International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 Vol. 1, No. 3, December 2013 Transient Analysis of Grid-Connected Photovoltaic System Based on Comparative Study of Maximum Power Point Tracking Techniques Almoataz Y. Abdelaziz1, Hadi M. El-Helw2, Basem Abdelhamed3 1 Department of Electrical Power & Machines, Faculty of Engineering, Ain Shams University, Cairo, Egypt e-mail: almoatazbdelaziz@hotmail.com 2 Electrical and Control Department, Arab Academy for Science, Technology and Maritime Transport, Cairo, Egypt e-mail: elhelw@cairo.aast.edu 3 Department of Electrical Power, El-Shorouk Academy, Cairo, Egypt e-mail: eng_basemabdelhamed@yahoo.com Abstract—In this paper the grid disturbances effects on a grid connected PV array were studied while considering different maximum power point tracking algorithms. The maximum power point tracking techniques included in this study are; the perturb & observe technique (P&O), incremental conductance technique (ICT), fuzzy logic based technique. The grid disturbances involved in this paper are the different types of faults, voltage sag, and voltage swell. A comparative study of the grid disturbances effect on the three maximum power point tracking algorithms is obtained. A 100 kW photovoltaic array connected to the grid via a voltage source inverter through a boost converter is modeled and simulated under the MATLAB/SIMULINK in order to accomplish this study. The simulation results show that the fuzzy logic based technique gives the best response under steady state and transient conditions. The output power of PV arrays is mainly influenced by the irradiance (amount of solar radiation) and temperature. Moreover for a certain irradiance and temperature, the output power of the PV array is function of its terminal voltage and there is only one value for the PV's terminal voltage at which the PV panel is utilized efficiently. The procedure of searching for this voltage called maximum power point tracking MPPT. Recently, several algorithms have been developed to achieve MPPT technique, such as; Perturb and Observe (P&O) [1], incremental conductance [2], open circuit voltage, short circuit current, fuzzy or neural network based algorithms, etc [3, 4]. The PV array generates DC power so power electronics converters are essential. Actually power electronics converters are required to converter the generated DC power to AC and to achieve the MPPT. The MPPT can be accomplished either in single stage or in a double stage. In single stage, the PV array is connected to the grid through an AC/DC converter and the converter is utilized to obtain both the MPPT and the conversion of the generated voltage to DC. In the double stage, the PV array is connected to the grid though an AC/DC converter via a DC/DC converter. In this case, the MPPT is obtained via the DC/DC converter by controlling its input voltage. The function of the inverter is to convert the output DC voltage of the PV into AC and to maintain the output voltage of the DC/DC converter constant. Index Terms — Maximum Power Point Tracking; Fuzzy Logic controller; Photovoltaic System; Transient Analysis O I. INTRODUCTION ver the past few years, the demand for renewable energy resources has increased significantly due to the fact that the fossil fuels will run out in the near future and the harmful environmental effects of the fossil fuels. Among various types of renewable energy resources, solar energy has become one of the most promising and attractive resource. Nodaway solar energy based photovoltaic is widely used in many applications as it owns the advantage of being maintenance and pollution free. Recently, the use of photovoltaic panel has grown consistently due to the following factors; the PV efficiency is enhanced, the manufacturing technology is improved and the PV panel's cost is decreased. Recently, a large number of PV modules are connected to utility grid in many countries. In this paper, maximum power point tracking for a grid connected PV array is executed and evaluated for three different MPPT algorithms. The evaluated methods are; (i) Perturb & Observe (P&O) (ii) Incremental conductance Technique (ICT) and (iii) Fuzzy logic based (FLC) [3-5]. Furthermore the effects of different grid disturbances on the PV array and the MPPT algorithms are studied. The considered disturbances in this study are; the different type of faults, 1 International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 voltage sag, and voltage swell. In order to accomplish this study the system shown in Fig. 1 is modeled and simulated using the MATLAB/SIMULINK. Vol. 1, No. 3, December 2013 N cs kT (4) q Where Ncs is the number of cells connected in series,q is electron charge, K is Boltzmann constant and, T is the temperature of the P-N junction in Kelvin`s Vt B. Model Verification In order to accomplish this paper, the mathematical model discribed in the previous sectionis is modeled under the MATLAB/Simulink.The developed Model is vrefied utilizing the parameters of a real PV module (Kyocera- KD 200GT) manufactured by Kyocera (see table I)[7]. Fig. 3 shows the IV and P-V characteristics which developed by the MATLAB model at different irradiance and constant temperature (25oC). Fig. 4 shows I-V and P-V characteristics which developed by the MATLAB model at constant irradiance (1000 W/m2) and variable temperature. The results shown in Fig. 3 and Fig. 4 are similar to that shown in the PV module datasheet [6]. Fig.1 Block diagram of the grid connected photovoltaic system discussed in this paper. II. THE PV MODEL TABLE I PRAMETERS OF PV MODULE A. Mathematical model A PV model based current source is illustreated in this section [6]. Fig. 2 shows the PV circuit diagram. Parameter I V Value Open circuit voltage (VOC) of a PV module 32.9.0 V Short circuit current (ISC) of a PV module 8.21 A Module voltage at maximum power point (Vm) 26.3 V Module current at maximum power point (Im) 7.61 A Maximum Power (Pm) of a PV module 200 W Reference temperature 25º C Reference solar radiation (1 sun) 1000W/m2 Fig. 2 Equivalent Circuit of photovoltaic cells. Where Np is the number of parallel modules , Ns is the number of series modules, Rp is the array parallel resistance, and Rs is the array series resistance. The module current Im can be calculated from: I 1 8 current (A) N V Rs s N p I m I PV N p I o N p exp Vt aN s PV module : Kyocera KC200GT at constant temperature (25°C) (1) 1000W/m2 800W/m2 6 600W/m2 4 400W/m2 2 200W/m2 0 0 5 10 15 20 25 30 35 Voltage (V) Ipv can be expressed by: G (2) I pv I pvn K i T Gn Where Ipvn is the generated output current at 1000 W/m2 and 25oC as nominal condition, ∆T is the difference between the real and the nominal temperatures in Kelvins, Ki is the current temperature coefficient, G is the irradiance and Gn is the irradiance at nominal conditions. While Io can be given by: I scn K i T (3) Io Vocn K v T 1 exp aVt Kv is the volatge temperature coefficient, Iscn , Vocn are the short circuit current and open circuit voltage at nominal condition respectively,a is the diode ideality constant and Vt is the thermal voltage of the array and can be calculated from: 1000 W/m2 Power (W) 200 800 W/m2 150 600 W/m2 100 400 W/m2 50 0 0 200 W/m2 5 10 15 20 25 30 Voltage (V) Fig. 3 I-V and P-V characteristics of the PV module at constant temperature 25°C and various irradiances 2 35 International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 the MPP and negative on the right.In this algorithm the duty cycle of the power electronics converter is changed and then the derivative of the array output power (slope) is caculated. Accordinding to the slop of the power curve the duty cycle of the converter is adjusted [2]. The MATLAB / SIMULINK model of the Incremental Conductance Algorithm (ICT) is shown in Fig. 6. PV module : Kyocera KC200GT at 1 kW/m2 current (A) 10 5 50°C 25°C 75°C 0 0 5 10 15 20 25 30 Vol. 1, No. 3, December 2013 35 Voltage (V) Power (W) 200 25°C 100 50°C 75°C 0 0 5 10 15 20 25 30 35 Voltage (V) Fig. 4 I-V and P-V characteristics of the PV module under constant irradiance and different temperature. III. MPPT ALGORITHMS As mentioned above the MPPT can be achieved eithier in single stage or in double stages. In this paper the double stages scheme is utilized. With the help of the DC/DC converter the maximum power can be excuted by controlling the duty cycle of the DC/DC converter in order to control the PV array terminal voltage. The tracking of the optimam termanl voltage can be performed by various algorithms.In this section the MPPTalgorithms[1-5], which are used for the comparative study in this paper, will be illustrated. Fig.6 MATLAB / SIMULINK model of ICT algorithm It is very rare for the ICT to reach exactly to the maximum power point MPP. Therefore, in this technique the MPP is considered reached when the operating voltage is within a certain error limit [2]. C. Proposed Fuzzy Logic Control (FLC) Algorithm A. Perturb and Observe (P&O)Algorithm. Maximum power point tracking based Fuzzy logic has the advantage of being robust and fast in response. In this paper, the input variables of the proposed fuzzy controller are ΔP(k) and ΔV(k), where P(k) is PV array output power and V(k) is PV array output voltage [5]. These variables are expressed in terms of seven linguistic fuzzy sets; Negative Big (NB), Negative Medium (NM), Negative Small (NS), Zero (ZO), Positive Big (PB), Positive Medium (PM) and Positive Small (PS) using basic fuzzy subset. The MATLAB / SIMULINK model of the proposed fuzzy logic controller (FLC) is shown in Fig.7. In this algorithm, a small perturbation is introduced in the duty cycle of the power electronics converter and then the output power of the PV module is observed. If the power increases due to this perturbation, then the perturbation is carried on in the same direction. On the other hand, if the PV output power is decreased then the direction of the perturbation has to be reversed. The P&O technique is the simplest MPPT algorithm; however it owns the disadvantage of oscillation around the final maximum power point (MPP) [8]. The MATLAB/SIMULINK model of Perturb and Observe (P&O) Algorithm is shown in Fig. 5. Fig. 5 MATLAB / SIMULINK model of P&O algorithm Fig. 7 MATLAB / SIMULINK model of the proposed fuzzy based algorithm B. Incremental Conductance (ICT) Algorithm The proposed fuzzy logic controller comprises three function blocks; fuzzification, Fuzzy rule base, and defuzzification. An error function (E) and a change of error (CH_E) are created during fuzzification. In the fuzzy rule base stage, these variables are then compared to a set of pre- The ICTalgotithm is built on the principle that the derivative of the PV array power curve is zero at the maximum power point(i.e. the slope of the power curve is zero)[4] . The slope of the power curve is positive on the left of 3 International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 designed value in order to determine the appropriate response. In the defuzzification block, the aggregated fuzzy set is employed to create the simple crisp value of output duty cycle D. The seven rules which used for tracking the MPP in the proposed technique are shown in Table II. via a voltage source inverter through a boost converter. The function of the boost converter is to control the terminal voltage of the PV array in order to accomplish the MPPT. ADC link capacitor is placed after the boost converter and acts as a temporary power storage device to provide the voltage source inverter with a steady flow of power. The capacitor's voltage is regulated using a DC link controller that balances input and output powers of the capacitor. The parameter of the system under study is shown in Table III. TABLE II. FUZZY RULES E↓ /CE→ NB NM NS ZE PS PM PB NB NM NS ZE PS PM PB ZE ZE NS NM PM PM PB ZE ZE ZE NS PS PM PB ZE ZE ZE ZE PS PM PB NB NM NS ZE PS PM PB NB NM NS ZE ZE ZE ZE NB NM NS PS ZE ZE ZE NB NM NS PM PS ZE ZE Vol. 1, No. 3, December 2013 TABLE III PRAMETERS OF THE SYSTEM UNDER STUDY Quantity Value 260V Gridvoltage The output of FLC is utilized to control the DC-to-DC converter. The membership functions of the input and output variables are shown in Fig. 8, Fig. 9 and Fig. 10 respectively. Fig. 8 Membership function for input variable (E) Frequency 60 Hz Switching frequency 5kHz DC link capacitor C 100µF DC link voltage 500V Boost converter inductance 5mH Boost converter capacitor 1.2mF Grid voltage 20kV Inverter voltage 260V Transformer 260V /20kV load at bus1 300 + j200 k VA load at bus 2 200kW The voltage source converter (VSC) is controlled utilizing vector control in order to provide a controllable three phase AC current to the grid. To attain unity power factor operation, current is injected in phase with the grid voltage. A phase locked loop (PLL) is utilized in order t o lock on the grid frequency and provide a reference synchronization signal for the inverter control system [10]. The MATLAB/SIMULINK model of the VSC is shown in Fig. 11. Fig. 9 Membership function for input variable (CH_E) Fig. 10 Membership function for output variable (D) The proposed fuzzy logic controller utilizes duty cycle with variable steps for controlling the boost converter and therefore provides quicker convergence to the maximum power point [9]. Fig. 11 MATLAB/SIMULINK model of the VSC V. SIMULATION RESULTS AND DISCUSSION In this section, the system shown in Fig. 1 is simulated using the MATLAB/SIMULINK under condition of applying three maximum power point tracking techniques. The simulation is run several times in order to study the effect of different disturbances on the three MPPT algorithms mentioned above. The MATLAB/SIMULINK model is shown in Fig. 12. IV. SYSTEM DISCRIBTION In this paper a 100 kW PV array is utilized. The 100 kW PV array is modeled using 63 parallel connected strings with each string having 8 series connected PV modules (KyoceraKD 200GT). The output of the array is connected to the grid 4 Vol. 1, No. 3, December 2013 Voltage (V) International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 PV Array : Kyocera KC200GT of 8 series modules; 63 parallel strings 400 200 0 0 0.5 1 1.5 2 1.5 2 1.5 2 Current (A) Time (S) 1000 500 0 0 0.5 1 Power (kW) Time (S) Fig. 12 The MATLAB/SIMULINK model of the grid connected PV system 200 100 0 A. STEADY STATE ANALYSIS 0.5 Voltage (V) B. FAULT ANALYSIS In this section the MATLAB/SIMULINK model shown in Fig.12 is simulated under different fault conditions. The simulation is accomplished under nominal condition (G = 2 1000 W/m and T=250 C). As shown in Fig. 12 the fault is applied on the grid side. The fault duration is 0.1s from 0.2 to 0.3 s. In this section all types of faults will be discussed under the same condition. Line-to-ground fault The model shown in Fig. 12 is simulated while applying single line to ground fault (1L-G) on phase A. the fault location is illustrated in Fig. 12 and the fault duration is 100 ms. The output voltage and current at the point of common coupling PCC are shown in Fig. 16. 1 1.5 2 1.5 2 1.5 2 Time (S) 1000 500 0 0.5 1 5 Time (S) 200 Voltage (V) 0 Power (kW) Current (A) MPPT Based FLC. PV Array : Kyocera KC200GT of 8 series modules; 63 parallel strings 0 100 0 0 0.5 1 Time (S) Fig.13 The output power, voltage and current of the PV array Grid Voltage at PCC With 1L-G 0 0.1 PV Array : Kyocera KC200GT of 8 series modules; 63 parallel strings 0.2 0.3 0.4 0.5 0.4 0.5 Grid Current at PCC With 1L-G 40 200 0 0 0.5 1 1.5 Current (A) Voltage (V) 4 Time (S) 2 Time (S) Current (A) x 10 -5 0 With MPPT Based P&O 400 1 Fig. 15 The output power, voltage and current of the PV array with 200 0 0.5 Time (S) First the simulation is accomplished while the PV array operates at nominal conditions (1000W/m2&25°C). The simulation is run three times; first, the Perturb and Observe algorithm is applied, second the incremental conductance algorithm is applied, and finally the fuzzy logic algorithm is applied. The output voltage, current, and power of the PV array while applying the three different algorithms P&O, ICT and FLC are shown in Fig.13, Fig. 14 and Fig. 15 respectively. It can be observed that PV array feeds100kW to the grid while utilizing the three algorithms but the proposed FLC gives a faster response when compared with the others. 400 0 1000 0 -20 500 0 20 0 0.5 1 1.5 0 0.1 0.2 0.3 Time (S) 2 Power (kW) Time (S) 200 Fig. 16 The output voltage and current at the PCC with single line to ground fault 100 0 0 0.5 1 1.5 2 The simulation was run three times under condition of applying a single line to ground fault while utilizing the three MPPT algorithms discussed in section III. Fig. 17 shows the output power at the array terminal for the three cases. Time (S) Fig. 14 The output power, voltage and current of the PV array With MPPT Based ICT 5 International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 different maximum power point tracking algorithms while applying this type of fault is shown in Fig. 21. P&O ICT FLC 50 0 0 0.5 1 1.5 x 10 5 Voltage (V) Power (kW) Output Power of the PV array using the three different algorithms at 1L-G 100 Vol. 1, No. 3, December 2013 2 Time (S) 4 Grid Voltage at PCC With L-L-G 0 -5 0 0.1 0.2 Fig.17 The output power of the PV array using the three different 40 Current (A) algorithms with 1LG fault Line-to-line Fault In this case the model shown in Fig. 12 is simulated with applying a line to line fault (L-LF) between phases A and B. The voltage and the current at the PCC are shown in Fig.18. x 10 4 0.4 0.5 0 0 0.1 0.2 0.3 Time (S) Grid Voltage at PCC With L-L Fig. 20 The output voltage and current at the PCC with L-L-G fault Output Power of the PV array using the three different algorithms at L-L-G 100 0 0.1 0.2 0.3 0.4 0.5 Time (S) Grid Current at PCC With L-L 20 Current (A) 0.5 -20 -40 0 -5 P&O ICT FLC 50 0 0 0.5 1 1.5 2 Time (S) Fig. 21 Output power of The PV array using the three different algorithms with L-L-G fault 0 -20 0 0.1 0.2 0.3 0.4 Three line to ground fault A three line to ground (L-L-L-G) fault is applied to the model shown in Figure 12. Fig. 22 shows the output voltage and current at the PCC. Fig. 23 shows the output power of the PV array while applying the three MPPT techniques for this type of fault.It noteworthy that the proposed FLC succeeds to sustain the stability of the MPPT during the fault while the other two conventional techniques fail. 0.5 Time (S) Fig.18 The output voltage and current at the PCC with L-L fault In order to compare the performance of the three MPPT algorithms mention above to this type of fault, the model is run three times; each time one algorithm is implemented. The output power of the PV array under the three cases is shown in Fig. 19. 5 P&O ICT FLC 50 0 Voltage (V) Output Power of the PV array using the three different algorithms at L-L 100 Power (kW) 0.4 20 Power (kW) Voltage (V) 5 0.3 Time (S) Grid Current at PCC With L-L-G 0 0.5 1 1.5 0 0.1 Current (A) 0.2 0.3 0.4 0.5 0.4 0.5 Time (S) Grid Current at PCC With L-L-L-G 0 -20 -40 Line-to-line-to-ground fault A line-to-line-to ground (L-L-G) fault is applied to the model shown in Fig. 12. The voltage and current waveforms for this case at the point of common coupling are shown in Fig. 20. Grid Voltage at PCC With L-L-L-G 20 2 Fig.19 The output power of the PV array using the three different algorithms with L-L fault 4 0 -5 Time (S) x 10 0 0.1 0.2 0.3 Time (S) Fig.22 The output voltage and current at point of common coupling (PCC) with L-L-L-G fault. The output power at the array terminal of the three 6 International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 Vol. 1, No. 3, December 2013 Output Power of the PV array using the three different algorithms at L-L-L-G 50 0 Output Power of the PV array using the three different algorithms at Voltage Dips P&O ICT FLC 0 0.2 0.4 0.6 0.8 P&O ICT FLC 100 Power (kW) Power (kW) 100 50 1 0 0 Time (S) 0.5 1 1.5 2 Time (S) Fig.23 The output power of the PV array using the three different algorithms with L-L-L-G fault Fig.26 Output power of The PV array using the three different algorithms under voltage sag C. VOLTAGE SAGS ANALYSIS D. VOLTAGE SWELLS The decrease in the RMS value of the voltage or current between 0.9 to 0.1 p.u. for duration of 0.5 cycle to 1 minute is defined as voltage sag. Voltage sags are generally caused by over loading or grid faults. The MATLAB/SIMULINK model shown in Fig. 24 is utilized to conduct the analysis in this section. The model shown in Fig. 24 is simulated under condition of voltage sag at the point of common coupling for a duration of 0.15 s. The increase in the RMS voltage or current between 1.1 to 1.8 p.u. for a duration of 0.5 cycle to 1 minute is defined as voltage swell. Voltage swells are normally initiated by the disconnection of a very large load, the energization of a large capacitor bank and voltage swells are usually associated with the system fault conditions. Fig. 24 shows the grid connected PV array MATLAB/SIMULINK model which utilizes in this section. The system is studied under voltage swells of 0.15 s duration. In order to studying the effect of voltage swells, the voltage at the PCC is increased from 20 kV to 26 kV as shown in Fig. 27. x 10 Voltage (V) 5 Fig. 24 Grid Connected PV system under sag Analysis Voltage (V) 5 x 10 4 Current (A) 0.2 0.3 0.4 0.5 0.4 0.5 Time (S) Grid Current at PCC With Voltage Swell 0 0.1 0.2 0.3 Time (S) Fig.27 The output voltage at the PCC in case of voltage increase by 30% 0 0.1 0.2 0.3 0.4 The PV array output power of the three MPPT algorithms in case of voltage swell is shown in Fig. 28. It can be observed that, the FLC has a good response and is not affected with the disturbances occurred on the grid side. 0.5 Time (S) Grid Current at PCC With Voltage Dips 0 Output Power of the PV array using the three different algorithms at Voltage Swell 0 0.1 0.2 0.3 0.4 100 0.5 Power (kW) -20 0.1 0 -20 Grid Voltage at PCC With Voltage Dips 20 Current (A) 0 20 0 -5 Grid Voltage at PCC With Voltage Swell 0 -5 In order to study the effect of voltage sag on the performance of the three MPPT algorithms under study in this paper, the voltage at the PCC is reduced from 20kV to 10kV. The output voltage and current at the PCC is shown in Fig. 25. 4 Time (S) Fig.25 The output voltage and current at PCC in case of voltage decrease by 50% 50 0 The simulation is run three times and each time one of the MPPT algorithms is employed while operating the PV array at nominal condition. Fig. 26 shows the output power of the PV array in the three cases. It can be observed that FLC has a faster response and is not affected with the disturbances occurred on the grid side. P&O ICT FLC 0 0.5 1 Time (S) 1.5 Fig.28 Output power of the PV Array using the three different algorithms under voltage swell condition 7 2 International Journal of Advances in Power Systems (IJAPS) ISSN: 2335-1772 VI. CONCLUSION In this paper, a 100 kW grid connected photovoltaic array is studied under steady state and transient conditions while utilizing three different maximum power point tracking algorithms. The three algorithms employed in this paper are: the perturb and observe (P&O) algorithm; the incremental conductance (ICT) algorithm and the fuzzy logic control (FLC) algorithm. The simulated results under steady state condition show the effectiveness of the MPPT on increasing the output power of the PV array for the three techniques. However the FLC algorithm offers accurate and faster compared to the conventional techniques. The simulation results under transient conditions show that the output power injected to grid from PV array is approximately constant while utilizing the proposed FLC and the PV system can still connect to grid and deliver power to grid without any damage to the inverter switches. REFERENCES [1] A.Durgadevi, S. Arulselvi, S. P. Natarajan, “Study and implementation of Maximum Power Point Tracking (MPPT) algorithm for Photovoltaic systems,” 1st International Conference on Electrical Energy Systems (ICEES), 2011, pp. 240 – 245. [2] A. Safri and S. Mekhilef, “Incremental conductance MPPT method for PV systems”, Canadian Conference on Electrical and Computer Engineering (CCECE), 2011, pp. 345 -347. [3] Sung-Jun Kang, Jae-Sub Ko, Jung-Sik Choi, Mi-Geum Jang, Ju-HuiMun ,Jin-Gook Lee, Dong-Hwa Chung, “A Novel MPPT Control of photovoltaic system using FLC algorithm,” 11th International Conference on Control, Automation and Systems (ICCAS), 2011, pp. 434 - 439. 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