COMPARATIVE ANALYSIS OF THE PERTURB-AND-OBSERVE AND INCREMENTAL CONDUCTANCE MPPT METHODS

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1 COMPARATIVE ANALYSIS OF THE PERTURB-AND-OBSERVE AND INCREMENTAL CONDUCTANCE MPPT METHODS Pratik U. Mankar 1 and 2 R.M. Moharil 1 PG student, Department of Electrical Engineering, Y.C.C.E., Nagpur 2 Professor, Department of Electrical Engineering, Y.C.C.E., Nagpur { 1 pratikmankar90@gmail.com} Abstract This paper presents a comparative study between two maximum power point tracking (MPPT) methods in Matlab/Simulink program that are perturb-and-observe method and incremental conductance method. Starting from the implemented model of the photovoltaic (PV) array together with MPPT controls have been simulated the PV systems with both MPPT algorithms at different solar radiation and finally, presents the simulations result depending on irradiance. Keyword - Photovoltaic systems, maximum power point tracking MPPT), Perturb-and-Observe (P&O), Incremental Conductance (INC). 1. Introduction The ever-increasing demand for low-cost energy and growing concern about environmental issues has generated enormous interest in the utilization of nonconventional energy sources such as the solar energy. The freely and abundantly available solar energy can be easily converted into electrical energy using photovoltaic (PV) cells. A PV source has the advantage of low maintenance cost, absence of moving/rotating parts, and pollution-free energy conversion process. However, a major drawback of the PV source is its ineffectiveness during the nights or low isolation periods or during partially shaded conditions. High initial capital cost has been another hurdle in the popularity of PV systems. These drawbacks notwithstanding, the PV systems have emerged as one of the most popular alternatives to conventional energy, thanks to the advancement in technology and favourable government policies in several countries. A major challenge in the use of PV is posed by its nonlinear Current voltage (I V) characteristics, which result in a unique maximum power point (MPP) on its power voltage (P V) curve. The matter is further complicated due to the dependence of these characteristics on solar irradiation and temperature. As these parameters vary continuously, MPP also varies. Considering the high initial capital cost of a PV source and its low energy conversion efficiency, it is imperative to operate the PV source at MPP so that maximum power can be extracted. Tracking the maximum power point (MPP) of a photovoltaic (PV) array is usually an essential part of a PV system. As such, there are at least 19 different algorithms of MPPT control with different ways on implementation and performance [1]. The best known MPPT classic algorithms are Perturb-and-observe (P&O) and incremental conductance (Inc. Cond.). These algorithms are based on the same technology, regulating PV array voltage by adjusting the optimal set point that represents the voltage at maxim power point (MPP) many MPP tracking (MPPT) methods have been developed and implemented. The methods vary in complexity, sensors required, convergence speed, cost, range of effectiveness, implementation hardware, popularity, and in other respects. Several tracking schemes have been proposed Among the popular tracking schemes are the perturb and observe (P&O) or hill climbing, incremental conductance, short circuit current, open-circuit voltage, and ripple correlation approaches etc. [2] This paper aims to implement in Matlab/Simulink the perturb-and-observe (P&O) and incremental conductance Algorithms that are published in the literature. This comparative analysis is designed to determine which of these two methods are the most suitable for MPPT in order to establish an optimal algorithm. 2. MPPT Implementation The objective of MPPT algorithm is to adjust the current (I mpp ) and voltage (V mpp ) of the PV array at which maximum output power (P mpp ) is obtained under a specific irradiation and temperature. As mentioned earlier the P&O and INC are the most popular techniques for tracking of maximum power. In the next sections these two methods are explained briefly. ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14

2 2.1 Perturb and Observe MPPT Method The P&O algorithms are widely used in control of MPPT thanks to their simple structure and reduced number of necessary measured parameters [6]. The flowchart implementation of P&O method is given in figure 1. As the name implies, the concept behind of this method is based on observation of PV array output power and its perturbation by changing the current or the voltage of PV array operation. The algorithm increments or decrements continuously the reference voltage or current based on the previous value of power until reaches the MPP. When dp/dv>0 and the operating voltage of PV array is perturbed in a specific direction, it known that perturbation moves the operating point of PV array to the MPP. P&O method will then continue to perturb the PV voltage in the same direction. When dp/dv<0, the perturbation moves the operating point of PV array away from the MPP and the P&O method reverses the direction of the perturbation. Although this method can result in oscillation of power output. It is referred to as a hill climbing method. Because, it depends on the rise of power against voltage below MPP and above MPP. [3] Fig.2 Shows typical P-V curve at 800 W/m 2. On the LHS of the MPP there is linear increase in power with respect to voltage (dp/dv>0). But on RHS of the MPP there is increase voltage with decrease in power (dp/dv<0). So, this P&O algorithm will try to maintain the maximum power point by perturbing voltage with respective power. At MPP change in power with respect to change in voltage is zero (dp/dv=0). Table I shows the direction of perturbation of voltage with respect to power. Table I shows that with same sign of ΔV and ΔP results in direction of step size is positive i.e. + C. and if either sign of ΔV and ΔP results in direction of step size is negative i.e. C. [4], [5]. From flowchart shown in Fig.1, dv and dp is calculated based on the difference between present values of voltage, power and previous samples of voltage and power. Based on the sign of dv,dp the perturbation is approached.table I xxxshows the sign of perturbation size (C) changing with respect to the sign of dv and dp. For the same sign of dv and dp the perturbation of step size is positive, alternate sign results in negative step size perturbation. This algorithm has the drawback that after reaching the maximum power point its starts deviating on the maximum power point continuously all the time results in the substantial amount of power loss at maximum power point. Although this algorithm is quite simple to implement and it requires only one voltage sensor so, the cost of implementation of this algorithm is low [6]. Fig.1 Flowchart of P&O MPPT Algorithm Fig.2 Typical P-V curve at 800 W/m Approaches for implementation of Perturb and Observe algorithm- There are two common approaches for implementing the P&O algorithm. Reference voltage perturbation Direct duty ratio perturbation ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14

3 Table 1 Truth Table of the Conventional P&O MPPT Method Sign of ΔV Sign of ΔP Direction of next step C C C C In Reference voltage perturbation, the PV array output voltage reference is used as the control parameter in conjunction with a controller (usually a PI controller) to adjust the duty ratio of the MPPT power converter. The PI controller gains are tuned while operating the system at a constant voltage equal to the standard test condition (STC) value of the MPP voltage. These gains are kept constant while the reference voltage is controlled by the MPPT algorithm. In direct duty ratio perturbation, the duty ratio of the MPPT converter is used directly as the control parameter. the duty ratio is perturbed directly eliminating the need for a PI controller which in turn reduces the complexity and enhances the stability of the system. This method also offers better energy utilization and better stability characteristics at a slower transient response and worse performance at rapidly changing irradiance [7]. In this paper P&O algorithm is implemented with Reference voltage perturbation. The algorithm is programmed in Matlab/Simulink. 2.2 Incremental Conductance MPPT Algorithm The incremental conductance method is based on the fact that the slope of the PV array power curve is zero at the MPP, positive on the left of the MPP, and negative on the right as given by, dp 0 left of MPP dv dp 0 right of MPP (1) dv dp 0 At MPP dv dp d IV di di Since, =I+V I+V (2) dv dv dv dv Equation (1) can be rewritten as, di -I right of MPP dv V di -I left of MPP dv V di -I = At MPP (3) dv V This method based on the whether the array voltage is greater than or less than peak power point voltage. Equation (10) shows that maximum power point can be tracked by comparing the instantaneous conductance to the incremental conductance. Incremental conductance method overcomes the drawback of Perturb and Observe method by using PV arrays incremental conductance to compute the sign of dp dv without perturbation. This helps to determine the Maximum power point technique has reached the maximum power point and stop perturbing the operating point. Although this method has the drawback is that it increases the complexity compared to Perturb & Observe method, requires more time for computation [8]. Fig.3 showing a typical PV curves with relationship between instantaneous conductance and incremental conductance. Equation (10) showing that at maximum power point both instantaneous and incremental conductance has same value. Thus maximum power point can be tracked. This method has the advantage over the Perturb & Observe of not oscillating around the maximum power point under rapidly varying environmental conditions [9]. P`ower in w att Voltage in volt Fig.3 Conductance showing Typical P-V curve The disadvantage of the Perturb and observe method to track the peak power under fast varying atmospheric condition is overcome by INC method. The INC can determine that the MPPT has reached the MPP and stop perturbing the operating point. If this condition is not met, the direction in which the MPPT operating point must be perturbed can be calculated using the relationship between dl/dv and I/V.This relationship is derived from the fact that dp/dv is negative when the MPPT is to the right of the MPP and positive when it is to the left of the MPP. This algorithm has advantages over P&O in that it can determine when the ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14

4 MPPT has reached the MPP, where P&O oscillates around the MPP. Also, incremental conductance can track rapidly increasing and decreasing irradiance conditions with higher accuracy than perturb and observe. One disadvantage of this algorithm is the increased complexity when compared to P&O [10]. Fig.5 Basic circuit diagram of MPPT method MPPT algorithm would be Perturb & Observe and Incremental conductance. Thus, by executing the Perturb & Observe and Incremental Conductance in Matlab/M-files under the different irradiation and keeping the PV panel temperature constant and obtained the output power. 4. Results and Discussions Fig.4 Flowchart of Incremental Conductance algorithm Fig.4 showing the flowchart of Incremental conductance, in which incremental conductance is compared with instantaneous conductance and thus maximum power point is tracked. Increment size determines the how fast maximum power point is tracked. Fast tracking can be achieved with bigger increments but the system might not operate exactly at the maximum power point and oscillate about instead. This method has complex circuitry; accuracy of the method depends on the iteration size, which is usually fixed for the conventional incremental conductance method [11]. 3. System under Study The basic circuit diagram shown in Fig. 5 represents the overall working of the MPPT method. It contains the current source which represents the photocurrent (I p ) i.e. the value of current when solar radiation falls on the panel and current flowing through the diode i.e. (I d ) which represents the diode saturation current. It is the value of the current when solar radiation absent on the solar panel. Fig. also shows the load current value is kept at zero value. the output current flowing through the panel (I) and output voltage across the panel (V oc ) is given as feedback signal to the MPPT algorithm. For the comparative analysis datasheet of KC GHT Solar Panel is considered shown in Table 2. In which 54 cells are connected in series. This section represents variation in irradiation will be done for particular range of iteration and thus tracking of power will be achieved. For the comparative analysis, three tests will be carried out. The first three test will be carried out for step size of (C=0.3 V) and later on, another three test will be carried out for step size of (C=0.5 V)). It is assumed that the cell temperature remains constant at T= K throughout the program. In the first test, The solar irradiation will start at a relative low level at W/m 2 for first and then rises to W/m 2 over 51 to 100. In the second test, The solar irradiation will start at a Relative low level at W/m 2 for first and then rises to W/m 2 over 51 to 100. In the third test, a fluctuated solar irradiation is applied in which each radiation level will last for less iteration over a complete 100. Fig.6 shows the result of test 1, in which P&O and INC output power plotted w.r.t. It is clear that, the output power is almost same around 70 of both the method. But later on there is slight increase in power output of P&O compared to INC for step size, C=0.3 V shown in table 3. Fig.6 also shows that after 70 th the power output of INC slightly gets fluctuated and gives less output at the end of 100. In the second test irradiation varies to W/m 2. From table 4, it clearly shows that for first power tracked by both the method is same i.e. for W/m 2. On 51 th irradiation changes to W/m 2, during this irradiation level same power tracked by INC method is quicker slightly as compared with P&O method. ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14

5 Table 2 Parameters of KCGHT-2 Solar Panel Constraints Icon Structured Values Maximum Power P [W] Tolerance % +10/-5 Maximum Power Voltage[V] V 26.3 Maximum Power Current[A] I 7.61 Open Circuit Voltage[V] V 32.9 Short Circuit Current [A] I 8.21 Temperature coefficient of V K [V/ C] Temperature coefficient of I [A/ C] K NOCT [ C] - 47 Test 1- Varying irradiation from to W/m 2 over 100 Table Table 4 Fig. 7 Power Vs Test 3- Varying different irradiation over 100 Table s Irradiat ion Fig. 6 Power Vs Test 2- Varying irradiation from to W/m 2 over 100 Fig.7 shows the graph between powers tracked by the both MPPT method and. From graph, it clearly indicates that power tracked by P&O and INC is same. Due to which curve of both the methods overlapped with each other. But required is slightly lesser by the INC method compared to P&O method. Table 5 shows the power tracked by the both methods for various irradiations level, from table it clearly shows that at various irradiation level power tracked by both methods is same but required at each level of irradiation is less of INC method as compared to P&O method. Fig.8 shows the graph Power Vs, it clearly indicates that power tracked of both the method is same, curves of the both methods is exactly same. Hence curves overlapped each other. ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14

6 Table 7 Fig. 8 Power Vs From test 2 and 3,it is clear that the power output performance of P&O and INC MPPT method is exactly the same for step size, C=0.3 V. Now, again three test will be carried out for Step size, C=0.5 V. In the first test, The solar irradiation will start at a relative low level at W/m 2 for first and then rises to W/m 2 over 51 to 100. In the second test, The solar irradiation will start at a Relative low level at W/m 2 for first and then rises to W/m 2 over 51 to 100. In the third test, a fluctuated solar irradiation is applied in which each radiation level will last for less iteration over a complete 100. From test 1 and 2 it can be seen that output power performance of both the MPPT method is same. But, there is slight curve after irradiation changes on th onwards of P&O method shown in Fig.9. Test 1- Varying irradiation from to W/m 2 over 100 Table 6 Fig. 9 Power Vs Fig. 10 Power Vs From table 6 and 7 clearly shows the for tracking the same power INC method requires less comparatively to the P&O method. Fig.10 shows output power comparison for irradiation changes from to W/m 2 for step size of C=0.5 V. it can be seen that, irradiation changes after th, that results in continuous deviation in output power. Because, the incremental (di/dv) and instantaneous conductance (-I/V) continuously more or less to each other. i.e. (di/dv) > (-I/V) or (di/dv) < (-I/V). That results in continuous variation in output power. Fig.10 also shows the output power of P&O method. This method also results in power variation after th iteration due to the continuous change in voltage (dv) and change in power (dp) less than or greater than zero. Test 3- Varying different irradiation over 100 Test 3 shows the rapid variation in irradiation, results dynamic variation of output power w.r.t. From Fig.11 it is clear that, due to less iteration for each irradiation results in initial power w.r.t irradiation doesn t reach to maximum power. i.e. not reaches to saturated value. In between the radiation level is 0 W/m 2. During this period of, the output power of incremental conductance is continuously fluctuated due to the, the incremental (di/dv) and instantaneous conductance (-I/V) continuously more or less to each other. i.e. (di/dv) > (-I/V) or (di/dv) < (-I/V).That results in continuous variation in output power. 76 th onwards there is stable power output of both the method w.r.t. irradiation. Test 2- Varying irradiation from to W/m 2 over 100 ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14

7 Table Fig. 11 Power Vs 5. Conclusion (Watt) In this study, the comparative analysis is made between P&O and INC. In the initial three tests step size (C=0.3 V) is kept at small value. That results in the less power during initial w.r.t. irradiation. But later on gives the rated maximum power. But due to small step size the power tracked by the both the method in initial is small compared to step size (C=0.5 V). From table 3 to 8 it is clear that the required by the INC MPPT method is less compared to P&O method. From that we can concluded that, the tracking speed of INC method is faster compared to P&O method. From different tests, it is clear that for different irradiations, the output power level of the both method i.e. P&O and INC w.r.t. irradiation is almost same. Although the both the method have different way to track the maximum power. References [1] D. Hohm and M. Ropp, Comparative study of MPPT algorithms using an experimental, programmable, MPPT test bed, in Proc. 28 th IEEE conf. Rec. Photovolt. Spec. Conf., 0, pp [2] T. Esram and P. Chapman, Comparison of photovoltaic array maximum power point tracking techniques, IEEE Trans. Energy Convers., vol. 22, no. 2, pp , Jun. 7. [3] Fan Zhang, Kary Thanapalan, Andrew Procter, Stephen Carr, and Jon Maddy, Adaptive Hybrid Maximum Power Point Tracking Method for a Photovoltaic System IEEE Trans. Energy Convers.(DOI, /TEC.), pp. 1-7,March 13. [4] S.K. Kollimalla and M.K. Mishra, Adaptive Perturb and Observe MPPT Algorithm for PV system, IEEE Conf. on Renewable energy, 13, pp [5] Jacob James Nedumgatt, Jayakrishnan K. B., Kothari D. P., Perturb and Observe MPPT Algorithm for Solar PV Systems-Modeling and Simulation, IEEE conf. on Power electronics application in renewable energy,13, pp. 1-6 [6] Hiren Patel and Vivek Agarwal, Maximum Power Point Tracking Scheme for PV Systems Operating Under Partially Shaded Conditions, IEEE Transactions On Industrial Electronics, Vol. 55, No. 4, pp ,April 8. [7] M. A. Elgendy, B. Zahawi and D. J. Atkinson, Evaluation Of Perturb And Observe Mppt Algorithm Implementation Techniques,IEEE conf. on Renewable energy conversion, May 9,pp [8] Ioan Viorel Banu, Răzvan Benigua, Marcel Istrate, Comparative Analysis of the Perturb-and-Observe and Incremental Conductance MPPT Methods, 8 th International Symposium On Advanced Topics In Electrical Engineering, May 23-25, 13, pp [9] Marcelo G. Villalva, Ernesto Ruppert F., Analysis and Simulation of the P&O MPPT Algorithm Using a Linearized PV Array Model, IEEE Conf. on renewable energy source, 9, pp [10] Fangrui Liu, Shanxu Duan, Fei Liu, Bangyin Liu, and Yong Kang, A Variable Step Size INC MPPT Method for PV Systems, IEEE Trans. Ind. Electronics, vol. 55, July 8, pp [11] Arjav Harjai, Abhishek Bhardwaj, Mrutyunjaya Sandhibigraha, Thesis on Study of Maximum Power Point Tracking (MPPT) Techniques in A Solar photovoltaic array, NIT Rourkela, pp. 1-42,May 11. Biographies R. M. Moharil: born on 06 th Oct at Nagpur, India. He has completed his B.E. (Electrical) engineering in 1989, M.Tech. (integrated power System) in 1998 and Ph.D. degree in 9 on the topic, Design and Reliability Analysis of Solar Photovoltaic and Wind Energy Generation Systems. He has published 9 papers in international Journals, 7 National Journals, 25 International Conferences and 15 National Conferences. Presently he is working as Professor and Head of Department of Electrical Engineering in Yeshwantrao Chavan College of Engg., Nagpur. Pratik U. Mankar: born on 29 th July 1990 at Nagpur, India. He has completed his B.E. (Electrical) engineering in 12 and currently pursuing M.Tech. (Integrated Power System) in Yeshwantrao Chavan College of Engg., Nagpur. ISSN (Print): ISSN (Online): IJREAS, Vol. 02, Issue 02, July 14