Simulation and Modeling of a Three-Phase Two- …In MATLAB/SIMULINK environment, the three-phase inverter can be modeled by six IGBTs switches [2]. The two main function of the dc-ac

International Journal on Power Engineering and Energy (IJPEE) Vol. (6) – No. (1)ISSN Print (2314 – 7318) and Online (2314 – 730X) January 2015

Reference Number: W13-P-0009 512

Simulation and Modeling of a Three-Phase Two-Stage Grid Connected Photovoltaic System

Almoataz Y. Abdelaziz, Ahmed M. Atallah and Raihan S. JumaahElectrical power & Machine Department, Faculty of Engineering

Ain Shams University, Cairo, EgyptEmail: almoatazabdelaziz@hotmail.com, atallah_eg@yahoo.com,

rjummah@yahoo.com

Abstract- Grid connected photovoltaic (PV) systems feedelectricity directly to the electrical network operatingparallel to the conventional source. This paper deals withthe design and simulation of a three phase inverter inMATLAB SIMULINK environment which can be a partof photovoltaic grid connected systems. This paper atfirst presents a control algorithm for a three-phase grid-connected photovoltaic system in which an inverterdesigned for grid-connected photovoltaic arrays cansynchronize a sinusoidal current output with a voltagegrid. The main points discussed here are the MPPtracking algorithm, the synchronization of the inverterand the connection to the grid. Tracking the dc voltageand current allows MPP calculation which gives theinverter to function efficiently. We apply the MPPequations to the PV array model and watch the inverterinput and output. In order to synchronize the simulatedinverter to the grid the waveforms from the grid areapplied to the pulse width modulation (PWM) input anddrive appropriately the inverter’s IGBT’s. AMATLAB/SIMULINK based simulation model isdeveloped for the system. Simulation results arepresented to show the overall system performance.

Keywords - Grid connected PV systems, DC-DC boostconverter, harmonic filter, MPPT and inverter.

I. Introduction

The increase of world energy demand, due to the modernindustrial society and population growth, is motivating a lotof investments in alternative energy solutions, in order toimprove energy efficiency and power quality issues. The useof photovoltaic energy is considered to be a primaryresource, because there are several countries located intropical and temperate regions, where the direct insolationdensity may reach up to 1000W/m. The conventional stand-alone photovoltaic systems have the advantages of simple

system configuration and control scheme. However, in orderto draw maximum power from PV arrays and store excessenergy, battery banks are required in these systems. Thesolar cell array produces only a small amount of current andvoltage. So, in order to meet a large load demand, the solarcell array has to be connected into modules and the modulesconnected into arrays. The output voltage from PV array ischangeable with solar radiation and ambient temperature. Soin order to connect the electrical grid the output voltage fromPV array should be fixed and converted to AC voltagecompatible with the grid ac voltage. Recently, energygenerated from clean, efficient and environmentally friendlysources has become one of the major challenges forengineers and scientists. This paper discusses the detailedmodeling of the whole system. PV array is connected to theutility grid by a boost converter to optimize the PV outputand DC/AC inverter to convert the DC output voltage of thesolar modules into the AC system. The DC input of theinverter must be constant and it is controlled by the use of aPI control circuit. An LC filter has been introduced to insurea clean current injection to the grid. The proposed model ofthe entire components and control system are all simulated inMATLAB/SIMULINK Software. Two different cases aresimulated; steady and transient states. All simulation resultshave verified the validity of models and effectiveness ofcontrol methods.

Figure (1): Two stage-grid connected PV blocks scheme [16]

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Reference Number: W13-P-0009 513

II. PV Generator Model

Photovoltaic generators are neither fixed current sources norvoltage sources but can be approximated as currentgenerators with dependant voltage sources. During darkness,the solar cell is not an active device. It produces neither acurrent nor a voltage. A solar panel cell essential is a p-nsemiconductor junction. When exposed to the light, a currentis generated (DC current).The generated current changelinearly with the solar irradiance. Figure 2 show theequivalent electrical circuit of an ideal solar cell.

Figure (2): Equivalent circuit of a solar cell

The I-V characteristics of the solar cell circuit can be sets bythe following equations [14]. The current through diode isgiven by:

ID = IO [exp (q(V + I RS)/KT)) – 1] (1)While, the solar cell output current:

I = IL – ID – Ish (2)I = IL – IO [exp (q(V + I RS)/KT)) – 1] – ( V + IRS )/ Rsh

(3)Where:I : Solar cell current (A)IL: Light generated current (A) [Short circuit value assumingno series/ shunt resistance]IO : Diode saturation current (A)q : Electron charge (1.6×10-19 C)K : Boltzman constant (1.38×10-23 J/K)T : Cell temperature in Kelvin (K)V : solar cell output voltage (V)Rs: Solar cell series resistance (Ω)Rsh: Solar cell shunt resistance (Ω)By implementing this mathematical model in Matlab forconstant conditions of temperature and solar irradiance, wetake the characteristics I-V and P-V curves of the PV-cell.The current-voltage and power-voltage characteristics

changes when irradiance and cell temperatures are changes.As the temperature is rising the efficiency is falling. Thisindicates the necessity of voltage, or current regulationpower electronic circuits (MPPT), and a system to enable themaximization of the generated power.

III. Model of the Boost DC-DC Converter

For grid-connected PV applications, two hardwaretopologies for MPPT have been mostly studied worldwide,known as one-stage and two-stage PV systems. In this work,it was selected the two-stage PV energy conversion system,because it offers an additional degree of freedom in theoperation of the system when compared with the one-stageconfiguration, Since the output voltage of PV cell is low, theuse of boost circuit will enable low-voltage PV array to beused, as a result, the total cost will be reduced. A capacitor isgenerally connected between PV array and the boost circuit,which is used to reduce high frequency harmonics. Theconfiguration of the boost circuit and its control system areillustrated in Figure 4. DC-DC converters boost step-up thePV voltage to the level of the allowable maximum linevoltage and to the stable required dc level without storageelements as battery. DC to DC converter is controlled totrack maximum power point of the PV array.

Figure (4): Boost converter circuit [7]

IV. Inverter Modeling

Inverter or power inverter is a device that converts the DCsources to AC sources. Inverters are used in a wide range ofapplications, from small switched power supplies for acomputer to large electric utility applications to transportbulk power. This makes them very suitable when the use ofAC power tools or appliances is required [10].Power inverters produce one of three different types of waveoutput:

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Reference Number: W13-P-0009 514

• Square Wave• Modified Square Wave (Modified Sine Wave)• Pure Sine Wave (True Sine Wave)The three different wave signals represent three differentqualities of power output. Square wave inverters result inuneven power delivery that is not efficient for running mostdevices.Square wave inverters were the first types of inverters madeand are obsolete.Modified square wave (modified sine wave) inverters deliverpower that is consistent and efficient enough to run mostdevices fine. Some sensitive equipment requires a sine wave,like certain medical equipment and variable speed orrechargeable tools [10].In MATLAB/SIMULINK environment, the three-phaseinverter can be modeled by six IGBTs switches [2].The two main function of the dc-ac converter is:• Efficiently generate AC output current in phase with theAC grid voltage• Balance the average power delivery from the PV array tothe grid

V. Control of Grid-Connected PV System

The control structure of the grid-connected PV system iscomposed of two structures Control [8]:1. The MPPT Control, the main property is to extract themaximum power from the PV generator.2. The inverter control, which have the main goal:- Control the active and regulate the reactive power injectedinto the grid;- Control the DC bus voltage;- Ensure high quality of the injected power. Table 1 showsthe standards of grid connected inverters.

V.1. MPPT Control

The maximum power that can be delivered by a PV paneldepends greatly on the insulation level and the operatingtemperature. Therefore, it is necessary to track the maximumpower point all the time. The weather and load changescause the operation of a PV system to vary almost all thetimes. A dynamic tracking technique is important to ensuremaximum power is obtained from the photovoltaic arrays.The Perturb and observe (P&O) technique are used.

Table (1): Standards of Grid Connected Inverters

This algorithm uses simple feedback arrangement and littlemeasured parameters. In this approach, the module voltage isperiodically given a perturbation and the correspondingoutput power is compared with that at the previousperturbing cycle [9]. In this algorithm a slight perturbation isintroduced to the system. This perturbation causes the powerof the solar module varies. If the power increases due to theperturbation then the perturbation is continued in the samedirection. After the peak power is reached the power at theMPP is zero and next instant decreases and hence after thatthe perturbation reverses. When the stable condition isarrived the algorithm oscillates around the peak power point.In order to maintain the power variation small theperturbation size is remain very small. The technique isadvanced in such a style that it sets a reference voltage of themodule corresponding to the peak voltage of the module. API controller then acts to transfer the operating point of themodule to that particular voltage level [9].

V.2. The Inverter Control

Inverter interfacing PV module(s) with the grid involves twomajor tasks. One is to ensure that the PV module(s) isoperated at the maximum power point (MPP). The other is toinject sinusoidal current into the grid. In grid-connected PVsystem, different inverter topologies and controllers areusually used for Interfacing the PVG and the utility grid[8].there are many methods of inverter control.

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Reference Number: W13-P-0009 515

V.2.1. Self-commutated inverters

Such inverters are more complicated and use switchingdevices (IGBT and MOSFET) that can control the switch-onand switch-off time and adjust the output signal to that of thegrid. The self-commutated inverters are the predominanttechnology in PV power sources because of their ability tocontrol the voltage and current output signal (AC side),regulate the power factor and reduce the harmonic currentdistortion. Especially, since the role of PV inverter hasbecome more vital, this operation principle is offering thecapability to cover the multiple services and increase theresistance to the grid disturbances. Depending on the type ofpulse they control, either voltage or current. Self-commutated inverters are divided to voltage source andcurrent source inverters [11].

V.2.1.1. Voltage source inverters (VSI)

VSI realize the DC side as a constant voltage source and theoutput current is changing with the load. For this reason it isnormally connected to the grid with an inductance so as notto supply with high current when there is no voltage or phasematch between inverter and grid

V.2.1.2. Current source inverters (CSI)

Respectively, CSI the DC source appears as a constantcurrent input and the voltage is changing with the load. Theprotection filter is normally a capacitance in parallel with theDC source.

Also self-commutated inverters produce very good sinewave outputs when PWM technique and low pass filters areused.

VI. MATLAB-SIMULINK Environment

The system model shown in Figure 1, demonstrates PV solarcell array connected to a 50 HZ, 400 V grid through aDC/DC boost converter and DC/AC inverter. The 400 Vobtained from DC/DC converter is applied to a signal dc toac inverter. The task of the boost DC/DC converter drainsthe power from the PV solar cell array and supplies the DClink capacitor with a maximum power point tracker obtainedfrom the MPPT controller. An LC filter is inserted after thedc-ac inverter in order to eliminate the harmonics containedin both the voltage and current of the inverter output.In Figure 5 the model of PV panel as a constant dc sourcecreated using the subsystem block from SIMULINK librarybrowser, which included all functions of PV panel. Themodel has three inputs irradiance, temperature and voltageinput that is coming as a feedback from the system and the

output of the block gives the current, Table 2 showsparameters of the PV model.

Table (2): Parameters of the PV model

Parameters Values

Referenced solar irradiance G ref G = 1000W/m²

Referenced cell temperature T ref T = 25◦C,

I mp 23.25 A

V mp 54.2 V

P mp 1260.2 kW

I sc 25.44 A

V oc 66

Figure (5): SIMULINK module of PV panel

Figures (6 & 7) show a SIMULINK® diagram of a boostconverter and Perturb and Observe maximum power pointtracking Algorithm.

Figure (6): SIMULINK® model of boost converter

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Reference Number: W13-P-0009 516

Figure (7): SIMULINK® model of P&O Algorithm.

Figures (8 & 9) show a SIMULINK® diagram of Inverter &LC filter and RL load and SIMULINK diagram of completephotovoltaic grid connected.

Figure (8): SIMULINK® model of Inverter & LC filter andRL load

VII. Results and Simulation

The simulation time was set for 5 seconds. The grid wassimulated according to the standards IEC with a 400 V ACRMS and a resistive load in series with an inductive load. Ina Self-commutated controlled scheme the inverter must beable to maintain the frequency and voltage of the gridwithout actually being connected to it. This helps in islandedsituations when the grid comes back online and the systemneeds to be synchronized before reconnecting. Based on theabove models and control methods, one simulation case isstudy, which is steady state operation, when there is nochange in atmospheric conditions, solar irradiance is 500W/m2, and temperature is 20˚ the output of PV systemrecorded in Table 3.

Figure (9): SIMULINK® model of complete photovoltaicgrid connected

Table (3): Parameters of the PV model

Parameters values

Referenced solarirradiance G ref

G = 400W/m²

Referenced celltemperature T ref

T = 20◦C

I mp 6.3 A

V mp 80 VP mp 400 W

Figure (10): PV output voltage, DC/DC converter outputcurrent and DC/DC converter output voltage

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Figure (11): Output Power of DC/DC converter

The MPPT algorithm, succeeded to track the maximumpower point and extracting this power from photovoltaicsystem. Through simulation it is observed that the systemcompletes the maximum power point tracking successfullydespite of fluctuations.

Figure (12): Input voltage before filtering

Figure (13): Input voltage after filter

The Simulation results of the inverter output Voltage beforeand after filtering which give a total harmonic distortion(THD) of 7% before filtering (Figure 12 & 13) , and only 4%after filtering by the LC filter, This percentage is within thelimits of 4% specified by the IEC as see in Table 1.

Figure (14): Vabc (three phase voltage) after filter

Figure (15): Vabc & Iabc (three phase voltage and threephase current) of Grid

Figure (16): Vabc & Vab and Iabc (three phase voltage &two phase voltage and three phase current) of inverter.

The output signal of the inverter can be observed. Byinspection of the graph there is no noticeable flicker and thiscomplies with IEC standards. Zooming in the area of interestbetween 0.8180 and 0.7981 seconds the response time can beobtained. The voltage control takes 0.037 seconds tosynchronize the output of the PV to the voltage provided bythe grid, the frequency can be calculated yielding the value

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50.25 HZ. This value is within the limits of 50±1 specifiedby the IEC. The peak value is 415 V. The RMS voltagevalue has to fall between 102% and 103% of 400 thusranging from 407 V to 415 V rms. This percentage is incompliance with the IEC 61727 in Table 1.

Figure (17): Vabc (three phase voltage)of load

The three phases Va, Vb, and Vc have a phase of 120 andhave no flicker. Their DC components are respectively 109.2V, 109.2V and 109.2V. The response time varies is the samefor each phase. The response time of Phase A at 0.81 with anRMS value of 407 corresponding to 103% of the rated valueand a peak value of 415V.

Figure (18): Vabc (three phase voltage of inverter afterfiltering)

Figure (19): Iabc (three phase current of inverter afterfiltering)

Figure (20): Active and Reactive power of the Grid (P & Q)

Figure (21): Active and Reactive power of inverter (P & Q)

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Figure (22): Active and Reactive power of load (P & Q)

The power requested by the load is 1 kW and 1 kVAR. Sincethe PV array generate only an active power of 500 W and noreactive power. Active power value of load and inverter isgreater than reactive power, but reactive power value of gridis a greater than active power.

VIII. CONCLUSION

In order to construct a PV grid connected system, a numberof parameters have to be taking into account and to beoptimized in order to achieve maximum power generation.The maximum power point tracking algorithm when appliedan accurate PV model has the ability to increase theefficiency of the system. In addition to that a controller hasto be used in order to achieve the synchronization to the gridand to perform the power management between the systemand the electrical grid. P&O MPPT method and PV gridconnected with its control are implemented with MATLAB-SIMULINK for simulation. The MPPT method simulated inthis paper is able to improve the dynamic and steady stateperformance of the PV system simultaneously. Throughsimulation, it is observed that the system completes themaximum power point tracking successfully. Moreover, thisstudy shows that the proposed control scheme offers asimple way to study the Performance for utility interfaceapplications. It is simple to implement and capable ofproducing satisfactory sinusoidal current and voltagewaveforms.

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