Prevention of islanding in grid-connected photovoltaic systems

Recently there has been a resurgence of concern about islanding of grid-con nected photovoltaic (PV) systems. This condition occurs when the PV system continues to energize a section of the grid after that section has been iso lated from the main utility voltage source. Generally, islanding is undesir able because it poses a safety hazard to utility service personnel, and als o because it can lead to asynchronous reclosure which can damage equipment. It is therefore important that PV systems incorporate methods to prevent i slanding. The purpose of this paper is threefold: (1) to critically review the literature on islanding prevention methods for PV systems and discuss t heir strengths and shortcomings; (2) to review and analyze the islanding be havior of four converters which are prominent in the literature in order to demonstrate the implementation and effectiveness of some islanding prevent ion methods and (3) to introduce a new islanding prevention scheme, active frequency drift with positive feedback, which overcomes many of the shortco mings of existing schemes. It is concluded that no 'perfect' islanding prev ention method yet exists, but also that many existing methods or combinatio ns thereof work very well in practical situations. Finally, it is noted tha n ana investigation of what constitutes 'sufficient' islanding prevention i s needed. Copyright (C) 1999 John Wiley & Sons, Ltd.