* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Comparative Analysis of Series and Parallel Photovoltaic Arrays Under Partial Shading Conditions Renan Diego de Oliveira Reiter, Leandro Michels, José Renes Pinheiro Rene Alfonso Reiter, Sérgio Vidal Garcia Oliveira, Adriano Péres Grupo de Eletrônica de Potência e Controle (GEPOC) Universidade Federal de Santa Maria (UFSM) Santa Maria, Brazil [email protected], [email protected], [email protected] Departamento de Engenharia Elétrica e Telecomunicações Universidade Regional de Blumenau (FURB) Blumenau, Brazil [email protected], [email protected], [email protected] Abstract—This paper presents a comparative analysis among different arrays of photovoltaic (PV) modules under partial shading conditions. This scenario is typical of a residential area where partial shading can occur due to trees or neighbor buildings. The main objective is to evaluate the power extraction under partial shading in parallel and series PV modules arrays, and compare the results. Simulation and experimental results are presented to validate the proposed analysis. Keywords—Photovoltaic systems, parallel arrangement, DC-DC converter. I. INTRODUCTION Photovoltaic (PV) systems have been the fastest growing energy technology over the last decade. This technology is been disseminated in residential applications employing underutilized spaces on rooftops . Many different topologies of PV systems have been developed for residential applications, such as central and multi-string inverters . Central inverter topology uses a single converter to process energy from PV models while multi-string uses more than one converter. Due to reduction of cables and protective devices among PV modules and the converter, most PV systems are based on central inverter topologies. Single-stage central inverter (CI) topologies have been widely used due to its simplicity and high converter efficiency, since it does not requires a step-up dc-dc converter. Normally these PV systems use a large number of modules in series for direct connection of the PV array to the inverter. The main drawback of series arrays is the lower utilization factor of the PV system under shading conditions -. This limitation is not a problem in most sites where residential PV systems are widely used. In sub-tropical and temperate climate sites such as in Germany, most residential buildings are designed to receive the maximum quantity of solar irradiation. Differently from many sub-tropical countries, most residential areas in Brazil present leafy trees. These trees are intentionally planted to reduce indoor heat due to the tropical climate. Although the removal of these trees could result in better performance of PV systems, most people are not likely to do it. As a result, there is a demand for residential PV systems optimized for partial shading conditions. The reduction of energy production due to partial shading can be mitigated in central inverters using parallel or seriesparallel arrangements of PV modules. However, the output voltage of these PV arrays is lower than the series one. As a result, series-parallel arrays of PV modules normally demand the use of an additional dc-dc converter to step-up voltage from PV array to inverter . The objective of this paper is to evaluate the energy production of series and series-parallel PV arrays under partial shading conditions. Two different arrays with twelve PV modules are analyzed. Simulation and experimental results are provided to determine the advantages and disadvantages of each PV array under this condition. II. CRITERIA OF COMPARISON This section describes the criteria used to investigate the impact of partial shading on energy production of central inverter PV systems . Irradiance, temperature, and partial shading pattern are the factors with greater influence in the maximum peak of available power from a PV array. It is worth mentioning that the influence of partial shading pattern is directly related to the arrangement of PV modules. Shaded modules decrease the maximum available power of the PV array and can produces multiple peaks in powervoltage curve. Moreover, it modifies the voltage of the PV array at maximum power point (MPP) and can change voltage range of the central inverter -. Assuming these factors, were considered the following criteria for comparison of different PV arrangements under partial shading conditions: VDC Array Utilization Factor (AUF): indicates the percentage of generation in relation to the maximum potential power. It is the ratio between the maximum output power of PV array in relation to the maximum power generation considering the array unshaded; DC V DC V DC V DC (a) V Fill-Factor (FF): indicates the percentage of generation of available power under shading. It is the ratio between the maximum output power of PV array in relation to the sum of maximum output power of each individual PV module ; DC (b) Voltage at Maximum Power Point (VMPP): indicate the PV array voltage at global maxima power generation; V Number of Maximum Power Peaks (NMPP): indicate the number of local maxima peaks of power versus voltage curve. III. V DC (c) MATERIALS AND METHODS This paper presents a comparison based on simulation and experimental results. Both analyses consider twelve Kyocera KD135SX-UPU PV modules , which results in a total PV installed power of 1620 Wp. PV modules were arranged in two different series-parallel configurations: i) four strings with three modules connected in series (array 3x4); and, ii) two strings of six modules connected in series (array 6x2). First configuration presents characteristic of series array while second one of parallel array. Four cases were chosen to represent typical residential roofs partial shaded by a building or a tree. Fig. 1 presents four different shading patterns considered in this analysis: i) Case I: no shading; ii) Case II: two modules are completely shaded. iii) Case III: three modules are completely shaded; and iv) Case IV: three modules are completely shaded and two are partially shaded. In these figures, white PV modules are not shaded, light gray modules are partially shaded, and dark gray modules are completely shaded. Both simulation and experimental curves were obtained connecting an adjustable resistor in parallel with the PV array. For each test condition this resistance was swept from zero (short-circuit) to infinite (open-circuit). Measuring the current and voltage for several points were plotted power versus voltage curves. V (d) Figure 1. PV array configuration: Left (Array 3x4), Right (Array 6x2). (a) Case I. (b) Case II. (c) Case III. (d) Case IV. in experimental tests. This difference is due to the characteristic of the pyranometer that measured the global horizontal irradiance, does not assuming that all PV modules are tilted. IV. RESULTS A. Simulation results Four different simulations were performed to determine the criteria of comparison defined in Sect. II, considering shading patterns shown in Fig. 1. The irradiances used in the simulation for the PV modules, according to Fig. 1, are 600 W/m² (white), 0 W/m² (dark gray), and 400 W/m² (light gray). Fig. 3 presents simulation results of power versus voltage curve for shading patterns. In this figure, the black plot is the response of the 3x4 PV array while the gray plot is the 6x2 PV array one. Fig. 3(a)-(d) show the results for shading patterns of cases I to IV presented in Fig. 1, respectively. Table I presents AUF, FF, VMPP, and NMPP obtained in these simulations. Fig. 2 presents the setup used for experimental tests. Fig. 2(a) shows the PV array, Fig. 2(b) show the adjustable resistors, Fig. 2(c) the data acquisition board for measurement of current, voltage and global horizontal irradiance, and Fig. 2(d) the pyranometer Apogee SP-110 used to measure the irradiance . Simulations were carried out using PSIM® software . The temperature of 48°C measured in experimental tests was considered for all simulations. On the other hand, solar irradiance used for simulations are lower than those obtained DC TABLE I. SIMULATION RESULTS Array 3x4 Case I Case II Case III Case IV AUF (%) FF (%) 100 75 75 68 100 90 100 97 Array 6x2 AUF VMPP NMPP (%) (V) 43 43 43 44 1 1 1 1 100 75 75 68 FF (%) 100 86 72 72 VMPP NMPP (V) 86 62 47 86 1 2 2 2 1200 Power (W) 1000 Array 3x4 881W Array 6x2 881W 800 600 400 200 0 0 20 40 60 80 Voltage (V) (a) (a) 661W Power (W) 800 400 200 20 40 Power (W) 60 80 Voltage (V) (b) 100 120 140 Array 3x4 Array 6x2 661W 600 475W 441W 400 200 0 20 40 800 Power (W) 140 441W 800 (c) 120 Array 3x4 Array 6x2 632W 600 0 (b) 100 60 80 Voltage (V) (c) 100 425W 140 Array 3x4 Array 6x2 597W 600 120 441W 400 200 0 20 40 60 80 Voltage (V) (d) 100 120 140 Figure 3. Simulation results: power versus voltage curve. (a) Case I (b) Case II. (c) Case III. (d) Case IV. modules were slightly changed due to small changes in the environmental conditions. (d) Figure 2. Experimental setup. (a) Photovoltaic Array. (b) Adjustable resistor. (c) Data acquisition system. (d) Pyranometer. B. Experimental results Experimental tests were performed considering similar condition of the simulation results presented in Sect. IV A. During these tests the temperature and irradiation levels of PV Fig. 4 presents experimental results of power versus voltage curve for shading patterns shown in Fig. 1. In these tests, the horizontal global irradiances were approximately 675 W/m² (white), 445 W/m² (light gray), and 0 W/m² (dark gray). The measured temperature of PV modules was 48°C. Fig. 4(a)-(d) show the results for shading patters of cases I to IV presented in Fig. 1, respectively. Table II presents AUF, VMPP, and NMPP obtained from experimental results. The AUF was calculated considering global maxima power obtained in this Case I. The FF can be considered 100% in this case because there is no shading. It is worth mentioning that FF was not measured experimentally due to complexity of setup, since it requires an individual converter for each PV module. EXPERIMENTAL RESULTS Array 3x4 Case I Case II Case III Case IV AUF (%) FF (%) 100 76 74 67 - AUF VMPP NMPP (%) (V) 39.4 39.4 39.4 37.8 1 1 1 1 100 66 51 53 Array 6x2 1200 FF (%) 1000 - VMPP NMPP (V) 88 56.5 83.6 83.6 1 2 2 2 Power (W) TABLE II. 800 600 400 0 A comparative analysis between 3x4 array and 6x2 array is presented in Fig. 5. This figure shows normalized results for Cases II, III and IV in relation to Case I, where there is no PV modules shaded. Fig. 5(a) presents the normalized Fill-Factor obtained from simulations results. This factor indicates the efficiency of the PV array under partial shading conditions. One can observe that arrays with few modules in series present high FF even under partial shading conditions. On the other hand, the FF of arrays with several modules in series decreases considerably if some modules are shaded. Therefore, parallelism of PV modules increases significantly the solar conversion efficiency. Fig. 5(b) shows the normalized array utilization factors calculated from experimental results. Observe that the energy production, in relation to unshaded case, decrease significantly in 6x2 arrays even when few modules are shaded. Fig. 5(c) presents the voltage changes of the maximum power point. Due to multiple local maximum power peaks, the voltage range of 6x2 array at MPP changes considerably. As a result, the converter used to process the power from this PV array must be designed to a wider input voltage range. On the other hand, the voltage range at MPP for 3x4 arrays is narrow. Finally, we conclude that PV arrays with few modules in series and several strings in parallel present advantageous characteristics, in relation to series arrays, if some modules are 20 Power (W) 40 60 80 Voltage (V) (a) 782W 100 120 140 Array 3x4 Array 6x2 549W 722W 600 400 200 0 0 20 40 60 80 Voltage (V) (b) 100 600 120 140 Array 3x4 Array 6x2 762W 800 Power (W) The shape of power vs. voltage curve is similar in all four cases for 3x4 array. On the other hand, under partial shading condition, 6x2 array presents two local power peaks: one around 90V and other around 50V. Depending on and environmental conditions, the MPPT algorithm may not converge to the global maxima, which reduce significantly the efficiency of the array. 0 800 557W 500W 400 200 0 0 20 40 60 80 Voltage (V) (c) 100 686W 800 576W Power (W) Simulation and experimental results for cases II, III and IV present a good correspondence between them. One can observe that global maxima of 3x4 array presents always higher power peak and AUF in relation to 6x2 array. Array 3x4 Array 6x2 200 C. Discussion This section presents the analysis and discussion of simulation and experimental results presented previously. Case I illustrate a test condition where no PV modules are shaded. In simulation, one can observe that global maxima power of both arrays are the same because was neglected the power losses in the circuit. On the other hand, experimentally, there is a difference of maximum power between arrays 3x4 and 6x2. This difference is due the higher wiring losses in 3x4 array. 1097W 1023W 600 120 140 Array 3x4 Array 6x2 479W 400 200 0 0 20 40 60 80 Voltage (V) (d) 100 120 140 Figure 4. Simulation results: power versus voltage curve. (a) Case I (b) Case II. (c) Case III. (d) Case IV. shaded. The complete system efficiency is normally increased even when is required an additional dc-dc converter to step-up the lower terminal voltage of parallel PV arrays. V. CONCLUSIONS This paper presents a comparative analysis among different arrays of photovoltaic modules under partial shading conditions. This scenario is typical of a residential area where partial shading can occur due to trees or neighbor buildings. Simulation and experimental results have demonstrated that PV systems with several modules connected in series can reduce significantly the solar conversion efficiency when some modules are partially shaded. On the other hand, solar conversion efficiency can be increased using several PV arrays in parallel with few modules in series. 1200 Power (W) 1000 800 90% 100% 600 86% 97% Case II Case III Case IV 72% 72% REFERENCES   400 200  0 Array 3x4 Array 6x2 (a) 1200  Case II Case III Case IV 66% 51% 53% Power (W) 1000 800 76% 74% 67% 600   400 200  0 Array 3x4 Array 6x2 (b) Voltage (V) 120 100 80 60 40 20 Case II Case III Case IV -5% -5%  -35% 0% 0% +4.8%  0 Array 3x4 Array 6x2 (c)  Figure 5. Normalized criteria of comparison in relation to Case I for 3x4 and 6x2 arrays. (a) FF. (b) AUF. (c) VMPP. ACKNOWLEDGMENT   The authors would like to thanks to FAPESC, FAPERGS and CAPES/PROCAD for financial support.    W. Li, X. He, ”Review of nonisolated high-step-up dc/dc converters in photovoltaic grid-connected applications,” IEEE Transactions on Industrial Electronics, vol. 58, no. 4, pp. 1239-1250, April 2011. Y. M. Chen, K. Y. Lo, Y. R. Chang, “Multi-string single-stage gridconnected inverter for PV system,” Energy Conversion Congress and Exposition (ECCE), pp. 2751-2756, Sept. 2011. R. D. O. Reiter, L. Michels, A. Péres, S. V. G. Oliveira, “Analysis and design of an isolated step-up dc-dc converter based on three-phase high-frequency transformer for pv aplications,” in XI COBEP 2011, 2011. Trends in Photovoltaic Applications: Survey report of Selected IEA Countries between 1992 and 2010. H. Yan, Z. Zhou and H. Lu, “Photovoltaic industry and market investigation,” SUPERGEN'09, pp. 1-4, April 2009. X. Wang, J. Gao, W. Hu, Z. Shi, B. Tang, “Research of effect on distribution network with penetration of photovoltaic system,” Universities Power Engineering Conference (UPEC), pp. 1-4, August/September 2010. Y. Ding, J. Ostergaard, W. Shen, P. Wang, P. C. Loh, “Expected energy production evaluation for photovoltaic systems,” 4th International Conference on Electric Utility Deregulation and Restructuring and Power Technologies (DRPT), pp. 1076-1080, 6-9 July 2011. M. Abdulazeez, I. Iskender, “Simulation and experimental study of shading effect on series and parallel connected photovoltaic PV modules,” 7th International Conference on Electrical and Electronics Engineering (ELECO), pp. I-28-I-32, 1-4 Dec. 2011. L. O'Callaghan, M. McKeever, B. Norton, “A simulation analysis of photovoltaic AC module integrated converters in parallel, under controlled edge shading conditions,” 11th International Conference on Environment and Electrical Engineering (EEEIC), pp. 699-705, 18-25 May 2012. H. Patel, V. Agarwal, “MATLAB-based modeling to study the effects of partial shading on pv array characteristics,” IEEE Transactions on Energy Conversion, vol.23, no.1, pp. 302-310. X. Qingshan, S. Jing, B. Haihong, K. Yukita, K. Ichiyanagi, “Analysis of photovoltaic array performance under shaded conditions,” Power and Energy Engineering Conference (APPEEC), pp. 1-4, 28-31 March 2010. B. A. Yount, F. F. Xiao, J. Zhao, J. W. Kimball, “Quantifying insolation in multiple shading scenarios,” Green Technologies Conference (IEEE-Green), 2011 IEEE, pp. 1-6, 14-15 April 2011. http://www.kyocerasolar.com/assets/001/5342.pdf, accessed August 2012. http://www.apogeeinstruments.com/specsheets/SP-110specs.pdf, accessed August 2012. http:// http://www.powersimtech.com, accessed August 2012.