Prevention of islanding in grid-connected photovoltaic systems

Recently there has been a resurgence of concern about islanding of grid-connected photovoltaic (PV) systems. This condition occurs when the PV system continues to energize a section of the grid after that section has been isolated from the main utility voltage source. Generally, islanding is undesirable because it poses a safety hazard to utility service personnel, and also because it can lead to asynchronous reclosure which can damage equipment. It is therefore important that PV systems incorporate methods to prevent islanding. The purpose of this paper is threefold: (1) to critically review the literature on islanding prevention methods for PV systems and discuss their strengths and shortcomings; (2) to review and analyze the islanding behavior of four converters which are prominent in the literature in order to demonstrate the implementation and effectiveness of some islanding prevention methods and (3) to introduce a new islanding prevention scheme, active frequency drift with positive feedback, which overcomes many of the shortcomings of existing schemes. It is concluded that no 'perfect' islanding prevention method yet exists, but also that many existing methods or combinations thereof work very well in practical situations. Finally, it is noted than ana investigation of what constitutes 'sufficient' islanding prevention is needed. Copyright (C) 1999 John Wiley & Sons, Ltd.