The paper describes the performances of a phasor measurement unit (PMU) prototype based on a synchrophasor estimation algorithm conceived for the monitoring of active distribution networks, as well as its experimental application during some intentional islanding and reconnection tests of an urban medium voltage power network. With respect to typical applications in transmission networks, the use of PMUs in distribution networks requires very low values of total vector error (TVE), which involves particular low values of phase errors of the synchrophasor estimates. These requirements are met by a specifically developed PMU, the characteristics and experimental characterization of which are illustrated in the paper. Three of these PMUs have been then used to monitor experimental tests carried out to assess the capability of a urban distribution network to operate autonomously when fed by a local 80 MW combined-cycle power plant. The information provided by the installed PMUs significantly facilitate the operator maneuvers and appear to be useful for the development of an improved control and management system of the active distribution network.