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ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)

REAL-TIME EMULATOR OF PHOTOVOLTAIC ARRAY IN PARTIAL SHADOW CONDITIONS BASED ON CLOSED-LOOP REFERENCE MODEL

Riad KADRI, Jean-Paul GAUBERT, Gérard CHAMPENOIS, Mohamed MOSTEFAÏ Laboratoire d’Automatique et d’Informatique Industrielle (LAII-ESIP) Université de Poitiers, France E-mail: [email protected]

Abstract: Solar photovoltaic systems technology have known can be simply adopted in existing buildings and can be an extensive research in recent times due to their suitability installed anywhere. In addition, manufacturers have for use in low, medium and high power generation. In recent designed various models, which can be placed at a years, the grid connected photovoltaic systems have become variety of different types of houses or buildings to more popular because they do not need battery back-ups to ensure maximum power point tracking (MPPT). However, achieve better performance. However, performance partial shading is one of the main causes that reduces energy analysis of this scheme in real conditions is generally a yield of photovoltaic array. Hence research activities have difficult task because several factors should be taken into mainly focused on the influences of array configuration on account such as the partial shading. the energy yield while in contrast very little attention has In a Classical grid connected PV system topology, a been drawn to the performance of the MPPT under shaded series connected of photovoltaic module is used Fig.1, array conditions. Consequently, photovoltaic array emulator performance is negatively affected if all its modules are is indispensable (essential) for the operational evaluation of inhomogeneous illuminated. All the modules in a series system components. The dynamic response of the array are forced to carry the same current even though a photovoltaic array emulator is of particular importance in order to avoid any significant impact on the maximum power few modules under shade produce less photocurrent. The point tracker and current control of the inverter. In shaded modules may get reverse biased, acting as loads numerous papers, the current and voltage vectors of the and dissipates power from fully illuminated modules in photovoltaic array are pre-loaded into a look-up table and the the form of heat. If the system is not appropriately system is iteratively converging to the solution. The purpose protected, hot-spot problem [1]-[2] can arise and in of this paper is to design and develop a new real-time several cases, the system can be irreversibly damaged. emulator of photovoltaic array output characteristics based on closed-loop reference model. The proposed system consists of a programmable power supply, which is controlled by a dSPACE DS1104 board under the Matlab/SimulinkTM environment. The control software uses feedback of the output voltage, current and reference model to regulate through PI regulator the actual operating point for the connected load. The experimental results show that the output characteristics of the emulator are very close to those of the actual photovoltaic modules with dynamic characteristics much lower than the possible climatic PV VSI L Grid variations. Keywords: photovoltaic, maximum power point tracking, Fig. 1. Classical grid connected PV system topology real-time PV emulator, shadow conditions. In the new trend of integrated PV arrays, it is difficult to avoid partial shading of array due to neighboring I. INTRODUCTION buildings throughout the day in all the seasons. This makes the performance study of partial shading of The importance of the renewable energy like modules an important issue. photovoltaic systems in the generation of electricity is Field testing is conducted to ensure the quality of rapidly increasing currently. Beside the plants in a large installation and the performance of the product, in fact, resource, one of the main focus areas in the introduction is costly, time consuming and strongly dependent on of photovoltaic (PV) as renewable energy power source actual climatic conditions. In addition, it is not without connected to the grid is the use of building surfaces for its risks since the direct employment of the PV modules photovoltaic installations. The PV systems are modular, for prototyping test can damage the source itself. A hence the major advantage of these systems is that they solution for developing experimentations without the

71 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) ISSN 1843-6188

need of real PV panels is thus very important, at least for current source is directly proportional to the light falling the first stage of testing. This has increased the interest on the cell (photocurrent). During darkness, the solar cell on the development of laboratory tools useful for is not an active device, it works as a diode, i.e. a p-n carrying out measurements and analyses, with no need to junction. It produces neither a current nor a voltage. perform field tests or to wait for particular atmospheric Thus the diode determines the I-V characteristics of the conditions [3]. cell. For this paper, the electrical equivalent circuit of a A wide range of photovoltaic array emulators based solar cell is shown in Fig. 2. The output current I and in power converters have been proposed and developed output voltage V of solar cell is given by (1) and (2). during last years. Some of them without galvanic isolation [4], based on structures with low frequency Vdo骣 q. V do V do transformer [5] or on HF transformers [6] and using I= Iph - I do - = I ph - I0 琪exp( ) - 1 - (1) Pulse Width Modulation (PWM) principle [7] or linear Rsh桫 n. k . T R sh converters to avoid EMC-measurements [8]. Trying to V= Vdo - Rs I (2) emulate the PV current-voltage curve (I-V curve) converters amplified the curve of a reference solar cell Here, I is the photocurrent, I is the reverse saturation [9] or obtained the I-V curve from a discrete table stored ph 0 current, I is the average current through diode, n is the in a memory and then interpolated the points [10] [11], d diode factor, q is the electron charge, q = 1.6*10-19, k is but most of them used mathematical models of panel’s I- Boltzmann’s constant, k = 1.38*10-23, and T is the solar V curve and calculated it from array‘s parameters [12] arrays panel temperature. R is the intrinsic series [13], making possible to modify and simulate the PV s resistance of the solar cell, this value is normally very curve under different situations easily. small. R is the equivalent shunt resistance of the solar In this paper, the development of real-time emulator sh array and its value is very large. In general, the output of photovoltaic array based on closed-loop reference current of solar cell is expressed by (3): model structure is presented. With this scheme it is possible to analyze at the laboratory the behavior of both the maximum power point tracking (MPPT) techniques 骣 q 骣V+ Rs I I= Iph - I0 琪exp( ( V + Rs I )) - 1 - 琪 (3) and the complete photovoltaic system under special 桫 n. k . T桫 Rsh conditions like partial shading. On the other hand, the emulation of the electrical behavior of photovoltaic Where the resistances can be generally neglected and generators allows performance analysis of photovoltaic (3) is simplified to (4). inverters to be carried out at the laboratory without requiring real photovoltaic energy sources. q In section 2, the characteristic I-V curve of a PV module 骣 I= Iph - I0 琪exp( . V ) - 1 (4) is explained, and the final model under partial shading 桫 n. k . T conditions used in this emulator is introduced while section 3 discusses ways of implementing a closed-loop If the circuit is opened, the output current I = 0 and reference model for regulating the output voltage as a the open-circuit voltage Voc is expressed by (5). function of the output current. Sections 4 and 5 present a simulation and experimental results, respectively. Finally 骣n. k . T骣Iph 骣 n . k . T 骣 I ph the conclusions are stated in section 6. Voc =琪 In琪 +1 琪 In 琪 (5) 桫q桫 I0 桫 q 桫 I 0 II. SOLAR CELL AND PV ARRAY MODEL If the circuit is shorted, the output voltage V = 0, the A photovoltaic generator is the whole assembly of average current through diode is generally neglected, and

solar cells, connections, protective parts, supports etc. In the short-circuit current Isc is expressed by using (6). the present modeling, the focus is only on cells. Solar cells consist of a p-n junction, various modeling of solar I ph cells have been proposed in the literature [14]-[17]. Isc = I = 骣 R (6) 琪1+ s 琪 R I 桫 sh ph I R I I s do r Finally, the output power P is expressed by (7). R V sh do V 骣 Vdo P= VI =琪 Iph - I do - V (7) 桫 Rsh

Fig. 2. Solar cell electrically equivalent circuit Two or more modules can be pre-wired together to be Thus the simplest equivalent circuit of a solar cell is a installed as a single unit called a PV or solar panel. current source in parallel with a diode. The output of the Additional PV panels can be added as electricity production needs increase. The entire PV system,

72 ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)

consisting of more panels in series connected, is known I ph I R I I s as an array Fig. 3. do r R V sh B I do d ph I R I I s do r R V sh do

I I ph I I R do r s V R V sh B I do d ph I R I I s do r R V sh do V I ph I R I I s do r R V sh B I I do d ph I I R do r s R V sh do Fig. 4. Equivalent circuit model of PV array bridged by bypass. III. DESCRIPTION OF THE PROPOSED SYSTEM

Fig. 3. Equivalent circuit model of PV array in series In order to obtain the I–V characteristics of the series- connection. connected modules (series assembly) conducting a current I, the voltages across these modules, V , V and Because the series connection of the PV generator forces 1 2 V , are added to determine the resultant output voltage. all modules to operate at the same current (string n The characteristics for series assembly are, thus, current), the shaded cell within a module becomes obtained internally by the software by applying similar reverse biased which leads to power dissipation and thus procedure at all the points on the I–V curve of the series- to heating effects. To avoid thermal overload and the connected modules. formation of hot spots, sub-strings of cells inside the interconnection circuit of modules are bridged by bypass diodes (Fig.4). This measure limits the bias voltage at if I  I T cc1 the shaded cell and thus the dissipated power. Another 1 V = f(I) 1 reason to use bypass diodes is to preserve more of the V G else 1 1 power output of the module in case of partial shading. V = V For economic reasons whole strings of cells are I 1 Bd V bypassed, and therefore, even if only one cell is shaded, pv end 2 the whole string is affected, and produce considerably V V pv less power than it would have done without the bypass n-2  diode. This phenomenon can move the maximum power V point to unexpected places. n-1 if I  I T ccn V n V = f(I) n n G n else V = V I n Bd pv end

Fig. 5. Configuration synoptic when the modules are bypassed.

IV. SIMULATION RESULTS

This section describes the procedure used for simulating the I–V and P–V characteristics of a partially shaded PV array. Based on the above equations, the PV model has been implemented using Matlab. The Graphical-User- Interface, allows to plot the I–V and P–V characteristics of a PV array. The user has access to choose the

73 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14) ISSN 1843-6188 irradiation intensity (0 to 1000 W/m2), the cell temperature, as well as the number of panels in series, Fig. 8. 3D typical P(V) characteristics of PV array under which form the string and the number of strings in homogeneous irradiation variation. parallel. These options offer a very high flexibility and a 4 wide range of different PV plant configurations with different voltage and current levels can be simulated. 3 . 5

Furthermore, the effects of the partial shading can be A (

2 . 5 visualized by choosing the Irradiation level. Modeling t n the partial shading effects has been made based on the e 2 r r assumption that every module has its own bypass diode. u 1 . 5 C Firstly, Figures 6, 7 and 8 shows the simulation 1 results of the emulating operation mode under 0 . 5 homogeneous illuminated. 0 The I–V and P–V characteristics of the PV (shaded and 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 unshaded modules) modules at the same temperature but at different irradiation levels are shown in Figures 9, 10 Fig. 9. I(V) characteristic of PV array with three and 11. 4 irradiation levels. 2 0 0 3 . 5 Voltage (V)

) 3 1 5 0 ) A (

r 1 0 0 r w o u 1 . 5 P C 1 5 0 0 . 5

0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0

Fig. 6. Typical I(V) of PV array. Fig. 10. P(V) characteristic of PV array with three irradiation levels. 4 0 0 Voltage (V) Voltage (V) 3 5 0

) 3 0 0 W

( 2 5 0

r e 2 0 0 w o

P 1 5 0 ) 1 0 0 W (

5 0 r e

0 w 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 o Voltage (V) P

Fig. 7. Typical P(V) characteristic of PV array.

Fig. 11. 3D P(V) characteristic of PV array under inhomogeneous irradiation variation. Voltage (V) Time ) W (

r e w o P

Voltage (V) Time 74 ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)

Programmable

) Power

W supply (

e dSPACE w DS1104 o P

Screen of ControlDesk Load

Fig. 12.Voltage Comparison (V) of 3D P(V) characteristics. Time

V. EXPERIMENTAL RESULTS Fig. 13. Photovoltaic experimental test bench structure.

The real-time emulator of photovoltaic experimental test 4 bench used was developed in LAII-ESIP laboratory (Fig. 3.5

13 and 16). It was achieved with a programmable power 3 supply: TDK-Lambda GEN300-11. The control strategy ) A 2.5 (

is implemented using a single-board DS1104 t

n 2 manufactured by dSPACE Company and developed e r

r 1.5 under the integrated development environment of u

TM C Matlab/Simulink RTW provided by The MathWorks 1

Inc. The control is achieved by two PI regulators with 0.5 reference model and uses feedback of outputs voltage and 0 current to regulate the operating point for the connected 20 40 60 80 100 120 load. Figures 14 and 15 show respectively the I(V) and P(V) Fig. 14. Experimental I(V) characteristics of PV array with characteristics for three irradiation levels or shadow three irradiation levels. conditions: 1000 W/m², 700 W/m² and 400 W/m². Red line corresponds to the reference model results and blue 2 0 0 Voltage (V) crosses are experimental points. In both cases, the real- 1 8 0 time emulator output follows perfectly the trajectories of 1 6 0 1 4 0

the statement PV arrays for the three shadow conditions. )

W 1 2 0

Other results are exposed on figures 17 and 18 where red (

r 1 0 0 line corresponds to the reference model also and blue line e

w 8 0 to the output of the emulator. On the right side of these o figures, shadow conditions are fixed through irradiation P 6 0 differences: for the first module irradiation is equal to 4 0 1000 W/m², for the second 700 W/m², for the third 400 2 0 0 W/m². On the other hand the temperature is identical for 2 0 4 0 6 0 8 0 1 0 0 1 2 0 the three cells since they are laid out at the same place.

Fig. 15. Experimental P(V) characteristics of PV array with three irradiation levels. Voltage (V)

Fig. 16. Control synoptic of the experimental test bench structure.

Fig. 17. Screen of real-time experiment environment ControDesk for I(V) characteristics.

76 ISSN 1843-6188 Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)

Fig. 18. Screen of real-time experiment environment ControDesk for P(V) characteristics.

VI. CONCLUSION [2] Klenk M, Keller S, Weber L, Marckmann C, Boueke A, Nussbaumer H, Fath P, Burkhart R, In this study, a real-time emulator of photovoltaic array Investigation of the hot-spot behavior and formation based on closed-loop reference model structure is in crystalline silicon POWER cells, PV in Europe, investigated. With this development, it is possible to From PV technology to energy solutions. analyse the behaviour of PV array on the one hand and Proceedings of the International Conference, 2002, on other hand to work out the maximum power point pp. 272–275. tracking (MPPT) techniques in case on partial shading at [3] Patel H, Agarwal V, MATLAB-based modeling to the laboratory test bench. Our solution allows evaluating study the effects of partial shading on PV array the maximum power point places and their number characteristics. IEEE Trans. Energy Convers., 2008, according to the atmospheric conditions as well in 32(1), pp. 302–310. simulation that an experimental. Thus, in function of [4] Sera D, Teodorescu R, Rodriguez P, PV panel different irradiation on PV arrays, we can plotted or model based on datasheet values. Proceedings IEEE emulated characteristics I(V) and P(V) and estimate the Intern. Symp. on Indus. Electron.-ISIE 07, Vigo, performances of the system under these conditions. The Spain, 2007, pp. 2392–2396. control circuit is implemented in real time by using a [5] Mukerjee AK, Dasgupta N, DC power supply used single-board dSPACE DS1104 with excellent results and as photovoltaic simulator for testing MPPT dynamic characteristic much lower than the climatic algorithms. Renew Energy 2007, 32, pp. 587–92. variations since its band-width is equal at 10 Hz. Now, [6] Enrique JM, Duran E, Sidrach-de-Cardona M, new algorithms are in progress in order to fit to its Andujar JM, Bohorquez MA, Carratero J, A new conditions and to detect the true point of maximum approach to obtain I-V and P-V curves of power. photovoltaic modules by using DC/DC converters. Rec. IEEE Photo. Spec. conf., 2005, pp. 1769–72. ACKNOWLEDGMENT [7] Di Piazza MC, Serporta C, Vitale G, A DC/DC converter based circuit model for a solar This work was supported in part by the local council of photovoltaic array. Rec. 21th Eur. Photovoltaic Poitou Charentes, France under research project N°: Solar Energy Conference and Exhibition, Dresden, 08/RPC-R-003. Germany, 2006, pp. 2726–2731. [8] Sanchis P, Echeverria I, Ursua A, Alonso O, Gubia VII. REFERENCES E, Marroyo L, Electronic converter for the analysis of photovoltaic arrays and inverters. Proceedings [1] W. Herrmann, W. Wiesner, W. Waassen, Hot spots IEEE 34th Power Electro. Spec. Conf., vol. 4, 2003, investigations on PV modules, new concepts for a pp. 1748–1753. test standard and consequences for module design [9] Zeng Q, Song P, Chang L, A photovoltaic simulator with respect to by-pass diodes, Proceedings of the based on DC chopper. Proceedings IEEE Canadian 26th IEEE Photovoltaic Specialists Conference, Conf. on Electro. and Comp. Engin., Winnipeg, 1997, pp. 1129–1132. Canada, 2002, pp. 257–61.

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