Risk of Unintentional Islanding in the Presence of Multiple Inverters or Mixed Generation Types

Generally, codes and standards require that distributed generators detect when they have been separated from the main grid, and cease to energize any part of the local electric power system in that case. Industry has responded by developing a variety of means for preventing unintentional islands, which are generally highly effective in cases with small numbers of inverters from a single manufacturer. However, as deployment levels of distributed generators increase, utilities are becoming concerned that islanding detection schemes may lose effectiveness on feeders on which there are inverters from multiple manufacturers, or if there is a mixture of inverters and rotating machines. This paper reviews the latest in islanding detection schemes, presents a number of study results in which these two problem cases are examined, and briefly discuss the future of islanding detection and how this problem will ultimately be solved. The two key conclusions of the paper are: a) that the two cases mentioned above sometimes do lead to elevated run-on times and concerns about these cases are legitimate; and b) the ultimate solution to the islanding detection problem is likely to be a communications-based method, backed up by advanced passive methods used in combination.