A Performance and Analysis of MPPT Controller Under Partial Shading Conditions Mr.Swapnil R. Borade M.E. (EPS), Student Electrical Engineering Dept SSGBCOET Bhusawal swapnilborade123@gmail.com Prof. Girish K. Mahajan Associate Professor Electrical Engineering Dept SSGBCOET Bhusawal girishmahajan_16@rediffmail.com Prof. N. M. Khandare Assistant Professor Electrical Engineering Dept SSGBCOET Bhusawal khandarenitin3@gmail.com ABSTRACT Maximum power point tracking (MPPT) is an integral part of a system of energy conversion using photovoltaic (PV) arrays. The issue of MPPT has been addressed in different ways in the literature but, especially for low-cost implementations, the perturb and observe (P&O) maximum power point tracking algorithm is the most commonly used method due to its ease of implementation. A drawback of P&O is that, at steady state, the operating point oscillates around the MPP giving rise to the waste of some amount of available energy. The powervoltage characteristic of PV arrays operating under partial shading conditions exhibits multiple local maximum power points (LMPPs). In this thesis, a new method has been presented to track the global maximum power point (GMPP) of PV. Compared with the past proposed global MPPT techniques, the method proposed in this project has the advantages of determining whether partial shading is present, calculating the number of peaks on P V curves, and predicting the locations of GMPP and LMPP. The new method can quickly find GMPP, and avoid much energy loss due to blind scan. The experimental results verify that the proposed method guarantees convergence to the global MPP under partial shading conditions. a MATLAB-based modeling and simulation scheme suitable for studying the I V and P V characteristics of a PV array under a non-uniform insolation due to partial shading. It can also be used for developing and evaluating new maximum power point tracking techniques, especially for partially shaded conditions. In order to limit the negative effects associated to the P&O algorithm, the P&O MPPT parameters must be customized to the dynamic behavior of the specific converter adopted. Thus, comparing with the conventional fixed step size method, the proposed approach can effectively improve the MPPT speed and accuracy simultaneously. Keywords: Maximum Power Point Tracking (MPPT), Global Maximum Power Point (GMPP), Local Maximum Power Point (LMPP), Multiple Maxima, Partial Shading, Photovoltaic (PV). 1. INTRODUCTION A Photovoltaic (PV) cell is an electrical device that converts the energy of light directly into electricity through PV effect. PV cells have a complex relationship between solar irradiation, temperature, and total resistance, and exhibit a nonlinear output efficiency characteristic known as the P V curve. Therefore, maximum power point tracking (MPPT) techniques should be developed in PV systems in order to maximize the output power of PV systems. Nowadays, there have been many MPPT methods reported in the literature, such as hill climbing, perturb and observe, incremental conductance (INC), and ripple correction. However, when there are multiple local power maxima, from partially shading or from installation on a curved surface, conventional MPPT techniques do not perform well. Multiple maxima may occur due to bypass diodes, which are used to avoid hot spots from forming when some cells in a module or some modules in a string receive less irradiance than others. Without the remediation of power electronics, the lost energy due to partial shading can be significant. Thus, it is imperative to utilize MPPT techniques that reliably track the unique global power maximum present in shaded arrays. Some researchers have proposed global maximum power point tracking (GMPPT) algorithms to address the partial shading condition. It is observed that the peaks follow a specific trend in which the power at a peak point continues to increase until it reaches the GMPP, and afterward, it continuously decreases. The proposed algorithm incorporates an online current measurement and periodic interruptions to address certain challenges associated with rapidly changing insolation and partial shading. A modified rule for updating the duty-cycle is presented, which is based on the first and second derivatives of the power as a function of the dutycycle. A smart P I curve scanning is devised to detect the presence of the global maxima. A dc dc converter controlled by a dc signal of adjustable amplitude is used to track the global MPP. Then the perturb and observe (P&O) MPPT algorithm is applied in order to continuously track the shortterm variations of the previously detected global MPP. It presents a novel model-based two-loop control scheme for a particular module-integrated PV and converter system, where bidirectional Cuk dc dc converters are used as the bypass converters and a terminal Cuk boost functioning as a whole system power conditioner. The system can increase power 237
generation compared to the conventional bypass diode structure. It uses a periodic scan sequence of the P V curve to detect local MPPs due to the long time required for the completion of this tracking process, and the PV energy production is reduced. The global MPPT methods that are presented are based on the measurements of the PV array open-circuit voltage and solar irradiation or short circuit current, respectively. within layers. Ideally, this is minimised to prevent efficiency losses due to increased charge carrier recombination. This can be achieved by ensuring good energy level alignment of the materials used in the solar cell. The shunt resistance (R sh ) accounts for the existence of alternate current pathways through a photovoltaic cell. Unlike the series resistance, this is ideally as high as possible to prevent current leakage through these alternate paths. 3. MPPT Figure 1. Structural Block Diagram of MPPT Controller 2. SOLAR CELL Figure 2. Solar cell operation for material with a low dielectric constant A solar cell is a device that converts light into electricity via the photovoltaic effect. They are also commonly called photovoltaic cells after this phenomenon, and also to differentiate them from solar thermal devices. The photovoltaic effect is a process that occurs in some semiconducting materials, such as silicon. At the most basic level, the semiconductor absorbs a photon, exciting an electron which can then be extracted into an electrical circuit by built-in and applied electric fields. Figure 3. Equivalent circuit of solar cell The series resistance (R s ) accounts for resistances that arise from energetic barriers at interfaces and bulk resistances Maximum power point tracking (MPPT or sometimes just PPT) is a technique used commonly with wind turbines and photovoltaic (PV) solar systems to maximize power extraction under all conditions. Although solar power is mainly covered, the principle applies generally to sources with variable power: for example, optical power transmission and thermo photo voltaic. Various methods of MPPT, 3.1 Perturb and observe In this method the controller adjusts the voltage by a small amount from the array and measures power; if the power increases, further adjustments in that direction are tried until power no longer increases. This is called the perturb and observe method and is most common, although this method can result in oscillations of power output. It is referred to as a hill climbing method, because it depends on the rise of the curve of power against voltage below the maximum power point, and the fall above that point. Perturb and observe is the most commonly used MPPT method due to its ease of implementation. Perturb and observe method may result in top-level efficiency, provided that a proper predictive and adaptive hill climbing strategy is adopted. 3.2 Incremental conductance In the incremental conductance method, the controller measures incremental changes in PV array current and voltage to predict the effect of a voltage change. This method requires more computation in the controller, but can track changing conditions more rapidly than the perturb and observe method (P&O). Like the P&O algorithm, it can produce oscillations in power output. This method utilizes the incremental conductance (di/dv) of the photovoltaic array to compute the sign of the change in power with respect to voltage (dp/dv). The incremental conductance method computes the maximum power point by comparison of the incremental conductance (I Δ / V Δ ) to the array conductance (I / V). When these two are the same (I / V = I Δ / V Δ ), the output voltage is the MPP voltage. The controller maintains this voltage until the irradiation changes and the process is repeated. The incremental conductance method is based on the observation that at the maximum power point dp/dv = 0, and that P = IV. 238
The current from the array can be expressed is a function of the voltage: P = I(V)V. Therefore, dp/dv = VdI/dV + I(V). Setting this equal to zero yields: di/dv = -I(V)/V. Therefore, the maximum power point is achieved when the incremental conductance is equal to the negative of the instantaneous conductance. 3.3 Current sweep The current sweep method uses a sweep waveform for the PV array current such that the I-V characteristic of the PV array is obtained and updated at fixed time intervals. The maximum power point voltage can then be computed from the characteristic curve at the same intervals. as the P&O tracking process can work well. Therefore, until the partial shading occurs, it maintains the operation at the GMPP by continuously implementing the P&O method (blocks 2 and 3). When an MPP is found, it will store the point information, i.e., array power and the voltage (block 4). A timer interrupt program is used to ensure regular checking of the shading condition (block 5). When a PV module voltage is greater than another one (V i >V j ) in the same point, it means that partial shading has occurred. 3.4 Constant voltage The term "constant voltage" in MPP tracking is used to describe different techniques by different authors, one in which the output voltage is regulated to a constant value under all conditions and one in which the output voltage is regulated based on a constant ratio to the measured open circuit voltage (V OC ). The latter technique is referred to in contrast as the "open voltage" method by some authors. If the output voltage is held constant, there is no attempt to track the maximum power point, so it is not a maximum power point tracking technique in a strict sense, though it does have some advantages in cases when the MPP tracking tends to fail, and thus it is sometimes used to supplement an MPPT method in those cases. In the "constant voltage" MPPT method (also known as the "open voltage method"), the power delivered to the load is momentarily interrupted and the open-circuit voltage with zero current is measured. The controller then resumes operation with the voltage controlled at a fixed ratio, such as 0.76, of the open-circuit voltage V OC. This is usually a value which has been determined to be the maximum power point, either empirically or based on modelling, for expected operating conditions. The operating point of the PV array is thus kept near the MPP by regulating the array voltage and matching it to the fixed reference voltage V ref =kv OC. The value of V ref may be also chosen to give optimal performance relative to other factors as well as the MPP, but the central idea in this technique is that V ref is determined as a ratio to V OC. 4. SYSTEM MODEL Fig. 5, 6, 7 shows the flowchart of the improved MPPT algorithm to track the GMPP under partial shading conditions. The execution of the algorithm always starts with a reference voltage (V ref ) value set to 85% of (block 1) as shown in the Main Program. In the meantime, it calculates the number of PV modules. When under uniform insolation, there is only one peak in the P V curve. Traditional MPPT methods such Figure 4. Matlab Model for case study When the absolute difference between V i and V j is greater than a predetermined constant (used to eliminate sample disturbance and minor differences due to slight changes in insolation), the Main Program calls the GMPP track subroutine (block 7). The GMPP track subroutine finds the truegmpp and then passes the control onto the Main Program, which maintains the operation at this new GMPP. The GMPP track subroutine determines the location of the lastmppon the P V curve (block 8).When the voltage of any PV module is less than zero (V i <0), it means that the last MPP is the left peak on the P V curve, and reference voltage of the right peak is set to about 80% of V OC (block 9). Then, a conventional MPP technique (such as P&O) is applied to track this peak (blocks 10 and 11). When any PV module voltage is not less than zero, the last MPP is the right peak on the P V curve. It divides the voltage of all PV modules into two groups according to their value and calculates the number of modules in the group with the smaller voltages (block 12). 239
These modules will be bypassed and will not output any power, so the reference voltage of the left peak is equal to about 80% of (1-M/N)*V OC (block 13). Afterward, the same MPP technique is applied to track this peak (blocks 14 and 15). By comparing the powers of this peak (new MPP) and the previous one (last MPP), the true GMPP is obtained (blocks 16 and 17). Finally, the reference voltage is set to the voltage of the true GMPP, and the control is passed onto the Main Program, which maintains the operation at this GMPP until the timer interrupt occurs again. Figure 6. GMPP Track Subroutine Flow Chart Figure 5. Main Program Flow Chart 240
Figure 8. P pv Scope When PV Curve Periodic Scanning Technique is Used 5. Results Figure 7. P&O Subroutine Flow Chart Figure 9. V pv Scope When PV Curve Periodic Scanning Technique is Used Figure 10. P pv Scope When Proposed Algorithm is Used 241
[2] Yang Chen, Keyue Ma Smedley, A Cost-Effective Single-Stage Inverter With Maximum Power Point Tracking, IEEE Transactions On Power Electronics, Vol. 19, No. 5, September 2004, PP-1289-1294 [3] Nicola Femia, Giovanni Petrone, Giovanni Spagnuolo, Massimo Vitelli, Optimization of Perturb and Observe Maximum Power Point Tracking Method, IEEE Transactions On Power Electronics, Vol. 20, No. 4, July 2005, PP-963-973 Figure 11. V pv Scope When Proposed Algorithm is Used 6. CONCLUSION In this project report, a new technique using an improved MPPT control was presented. The proposed technique enhances the output capacity of solar PV array. The results obtained by means of such approach clearly show that in the design of efficient MPPT regulators the easiness and flexibility of P&O MPPT control technique can be exploited by optimizing it according to the specific system s dynamic characteristics. From above section, it is concluded that, proposed algorithm for partial shading condition is more efficient. The technique used will avoid blind scanning & reduce energy loss due to blind scanning. Thus, it can easily detect partial shading condition by comparing voltages of every PV module. The results obtained by means of such approach clearly show that in the design of efficient MPPT regulators the easiness and flexibility of P&O MPPT control technique can be exploited by optimizing it according to the specific system s dynamic characteristics. Also, it can easily calculate the number of peaks on P-V curves by separating the voltages of all PV modules into various groups & due to this it can easily locate the GMPP & LMPPs. The results obtained by means of such approach clearly show that in the design of efficient MPPT regulators the easiness and flexibility of P&O MPPT control technique can be exploited by optimizing it according to the specific system s dynamic characteristics. Also, it can easily predict the locations of GMPP & LMPPs by calculating the number of PV modules in every group & due to which, tracking time is reduced. Due to this algorithm, it can check partial shading condition after specific time interval. This makes an advantage over the other methods. REFERENCES [1] Kai Chen, Shulin Tian, Yuhua Cheng, Libing Bai, An Improved MPPT Controller for Photovoltaic System Under Partial Shading Condition, IEEE Transactions On Sustainable Energy, Vol. 5, No. 3, July 2014, PP-978-985 [4] Fangrui Liu, Shanxu Duan, Fei Liu, Bangyin Liu, Yong Kang, A Variable Step Size INC MPPT Method for PV Systems, IEEE Transactions On Industrial Electronics, Vol. 55, No. 7, July 2008, PP-2622-2628 [5] Jonathan W. Kimball, Philip T. Krein, Discrete-Time Ripple Correlation Control for Maximum Power Point Tracking, IEEE Transactions On Power Electronics, Vol. 23, No. 5, September 2008, PP-2353-2362 [6] Hiren Patel, Vivek Agarwal, MATLAB-Based Modeling to Study the Effects of Partial Shading on PV Array Characteristics, IEEE Transactions On Energy Conversion, Vol. 23, No. 1, March 2008, PP-302-310 [7] Niket Thakkar, Daniel Cormode, Vincent P.A. Lonij, Steve Pulver, Alexander D. Cronin, A SIMPLE NON- LINEAR MODEL FOR THE EFFECT OF PARTIAL SHADE ON PV SYSTEMS, IEEE, 2010, PP-2321-2326 [8] N. Femia, D. Granozio, G. Petrone, G. Spagnuolo, M. Vitelli, Predictive & Adaptive MPPT Perturb and Observe Method, IEEE Transactions On Aerospace And Electronic Systems, Vol. 43, No. 3, July 2007, PP-934-950 [9] Hiren Patel, Vivek Agarwal, Maximum Power Point Tracking Scheme for PV Systems Operating Under Partially Shaded Conditions, IEEE Transactions On Industrial Electronics, Vol. 55, No. 4, April 2008, PP-1689-1698 [10] Gianluca Acciari, Davide Graci, Amedeo La Scala, Higher PV Module Efficiency by a Novel CBS Bypass, IEEE Transactions On Power Electronics, Vol. 26, No. 5, May 2011, PP-1333-1336 [11] Evagelia V. Paraskevadaki, Stavros A. Papathanassiou, Evaluation of MPP Voltage and Power of mc-si PV Modules in Partial Shading Conditions, IEEE Transactions On Energy Conversion, Vol. 26, No. 3, September 2011, PP-923-932 [12] Eftichios Koutroulis, Frede Blaabjerg, A New Technique for Tracking the Global Maximum Power Point of PV Arrays Operating Under Partial-Shading Conditions, IEEE Journal Of Photovoltaics, Vol. 2, No. 2, April 2012, PP- 184-190 [13] Ali Bidram, Ali Davoudi, Robert S. Balog, Control and Circuit Techniques to Mitigate Partial Shading Effects in Photovoltaic Arrays, IEEE Journal Of Photovoltaics, Vol. 2, No. 4, October 2012, PP-532-546 242
[14] Pooja Sharma, Vivek Agarwal, Exact Maximum Power Point Tracking of Grid Connected Partially Shaded PV Source Using Current Compensation Concept, IEEE Transactions On Power Electronics, Vol. 29, No. 9, September 2014, PP- 4684-4692 AUTHOR PROFILE Swapnil R. Borade received the B.E. degree from Governrnent College of Engg, Jalgaon in 2013, the M.E. pursing from SSGBCOET, Bhusawal, Jalgaon. G. K. Mahajan received the B.E. degree from Shri Sant Gajanan Maharaj College of Engineering, Shegaon in 1999 and M.E. degree from Government College of Engineering, Amravati in 2012. Total teaching experience of 16 years. N. M. Khandare received the B.E. degree from Shri Sant Gadgebaba College of Engineering, Bhusawal in 2006 and M.E. degree from Shri Sant Gajanan Maharaj College of Engineering, Shegaon. Total teaching experience of 11.5 years. 243