APPROXIMATION ALGORITHM FOR CURRENT-VOLTAGE CHARACTERISTICS OF PV MODULES UNDER SHADING CONDITIONS
This work is devoted to modeling of current-voltage characteristic (СVC) of a PV module on the base of experimental data obtained using a measuring device with a variable load. The class of approximating functions is defined and the approximation algorithm is constructed. The developed method of processing data obtained from the microcontroller's analog-digital converter (ADC) via a PC serial interface is intended for building CVC and power characteristic under shading conditions. The technique is based on the construction of a model function, which approximate a multistage CVC of shaded PV modules (the CVC curves have been obtained for four levels of shading). The number of steps of the obtained experimental curve and the number of maximum power points are determined by the number of bypass diodes in PV module. As an example, the effect of vertical partial shading on the operating mode of a polycrystalline PV module in album orientation with two diodes is investigated. The total experimental curve is modeled by a corresponding piecewise polynomials: the CVC of the partially shaded module is represented as a superposition of two single-step I–V curves, which correspond to a single-diode equivalent circuit. The proposed algorithm for polynomial approximation of a multi-step CVC makes it possible to simulate the characteristics of shaded PV modules with high accuracy. The proposed class of approximating functions was tested on large arrays of raw experimental data. The obtained CVC allows one to estimate the real parameters of PV module. The speed and simplicity of the developed method give the possibility for calculating the PV module parameters on the base of experimental data obtained in field under shading conditions. The method allows to determine in real-time the operating maximum power point in changing of shading conditions, as well as predict the yield of PV module arrays and carry out their diagnostics. Ref. 24, fig. 6.
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