Quantification of Photovoltaic Modules Degradation in a String Using Model Based Indicators

Photovoltaic (PV) modules usually have power warranties of 25 years; however, recent studies have shown that they may present early degradation due to the continuous outdoor operation. Those degradations reduce the power generation and extend the return-of-investment time. This paper proposes a procedure for the calculation of two indicators that quantify the degradation of each PV module in a string. The method starts measuring the current vs. voltage (IV) curve of the string and identifying the short-circuit, open-circuit and inflection points, which are used to divide the string IV curve in segments. From each segment it is possible to identify the number of active modules (bypass diodes inactive) and apply a curve-fitting procedure to identify the single-diode model parameters of each module. In this way, it is possible to obtain an approximated IV curve of each module of the string. Finally, two indicators, based on series and parallel resistance variations, are calculated for each module in the string using their estimated IV curves. The proposed method only requires one current sensor for each string and one voltage sensor for the array; moreover, it is validated with simulation results, which show the capacity of the proposed method to quantify the degradation in each module from the string IV curve.