CHAPTER 1INTRODUCTION 1.1GENERALRenewable energy sources also called non-conventional type of energy are the sources which are continuously replenished by natural processes. Solar energy, bio-energy, bio-fuels grown sustainably, wind energy and hydropower are some of the examples of renewable energy sources. A renewable energy system converts the energy found in sunlight, falling-water, wind, sea-waves, geothermal heat, or biomass into a form, which we can use in the form of heat or electricity. The majority of the renewable energy comes either directly or indirectly from the sun and can never be exhausted, and therefore they are called renewable.However, the majority of the world's energy sources come from conventional sources-fossil fuels such as coal, natural gas and oil. These fuels are often termed non-renewable energy sources. Though, the available amount of these fuels are extremely large, but due to decrease in level of fossil fuel and oil level day by day after a few years it will end. Hence renewable energy source demand increases as it is environmental friendly and pollution free which reduces the greenhouse effect .1.2 MOTIVATION The Conventional sources of energy are rapidly depleting. Moreover the cost of energy is rising and therefore photovoltaic and wind energy conversion systems are promising alternatives. They are abundant, pollution free, distributed throughout the earth and recyclable. The hindrance factor is its high installation cost and low conversion efficiency. Therefore our aim is to increase the efficiency and power output of the system by implementing Maximum Power Point tracking (MPPT) for both PV system and WECS. It is also required to model PV system and WECS with appropriate power electronic interfacing with the grid and load under varying solar irradiance,wind speed and load variations.

1.3 STATE OF THE ARTPV system and Wind energy conversion system are likely to contribute to significant portions of grid electricity, they also bring new challenges in system operation such as meeting grid connection requirements, security, power quality of supply, reactive power control and maximum power capture.1.3.1 PV AND WECS AND ASSOCIATED CONTROL ASPECTSThere are different Control Techniques for DC-DC Converters, inverters for the PV System. The Boost converter is highly relevant to DC-DC Converter today and they may be used. There are many MPPT methods among which Incremental conductance method with direct control is used. There are different design philosophies for rectifiers and inverters for the variable speed WECS. During recent years different converter topologies have been investigated for the applicability in WECS. The back to back converter is highly relevant to WTs today and they may be used for benchmarking the other converter topologies.Typical self-commutated converters are dominating the market because they have high switching frequencies and harmonics can be filtered out easily. Today, the most commonly used device is an IGBT having a typical switching frequency of 2 to 20 KHZ. Self- commutated converters are either Voltage Source Converters (VSCs) or Current Source Converters (CSCs) and can control both frequency and the voltage. VSCs and CSCs supply a relatively well defined switched voltage and current wave form, respectively. They can be implemented in several ways: six step, pulse amplitude modulation (PAM) and pulse width modulation (PWM). By using the PWM technique, lower frequency harmonics can be eliminated; the frequencies of the first higher order harmonics lie at about the switching frequency of the inverter.There are different control strategies for the Machine side converter ( MSC) such as Flux oriented control (FOC) and Direct Torque Control (DTC) that allow very efficient and controlled variable speed operation enabling MPPT [24] . In this work, a rotor flux oriented vector control strategy is used for generator control to achieve high dynamic performance.A vector control approach is generally used for GSC with the reference frame oriented along with the grid voltage sector, where an independent control of active and reactive power is achieved. Hysteresis Current Control (HCC) and Synchronous Voltage Oriented Control (VOC) are the state of the art control techniques being used presently for the inner current control loop [23]. In synchronous reference frame control, all the voltage and current variables are transformed to a synchronously rotating reference frame so that control variables become DC quantities and hence one can use traditional PI controllers to control the same. Phase angle detection has a significant role in control of GSC. several algorithms capable of detecting the grid voltage phase angle like zero crossing detection, use of arc tangent function or Phase Locked Loop (PLL).We are using Phase locked loop technique [25] in this project.1.4 ORGANIZATION OF THE THESIS Chapter 1 gives an introduction to the renewable energy sources, state of art with regard to control implementation for PV system and WECS and Organization of the thesis chapters are also included in this chapter.Chapter 2 is about modelling and simulation of PV system, PV arrangements, materials used in PV cells, PV characteristics, boost converter and its operation, hysteresis band current control PWM , incremental conductance method with direct control, GSC controller, simulink model of PV System, simulation results of PV system.Chapter 3 gives an introduction to variable speed wind energy system,describes the WT aerodynamics and various parameters that affect the WT performance.This chapter also describes the aerodynamic control and various methods used for achieving optimal power control, modelling of SCIG based WECS, simulation of SCIG based WECS, simulation results of SCIG based WECS.Chapter 4 is simulation of grid connected hybrid Wind-PV system, simulation results of grid connected hybrid Wind-PV system.Chapter 5 is the conclusion of the thesis, which summarizes the main work done.

CHAPTER 2MODELLING AND SIMULATION OF PHOTO-VOLTAIC SYSTEM2.1 INTRODUCTIONMany renewable energy technologies today are well developed, reliable and cost competitive with the conventional fuel generators. Among various renewable energy technologies, the solar energy has several advantages like clean power, unlimited, and provides sustainable electricity. However, the solar energy produces DC power, and hence power electronics and control equipment are required to convert DC to AC power. The performance of the power inverter depends on the control strategy adopted to generate the gate pulses. To control the inverters, current control methods are normally used.There are several current control strategies. Among the various current control techniques, hysteresis control is the most popular one for voltage source inverter, most of photovoltaic inverters are voltage-source inverters. The hysteresis band current control is very simple, has robust current control performance with good stability, very fast response, an inherent ability to control peak current and easy to implement. MATLAB simulations are carried out for modelling solar photovoltaic array based on its mathematical equations and the developed model is used to interconnect DC to DC converter, hysteresis current controlled DC to AC converter with the grid. By implementing MPPT to DC-DC converter we can track maximum power from PV array [10]. Simulation results of PV system are presented in this chapter. 2.2 MODELLING OF PV SYSTEMDEFINITIONA photovoltaic system is a system which uses one or more solar panels to convert solar energy into electricity. It consists of multiple components, including the photovoltaic modules, mechanical and electrical connections and mountings and means of regulating and/or modifying the electrical output. 2.2.1 SOLAR ENERGY Solar energy is a non-conventional type of energy. Solar energy has been harnessed by humans since ancient times using a variety of technologies. However a small fraction of the available solar energy is currently being exploited. Solar power is possible through photovoltaic system or through solar thermal power generation system. Solar energy's uses are limited only by human creativity. To harvest the solar energy, the most common way is to use photo voltaic panels which receive photon energy from the sun and convert it to electrical energy. Solar technologies are broadly classified as either passive solar or active solar depending on the way they detain, convert and distribute solar energy. Active solar techniques include the use of PV panels and solar thermal collectors to strap up the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties and design spaces that naturally circulate air. Solar energy has a vast area of application such as electricity generation for distribution, heating water, lightening building, crop drying etc.2.2.2 SOLAR RADIATION REACHING EARTH SURFACE The intensity of solar radiation reaching earth surface which is 1369 watts per square meter is known as Solar Constant. It is important to realize that it is not the intensity per square meter of the Earths surface but per square meter on a sphere with the radius of 149,596,000 km and with the Sun at its centre.The total amount of solar radiation intercepted by the Earth is the Solar Constant multiplied by the cross section area of the Earth. If we now divide the calculated number by the surface area of the Earth, we shall find how much solar radiation is received in an average per square meter of the Earth's surface . Hence the average solar radiation R per square meter of the Earth surface is

where S is the solar constant (1369 w/m^2 ), r is the earth radius.The Handy formula which is used to calculate solar energy received by earth

where E is the solar energy in EJ. S is the Solar Constant in w/m2. n is the number of hours. r is the Earth's radius in km .2.2.3 STANDARD TEST CONDITIONS (STC) The comparison between different photovoltaic cells can be done on the basis of there performance and characteristic curve. The parameters are always given in datasheet. The datasheet make available the parameter regarding the characteristics and performance of PV cells with respect to standard test condition. Standard test conditions are as follows: Temperature (Tn) = 25Irradiance (Gn) = 1000 w/2.2.4 EFFICIENCY OF A PV CELLThe efficiency of a PV cell is defined as the ratio of peak power to input solar power. Where, Vmp is the voltage at peak power, Imp is the current at peak power, I is the solar intensity per square metre, A is the area on which solar radiation fall. The efficiency will be maximum if we track the maximum power from the PV system at different environmental condition such as solar irradiance and temperature by using different methods for maximum power point tracking.2.2.5 PHOTOVOLTAIC ARRANGEMENTS 2.2.5.1 PHOTOVOLTAIC CELL PV cells are made of semiconductor materials, such as silicon. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current - that is, electricity. This electricity can then be used to power a load. A PV cell can either be circular or square in construction Fig.2.1 Basic structure of a PV cell 2.2.5.2 PHOTOVOLTAIC MODULE Due to the low voltage generated in a PV cell (around 0.5V), several PV cells are connected in series (for high voltage) and in parallel(for high current) to form a PV module for desired output. Separate diodes may be needed to avoid reverse currents, in case of partial or total shading, and at night. The p-n junctions of mono-crystalline silicon cells may have adequate reverse current characteristics. Reverse currents waste power and can also lead to overheating of shaded cells. Solar cells become less efficient at higher temperatures and installers try to provide good ventilation behind solar panels.2.2.5.3 PHOTOVOLTAIC ARRAY The power that one module can produce is not sufficient to meet the requirements of home or business. Most PV arrays use an inverter to convert the DC power into alternating current that can power the motors, loads, lights etc. The modules in a PV array are usually first connected in series to obtain the desired voltages; the individual modules are then connected in parallel to allow the system to produce more current.

Fig.2.2 Photovoltaic system2.2.6 MATERIALS USED IN PV CELL The materials used in PV cells are as follows: Single-crystal silicon Single-crystal silicon cells are the most common in the PV industry. The main technique for producing single-crystal silicon is the Czochralski (CZ) method. High-purity polycrystalline is melted in a quartz crucible. A single-crystal silicon seed is dipped into this molten mass of polycrystalline. As the seed is pulled slowly from the melt, a single-crystal ingot is formed. The ingots are then sawed into thin wafers about 200-400 micrometers thick (1 micrometer = 1/1,000,000 meter). The thin wafers are then polished, doped, coated, interconnected and assembled into modules and arrays . Polycrystalline silicon Consisting of small grains of single-crystal silicon, polycrystalline PV cells are less energy efficient than single-crystalline silicon PV cells. The grain boundaries in polycrystalline silicon hinder the flow of electrons and reduce the power output of the cell. A common approach to produce polycrystalline silicon PV cells is to slice thin wafers from blocks of cast polycrystalline silicon. Another more advanced approach is the ribbon growth method in which silicon is grown directly as thin ribbons or sheets with the approach thickness for making PV cells . Gallium Arsenide (GaAs) A compound semiconductor made of two elements: Gallium (Ga) and Arsenic (As). GaAs has a crystal structure similar to that of silicon. An advantage of GaAs is that it has high level of light absorptivity. To absorb the same amount of sunlight, GaAs requires only a layer of few micrometers thick while crystalline silicon requires a wafer of about 200-300 micrometers thick. Also, GaAs has much higher energy conversion efficiency than crystal silicon, reaching about 25 to 30%.The only drawback of GaAs PV cells is the high cost of single crystal substrate that GaAs is grown on .

Cadmium Telluride (CdTe) It is a polycrystalline compound made of cadmium and telluride with a high light absorbility capacity (i.e a small thin layer of the compound can absorb 90% of solar irradiation).The main disadvantage of this compound is that the instability of PV cell or module performance. As it a toxic substance, the manufacturing process precaution Copper Indium Diselenide (CuInSe2) It is a polycrystalline compound semiconductor made of copper, indium and selenium. It delivers high energy conversion efficiency without suffering from outdoor degradation problem. It is one of the most light-absorbent semiconductors. As it is a complex material and toxic in nature so the manufacturing process face some problem.2.2.7 CHARACTERISTICS OF PV CELL Fig.2.3 Equivalent circuit of a PV cellAn ideal PV cell is modeled by a current source in parallel with a diode. However no solar cell is ideal and there by shunt and series resistances are added to the model as shown in the PV cell diagram above. is the intrinsic series resistance whose value is very small. is the equivalent shunt resistance which has a very high value .Applying Kirchoffs law to the node where , diode, and meet, we get ++IWe get the following equation for the photovoltaic current: Where, Iph is the Insolation current, I is the Cell current, Io is the Reverse saturation current, V is the Cell voltage, is the Series resistance, is the Parallel resistance, is the Thermal voltage ( kT/q), K is the Boltzman constant, T is the Temperature in Kelvin, q is the Charge of an electron.MATLAB simulations are carried out for modelling solar photovoltaic array based on its mathematical equation and Figure 2.4 shows the simulation diagram for a PV cell. Simulation diagram for diode equivalent circuit is also shown in Figure 2.5.

Fig.2.4 Simulation diagram for PV cell equivalent circuit

Fig 2.5 Simulation diagram for diode equivalent circuit2.2.8 PARAMETERS USED IN THE MATLAB CODE The values of the parameters used in developing the MATLAB code for the Photovoltaic array have been tabled below Table 1 : PARAMETERS USED IN THE MATLAB CODE ParametersValues

11

3

64.2 V

5.96 A

54.7 V

5.58 A

305.2W

2.2.9 MATLAB CODE FOR PV ARRAY MODELLING SUNPOWER SPR -305-WHT 3*11*305=10KWSolar ModuleSpec(Type).Desc=SunPower SPR-305-WHT;Solar ModuleSpec(Type).ncells=96; %Number of cells in seriesSolar ModuleSpec(Type).Pmp=305.2; % Maximum Power (W)Solar ModuleSpec(Type).Vmp=54.70;%Maximum Power Voltage(V)Solar ModuleSpec(Type).Imp=5.58;%Maximum Power Current(A)Solar ModuleSpec(Type).Voc=64.20;%open circuit voltage(V)Solar ModuleSpec(Type).Isc=5.96;%short circuit current(A)Solar ModuleSpec(Type).Tempc_Pmp=-1.154e+000;%Maximum power temp.coefficient(w/deg.c)Solar ModuleSpec(Type).Tempc_Vmp=-1.860e-001;%Maximum power Voltage temp.coefficientSolar ModuleSpec(Type).Tempc_Imp=-2.120e-003;%Maximum Power Current temp.coefficientSolar ModuleSpec(Type).Tempc_Voc=-1.770e-001;%Open Circuit Voltage temp.coefficientSolar ModuleSpec(Type).Tempc_Isc=3.516e-003;Short Circuit Current temp.coefficientSolar ModuleSpec(Type).Rs=0.037998;%Series resistance of PV model(ohms)Solar ModuleSpec(Type).Rp=993.51;%Parallel resistance of PV model(ohms)Solar ModuleSpec(Type).Isat=1.1753e-08;%Diode Saturation Current of PV model(A)Solar ModuleSpec(Type).Iph=5.9602;%Light generated photo-current of PV model(A)Solar ModuleSpec(Type).Qd=1.3;%Diode quality factor of PV model2.2.10 PV ARRAY CHARACTERISTIC CURVESThe current to voltage characteristic of a solar array is non-linear, which makes it difficult to determine the MPP. The power to voltage curves for various irradiance but a fixed temperature (C) is shown below in Figure 2.6 . Irradiance, temperature plays an important role in predicting the PV characteristic, and effects of both factors have to be considered while designing the PV system. Whereas the irradiance affects the output, temperature mainly affects the terminal voltage.

Fig.2.6 P-V Characteristic of a Solar Array for a fixed temperature but varying irradianceRed dots on blue curves indicate module specifications (Voc, Isc, Vmp, Imp) under standard test conditions (25 degrees Celsius, 1000 W/m2).2.2.11 DC-DC CONVERTERS DC-DC converters can be used as switching mode regulators to convert an unregulated dc voltage to a regulated dc output voltage. The regulation is normally achieved by PWM at a fixed frequency and the switching device is generally BJT, MOSFET or IGBT. The PWM control signal for the dc converter is generated by comparing duty cycle with a sawtooth voltage Vr. There are four topologies for the switching regulators: buck converter, boost converter, buck-boost converter, ck converter. However my project work deals with the boost regulator. 2.2.11.1 BOOST CONVERTER AND ITS OPERATION The Figure 2.7 below shows a step up or PWM boost converter. It consists of a dc input voltage source Vg, boost inductor L, controlled switch S, diode D, filter capacitor C, and the load resistance R. When the switch S is in the on state, the current in the boost inductor increases linearly and the diode D is off at that time. When the switch S is turned off, the energy stored in the inductor is released through the diode to the output RC circuit.

Fig.2.7 Circuit diagram of boost converter2.2.11.2 STEADY STATE ANALYSIS OF THE BOOST CONVERTEROFF STATE:

Fig.2.8 The off state diagram of the boost converter When the switch is off, the sum total of inductor voltage and input voltage appear as the load voltage.ON STATE:

Fig.2.9 The on state diagram of the boost converter When the switch is ON, the inductor is charged from the input voltage source Vg and the capacitor discharges across the load. The duty cycle, D=/T where T=1/f Fig.2.10 Inductor current waveform Fig.2.11 Inductor voltage waveform

From the inductor voltage balance equation, we have:-

Vg(DTs) +(Vs-Vo)(1-D)Ts=0

Vg(DTs)-Vg(DTs)-VgTs+VoDTs-VoTs=0

Vo=Vg/(1-D)

Conversion ratio, M=Vo/Vg=1/(1-D)

2.2.12 MAXIMUM POWER POINT TRACKING ( MPPT)There is a large number of algorithms that are able to track MPPs. Some of them are simple, such as those based on voltage and current feedback, and some are more complicated, such as perturbation and observation (P&O) or the incremental conductance (IncCond) method[10]. They also vary in complexity, sensor requirement, speed of convergence, cost, range of operation, popularity, and their applications. Having a curious look at the recommended methods, hill climbing and P&O are the algorithms that were in the center of consideration because of their simplicity and ease of implementation. Hill climbing is perturbation in the duty ratio of the power converter, and the P&O method is perturbation in the operating voltage of the PV array. However, the P&O algorithm cannot compare the array terminal voltage with the actual MPP voltage, since the change in power is only considered to be a result of the array terminal voltage perturbation. As a result, they are not accurate enough because they perform steady-state oscillations, which consequently waste the energy . By minimizing the perturbation step size, oscillation can be reduced, but a smaller perturbation size slows down the speed of tracking MPPs. Thus, there are some disadvantages with these methods, where they fail under rapidly changing atmospheric conditions . Fig.2.12 Basic idea of Inc cond method on a PV curve of a solar moduleThe Incremental Conductance method is the one which overrides over the aforementioned drawbacks. In this method, the array terminal voltage is always adjusted according to the MPP voltage. It is based on the incremental and instantaneous conductance of the PV module shows that the slope of the PV array power curve is zero at the MPP, increasing on the left of the MPP and decreasing on the right-hand side of the MPP. The basic equations of this method are as follows : Where I and V are the PV array output current and voltage, respectively. The left-hand side of the equations represents the IncCond of the PV module, and the right-hand side represents the instantaneous conductance. From (1-3) it is obvious that when the ratio of change in the output conductance is equal to the negative output conductance, the solar array will operate at the MPP. In other words, by comparing the conductance at each sampling time, the MPPT will track the maximum power of the PV module.2.2.12.1 INCREMENTAL CONDUCTANCE METHOD WITH DIRECT CONTROL Conventional MPPT systems have two independent control loops to control the MPPT. The first control loop contains the MPPT algorithm, and the second one is usually a proportional integral (PI) controller. The IncCond method makes use of instantaneous and IncCond to generate an error signal, which is zero at the MPP; however, it is not zero at most of the operating points. The main purpose of the second control loop is to make the error from MPPs near to zero . Simplicity of operation, ease of design, inexpensive maintenance, and low cost made PI controllers very popular in most linear systems. However, the MPPT system of standalone PV is a nonlinear control problem due to the nonlinearity nature of PV and unpredictable environmental conditions, and hence, PI controllers do not generally work well . In this project, the IncCond method with direct control is selected [10]. The PI control loop is eliminated, and the duty cycle is adjusted directly in the algorithm. The control loop is simplified, and the computational time for tuning controller gains is eliminated. To compensate the lack of PI controller in the proposed system, a small marginal error of 0.002 was allowed. MATLAB function generates pulse width modulation (PWM) waveform to control the duty cycle of the converter switch according to the IncCond algorithm. It is clear that the MPP is located at the knee of the PV curve, where the resistance is equal to the negative of differential resistance the slope of the PV curve at the MPP is equal to zero dp/dv=0 (7)Equation (8) can be rewritten as follows: / = / + . / (8) / = + . /dv (9) And hence + . / = 0 (10) This is the basic idea of the IncCond algorithm. One noteworthy point to mention is that (7) or (8) rarely occur in practical implementation, and a small error is usually permitted . The size of this permissible error (e) determines the sensitivity of the system. This error is selected with respect to the swap between steady-state oscillations and risk of fluctuating at a similar operating point. It is suggested to choose a small and positive digit . Thus, (10) can be rewritten as + . = e. In this project, the value of e was chosen as 0.002 on the basis of the trial-and-error procedure. The flowchart of the IncCond algorithm within the direct control method is shown in Figure2.13. According to the MPPT algorithm, the duty cycle (D) is calculated. This is the desired duty cycle that the PV module must operate on the next step. Refer to Figure 2.14 which shows simulation diagram for incremental conductance method with direct control.

Fig.2.13 Flow chart of incremental conductance method with direct control

Fig 2.14 Simulation diagram for incremental conductance method with direct control

Fig 2.15 Simulation diagram for giving pulses to boost converter2.2.13 HYTERESIS BAND CURRENT CONTROL PWM (HBPWM) 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. Inverters employing PWM principle are called PWM inverters. PWM techniques are characterized by constant amplitude pulses. The width of these pulses is modulated to obtain inverter output voltage control and to reduce its harmonic content. The advantages possessed by PWM technique are (i)The output voltage control with this method can be obtained without any additional components.(ii)With this 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.This is the most popular method of controlling the output voltage of an inverter in industrial applications. The hysteresis band current control PWM has been used because of its simple implementation, fast transient response, direct limiting of device peak current and practical insensitivity of dc link voltage ripple that permits a lower filter capacitor.2.2.13.1 HBPWM CURRENT CONTROL The HBPWM is basically an instantaneous feedback current control method of PWM where the actual current continually tracks the command current within a specified hysteresis band[12]. Fig.2.16 Simple voltage source inverter

Fig 2.17 Principle of hysteresis band current control

The Fig 2.17 explains the operation principle of HBPWM for a half bridge inverter. The control circuit generates the sine reference current wave of desired magnitude and frequency, and it is compared with the actual phase current wave. As the current exceeds a prescribed hysteresis band, the upper switch in the half-bridge is turned off and the lower switch is turned on. As a result the output voltage transitions from +0.5Vd to -0.5Vd, and the current starts to decay. As the current crosses the lower band limit, the lower switch is turned off and the upper switch is turned on. The actual current wave is thus forced to track the sine reference wave within the hysteresis band by back- and-forth switching of the upper and lower switches. The inverter then essentially becomes a current source with peak to peak current ripple, which is controlled within the hysteresis band irrespective of Vd fluctuations. The peak-to peak current ripple and the switching frequency are related to the width of the hysteresis band. The HBPWM inverter control method is shown in the Fig 2.18. The inputs to the HBPWM controller are three phase current errors and the outputs are the switching patterns to the PWM inverter. Fig.2.18 Hysteresis current controller model The hysteresis current controller gives output pulses to the inverter accordingly to this rule | - | < keeps the output pulse at the same state - > let output pulse = 1 (high) - < - let output pulse = 0(low).Where m=a, b, c phases and is the hysteresis band .The algorithm for this scheme is: (t) = Sin (

Upper band = (t) + (1)

Lower band = (t) - (2)

Where If >, = - (3)If >, = (4) Else, maintain the same state. Where m=a, b, c phases i is inverter output current and is the dc link voltage of the inverter.

Fig.2.19 Simulation diagram for hysteresis current controller PWM2.2.14 CONTROL OF GRID SIDE CONVERTERThe major functions of the grid side controller are: Control of active power injected to the grid to maintain a constant DC link Voltage Control of reactive power transfer between the converter and the grid Ensure acceptable power quality at the grid interface point Grid Synchronisation The control strategy applied to the GSC consists mainly of two cascaded loops. Usually, there is a fast internal current loop, which regulates the converter current, and an external voltage loop, which controls the dc-link voltage . The current loop is responsible for maintaining sinusoidal currents and also for protection against over current. The DC Link voltage controller is designed for balancing the power flow in the system. A division of the control strategies with respect to the reference frame using which they are implemented is given below Classification of Grid Converter Control StrategyIn synchronous reference frame control, all the voltage and current variables are transformed to a synchronously rotating reference frame so that the control variables become DC quantities and hence one can use traditional PI controllers to control the same. But in case of Stationary reference frame control the control variables are time varying quantities and hence one cannot use normal PI controllers. To control the system on stationary reference frame, one needs to use Proportional Resonant (PR) Controllers, which makes the control complex. Natural reference frame control, also known as abc control, is normally a structure where nonlinear controllers like hysteresis or dead beat are preferred due to their high dynamics . It is well known that the performance of these controllers has a direct relation to the sampling frequency; hence, the use of high speed digital control systems such as digital signal processors (DSPs) or field-programmable gate array (FPGA) is an advantage for such an implementation. In synchronous voltage oriented control (VOC), the switching frequency of the inverter remains constant and hence the harmonics have specific pattern depending on the switching frequency whereas in hysteresis control it varies over a wide range. So, output filter design is difficult in a hysteresis controlled inverter. The necessity of voltage feed-forward and cross coupling terms is the major drawback of the control structure implemented in synchronous reference frame. All these together makes synchronous VOC highly complex compared to hysteresis controller. Hysteresis control does not require any information about the system parameters[23] 2.2.14.1 GRID SIDE CONVERTER CONTROLA conventional PLL is used to monitor the grid voltage angle which is used for transformation of the 3-phase voltages to synchronous frame. Figure 2.20 shows Schematic diagram a grid connected PWM inverter . Let eA, eB, eC be the grid voltage, VA, VB, VC be the voltage at the terminals of inverter and IA, IB, IC be the line currents and L is the filter inductance.

Fig.2.20 Schematic of grid connected PWM inverterGrid side current equations on stationary frame can be represented as follows: iA = Im sin t iB = Im sin (t - 2/3) iC = Im sin (t + 2/3)Transforming these equations to the synchronous frame (d axis coincides with A-phase)The control law for the PI current controllers along the d and q-axes are: These currents can be transformed back to stationary 3-phase (abc) frame as shown below and can be used as the reference current to control the inverter. After transformation the real and reactive power can be given by following equations,P = 1.5 (Vd Id + Vq Iq) P = 1.5 Vd IdQ = 1.5 (Vq Id Vd Iq) Q = - 1.5 Vd Iq It is clear that transforming all voltages and currents to synchronous reference frame enables decoupled control of real and reactive power by controlling Id and Iq current components respectively.2.2.14.2 GRID SIDE CONVERTER CONTROLLER MODELLINGSynchronous reference frame control, also called d-q control, uses a reference frame transformation module, abc dq, to transform the grid current and voltage waveforms into a reference frame By. means of this, the control variables become DC values; thus, filtering and controlling can be easily achieved. A schematic of the dq control is represented in Figure 2.21. In this structure, the DC-link voltage is controlled to obtain the necessary output power. A standard PI controller is used for the control of DC voltage and its output act as the reference for the active current id. Another PI controller processes the error in the reactive power to set the reference for Iq. The reactive power reference is set to zero to have unity power factor at the grid interface [23]..

Fig.2.21 Block diagram of grid side converter controller Since the controlled current has to be in phase with the grid voltage, the phase angle used by the abc dq transformation module has to be extracted from the grid voltages. As a solution, filtering of the grid voltages and using phase locked loop (PLL) technique to extract the phase angle are incorporated.The dq control structure is normally associated with PI controllers since they have a satisfactory behavior when regulating DC variables. 2.2.14.3 PHASE LOCKED LOOP Fig.2.22 Block diagram of 3-phase PLLFigure 2.22 shows block diagram of 3-phase PLL.The output of the GSC is connected to the grid by synchronization for which the phase angle of the grid voltage space phasor is needed. This can be derived using different techniques such as zero crossing detection, and PLL technique. The latter one is implemented in d q synchronous reference frame As it can be noticed, this structure needs the coordinate transformation from abc to dq and here it is realized by setting the reference Vq,ref to zero. A controller, usually PI, is used to control this variable and the output of this regulator is the grid frequency. To improve the system performance and fast settling grid frequency (ff) is feed forwarded [25]

Refer to Figure 2.23 and Figure 2.24 which shows the simulation diagram for hysteresis current control PWM and simulation diagram for grid side converter controller.

Fig 2.23 Simulation diagram for hysteresis current control PWM

Fig 2.24 Simulation diagram for grid side converter controller2.2.15 SIMULINK MODEL FOR PV SYSTEMMATLAB simulations are carried out for modelling solar photovoltaic array based on its mathematical equations and the developed model is used to interconnect DC to DC converter, hysteresis current controlled DC to AC converter with the grid..By implementing MPPT to DC-DC converter we can track maximum power from PV array and feed to the load. Figure 2.25 shows the simulink model for PV system.

Fig 2.25 Simulation diagram for PV system

2.3 SIMULATION RESULTS OF PV SYSTEM AT 1000W/m^2

Fig 2.26 (a) PV Array output voltage(V)

Fig 2.26 (b) PV Array output current(A)

Fig 2.26 (c) PV Array diode current(A)

Fig 2.26 (d)Functioning of MPPT controller

Fig 2.26 (e) Duty cycle of boost converter

Fig 2.26 (f) Solar irradiance(w/m^2)

Fig 2.26 (g) PV Array output(kW)

Fig 2.26 (h) Active Power delivered by the PV system (kW)

Fig 2.26 (i) Voltage at the point of grid interconnection (V)

Fig 2.26 (j) Current through the point of grid interconnection (A) Fig 2.26 (k) Reactive power curve for maintaining unity power factor

Constant DC link voltage maintained by GSC controller as shown in figure 2.26(i)

Fig 2.26 (l) Constant DC link voltage maintained by GSC controller(V)

Fig.2.26 Simulation Results of PV System at 1000W/m^2

Table 2: Active Power Harnessed at Different Irradiance by PV system

Irradiance(W/m^2)Active Power Harnessed(kW)

250

2.5

5005

7507

100010

The PV system is operated for irradiance from 250w/m^2 to 1000 w/m^2 .By implementing MPPT algorithm to boost converter we can track maximum power from PV array under various irradiance. The generated power increases with the increasing irradiance from 250W/m^2 to 1000W/m^2 as shown in Table 2. Also, it is observed that generation is possible even at very low irradiance.2.4 CONCLUSION: In this Chapter modelling of the PV system of 10 Kw capacity has been done.The PV system is operated for irradiance from 250w/m^2 to 1000 w/m^2 .By implementing MPPT algorithm to boost converter we can track maximum power from PV array under various irradiance. Simulation results are analyzed to observe the changes in active power harnessed under varying irradiance, the generated power increases with the increasing irradiance. Also, it is observed that generation is possible even at very low irradiance and constant dc link voltage is maintained by GSC controller.

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