Photovoltaic (PV) systems and other dispersed power systems need to be connected to a utility power grid for the systems to work effectively as energy sources. Protection against islanding is one of the most important techniques to develop for the dispersed power systems supplying surplus power back to the utility power grid. It is considered that active methods where the outputs of the power systems are actively perturbed are necessary to detect islanding. Various active methods have been developed. It has been shown that these methods work when a single power system is connected to one power distribution line. However, most of these methods have a limit in detecting islanding when multiple power systems are connected to one distribution line. In this paper, we compare the detection characteristics of typical active methods, frequency shift methods, a pulse perturbation method and a method using a correlation technique by simulation analysis for the case of multiple PV power systems. The study shows that the method using the correlation technique is the most promising when multiple power systems are operated on one distribution line.