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

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1 Simulation and Modeling of a Three-Phase Two- Stage Grid Connected Photovoltaic System Almoataz Y. Abdelaziz, Ahmed M. Atallah and Raihan S. Jumaah Electrical power & Machine Department, Faculty of Engineering Ain Shams University, Cairo, Egypt almoatazabdelaziz@hotmail.com, atallah_eg@yahoo.com, rjummah@yahoo.com Abstract- Grid connected photovoltaic (PV) systems feed electricity directly to the electrical network operating parallel to the conventional source. This paper deals with the design and simulation of a three phase inverter in MATLAB SIMULINK environment which can be a part of photovoltaic grid connected systems. This paper at first presents a control algorithm for a three-phase gridconnected photovoltaic system in which an inverter designed for grid-connected photovoltaic arrays can synchronize a sinusoidal current output with a voltage grid. The main points discussed here are the MPP tracking algorithm, the synchronization of the inverter and the connection to the grid. Tracking the dc voltage and current allows MPP calculation which gives the inverter to function efficiently. We apply the MPP equations to the PV array model and watch the inverter input and output. In order to synchronize the simulated inverter to the grid the waveforms from the grid are applied to the pulse width modulation (PWM) input and drive appropriately the inverter s IGBT s. A MATLAB/SIMULINK based simulation model is developed for the system. Simulation results are presented to show the overall system performance. Keywords - Grid connected PV systems, DC-DC boost converter, harmonic filter, MPPT and inverter. system configuration and control scheme. However, in order to draw maximum power from PV arrays and store excess energy, battery banks are required in these systems. The solar cell array produces only a small amount of current and voltage. So, in order to meet a large load demand, the solar cell array has to be connected into modules and the modules connected into arrays. The output voltage from PV array is changeable with solar radiation and ambient temperature. So in order to connect the electrical grid the output voltage from PV array should be fixed and converted to AC voltage compatible with the grid ac voltage. Recently, energy generated from clean, efficient and environmentally friendly sources has become one of the major challenges for engineers and scientists. This paper discusses the detailed modeling of the whole system. PV array is connected to the utility grid by a boost converter to optimize the PV output and DC/AC inverter to convert the DC output voltage of the solar modules into the AC system. The DC input of the inverter must be constant and it is controlled by the use of a PI control circuit. An LC filter has been introduced to insure a clean current injection to the grid. The proposed model of the entire components and control system are all simulated in MATLAB/SIMULINK Software. Two different cases are simulated; steady and transient states. All simulation results have verified the validity of models and effectiveness of control methods. I. Introduction The increase of world energy demand, due to the modern industrial society and population growth, is motivating a lot of investments in alternative energy solutions, in order to improve energy efficiency and power quality issues. The use of photovoltaic energy is considered to be a primary resource, because there are several countries located in tropical and temperate regions, where the direct insolation density may reach up to 1000W/m. The conventional standalone photovoltaic systems have the advantages of simple Figure (1): Two stage-grid connected PV blocks scheme [16] Reference Number: W13-P

2 II. PV Generator Model Photovoltaic generators are neither fixed current sources nor voltage sources but can be approximated as current generators with dependant voltage sources. During darkness, the solar cell is not an active device. It produces neither a current nor a voltage. A solar panel cell essential is a p-n semiconductor junction. When exposed to the light, a current is generated (DC current).the generated current change linearly with the solar irradiance. Figure 2 show the equivalent 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 by the following equations [14]. The current through diode is given by: changes when irradiance and cell temperatures are changes. As the temperature is rising the efficiency is falling. This indicates the necessity of voltage, or current regulation power electronic circuits (MPPT), and a system to enable the maximization of the generated power. III. Model of the Boost DC-DC Converter For grid-connected PV applications, two hardware topologies 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 the operation of the system when compared with the one-stage configuration, Since the output voltage of PV cell is low, the use of boost circuit will enable low-voltage PV array to be used, as a result, the total cost will be reduced. A capacitor is generally connected between PV array and the boost circuit, which is used to reduce high frequency harmonics. The configuration of the boost circuit and its control system are illustrated in Figure 4. DC-DC converters boost step-up the PV voltage to the level of the allowable maximum line voltage and to the stable required dc level without storage elements as battery. DC to DC converter is controlled to track maximum power point of the PV array. I D = I O [exp (q(v + I R S )/KT)) 1] (1) While, the solar cell output current: I = I L I D I sh (2) I = I L I O [exp (q(v + I R S )/KT)) 1] ( V + IR S )/ R sh (3) Where: I : Solar cell current (A) I L : Light generated current (A) [Short circuit value assuming no series/ shunt resistance] I O : Diode saturation current (A) q : Electron charge ( C) K : Boltzman constant ( J/K) T : Cell temperature in Kelvin (K) V : solar cell output voltage (V) R s : Solar cell series resistance (Ω) R sh : Solar cell shunt resistance (Ω) By implementing this mathematical model in Matlab for constant conditions of temperature and solar irradiance, we take the characteristics I-V and P-V curves of the PV-cell. The current-voltage and power-voltage characteristics Figure (4): Boost converter circuit [7] IV. Inverter Modeling Inverter or power inverter is a device that converts the DC sources to AC sources. Inverters are used in a wide range of applications, from small switched power supplies for a computer to large electric utility applications to transport bulk power. This makes them very suitable when the use of AC power tools or appliances is required [10]. Power inverters produce one of three different types of wave output: Reference Number: W13-P

3 Square Wave Modified Square Wave (Modified Sine Wave) Pure Sine Wave (True Sine Wave) The three different wave signals represent three different qualities of power output. Square wave inverters result in uneven power delivery that is not efficient for running most devices. Square wave inverters were the first types of inverters made and are obsolete. Modified square wave (modified sine wave) inverters deliver power that is consistent and efficient enough to run most devices fine. Some sensitive equipment requires a sine wave, like certain medical equipment and variable speed or rechargeable tools [10]. In MATLAB/SIMULINK environment, the three-phase inverter 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 the AC grid voltage Balance the average power delivery from the PV array to the grid Table (1): Standards of Grid Connected Inverters V. Control of Grid-Connected PV System The control structure of the grid-connected PV system is composed of two structures Control [8]: 1. The MPPT Control, the main property is to extract the maximum power from the PV generator. 2. The inverter control, which have the main goal: - Control the active and regulate the reactive power injected into the grid; - Control the DC bus voltage; - Ensure high quality of the injected power. Table 1 shows the standards of grid connected inverters. V.1. MPPT Control The maximum power that can be delivered by a PV panel depends greatly on the insulation level and the operating temperature. Therefore, it is necessary to track the maximum power point all the time. The weather and load changes cause the operation of a PV system to vary almost all the times. A dynamic tracking technique is important to ensure maximum power is obtained from the photovoltaic arrays. The Perturb and observe (P&O) technique are used. This algorithm uses simple feedback arrangement and little measured parameters. In this approach, the module voltage is periodically given a perturbation and the corresponding output power is compared with that at the previous perturbing cycle [9]. In this algorithm a slight perturbation is introduced to the system. This perturbation causes the power of the solar module varies. If the power increases due to the perturbation then the perturbation is continued in the same direction. After the peak power is reached the power at the MPP is zero and next instant decreases and hence after that the perturbation reverses. When the stable condition is arrived the algorithm oscillates around the peak power point. In order to maintain the power variation small the perturbation size is remain very small. The technique is advanced in such a style that it sets a reference voltage of the module corresponding to the peak voltage of the module. A PI controller then acts to transfer the operating point of the module to that particular voltage level [9]. V.2. The Inverter Control Inverter interfacing PV module(s) with the grid involves two major tasks. One is to ensure that the PV module(s) is operated at the maximum power point (MPP). The other is to inject sinusoidal current into the grid. In grid-connected PV system, different inverter topologies and controllers are usually used for Interfacing the PVG and the utility grid [8].there are many methods of inverter control. Reference Number: W13-P

4 V.2.1. Self-commutated inverters Such inverters are more complicated and use switching devices (IGBT and MOSFET) that can control the switch-on and switch-off time and adjust the output signal to that of the grid. The self-commutated inverters are the predominant technology in PV power sources because of their ability to control the voltage and current output signal (AC side), regulate the power factor and reduce the harmonic current distortion. Especially, since the role of PV inverter has become more vital, this operation principle is offering the capability to cover the multiple services and increase the resistance to the grid disturbances. Depending on the type of pulse they control, either voltage or current. Selfcommutated inverters are divided to voltage source and current source inverters [11]. V Voltage source inverters (VSI) VSI realize the DC side as a constant voltage source and the output current is changing with the load. For this reason it is normally connected to the grid with an inductance so as not to supply with high current when there is no voltage or phase match between inverter and grid V Current source inverters (CSI) Respectively, CSI the DC source appears as a constant current input and the voltage is changing with the load. The protection filter is normally a capacitance in parallel with the DC source. Also self-commutated inverters produce very good sine wave outputs when PWM technique and low pass filters are used. VI. MATLAB-SIMULINK Environment The system model shown in Figure 1, demonstrates PV solar cell array connected to a 50 HZ, 400 V grid through a DC/DC boost converter and DC/AC inverter. The 400 V obtained from DC/DC converter is applied to a signal dc to ac inverter. The task of the boost DC/DC converter drains the power from the PV solar cell array and supplies the DC link capacitor with a maximum power point tracker obtained from the MPPT controller. An LC filter is inserted after the dc-ac inverter in order to eliminate the harmonics contained in both the voltage and current of the inverter output. In Figure 5 the model of PV panel as a constant dc source created using the subsystem block from SIMULINK library browser, which included all functions of PV panel. The model has three inputs irradiance, temperature and voltage input that is coming as a feedback from the system and the output of the block gives the current, Table 2 shows parameters of the PV model. Table (2): Parameters of the PV model Parameters Referenced solar irradiance G ref Referenced cell temperature T ref Values G = 1000W/m² T = 25 C, Reference Number: W13-P I mp V mp P mp I sc A 54.2 V kw A V oc 66 Figure (5): SIMULINK module of PV panel Figures (6 & 7) show a SIMULINK diagram of a boost converter and Perturb and Observe maximum power point tracking Algorithm. Figure (6): SIMULINK model of boost converter

5 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 complete photovoltaic grid connected. Figure (9): SIMULINK model of complete photovoltaic grid connected Parameters Table (3): Parameters of the PV model values Referenced solar irradiance G ref Referenced cell temperature T ref I mp V mp P mp G = 400W/m² T = 20 C 6.3 A 80 V 400 W Figure (8): SIMULINK model of Inverter & LC filter and RL load VII. Results and Simulation The simulation time was set for 5 seconds. The grid was simulated according to the standards IEC with a 400 V AC RMS and a resistive load in series with an inductive load. In a Self-commutated controlled scheme the inverter must be able to maintain the frequency and voltage of the grid without actually being connected to it. This helps in islanded situations when the grid comes back online and the system Figure (10): PV output voltage, DC/DC converter output needs to be synchronized before reconnecting. Based on the current and DC/DC converter output voltage above models and control methods, one simulation case is study, which is steady state operation, when there is no change in atmospheric conditions, solar irradiance is 500 W/m 2, and temperature is 20 the output of PV system recorded in Table 3. Reference Number: W13-P

6 Figure (11): Output Power of DC/DC converter The MPPT algorithm, succeeded to track the maximum power point and extracting this power from photovoltaic system. Through simulation it is observed that the system completes the maximum power point tracking successfully despite of fluctuations. Figure (14): Vabc (three phase voltage) after filter Figure (15): Vabc & Iabc (three phase voltage and three phase current) of Grid Figure (12): Input voltage before filtering Figure (13): Input voltage after filter The Simulation results of the inverter output Voltage before and 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 the limits of 4% specified by the IEC as see in Table 1. 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. By inspection of the graph there is no noticeable flicker and this complies with IEC standards. Zooming in the area of interest between and seconds the response time can be obtained. The voltage control takes seconds to synchronize the output of the PV to the voltage provided by the grid, the frequency can be calculated yielding the value Reference Number: W13-P

7 50.25 HZ. This value is within the limits of 50±1 specified by the IEC. The peak value is 415 V. The RMS voltage value has to fall between 102% and 103% of 400 thus ranging from 407 V to 415 V rms. This percentage is in compliance with the IEC in Table 1. Figure (19): Iabc (three phase current of inverter after filtering) Figure (17): Vabc (three phase voltage)of load The three phases V a, V b, and V c have a phase of 120 and have no flicker. Their DC components are respectively V, 109.2V and 109.2V. The response time varies is the same for each phase. The response time of Phase A at 0.81 with an RMS value of 407 corresponding to 103% of the rated value and a peak value of 415V. Figure (20): Active and Reactive power of the Grid (P & Q) Figure (18): Vabc (three phase voltage of inverter after filtering) Figure (21): Active and Reactive power of inverter (P & Q) Reference Number: W13-P

8 Figure (22): Active and Reactive power of load (P & Q) The power requested by the load is 1 kw and 1 kvar. Since the PV array generate only an active power of 500 W and no reactive power. Active power value of load and inverter is greater than reactive power, but reactive power value of grid is a greater than active power. VIII. CONCLUSION In order to construct a PV grid connected system, a number of parameters have to be taking into account and to be optimized in order to achieve maximum power generation. The maximum power point tracking algorithm when applied an accurate PV model has the ability to increase the efficiency of the system. In addition to that a controller has to be used in order to achieve the synchronization to the grid and to perform the power management between the system and the electrical grid. P&O MPPT method and PV grid connected with its control are implemented with MATLAB- SIMULINK for simulation. The MPPT method simulated in this paper is able to improve the dynamic and steady state performance of the PV system simultaneously. Through simulation, it is observed that the system completes the maximum power point tracking successfully. Moreover, this study shows that the proposed control scheme offers a simple way to study the Performance for utility interface applications. It is simple to implement and capable of producing satisfactory sinusoidal current and voltage waveforms. REFERENCES photovoltaic system», JATIT & LLS. Vol. 37 No.2, 31st March [3] C. Meza Benavides «Analysis and Control of a Single-Phase Single-Stage Grid-Connected Photovoltaic Inverter», PhD Thesis, PhD Program: Advanced Automatics and Robotics, University of Politecnica de Catalunya. [4] G. Ertasgin «Low-Cost Current-Source 1-ph Photovoltaic Grid-Connected Inverter», Ph. D. Thesis, University of Adelaide, Faculty of Engineering, Computer and Mathematical Science, School of Electrical and Electronic Engineering. August [5] Mahdi Salimi «Low-Cost Grid Connected Photovoltaic System», 2nd International Conference on Environmental Science and Technology. IPCBEE vol.6, 2011, Singapore. [6] L. M. Daud, S.A. Ghani and N.Z. A. Naharuddin «Simulation Study Using SIMULINK/MATLAB on THD for PV Grid Connected System», Faculty of Electrical and Electronics Engineering, University Malaysia Pahang Pekan, Pahang, Malaysia. [7] P. Dharmaraj«Modeling of Three Phase Inverter for Photovoltaic Application» M. S. Thesis, University of Tun Hussein On Malaysia Faculty of Electrical and Electronic Engineering. July, [8] Z. Ahmad, S.N. Singh «Modeling and Control of Grid Connected Photovoltaic System-A Review» International Journal of Emerging Technology and Advanced Engineering. Volume 3, Issue 3, March [9] Vikrant A. Chaudhari, «Automatic Peak Power Traker for Solar PV Modules Using dspacer Software» in Maulana Azad National Institute of Technology. Degree of Master of Technology In Energy. Bhopal: Deemed University, 2005, pp. 98. [10] A. Bin Alias «Modeling and simulation of single phase Inverter with PWM using MATLAB/SIMULINK», B. Sc. Electrical Engineering (Power System), Faculty of Electrical & Electronic Engineering University Malaysia Pahang, November, [11] A.S. Abu Hasim, Z. Ibrahim, M.H. Nizam Talib, S.N. Mat Isa, J. Mat Lazi and N. Mohd. Yakop «Photovoltaic System Connected to Three Phase Grid Connected System Incorporating With Active Power Filter», Department of Electrical, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, Kuala Lumpur, Malaysia. Australian Journal of Basic and Applied Sciences, 6(7): , [12] P. C. Tan, Student Member, IEEE, and Z. Salam, [1] M. Makhlouf, F. Messai, H. Benalla, «Modeling and Simulation of grid-connected photovoltaic distributed generation system», JATIT & LLS. Vol. 45 No.2, 30th November [2] M. Makhlouf, F. Messai, H. Benalla, «Modeling and control of a single-phase grid connected Reference Number: W13-P

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