This paper proposes an active frequency with a positive feedback in the d-q frame anti-islanding method suitable for a robust
phase-locked loop (PLL) algorithm using the FFT concept. In general, PLL algorithms for grid-connected PV PCS use d-q
transformation and controllers to make zero an imaginary part of the transformed voltage vector. In a real grid system, the grid
voltage is not ideal. It may be unbalanced, noisy and have many harmonics. For these reasons, the d-q transformed components
do not have a pure DC component. The controller tuning of a PLL algorithm is difficult. The proposed PLL algorithm using
the FFT concept can use the strong noise cancelation characteristics of a FFT algorithm without a PI controller. Therefore, the
proposed PLL algorithm has no gain-tuning of a PI controller, and it is hardly influenced by voltage drops, phase step changes and
harmonics. Islanding prediction is a necessary feature of inverter-based photovoltaic (PV) systems in order to meet the stringent
standard requirements for interconnection with an electrical grid. Both passive and active anti-islanding methods exist. Typically,
active methods modify a given parameter, which also affects the shape and quality of the grid injected current. In this paper,
the active anti-islanding algorithm for a grid-connected PV PCS uses positive feedback control in the d-q frame. The proposed
PLL and anti-islanding algorithm are implemented for a 250kW PV PCS. This system has four DC/DC converters each with
a 25kW power rating. This is only one-third of the total system power. The experimental results show that the proposed PLL,
anti-islanding method and topology demonstrate good performance in a 250kW PV PCS.