switches.PWM is commonly used in applications like motor speed control, converters, audio amplifiers, etc. For example, it is used to reduce the total power delivered to a load without losses, which normally occurs when a power source is limited by a resistive element.PWM is used to adjust the voltage applied to the motor. Changing the duty ratio of the switches changes the speed of the motor. The longer the pulse is closed compared to the opened periods, the higher the power supplied to the load is. The change of state between closing (ON) and opening (OFF) is rapid, so that the average power dissipation is very low compared to the power being delivered. PWM amplifiers are more efficient and less bulky than linear power amplifiers. In addition, linear amplifiers that deliver energy continuously rather than through pulses have lower maximum power ratings than PWM amplifiers. There is no single PWM method that is the best suited for all applications and with advances in solid-state power electronic devices and microprocessors, various pulse-width modulation (PWM) techniques have been developed for industrial applications. For these reasons, the PWM techniques have been the subject of intensive research since 1970s.The SPWM technique is the easiest modulation scheme to understand and to implement in software or hardware but this technique is unable to fully utilize the DC bus supply voltage available to the voltage source inverter. This drawback led to the development of THIPWM and SVPWM. THIPWM is a technique that adds a third-order harmonic content to a sinusoidal reference signal thereby increasing the utilization rate of the DC bus voltage by 15.5%.The implementation of the conventional SVPWM is especially difficult because it requires complicated mathematical operations. In the under-modulation region, this algorithm provides 15.5% higher output voltages compared to the SPWM technique. Moreover, the utilization of the DC bus voltage can be further increased when extending in to the over-modulation region of SVPWM. Three-phase voltage source pulse-width modulation inverters have been widely used for DC to AC power conversion since they can produce outputs with variable voltage magnitude and variable frequency.5.3 Different types of PWM techniques

PWM TECHNIQUESSinusoidal PWMThird-Harmonic-Injection PWMSpace Vector Pulse Width Modulation for 3-phase VSI:FIG.5.1 TYPES OF PWM TECHNIQUES5.3.1 Sinusoidal PWMThe sinusoidal pulse-width modulation (SPWM) technique produces a sinusoidal waveform by filtering an output pulse waveform with varying width. A high switching frequency leads to a better filtered sinusoidal output waveform. The desired output voltage is achieved by varying the frequency and amplitude of a reference or modulating voltage. The variations in the amplitude and frequency of the reference voltage change the pulse-width patterns of the output voltage but keep the sinusoidal modulation. A low-frequency sinusoidal modulating waveform is compared with a high-frequency triangular waveform, which is called the carrier waveform. The switching state is changed when the sine waveform intersects the triangular waveform. The crossing positions determine the variable switching times between states. In three-phase SPWM, a triangular voltage waveform (VT ) is compared with three sinusoidal control voltages (Va, Vb, and Vc), which are 120 degrees out of phase with each other and the relative levels of the waveforms are used to control the switching of the devices in each phase leg of the inverter.5.3.2 Third-harmonic-injection PWMThe sinusoidal PWM is the simplest modulation scheme to understand but it is unable to fully utilize the available DC bus supply voltage. Due to this problem, the third-harmonic injection pulse-width modulation (THIPWM) technique was developed to improve the inverter performance.5.3.3 Space vector pulse width modulation for 3-phase VSIAnother method for increasing the output voltage about that of the SPWM technique is the space vector PWM (SVPWM) technique. Compared to THIPWM, the two methods have similar results but their methods of implementation are completely different. In the SVPWM technique, the duty cycles are computed rather than derived through comparison as in SPWM. The SVPWM technique can increase the fundamental component by up to 27.39% that of SPWM. The fundamental voltage can be increased up to a square wave mode where a modulation index of unity is reached. SVPWM is accomplished by rotating a reference vector around the state diagram, which is composed of six basic non-zero vectors forming a hexagon. .The topology of a three-leg voltage source inverter is shown in Fig.5.2. Because of the constraint that the input lines must never be shorted and the output current must always be continuous a voltage source inverter can assume only eight distinct topologies. These topologies are shown on Fig. 5.3. Six out of these eight topologies produce a nonzero output voltage and are known as non-zero switching states and the remaining two topologies produce zero output voltage and are known as zero switching states.A three-phase mathematical system can be represented by a space vector. For example, given a set of three-phase voltages, a space vector can be defined by

Where Va(t), Vb(t), and Vc(t) are three sinusoidal voltages of the same amplitude &Frequency .But phase shift by 120 degrees. The space vector at any given time maintains its magnitude. As time increases, the angle of the space vector increases, causing the vector to rotate with a frequency equal to that of the sinusoidal waveforms. When the output voltages of a three-phase six-step inverter are converted to a space vector and plotted on the complex plane, the corresponding space vector takes only on one of six discrete angles as time increases. The central idea of SVWPM is to generate appropriate PWM signals so that a vector with any desired angle can be generated.In the space-vector modulation, a three-phase two-level inverter can be driven to eight switching states where the inverter has six active states (1-6) and two zero states (0 and 7).A typical two-level inverter has 6 power switches (labeled S1 to S6) that generate three phase voltage outputs. A detailed drawing of a three-phase bridge inverter is shown in Figure 5.2. The circuit has a full-bridge topology with three inverter legs, each consisting of two power switches. The circuit allows only positive power flow from the supply system to the load via a full-bridge diode rectifier. Negative power flow is not possible through the rectifier diode bridge.The six switching power devices can be constructed using power BJTs, GTOs, IGBTs, etc. The choice of switching devices is based on the desired operating power level, required switching frequency, and acceptable inverter power losses. When an upper transistor is switched on, the corresponding lower transistor is switched off. Therefore, then ON and OFF states of the upper transistors S1; S3; S5 can be used to determine the current output voltage. The ON and OFF states of the lower power devices are complementary to the upper ones. Two switches on the same leg cannot be closed or opened at the same time. The basic principle of SVPWM is based on the eight switch combinations of a three phase inverter. The switch combinations can be represented as binary codes that correspond to the top switches S1, S3, and S5 of the inverter as shown in Figure 5.2. Each switching circuit generates three independent pole voltages Vao, Vbo, and Vco, which are the inverter output voltages with respect to the mid-terminal of the DC source marked as O on the same figure. These voltages are also called pole voltages.The pole voltages that can be produced are either or. For example,When switches S1, S6, and S2 are closed, the corresponding pole voltages are ,, and. This state is denoted as (1, 0, 0).The three-phase inverter is therefore controlled by six switches & eight inverter configurations. The eight inverter states can be transformed into eight corresponding space vectors. In each configuration, the vector identification uses a 0 to represent the negative phase voltage level and a 1 to represent the positive phase voltage level. The relationship between the space vector and the corresponding switching states is given in Table 5.1 and Figure 5.2. In addition, the switches in one inverter branch are in controlled in a complementary fashion (1 if the switch is on and 0 if it is off). Therefore,

FIG. 5.2: TOPOLOGY OF A THREE-LEG VOLTAGE SOURCE INVERTER

FIG. 5.3: EIGHT SWITCHING CONFIGURATION OF A THREE-PHASE INVERTER.The reference voltage vector rotates in space at an angular velocity w = 2f, Where f is the fundamental frequency of the inverter output voltage. When the reference voltage vector passes through each sector, different sets of switches in Table 5.1 will be turned on or off. As a result, when the reference voltage vector rotates through one revolution in space, the inverter output varies one electrical cycle over time. The inverter output frequency coincides with the rotating speed of the reference voltage vector. The zero vectors ( & ) and active vectors ( to ) do not move in space. They are referred to as stationary vectors.TABLE 5.1: SPACE VECTORS, SWITCHING STATES, AND ON STATE SWITCHES

5.3.3(A) Voltage space vectorsSpace vector modulation (SVM) for three-leg VSI is based on the representation of the three phase quantities as vectors in a two-dimensional ( ) plane. This is illustrated here for the sake of completeness. Considering topology 1 of Fig. 10, which is repeated in Fig. 11(a) we see that the line voltages Vab, Vbc, and Vca are given byVab = Vg Vbc = 0 Vca = -Vg 1This can be represented in the , plane as shown in Fig. 5.4, where voltages Vab, Vbc, and Vca are three line voltage vectors displaced 120 degrees in space. The effective voltage vector generated by this topology is represented as V1 (pnn) in Fig. 5.4. Here the notation pnn refers to the three legs/phases a,b,c being either connected to the positive dc rail (p) or to the negative dc rail (n). Thus pnn corresponds to phase a being connected to the positive dc rail and phases b and c being connected to the negative dc rail.

FIG. 5.4 TOPOLOGY 1-V1 (PNN) OF A VOLTAGE SOURCE INVERTER.

FIG. 5.5 REPRESENTATION OF TOPOLOGY 1 IN , PLANE.Proceeding on similar lines the six non-zero voltage vectors (V1 - V6) can be shown to assume the positions shown in Fig.5.6. The tips of these vectors form a regular hexagon (dotted line in Fig. 5.6). We define the area enclosed by two adjacent vectors, within the hexagon, as a sector. Thus there are six sectors numbered 1 - 6 in Fig. 5.6.

FIG. 5.6 NON-ZERO VOLTAGE VECTORS IN THE PLANE.Considering the last two topologies of Fig. 5.3 which are repeated in Fig. 5.7 for the sake of convenience we see that the output line voltages generated by this topology are given by

Vab = Vg Vbc = 0 Vca = -Vg 2These are represented as vectors which have zero magnitude and hence are referred to as zero-switching state vectors or zero voltage vectors. They assume the position at origin in the , plane as shown in Fig. 5.8. The vectors V1-V8 are called the switching state vectors (SSVs).

FIG 5.7 ZERO OUTPUT VOLTAGE TOPOLOGIES.

FIG. 5.8 REPRESENTATION OF THE ZERO VOLTAGE VECTORS IN THE PLANE.

5.3.3(B) Space vector modulationThe desired three phase voltages at the output of the inverter could be represented by an equivalent vector V rotating in the counter clock wise direction as shown in Fig. 5.9. The magnitude of this vector is related to the magnitude of the output voltage (Fig. 5.10)) and the time this vector takes to complete one revolution is the same as the fundamental time period of the output voltage.

FIG. 5.9 OUTPUT VOLTAGE VECTOR IN THE PLANE.

FIG. 5.10 OUTPUT LINE VOLTAGES IN TIME DOMAIN.Let us consider the situation when the desired line-to-line output voltage vector V is in sector 1 as shown in Fig. 5.11. This vector could be synthesized by the pulse-width modulation (PWM) of the two adjacent SSVs V1 (pnn) and V2 (ppn), the duty cycle of each being d1 and d2, respectively, and the zero vector (V7 (nnn) / V8 (ppp)) of duty cycle d0:

d1 V1 + d2 V2 = V = m Vg eje 3

d1 + d2 + d3 = 1 4

Where, 0 m 0.866, is the modulation index. This would correspond to a maximum line-to-line voltage of 1.0Vg, which is 15% more than conventional sinusoidal PWM as shown.All SVM schemes and most of the other PWM algorithms use Eqns. (3) and (4) for the output voltage synthesis. The modulation algorithms that use non-adjacent SSVs have been shown to produce higher THD and/or switching losses and are not analyzed here, although some of them, e.g. hysteresis, can be very simple to implement and can provide faster transient response. The duty cycles d1, d2, and d0, are uniquely determined from Eqns. (3) and (4) , the only difference between PWM schemes that use adjacent vectors is the choice of the zero vector(s) and the sequence in which the vectors are applied within the switching cycle.

FIG. 5.11 SYNTHESIS OF REQUIRED OUTPUT VOLTAGE VECTOR IN SECTOR 1.The degrees of freedom we have in the choice of a given modulation algorithm is:1) The choice of the zero vector; whether we would like to use V7(ppp) or V8(nnn) or both. 2) Sequencing of the vectors.3) Splitting of the duty cycles of the vectors without introducing additional commutations.5.3.3 (C) Implementing SVPWMThe SVPWM can be implemented by using wither sector selection algorithm or by using a carrier based space vector algorithm. The types of SVPWM implementations are:- a) Sector selection based space vector modulation b) Reduced switching Space vector modulation c) Carrier based space vector modulation d) Reduced switching carrier based space vector modulation. 5.4 SummaryThis chapter explains about the modulation topologies for three phase voltage source inverter in brief and chapter6 explains about MPPT.

CHAPTER 6MAXIMUM POWER POINT TRACKING

6.1 IntroductionIn chapter5 the modulation topologies for three phase voltage source inverter was explained in brief. This chapter explains about maximum power point tracking.6.2 Requirement for maximum power point tracking (MPPT)Maximum power point tracking, frequently referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in a manner that allows the modules to produce all the power they capable of. MPPT is not a mechanical tracking system that physically moves the modules to make them point more directly at the sun. Since the power output of a PV array varies according to the sunlight conditions, atmospheric conditions, including cloud cover local surface reflectivity and temperature. So MPPT is necessary in order to extract the maximum power from the array at all times. MPPT is a fully electronic system that varies the electrical operating point of the modules so that the modules are able to deliver maximum available power. the MPPT varies the ratio between the voltage and current delivered to the battery, in order to deliver maximum power. If there is excess voltage available from the PV, then it converts that to additional current to the battery. Furthermore, it is like an automatic transmission. As the Vpp of the PV array varies with temperature and other conditions, it "tracks" this variance and adjusts the ratio accordingly.

FIG.6.1: PV MODULE POWER/VOLTAGE/CURRENT CHARACTERISTICS

The function of a MPPT is analogous to the transmission in a car. When the transmission is in the wrong gear, the wheels do not receive maximum power. That's because the engine is running either slower or faster than its ideal speed range. The purpose of the transmission is to couple the engine to the wheels, in a way that lets the engine run in a favorable speed range in spite of varying acceleration and terrain. Let's compare a PV module to a car engine. Its voltage is analogous to engine speed. Its ideal voltage is that at which it can put out maximum power. This is called its maximum power point. (It's also called peak power voltage, abbreviated Vpp). Vpp varies with sunlight intensity and with solar cell temperature. The voltage of the battery is analogous to the speed of the car's wheels. It varies with battery state of charge, and with the loads on the system (any appliances and lights that may be on). For a 12V system, it varies from about 11 to 14.5V. In order to charge a battery (increase its voltage), the PV module must apply a voltage that is higher than that of the battery. If the PV module's Vpp is just slightly below the battery voltage, then the current drops nearly to zero (like an engine turning slower than the wheels). So, to play it safe, typical PV modules are made with a Vpp of around 17V when measured at a cell temperature of 25C. They do that because it will drop to around 15V on a very hot day. However, on a very cold day, it can rise to 18V!

FIG.6.2: SYSTEM CONFIGURATION OF PV SYSTEM6.3 System configurationThe system configuration for the topic is as shown. Here the PV array is a combination of series and parallel solar cells. This array develops the power from the solar energy directly and it will be changes by depending up on the temperature and solar irradiances. So we are controlling this to maintain maximum power at output side we are boosting the voltage by controlling the current of array with the use of PI controller. The relevant circuit is as shown. After we are getting the maximum power we are applying to the utility grid.6.3.1 PV arrayA photovoltaic cell is nothing but a Solar cell which consists of a p-n junction fabricated in a thin wafer or layer of semiconductor (usually silicon). In the dark, the I-V output characteristic of a solar cell has an exponential characteristic similar to that of a diode. When solar energy (photons) hits the solar cell, with energy greater than band gap energy of the semiconductor, electrons are knocked loose from the atoms in the material, creating electron-hole pairs. These carriers are swept apart under the influence of the internal electric fields of the p-n junction and create a current proportional to the incident radiation. The voltage and current relationship of the simplified solar cell based on Kirchhoffs current law.6.3.2 Boost converterThe boost converter is nothing but a DC/DC converter which has boosting the voltage to maintain the maximum output power constant for all the conditions of temperature and solar irradiance variations. Shown in fig.6.3

FIG.6.3: BOOST TOPOLOGYWhen the switch S is on, the current builds up in the inductor L due to the positive inductor voltage is equal to the input voltage. When S is off, the voltage across L reverses and adds to the input voltage, thus makes the output voltage greater than the input voltage. For steady state operation, the average voltage across the inductor over a full period is zero. By designing this circuit we can also investigate performance of converters which have input from solar energy.6.3.3 Maximum power point tracking (MPPT)Maximum Power Point Tracker (MPPT) has been used to force the PV array to work around the maximum power point. For this reason, the MPPT is required to track the maximum power available in the PV array. The MPPT operates by periodically incrementing the terminal voltage of the PV array and continuously seek. The radiation and temperature are used to calculate the maximum PV array output power and PV array terminal voltages. The MPPT operates by periodically incrementing the terminal voltage of the PV array and continuously seek the peak power point. The control system adjusts the boost converter to seek maximum power point of PV array. A comparison between the terminal voltages (actual and optimum) will control the duty ratio of boost converter. Changing the duty ratio according to the error signal between the maximum and actual power will pass the maximum power available from PV to the electric utility. A comparison between actual and reference values for PV terminal voltage and maximum power available from PV array will control the duty ratio of boost converter. The PV simulator uses the radiation, temperature and output current from PV to determine the corresponding PV curve by using equations. The output power from PV is the result from multiplying PV terminal voltage and PV output current.Different algorithms help to track the peak power point of the solar pv module automatically. The various algorithms used are.A. Perturb and observe.B. Incremental Conductance.C. Parasitic Capacitance.D. Voltage Based Peak Power Tracking.E. Current Based peak power Tracking.A perturb and observe: In this algorithm a slight perturbation is introduce system. Due to perturbation, power of the module changes. If the power increases due to the perturbation then the perturbation is continued in that direction. After the peak power is reached the power at the next instant decreases and hence after that the perturbation reverses.

FIG.6.4. PERTURB AND OBSERVE ALGORITHMWhen the steady state is reached the algorithm oscillates around the peak point. In order to keep the power variation small the perturbation size is kept very small. The algorithm is developed in such a manner that it sets a reference voltage of the module corresponding to the peak voltage of the module. A PI controller then acts moving the operating point of the module to that particular voltage level. It is observed that there some power loss due to this perturbation also the fails to track the power under fast varying atmospheric conditions. But still this algorithm is very popular and simple.B. Incremental conductance: The disadvantage of the perturb and observe method to track the peak power under fast varying atmospheric condition is overcome by Incremental conductance method. The algorithm makes use of the equation P=V*I 1(Where P= module power,V=module voltage, I=module current);Diff with respect to dVdP/dV = I+V*dI/dV 2 Depending on this equation the algorithm works.At peak power point dP/dV = 0 3 dI/dV = -I/V 4

FIG.6.5.INCREMENTAL CONDUCTANCE METHOD.If the operating point is to the right of the Power curve then we have dP/dV < 0 5 dI/dV < I/V 6If operating point is to the left of the power curve then we have dP/dV > 0 7 dI/dV > I/V 8 Using equations 7, 9& 10 the algorithm works. The incremental conductance can determine that the MPPT has reached the MPP and stop perturbing the operating point. If this condition is not met, the direction in which the MPPT operating point must be perturbed can be calculated using the relationship between dl/dV and I/V This relationship is derived from the fact that dP/dV is negative when the MPPT is to the right of the MPP and positive when it is to the left of the MPP. This algorithm has advantages over perturb and observe in that it can determine when the MPPT has reached the MPP, where perturb and observe oscillates around the MPP. Also, incremental conductance can track rapidly increasing and decreasing irradiance conditions with higher accuracy than perturb and observe. One disadvantage of this algorithm is the increased complexity when compared to perturb and observe.The solar array terminal voltage can be adjusted relative to the maximum power point voltage by measuring the incremental and instantaneous array conductance (dI/dV and I/V, respectively). Although the incremental conductance method offers good performance under rapidly changing atmospheric conditions, four sensors are required to perform the computations. The drawback is that sensor devices require more conversion time thus result in a large amount of power loss. Figure 6.7 views the algorithm of incremental conductance and the operation of incremental conductance algorithm is shown in Figure 6.6

FIGURE 6.6 THE OPERATION OF THE INCREMENTAL CONDUCTANCE METHOD.

FIGURE 6.7: CONDUCTANCE INCREMENTAL ALGORITHM FLOWCHARTC. Parasitic capacitances: The parasitic capacitance method is a refinement of the incremental conductance method that takes into account the parasitic capacitances of the solar cells in the PV array. Parasitic capacitance uses the switching ripple of the MPPT to perturb the array. To account for the parasitic capacitance, the average ripple in the array power and voltage, generated by the switching frequency, are measured using a series of filters and multipliers and then used to calculate the array conductance. The incremental conductance algorithm is then used to determine the direction to move the operating point of the MPPT. One disadvantage of this algorithm is that the parasitic capacitance in each module is very small, and will only come into play in large PV arrays.Where several module strings are connected in parallel. Also, the DC-DC converter has a sizable input capacitor used filter out small ripple in the array power. This capacitor may mask the overall effects of the parasitic capacitance of the PV array.D. Voltage control maximum point tracker: It is assumed that a maximum power point of a particular solar PV module lies at about 0.75 times the open circuit voltage of the module. So by measuring the open circuit voltage a reference voltage can be generated and feed forward voltage control scheme can be implemented to bring the solar pv module voltage to the point of maximum power. One problem of this technique is the open circuit voltage of the module varies with the temperature. So as the temperature increases the module open circuit voltage changes and we have to measure the open circuit voltage of the module very often. Hence the load must be disconnected from the module to measure open circuit voltage. Due to which the power during that instant will not be utilize.E. Short-circuit current MPPT algorithmCurrent control maximum power point tracker: The peak power of the module lies at the point which is at about 0.9 times the short circuit current of the module. In order to measure this point the module or array is short-circuited. And then by using the current mode control the module current is adjusted to the value which is approx 0.9 times the short circuit current. The problem with this method is that a high power resistor is required which can sustain the short-circuit current. The module has to be short circuited to measure the short circuit current as it goes on varying with the changes in isolation level.This method exploits the assumption of linear relationship between the cell current corresponding to maximum power (IMP) and the cell-short circuit current (ISC). This relationship can be expressed as:IMP = K . ISCWhere K is called the current factorPeak Power of the module lies at about 90% of its short circuit current. Measuring the short circuit current Isc and adjusting the operating the converter at 90% of Isc the module can be made to operate at Peak power.The Flowchart of Short-circuit current MPPT is shown below

FIG .6.8. SHORT CIRCUIT CURRENT MPPT FLOWCHART

6.3.4 Comparison of incremental conductance and short circuit current MPPT methods

TABLE.6.1 COMPARISON OF INCREMENTAL CONDUCTANCE AND SHORT CIRCUIT CURRENT MPPT METHODS

Sl.No.SPECIFICATIONINCREMENTALCONDUCTANCESHORT-CIRCUIT CURRENT

1EFFICIENCYHigh about 98%Low about 90%

2COMPLEXITYDifficultyVery simple

3REALIZATIONMore complexEasy

4COSTHigh costRelatively lower

5RELIABILITYAccurateNot accurate

6RAPIDLY CHANGING ATMOSPHEREGood and automatically adjusts modules operating voltageFaster response

TABLE 6.2 SHOWING THE MAXIMUM POWER, VOLTAGE, CURRENT AT DIFFERENT IRRADIANCE:

Sl.No.IRRADIENCE(W/M2)POWER(W)AVAILABESHORT-CIRCUIT CURRENTIMAXVMAX

110001169109.3125

280091987.4123

TABLE 6.3 SHOWING THE POWER TRACKED AT DIFFERENT IRRADIANCE.METHODSINCREMENTALCONDUCTANCESHORT-CIRCUIT CURRENT

IRRADIENCEVoltage(v)Current(A)Power tracked(W)Voltage(v)Current(A)Power tracked(W)

1000124.39.3311621179.6761132

800105.27.89831.3138.75.343741.3

6.4 SummaryIn this chapter maximum power point tracking was explained in brief .Chapter7 explains about explains about simulation of simulation of SSICC.

CHAPTER 7SIMULATION CIRCUIT

7.1 IntroductionThe above chapter explains the modulating topologies & its types. In this chapter simulation of SSICC& results are shown. It integrates computation, visualization & programming in an easy to use environment where problems & solutions are expressed in familiar mathematical notation. Typical uses include: Math & computation Algorithm development Data acquisition Modeling, simulation & prototyping Data analysis, exploration & visualization Scientific & engineering graphics Application development like graphical user interface building MATLAB is an interactive system whose basic data element is an array that does not require dimensioning. This allows you to solve many technical computing problems especially those with matrix & vector formulations in a fraction of the time it would take to write a program in a scalar non interactive. The name MATLAB software developed by the LINPAC & EISPACK projects .Today, MATLAB engines incorporate the LAPACK & BLAS libraries, embedding the state of art in software for matrix computation.MATLAB has evolved over a period of years with input from many users. In university environments, it is standard instructional tool for introductory & advanced courses in mathematics, engineering & science. In industry MATLAB is the tool of choice for high productivity research, development & analysis .MATLAB features a family of add-on application-specific solutions called toolboxes. Very important to more users of MATLAB toolboxes allow you to learn & specialized technology .Toolboxes are comprehensive collections of MATLAB functions (M-Files) that extend the MATLAB environment to solve particular problems .Areas in which toolboxes are available include signal processing ,control systems ,neural networks ,fuzzy logic , wavelets, simulation & many others.7.2 Simulation circuit for dc/dc operation mode

FIG.7.1 SIMULINK MODEL IN DC/DC CONVERSION MODEThe fig 7.1 shows simulink model of SSICC in dc/dc operation mode .In this mode of operation SSICC converts fixed DC from PV cells to variable DC in order to connect PV cells with the batteries.

7.3 Controller block for dc - dc operation FIG.7.2 SIMULINK MODEL OF CONTROLLER BLOCK FOR DC-DC OPERATION

7.4 MPPT block for DC DC operation

FIG.7.3 SIMULINK MODEL OF MPPT CONTROLLER BLOCK FOR DC-DC OPERATION

7.5 Simulation circuit for dc/ac operation mode

FIG.7.4 SIMULINK MODEL OF CONTROLLER BLOCK FOR DC-AC OPERATION

7.6 Controller blocks for DC - AC operation

FIG.7.5 SIMULINK MODEL OF CONTROLLER BLOCK FOR DC -AC OPERATION

7.7 MPPT block for DC AC operation

FIG.7.6 SIMULINK MODEL OF MPPT CONTROLLER BLOCK FOR DC -AC OPERATION

The fig 7.4 shows simulink model of SSICC in DC/AC operation mode .In this mode of operation SSICC converts fixed DC from PV cells to variable AC in order to connect PV cells with the grid, battery to the grid.

7.8 Simulation out puts in DC- DC mode7.8.1PV voltage (DC-DC)

7.8.2 PV Current (DC-DC)

7.8. Voltage across the capacitor (DC-DC)

7.8.4 Battery State of charge mode (SOC) (DC-DC)

7.8.5 Voltage across Battery (DC-DC)

7.9 Simulation out puts in DC-AC MODE

7.9.1 Voltage across the Grid (DC AC)

7.9.2 Current across Grid (DC AC)

CHAPTER 8CONCLUSION

The single stage inverter cum chopper for PV- battery application works efficiently for generating the required output. The SSICC power conversion system performs different operating modes such as PV to grid (dc to ac), PV to battery (dc to dc), battery to grid (dc to ac), and battery/PV to grid (dc to ac) in different control strategies for solar PV systems with energy storage. Test results for various inputs shown in the above figures confirm that the proposed system is an optimal solution for PV-battery power conversion systems. The proposed solution requires minimal complexity and modifications to the conventional three-phase solar PV converters for PV-battery systems. Therefore the solution is very attractive for PV-battery applications because it minimizes the number of conversion stages, thereby improving effici