Islanding is an undesirable condition that is potentially dangerous to the maintenance or repair staff and may cause damages to sensitive equipments. Therefore, islanding detection is a mandatory feature for distributed generators (DGs), including grid-connected photovoltaic systems (GCPVSs). This article presents a new active islanding detection method for GCPVSs with string inverter based on sliding mode controller. In the proposed method, a feedback of point of common coupling (PCC) voltage has been inserted to the reference signal of sliding mode controller as a new maximum power point tracking (MPPT) technique. Through applying this voltage positive feedback (VPF), the inverter output power and consequently, the PCC voltage decreases in islanding condition. This procedure is repeated until the PCC voltage falls below the under voltage (UV) relay setting. The conventional VPF technique attempts to increase and decrease the output power in elevated and declined PCC voltage cases after grid disconnection, respectively. However, due to the input power limitation and MPPT in GCPVSs in the PCC voltage raised cases, the output active power cannot be elevated any further. In this study, the active power perturbation has been only applied in deceleration mode, considering the disturbance definition in the inverter voltage control loop. Therefore, unlike the conventional VPF scheme, it has the advantage of not violating the "conservation of energy law". The performance of the proposed method is evaluated by simulating a GCPVS with string inverter supplying a local load, connected to the grid. Simulation results show troubleshooting the conventional VPF scheme drawbacks as well as successful operation of the proposed method in islanding and non-islanding conditions even in multi-inverter systems. (C) 2016 Elsevier Ltd. All rights reserved.