Under the partial-shading condition, the voltage-current characteristics of a series-connected solar panel strings are different from that of a single panel. Instead of having a single maximum power point (MPP), they exhibit multiple MPPs. It is thus crucial to operate the string at the global MPP (GMPP). A large body of literature has been devoted to techniques for locating GMPP. However, those techniques typically require knowing the panel characteristics, adding extra sensing components and circuits, and/or causing output power interruption. In order to deal with these issues, a two-step algorithm for tracking the GMPP is proposed. The first step determines the string voltage at which the GMPP appears. It is based on extracting the power-voltage characteristics of the string through varying the input impedance of the power processing unit to determine the approximate location of the GMPP. The second step determines the exact location of the GMPP. It is based on formulating two quadratic functions, namely static and fictitious power functions, and then calculating the intersection point of the two functions to determine the GMPP. The algorithm has been implemented on a 3 kW grid-connected inverter. As the proposed method does not require modifying the hardware and knowing pane) characteristics, it can be immediately integrated into existing photovoltaic systems. (C) 2016 Elsevier Ltd. All rights reserved.