Global MPPT Method for Partially Shaded Photovoltaic Modules Stefano Bifaretti, Vincenzo Iacovone, Lucio Cinà, Emilio Buffone Dept. of Electronic Engineering University of Rome “Tor Vergata” Rome, Italy email:
[email protected]
Abstract- Among Renewable Energy Sources, the solar energy represents an attractive alternative solution to traditional sources because is available everywhere; moreover, photovoltaic (PV) modules are nowadays becoming more and more attractive as the price per watt of the photovoltaic modules is decreasing. The output voltage furnished by a PV module is quite low; therefore, a series connection of different modules has to be employed in order to obtain a sufficient output dc voltage. In non-uniform irradiation conditions of a PV array, traditional Maximum Power Point Tracking (MPPT) algorithms can converge to a relative maximum point, thus output power becomes lower than the absolute maximum. Since the conversion efficiency of commercial photovoltaic modules is still rather low, to increase the power produced, also under partially shaded conditions, is necessary to employ an efficiently, fast and simple MPPT. The paper proposes, for a Switched CapacitorBoost converter structure, a non-conventional MPPT method able to track the absolute Maximum Power Point (MPP). I.
INTRODUCTION
The continuing decrease in Photovoltaic (PV) modules costs and the increase in their efficiency, imply a promising role for PV generation systems in the near future [1]. The performance of any PV system depends also on the electric load operating conditions. When designing the PV system, the combined effect of the electric load, ambient temperature and solar irradiation variations should
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all be considered. Because the output voltage furnished by a PV module is quite low (20-40V), usually a series connection of different modules, denoted as string, have to be employed in order to obtain a sufficient output dc voltage. The conversion efficiency of commercial PV modules is still rather low; therefore it is particularly important to achieve, in every operating situation, the maximum possible energy. To this aim, a Maximum Power Point Tracking (MPPT) is necessary to maximize the photovoltaic energy utilization for any given ambient/junction temperature and solar irradiation levels [2]-[6]. Among MPPT, Perturb and Observe (P&O) method tracks MPP of PV module by repeatedly increasing or decreasing the output voltage at MPP of PV module. This method, quite simple to implement, can perform an accurate power tracking close to MPP when atmospheric conditions are constant or slowly changing but, when those are rapidly changing, an improved method has to be used [6]. On the contrary the incremental conductance (INC) method overcomes such problem and tracks MPP very well, but the response speed of finding MPP is quite slow due to the increased complexity of the control algorithm. However, shadows created by obstacles, i.e. neighboring houses, utility poles or trees, can sometimes partially cover one of the string modules. In such situation, which produces an unbalanced generation, a significant reduction of the total output power of the PV system is observed. Under shading condition, multiple relative MPP can arise and the traditional MPPT, mentioned above, can fail to track the absolute MPP. To overcome this problem, more complex algorithms have been developed [7]-[12].
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II. UNBALANCED GENERATION The current generated by a PV module can be expressed, as function of voltage, by the simplified expressions [13]: =
∙ 1−
∙
∙
−1
(1)
where = 1−
=
∙
∙
(2)
(3)
Coefficients C1 and C2 depends on short circuit current ISC, open circuit voltage VOC, maximum power point voltage VMPP, maximum power point current IMPP. Such parameters depend on the illumination intensity G and the module temperature T. The dependence can be shown using different expressions [14] that account parameters ISCS, VOCS, Impps and Vmpps at standard conditions, STC (Gs=1000W/m2 and Ts=25°C). The PV model considered in [14] has been accounted in this paper, to simulate the behavior, in Matlab-Simulink environment, of a Photowatt PW1650 module. Series connections of several modules, denoted as string or array, have to be employed in order to obtain a sufficient output dc voltage. If all modules have the same solar
irradiation, MPP supplied by the array is equal to the sum of MPP of all modules; on the contrary, in shadowing condition of one or more modules, multiple MPPs arise. In order to clarify this problem, a series of two modules with different irradiations are considered; the first one is at STC while the shaded module has G= 700 W/m2 and T=25°C. When the total array current is less than short circuit current ISC2 of the shaded module, both modules produce energy and the total array voltage is the sum of two modules’ voltage. If the total array current becomes greater than ISC2, the shaded module absorbs energy; such condition can lead the module to crack. To solve this problem, a bypass diode, connected in parallel to every module and turned on when the shaded module’s voltage becomes negative, is inserted. Using a bypass diode, when the total array current becomes greater than ISC2, the total array voltage is given by the non-shaded module’s voltage value. Figure 1 shows I-V curves of the two distinct modules (dotted lines) and the array (continuous line) obtained with the same model’s parameters described above. P-V curves of the two distinct modules (dotted lines) and the array (continuous line) are illustrated in Figure 2. As it can be noted, the array P-V curve presents two power peaks. In general, a series connection of N modules, which M
