Accurate and less-disturbing active anti-islanding method based on PLL for grid-connected PV Inverters

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Abstract
Islanding prediction is a necessary feature of inverter-based photovoltaic (PV) system in order to meet stringent standard requirements for interconnection with the 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 injected current. In this paper, an accurate and less-disturbing active method suitable for phase-locked loop (PLL) based controllers is proposed. The PLL structure is built using a second order generalized integrator (SOGI). Each output cycle the inverter current reference is slightly modified by an injected signal thus affecting the generated current in a negligible way. Therefore, a feedback signal can be extracted from the voltage of the point of common coupling (PCC) as a consequence of the injected signal. When the grid becomes unavailable, the feedback signal extracted from the voltage at PCC moves outside of a preset threshold value. This new active anti-islanding method meets both standard requirements IEEE 929-2000, IEEE 1547.1 and VDE 0126.1.1. The disturbance used by this method is small compared to other active anti-islanding methods, such as active frequency drift or frequency shift up/down. Moreover, this method does not present non-detection-zones (NDZ), it does not affect the zero-crossing of the injected current and it can also estimate the grid impedance. In addition, the method is highly robust to different grid disturbances and stiffness, and is effective for multiple inverters running in parallel. The performance of the proposed method has been studied through extensive simulations using MATLAB/Simulink for several operating conditions and load types and then successfully tested on an experimental setup. Selected results are presented in order to confirm the validity of the proposed method.

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