LabVIEW/MATLAB Based Simulator for Grid Connected ... - IEEE Xplore

4th International Conference on Power Engineering, Energy and Electrical Drives

Istanbul, Turkey, 13-17 May 2013

LabVIEW/MATLAB Based Simulator for Grid Connected PV System Necmi ALTIN

Tevfik YILDIRIMOGLU

Department of Electrical and Electronics Engineering Faculty of Technology, Gazi University Ankara, Turkey [email protected]

Department of Electrical Education, Institute of Science and Technology, Gazi University Ankara, Turkey [email protected]

Abstract— In this study, LabVIEW and MATLAB/Simulink programs has used together to develop a simulator of grid connected photovoltaic system which has the maximum power point tracking ability. System consists of a photovoltaic module model, a DC/DC boost converter, a voltage source inverter, a maximum power point tracking algorithm and current controller. The photovoltaic panel and maximum power point tracking algorithm are modeled in LabVIEW and a string with ten serial connected modules is used as photovoltaic supply. The two-stage converter composed of a DC / DC and a voltage source inverter and inverter control structure have modeled in MATLAB/Simulink. These components of the system have communicated by using the simulation interface toolkit. PV panel has set according to the datasheets and has been tested by using maximum power point tracking algorithm for different irradiation and temperature conditions. Maximum power point and the instantaneous operating points have been stated on the current-voltage and power-voltage graphs plotted during the operation. This prepared simulator is sufficient for the preassessment about the amount of energy to be produced for the geographical planned to invest as well as it can be used for educational purposes. Keywords-PV simulator, MPPT, grid interactive inverter, boost converter.

I.

INTRODUCTION

Although the main energy sources are fossil fuels such as coal, oil and natural gas, it is difficult for many countries to meet the energy demand, because energy demand is increasing day by day due to industrial development, increasing population and demand for a better life quality. Also environmental effects of these fossil sources are seen as a problem. In order to solve these problems, clean and inexhaustible alternative sources have been and as a result of these studies, renewable energy sources such as solar, wind, hydrogen, biomass has gained importance. The solar energy is a clean, reliable and environmentally friendly source and it seems as a major candidate to have significant share in energy production in the future [1-5]. Solar energy is converted into DC electrical energy using photovoltaic (PV) modules. Although PV modules have many advantages; they have some disadvantages such as high cost and low energy conversion efficiency. In the literature, there are different studies on mitigating some of these disadvantages and to provide the maximum benefit of the investment.

Different materials are used to increase the efficiency of PV system, but limited improvement has been obtained yet. Because the PV modules have maximum efficiency with a 90° angle to the sun light, sun tracking systems are designed which follow the sun in two or single axis [6, 7, 8]. PV modules, even in conditions of constant temperature and radiation, show a nonlinear current-voltage (I-V) and/or power-voltage (P-V) characteristic. There is a unique point on these curves that the PV modules generates maximum power called maximum power point (MPP). Because the MPP of the PV module varies with some parameters such as irradiation, temperature and load level, the MPP of PV system has to be tracked during the operation. Usually, maximum power point tracking (MPPT) is carried out via static converters. A simple PV system which includes a PV module, DC-DC converter, inverter and loads is given in Fig.1. It is design to provide maximum power transfer to the load/loads under any circumstance. In this structure there are two power conversion stages so it is called two-stage system. DC-DC converter tracks the MPP and regulates the DC load voltage. In case of AC power demand or grid connection, the power produced by the PV module is converted into AC power by using DC/AC inverters. On the other hand, both DC/AC conversion and MPPT can be performed by using only one DC/AC inverter which is known as single-stage systems [9-12]. In Fig. 1, DC/DC converter is desired to track the MPP of the system. So, a special algorithm to determine the MPP of the system and vary the operating point of the system to the MPP all the time. This algorithm provides the MPPT. Different MPPT algorithms and methods are proposed which can be classified into two groups, called passive methods and active methods [13, 14].

Fig. 1. Block diagram of solar energy conversion system.

This work is supported by Gazi University Academic Research Projects Units, under grant 07/2011-45 project number.

978-1-4673-6392-1/13/$31.00 ©2013 IEEE POWERENG 2013

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Passive methods use some parameters such as the irradiation level, panel temperature, short-circuit current, open-circuit voltage and some other module parameters directly or by utilizing mathematical equations. Firstly, the parameters for the selected module are calculated and then the obtained data is used for MPPT. Although these methods are simple, low cost and removes the complex calculations, they cannot provide real MPPT, because the module parameters change with pollution or aging of the module [13, 15, 16]. The characteristics of the modules do not be taken into consideration while applying the active methods, so module independent MPPT is obtained. Such parameters like output current, voltage or power of the module and / or the converter circuit is to be monitored continuously to determine the operation point and if it is MPP or not. Since the active methods provide more actual results compared or passive methods, they are widely used [14, 17]. In the literature, different type of MPPT methods has been proposed. Perturb & Observe (P&O) method, Incremental conductance (IC) method, fuzzy logic control method, neural networks, parasitic capacitance method and ripple correlation control method can be given as example to these methods. Because of the advantage of being easy to implement and the low costs, the use of P&O and IC are more common among these methods [18]. LabVIEW is a graphical programming language and it is capable of many applications such as automation, data acquisition, control, test and measurement. It is possible to design easily without the need for writing code by using the visualized design icons defined libraries of LabVIEW. LabVIEW is composed of two worksheets. The first of these is called as “Front Panel” which provides graphical user interface. The other one is called as the “Block Diagram” and it carries out the data flow to run the designed system. Besides of ability to simulate of any system, real time monitoring and control actions can be performed by using necessary equipment [19, 20]. In this study, a grid connected PV system simulator is developed by using MATLAB/Simulink and LABVIEW design programs. Proposed system is composed of three parts; the PV module, the DC/DC converter and DC/AC inverter. The inverter provides grid interactive operations while DC-Dc converter provides MPPT. DC/DC converter and DC/AC inverter and inverter controller are modeled with MATLAB/Simulink environment. The other parts of the system are designed with the LabVIEW. Data flow between these programs is performed by using Simulation Interface Toolkit (SIT). Simulator is designed to simulate different panels by using some characteristic values which are obtained from the datasheets. P&O method which runs in the LabVIEW environment is used for MPPT. System has tested with serial connected 10 modules with 200 W output power for each, manufactured by KYOCERA. Results obtained by simulation have showed that proposed system is able to perform tracking the MPP for different irradiation, temperature and load conditions, and grid interactive operation. The operating point and MPP of the system, input-output current and voltage waveforms of the DC/DC converter and inverter has been

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Istanbul, Turkey, 13-17 May 2013

shown on the simulator, according to the change of irradiation and temperature values depending on the selected panel. II.

PV CELL AND MATHEMATICAL MODEL

PV cells are manufactured using combination of p-n semiconductor materials. Usually, germanium (Ge) and silicon (Si) elements are used in production. However, there are PV cells made of different semiconductor materials such as Gallium arsenide (GaAs), cadmium telluride (CdTe), etc. Furthermore single-crystalline and polycrystalline silicon cells have been used currently in commercial modules. PV cells convert solar energy into DC electricity directly. As the sun's irradiance hit the surface of the pn junction of the PV cell, the minority charge carriers (free electrons) is caused by the breaking of one another. If the cell output is short-circuit or is connected to a load, the current would flow. The equivalent model of PV cells is given in Fig. 2 [20].

Fig. 2. The equivalent model of PV cells

If the PV cell in Fig.2 is assumed to be an ideal, the relationship between the output current and voltage can be written as Eq.1. ౧Ǥ౒

I = IPV – I0 ( ‡ሺ౗ǤౡǤ౐ሻ – I )

(1)

where, IPV refers to the generated current by the effect of irradiation, I0 refers to diode saturation current (or the holding current in the opposite direction), q is amount of the unit electron charge (1.60217646x10-19 C), k is the Boltzmann constant (1.3806503 x10-23 J/K), T is the temperature of the pn junction (K), Į indicates the diode ideality constant. According to the characteristics of the semiconductor material used in PV, some parameters change with time and if they are not taken into consideration, variation of these parameters may cause incorrect results in Eq. 1, In order to create a real model, some parameters should be added to the model as seen in Fig.2. In this case,, the equivalent of an ideal relationship between the output current and voltage can be written as in Eq.2. ౧

I = IPV –I0 (‡౗ǤౡǤ౐Ǥሺ୚ା୍Ǥୖ౩ሻ –1 )

୚ା୍Ǥୖ౩ ୖ౦

(2)

where, Rs is the equivalent series resistance. RP refers to the equivalent parallel resistor of PV cell. In addition, solar cells connected in parallel. In this case, Eq. 3 and Eq. 4 are used for cell current and diode current. Ipv = Ipv.Np

(3)

I0 = I0 .Np

(4)

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If Eq. 3 is used for the selected PV cell, non-linear currentvoltage (I-V) and/or power-voltage (P-V) relation given in Fig. 3 occurs. In Fig.3, Voltage-Current (IV) and Power-Voltage (PV) curves are given, where VOC is open-circuit voltage; ISC is short-circuit current; PMPP is power at MPP; VMPP and IMPP are maximum power point voltage and current.

(a)

(b)

Fig. 3. For PV module (a) Power-Voltage (P-V) graphics (b) Current-Voltage (I-V) graphics

III.

DC/DC CONVERTER

Equivalent model of the DC/DC boost converters is shown in Fig. 4, this circuit generates output voltage by increasing the input voltage with certain rates. The boost converters are widely used in PV systems because of several advantages such as simple implementation, cost-effective structure and drawing current from supply in any case when compared with some other DC/DC converters.

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L

dI L = Vg − V0 dt

C

dV0 V = IL − 0 dt R

(9) (10) IV.

DC/AC INVERTER

A single phase voltage source gird interactive inverter is depicted in Fig. 1. As it is seen, the system consists of a DC supply which is represented by VDC, a DC-AC voltage source inverter (VSI) and a LCL output filter. The output filter is employed to reduce the high frequency harmonic components in current waveform due to PWM switching and to reduce the output current THD. Since the resonant frequency of the LCL filter is related with only values of L1, L2 and C, it is preferred in grid interactive inverter applications. The state equations of the system, depicted in Fig. 5 can be written in matrix form as given in Eq. 4 and Eq. 5 [21]: ª «0 ª I1 º « « » « « I inv » = « 0 « VC » « ¬ ¼ 1 « ¬« C

0 0 −1 C

− 1º ª1º ª 0º L1 » ª I º « » » 1 L « − 1» 1 »« » « 1 » I inv » + « 0 »Vinv + « »Vgrid « » L2 « L2 » » «¬ VC »¼ « 0 » « » ¬« 0 ¼» 0» ¬ ¼ »¼

ª IL º y = [0 1 0]«« I inv »» «¬ VC »¼

(11)

(12)

Fig. 4. Equivalent model of boost controller

Depending on the equivalent circuit in Fig. 4, for DC/DC converter equations can be written as the following [20]. dI L L = Vg − (1 − d )V0 dt C

dV0 V = (1 − d )I L − 0 dt R

(5) (6)

where, d is a control signal and its value is”1” when switch is on and “0” when switch is off. According to Eq. 5 and Eq. 6, Eq. 10 and Eq. 11 can be rewritten as follows: Switch is on: L

dI L = Vg dt

(7)

C

dV0 V =− 0 dt R

(8)

Switch is off:

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Fig.5. Grid interactive inverter

V.

PERTURB & OBSERVE METHOD

Transferring maximum power to the load and/or grid from PV modules at any time is possible when MPPT methods which can continuously track output of modules are used. Many MPPT methods is developed and applied. P&O method is designed and one of the most commonly used MPPT methods. In this method, reference signal is changed and the effect of this variation on power is monitored, according the rate of change in power, new reference signal is generated. The flowchart of P&O algorithm is shown in Fig. 6. In Fig. 3, I-V and P-V graphics of a PV module are given. As it is seen from figure, there is unique MPP for any condition, and this point tracked by designed MPPT algorithm via changing power drawn from PV panel by DC-DC converter. According to this graphics, change direction of the reference signal determines the position of the operating point by MPP. In Table 1, relation between instantaneous changes of the reference signal and changes of output power is shown.

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TABLE 1. Changes of output power depending on instantly changes of reference signal

Reference

Change in Power

Direction of change

+ + -

+ + -

+ +

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values by using obtained values. In Fig. 8, block diagrams of PV panel model and output I-V and P-V curves. Maximum power (PMPP), maximum power voltage (VMPP) and maximum power current (IMPP) which calculated from PV panel model, and results are shown in “Maximum PV Parameters” part of front panel. Operation point of PV system determined by using these calculated values and is marked as a point on I-V and P-V curves of PV panel.

Although P&O is simple, non complex, low cost and a measurement of a small number of parameters is sufficient method, it has a disadvantage as to make oscillation around MPP. Variable step size P&O methods are proposed to prevent this oscillation [22].

Fig. 7.The front panel of the simulator PV

Fig. 6. Flowchart of P&O algorithm

VI.

DESIGN OF PV SYSTEM SIMULATOR

In this study, a PV system simulator is design and cosimulations are carried out by using LabVIEW and MATLAB/Simulink. In this study, linear and nonlinear dynamic systems is transferred from LabVIEW to MATLAB/Simulink and input/output current and voltage values of DC/DC converter and DC/AC inverter are transferred from Simulink to LabVIEW. Communication between these two programs is provided by the SIT which operates in LabVIEW. Front panel of PV simulator is shown in Fig. 7 [19]. PV panel is modeled by using parameters which are given in datasheets for standard operating conditions such Voc, Isc, number of cells in series (Ns) parameters in the field shown as "PV Module Parameters" on PV simulator screen. Thereby, designed system is allowed to be used different PV modules, where A is an optimization coefficient used in module model. Also, scroll buttons is used to change the atmospheric conditions such as irradiation and temperature. Irradiation (G) is defined within the range from 0 to 1000 W/m2, temperature (T)is in the range of 0-100°C. LabVIEW calculates current and voltage of panel by using given parameters, designed model by using Eq.2 and draws I-V and P-V curves with the

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Fig. 8.Block diagram of PV panel model and the output curves

In this study, P&O method is designed in LabVIEW and MPP of the PV modules is tracked continuously. The block diagram of P&O algorithm is shown in Fig. 9. The designed P&O algorithm generates control signal for PWM generator which is designed in MATLAB/Simulink. The DC/DC converter is controlled to obtain maximum power from PV system. At the same time, operating point of system is shown on V-I and P-V graphics on simulator screen. Thus, operation point of the system visualized. MATLAB/Simulink model of two-stage contverter, which consists of DC/DC boost converter and DC/AC VSI is shown in Figure 10. Reference current of grid interactive inverter is generated by voltage controller and phase locked loop circuit. A PI regulator is shaped the inverter output current. A line frequency transformer (LFT) is used to step up the output

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4th International Conference on Power Engineering, Energy and Electrical Drives

Istanbul, Turkey, 13-17 May 2013

P&O algorithm calculates the reference signal and tracks the MPP of the system. In Fig. 14, shown I-V and P-V curves when PV panel operates at MPP is given. The output current and output voltage waveforms of DC/DC converter and output current and output voltage waveforms of VSI when system operates at MPP are depicted in Fig. 15 and Fig. 16, respectively. Operation point of the system is visualized, and it can be easily defined if the system operates at MPP or at the left side of MPP or at the right side of the MPP from simulator screen.

Fig. 9. LabVIEW block diagram of P&O algorithm.

(a) (b) Fig. 11. a) I-V curve of PV modules for 1665W, b) P-V curves of PV modules for 1665W.

Fig. 10. MATLAB/Simulink model of DC/DC boots converter and DC/AC VSI.

voltage level to the grid voltage. The LFT also provides the galvanic isolation between the AC grid and PV modules and prevents DC current injection. Voltage and power values of DC/DC converter is shown on Simulator screen in “Converter Output Parameter” part by placing “out” output commands on Simulink model. Also output current, voltage and power values of VSI inverter is shown on Simulator screen in “Inverter Output Parameter” part. Both converter and inverter current and voltage waveforms are shown in front panel of simulator. In addition, implemented Simulink model can be controlled by using “Start”, “Stop” and “Pause” buttons which are placed “Simulink Controls” part on Simulator screen. VII. SIMULATION RESULTS In this study, simulation studies are realized by using parameters of 200W PV panel which produced KYOCERA. System operating and performance is tested for different radiation and temperature levels. In Figure 11 is shown I-V and P-V graphics for 1665W panel power. The operating point and MPP of the PV system is indicated in these curves. As can be seen operating point is far from MPP. In this case, P&O algorithm generates reference signal to reach the MPP by controlling DC/DC converter. DC/AC inverter is connected to the output of DC/DC converter and to convert DC energy to AC energy and transfer to the grid. The output current and output voltage waveforms of DC/DC converter and output current and output voltage waveforms of VSI are shown in Fig. 12 and Fig. 13, respectively.

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(a)

(b)

Fig. 12. a) DC/DC converter output voltage waveform for 1665W, b) DC/DC converter output current for 1665W.

(a) (b) Fig. 13. a) Grid voltage and inverter output current for 1665W, b) Reference and inverter output currents for 1665W.

(a) (b) Fig. 14. a) I-V curve of PV modules at MPP a) b) P-V curves at MPP.

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[5]

[6]

[7] (a) (b) Fig. 15. a) Output voltage of DC/DC converter at MPP, b) Output current of DC/DC converter at MPP.

[8]

[9]

[10]

[11] (a) (b) Fig. 16. a) Grid voltage and inverter output current at MPP, b) Reference and inverter output currents at MPP.

[12]

VIII. CONCLUSIONS

[13]

In this study, a grid connected PV system simulator that is capable of MPPT is designed by using both of MATLAB/Simulink and LabVIEW programs. Data transfer between programs is provided by using SIT which operated LabVIEW. Different type of PV modules can be modeled by using parameters given in datasheets. Designed PV simulator is tested by using ten serial connected modules produced by KYOCERA. A two-stage inverter system which consists of a DC/DC converter which tracks maximum power point and DC/AC grid interactive inverter is obtained. Proposed system simplifies the operation principles and specifications of grid interactive PV systems by visualizing system. Proposed simulator can be used not only for educational purposes, but also investigating of reliability any territory of energy investment and in order to make a preliminary assessment of properties.

[14]

[15]

[16]

[17]

[18]

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Istanbul, Turkey, 13-17 May 2013

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