A global MPPT algorithm for PV system under rapidly fluctuating irradiance

A two-step global maximum power point (GMPP) tracking algorithm for photovoltaic (PV) system powered by a string of series-connected solar panels will be presented. In the first step, the power-voltage characteristics of the string is extracted by changing the input impedance of the inverter through three phases, so as to determine the number of panels in operation at the approximated GMPP. In the second step, the exact location of the GMPP is determined by formulating two second-order power functions, namely actual string power function and fictitious power function, and finding the intersection point of the two functions. The proposed algorithm has been evaluated on a 3kW grid-tie PV system with a PV simulator connected. The two different power-voltage characteristics are programmed and switched in order to test and compare the tracking performances of traditional perturb-and-observe (P&O) and proposed MPPT algorithms. Results show that the proposed algorithm takes 2.5 seconds for the step-1 process and 1 second for the step-two process to locate the GMPP. Without the first step, the speed of the second step is three times faster than the P&O algorithm.