2015 12th International Multi-Conference on Systems, Signals & Devices 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 60 61 62 63 64 65
Design verification based on Hardware-In-the-Loop simulation for photovoltaic system Hanen Abbes*, Mossaad Ben Ayed*, Hafedh Abid**, Mohamed Abid* Department of Electrical Engineering National Engineering School of Sfax * : Laboratory of Computer and embedded Systems (Lab-CES), University of Sfax, PB 1173, 3038, Sfax, Tunisia ** : Laboratory of Sciences and techniques of Automatic control & computer engineering (Lab-STA ) E-mail:
[email protected] [email protected]
of this latest algorithm is based on a linear relationship which linked the optimal point to the open-circuit voltage through a K factor. Determining the optimal value of this K parameter is very difficult. Therefore, this method seems to be just an approximation, and it does not have sufficient precision. Owing to the complexity of photovoltaic systems and their specific behavior, researchers are greatly prompted by the FPGA device for prototyping and testing the PV system [8][9]. Since, the design of the system controller requires verification step to ensure an effective operation of the system, simulation tools are used to fulfill verification task. The simulation platform needs to be accurate, flexible to change, reliable and with minimum risk of fault. Classical approaches of simulation aim to describe physical processes in mathematical or physical models. In some cases, the user have to interact with the simulation to build up a hardware in the loop simulation environment. The integration of target architecture in the simulation loop, known as Hardware-In-the-Loop (HIL), provides a realistic verification and therefore enhances the quality of testing [1]. It allows simulation of the complete model on a computer while some of its functions are implemented in real hardware. Thus, real-time HIL simulation replaces the emulated hardware under test and allows testing dangerous situations. This reduces risks of discovering errors in last stages of on-the-field testing and therefore minimizes the gap between design and implementation. The DSP builder software is the tool which is used to highlight HIL simulation method on the photovoltaic (PV) system. This software interfaces the model designed in Simulink tool with Altera Quartus development software. In fact, it relies on a seamless design flow which allows system integration in Simulink and later on generates HDL files to use in the Quartus II software. This promising HIL method enables chip programming for validation [7]. The experiment design verification is achieved using Altera Startix-II platform as a suitable selected device. Since this programmable logic device is intended for a rapid prototyping, it looks to be an efficient hardware to verify the functionality of the photovoltaic system. Despite of the aforementioned drawbacks of the opencircuit voltage, this type of algorithm is chosen thanks to its high simplicity and its easy implementation. In this paper, the
Abstract—Due to the increasing complexity of photovoltaic systems and problems linked to development and design, verification of the entire system operation is essential before real implementation. Besides, high requirements of a real-time simulation and control circuit prototyping before application increases safeness, and can reduce time and costs of implementation. For this end, the purpose of this work is to achieve photovoltaic system development and its design verification through system simulation using FPGA device. The verification method used in this study is the “Hardware-In-the-Loop” (HIL) simulation method. It provides an effective platform for developing and testing real time embedded systems. In this paper, we design the power circuit and we develop the open-circuit voltage controller which tracks the maximum power point. Then, the HIL simulation process is performed for the photovoltaic system. The efficiency of the photovoltaic system is validated by the simulation results. Keywords—Photovoltaic; MPPT; open-circuit Hardware-in-the-Loop; simulation; FPGA.
voltage;
I. INTRODUCTION The increasing demand of the energy as well as the growing attention to environment pollution, lead researchers to the development of renewable energy sources. For these reasons, photovoltaic source, as a kind of a free and clean energy seems to be a very auspicious solution. In most photovoltaic applications, maximum power point tracking (MPPT) is an essential issue that increases the efficiency of photovoltaic arrays. Therefore, a great effort has been spent with MPPT techniques while considering the variation of parameters such as temperature, solar irradiation or the load of the photovoltaic system. In the literature, several algorithms have been elaborated to track the maximum power point. Among all these algorithms, Perturb and Observe and Incremental Conductance are very popular and broadly used [10][11]. We found also very simple algorithms which are based on linear equations such as short-circuit current and open-circuit voltage [12]. The principle
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open-circuit voltage method is considered to control the PV system. Afterward, this paper is organized as follows: In section 2; the entire photovoltaic system is designed and modeled. In section 3, the HIL simulation method is described. In section 4, we develop the fractional open-circuit voltage MPPT controller using DSP Builder tool and then we perform the HIL simulation to the system. In section 5, we present simulation results and we make discussions. Finally, we have a conclusion.
Electrical characteristics (I-V) and (P-V) of photovoltaic module are given by Fig. 3. It is clear that the photovoltaic module has a nonlinear characteristic with a single point where the power is maximal. Since, these characteristics vary greatly in terms of temperature and irradiation, the optimal power point varies. Thereby, it is more efficient when the power converter switch is controlled by a specific algorithm to track the MPP.
II. DESIGN MODEL OF PHOTOVOLTAIC SYSTEM The photovoltaic system is made up of three main blocks: photovoltaic module which represents the source of energy, the static DC-DC power converter and the MPPT controller. The block diagram of photovoltaic system is shown in Fig. 1.
Fig. 3. I-V and P-V characteristics of photovoltaic module
B. Boost Converter The power converter is an interface which allows the adaptation between the PV module and the load in order to extract the maximum power of PV module. The static DC-DC converter type depends on the role that we want to achieve through the photovoltaic system. There are many types of DCDC power converter such as Buck, Boost or Buck-Boost. We are interested in a Boost type that its electrical model is given by Fig. 4.
Fig. 1. Block diagram of phototvoltaic system
A. Photovoltaic module The photovoltaic cell is the basic component of photovoltaic module. It is commonly considered as a current source [14]. The current generated by this cell varies with temperature (T) and irradiation (G). Its expression is: ୮୦ ൌ ሺ ୱ୦ ୍ οሻ
ୋ
(1)
ୋ
For t[0,D7@, the transistor is on. Therefore, in this stage the converter is modeled by the following equations:
Therefore, the model of the PV cell can be draw up as shown in Fig. 2 and the appropriate equations are given as follows:
ௗಽ ௗ௧
The expression of photovoltaic current is equal to:
ௗమ
ܫ ൌ ܫ െ ܫௗ െ ܫோ
ௗ௧
(2)
ುೇ ାோೞ ூುೇ
ቁ െ ͳቃ
(3)
ௗಽ ௗ௧
The resistance current of Rp is equal to: ܫோ ൌ
ௗమ
ುೇ ାோೞ ூುೇ
Temperature Irradiation
Iph
ௗ௧
(4)
ோ
Rs Id
IR
D
Rp
భ
ൌെ
(5)
మ ோಽ ೞ
(6)
For t[D77], the transistor is off. Therefore, the equations of the converter in this stage are given as follows:
The current expression of P-N junction is given by: ܫௗ ൌ ܫ௦ ቂ ቀ
ൌ
ൌ ൌ
భ ିమ
(7)
ಽ ࢙
െ
మ ೞ ோಽ
(8)
T is the switch time of the power converter and α is the duty ratio.
IPV
Where : Gn : reference irradiation (1000W/m2), Ish : short-Circuit current, Is: saturation current of the cell junction, n: quality factor of the diode which is between 1 and 2, KI : short-circuit current/temperature coefficient, VT : thermodynamic potential.
VPV
Fig. 2. Equivalent circuit of photovoltaic cell
2
L
V1
D
K
CS
RL
V2
Fig. 4. Electrical schema of BOOST converter
C. Open-circuit voltage MPPT algorithm The Open-Circuit voltage algorithm is the simplest MPPT control technique. The open circuit voltage VOC of the PV panel depends on the solar cells property. The relationship between VOC and the optimal point VMPP can be described by the following equation: VMPP=K*Voc (9). Detailed flowchart of the open-circuit voltage algorithm is depicted in Fig. 5. The K factor varies between 0.73 and 0.8. When the PV output voltage is approximately 80% of the open circuit voltage, photovoltaic module operates at its MPP. Indeed, duty ratio (D) is varied until panel attains the optimal value [13].
Fig. 6.
HIL proceedings
Consequently, the HIL method ensures communication between the digital control algorithm (implemented into the target FPGA) and the design environment (emulated by the computer within Matlab/Simlink software). In next section, we practice HIL simulation process on the photovoltaic system. IV. SIMULATION OF PHOTOVOLTAIC SYSTEM USING HARDWARE IN-THE-LOOP METHOD The photovoltaic system is modeled then simulated using Matlab-Simulink, as shown in Fig. 7: First, the PV array is modeled. Second, the DC-DC Boost converter is designed and linked to load resistance (RL) and finally, the MPPT Controller is developed.
Start
Measure of VOC, V(k)
VMPP =K*VOC
V(k)< VMPP N
Y
D =D+dD
D =DdD
Fig. 7. Complete Simulink model of Photovoltaic system Fig. 5. Flowchart of Open-Circuit Voltage algorithm
Hardware Simulation for PV system is performed using HIL simulation method as shown in Fig. 9. The first step of this method is destined to design the PV module as well as the Boost converter power circuit by the means of Matlab/Simulink tool. In the second step, the open-circuit voltage MPPT controller is developed using Altera DSP builder tool. In this stage, the controller part should be replaced by blocks from Altera DSP Builder Blockset, as seen in Fig.8. We noted that in many cases, it is too hard to found suitable blocks from DSP Builder library to replace some Simulink blocks. As a result, the designer should try to build these functions from the basic DSP Builder blocks. Hence, the implementation of the Open-Circuit voltage algorithm is carried by performing these following steps: first the V(k) voltage is measured, then we compute the optimal voltage value using the linear equation. Second, we compare V(k) to VMPP value. In fact, we use “IF Statement” block and “Multiplexer” block available on Altera Blockset to
III. HIL SIMULATION METHOD In many cases, the most effective way to develop an embedded system is to connect the embedded system to the real plant. Therefore, Hardware-In-the-Loop (HIL) simulation seems to be an efficient solution that enables you to test your embedded system before deploying it in a production environment. For verification reasons and early detection of system errors, HIL simulation method is currently used [4][5]. It is an important feature provided by DSP Builder Software. This latest is a development tool that integrates into a single environment a design model based on Matlab/Simulink and FPGA platform. The use of this function plays an important role in the development process. The steps of this function, as shown in Fig. 6, can be summarized as follows: configure, compile/program the target architecture and simulate the whole system [15].
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perform this test: if the V(k) is less than VMPP, we increase duty cycle, else we decrease the duty cycle.
V.
SIMULATION RESULTS AND DISCUSSIONS
The block diagram of the proposed PV system using FPGA is seen in Fig. 10. The operation of the proposed system using HIL block is described as follows: At each time step, system model is simulated using Simulink software, and signals (I and V) are sent to the target FPGA. When FPGA device receives signals from Simulink, it executes implemented program for one sample interval and after returns the control signal.
DC/DC Converter
PV I
PWM signal
MPPT Controller (HIL Block)
Fig. 8. Implementation of Open-circuit volatge MPPT algorithm using Altera DSP Builder Blockset
In this way, the integration of Matlab/Simulink software together with Altera DSP Builder brings out the hardware cosimulation concept. VHDL code and TCL scripts are generated by the means of “Signal Compiler” block to perform synthesis and compilation in Simulink. The complete Altera controller block is then converted into a single HIL block which represents FPGA in Simulink environment. At each simulation step, photovoltaic current Ipv and photovoltaic voltage Vpv are exported to FPGA, process in the controller and send output pulses to the Simulink through JTAG connection [6].
Photovoltaic array and Boost Converter modeled on Matlab/Simulink
V
Load
Matlab/Simulink environment
Altera/ Startix II FPGA Fig. 10. Block diagram of the proposed PV system
Simulation results of the proposed system with MPPT controller using HIL block and FPGA device is given by Fig. 11. In fact, the optimal outputs power, voltage and current are illustrated.
MPPT controller designed on Altera DSP Builder
Co-simulation
(a) Automatic code generation Synthesis and Fitter (Quartus II)
HIL block
Fig. 9. Photovoltaic system using HIL simulation process (b)
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(c) (b)
Fig. 11. Optimal outputs of : (a) power, (b) voltage, and (c) current of the proposed PV system using HIL simulation
Fig. 13. Simulation results of : (a) PV output power and (b) voltage for simulink model and DSP builder compared to HIL simulation
Simulation result of the duty cycle generated by FPGA circuit controller is given by Fig. 12.
These results prove that PV system operates correctly at the optimal values in the three stages of HIL simulation process. Therefore, algorithm feasibility is tested and verified using Hardware FPGA device before its real implementation. We note so that we developed an appropriate design for the photovoltaic system and consequently we validate the efficiency of the proposed system. To show performances of PV system, we test its operation under temperature and irradiation variations. We suppose variations given by Fig. 14. Irradiation variation can be occur due to clouds. Temperature is kept at 25°C. The results of simulation of PV output power and voltage of the proposed photovoltaic system is shown in Fig. 15.
Fig. 12. Duty cycle given by FPGA device
In each stage of the HIL simulation flow, the proposed PV system is tested. Simulation results show validated design and implementation of MPPT controller. Fig. 13 depicts matched results given by the simulation of the PV system designed first in Simulink, then using DSP builder tool and finally the HIL method.
Fig. 14. Irradiation variations
Fig. 15. Output power and voltage of PV system under temperature and irradiation variations
(a)
5
In addition to the verification feature of HIL simulation method, compute the simulation time is a motivate reason to use this tool. In fact, HIL method spends less time compared to traditional Simulink model simulation (table 1). TABLE I.
SIMULATION TIME OF PV SYSTEM MODELED ON SIMULINK, DSP BUILDER TOOL AND HIL METHOD
Simulink model
DSP Builder method
HIL simulation
498.6 s
462.4 s
420.3 s
[5]
Marco Mauri, “Hardware in the loop simulation of renewable distributed generation systems”, Departement of Mechanics, Politecnico di Milano Italy
[6]
Rajesh P. , Rajasekar S., Rajesh Gupta, Paulson Samuel, “Solar array system simulation using FPGA with Hardware Co-simulation”, Industrial Electronics (ISIE), 2014 IEEE 23rd International Symposium on
[7]
N. F. Guerrero-Rodríguez, A. B. Rey-Boué, S. de Pablo-Gómez, “Design of the Control Algorithms for Photovoltaic Grid-Connected Renewable Agents using the Hardware-in-the-loop Simulation Technique”, International Conference on Power System Transients (IPST2013) in Vancouver, Canada July 18-20, 2013
[8]
S. PIRÓG, R. STALA, and L. STAWIARSKI, “Power electronic converter for photovoltaic systems with the use of FPGA-based real-time modeling of single phase grid-connected systems”, ULLETIN OF THE POLISH ACADEMY OF SCIENCES TECHNICAL SCIENCES Vol. 57, No. 4, 2009
[9]
Basil M. Hamed, Mohammed S. El-Moghany, “Fuzzy Controller Design Using FPGA for Photovoltaic Maximum Power Point Tracking”, International Journal of Advanced Research in Artificial Intelligence, Vol. 1, No. 3, 2012
CHARACTERISTICS OF PHOTOVOLTAIC PANEL: Characteristics of the used photovoltaic panel are summarized in table 2: TABLE II.
PV PANEL CHARACTERISTICS [10] A.Pradeep, Kumar Yadav, S.Thirumaliah, G.Haritha, “Comparison of MPPT Algorithms for DC-DC Converters Based PV Systems”, IJAREEIE, Vol. 1, Issue 1, July 2012
NP
NS
A
Eg
q
VCO
1
36
1.9
1.12
1.6e-19 c
21.38V
ISH
RS
RL
RSH
KI
Pmax
4.8A
0.09 Ω
30 Ω
100 Ω
0.00171A°/C
81.4W
[11] Balakrishna S, Thansoe, Nabil A, Rajamohan G, Kenneth A.S., Ling C. J., “The Study and Evaluation of Maximum Power Point Tracking Systems”, International Conference on Energy and Environment”, 2006 [12] Burri Ankaiah, Jalakanuru Nageswararao,"Enhancement of Solar Photovoltaic Cell by Using Short-Circuit Current Mppt Method”, IJESI, Volume 2 Issue 2, PP.45-50, February 2013 [13] J. Surya Kumari, Ch. Sai Babu, “Comparison Of Maximum Power Point Tracking Algorithms For photovoltaic System”, IJAET, Vol. 1, Issue 5, pp.133-148 Nov 2011 [14] Ali Chermitti, Omar Boukli-Hacene, Samir Mouhadjer, “Design of a Library of Components for Autonomous Photovoltaic System under Matlab/Simulink”, International Journal of Computer Applications (0975 – 8887), Volume 53– No.14, September 2012
CONCLUSION: In this paper, FPGA based photovoltaic system is set using HIL simulation method. Power circuit is modeled in Simulink/Matlab environment. Afterwards, Altera DSP builder tool is harnessed to develop open-circuit voltage MPPT controller and to test photovoltaic system performances. Experimental results of output power obtained from the proposed HIL simulation system are closely matched with results of the photovoltaic system model based on Simulink software. Results show that PV system feasibility is verified using FPGA circuit before real implementation. Therefore, HIL simulation is an accurate method and thus an efficient way used to validate the photovoltaic system design.
[15] DSP Builder Handbook : “Volume 2 : DSP Builder Standard Blockset”, www.altera.com, Document for Altera, June 2014
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Carlos Vladimir Paiz Gatica, “Dynamically Reconfigurable Hardware for Embedded Control Systems”, dissertation, December 2011.
[2]
Shahram Karimi, Philippe Poure, and Shahrokh Saadate, “An HIL-Based Reconfigurable Platform for Design,Implementation, and Verification of Electrical System Digital Controllers”, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 4, APRIL 2010
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