An Improved Equivalent Model for a Long PV String under Partial Shading Conditions

An accurate model for a long Photovoltaic (PV) string is essential to predict the position of maximum power point under partial shading conditions. Normally it divides a PV string into three parts, namely a diode forward voltage drop, an equivalent PV source and a linear voltage source, under different operating stages. However, significant error may occur when this model is applied on a long PV array or under low solar irradiance condition. Therefore, a new algorithm is proposed to improve the performance of this equivalent model. The proposed method collects and takes advantage of irradiance information in calculation, together with curve fitting method to obtain PV model of higher frequency. To validate this algorithm, error analysis is presented on results of both original and proposed algorithm. It illustrates that there is little deviation on each calculated MPP by applying the proposed algorithm even in a long PV string under low irradiance condition.