Islanding of Grid-connected PV Inverters: Test Circuits and some Test Results

Abstract

Many grid connected PV inverters are self-commutated. Therefore they have a certain design inherent tendency for islanding. For safety reasons islanding is a major concern of many utilities. Several different methods to prevent islanding are currently used. Passive monitoring of essential line parameters like voltage and frequency (in Japan often also harmonics) are not sufficient. Therefore also active methods (e.g. frequency shifting, continuous line impedance monitoring or power variations) are used. There are also many different proposals for tests circuits. The PV laboratory of HTA Burgdorf has carried out many islanding tests with different inverters and test circuits. In this paper some of these methods, their advantages and drawbacks and some test results are presented. At present there is no international consent how to prevent islanding of grid-connected inverters. However, at least a common test circuit seems feasible, where only one component has to be modified to perfom islanding test in countries requiring line impedance monitoring (e.g. Germany) or in countries that allow also other protecting schemes (e.g. frequency shifting).