Testing of anti-islanding protections for grid-connected inverters

Nowadays, almost all photovoltaic and grid-connected inverters, have internal protections against islanding. Usually, these protections perform a disconnection from the grid when process variables go beyond over-voltage or under-voltage thresholds, as well as over-frequency or under-frequency thresholds. Furthermore, other schemes are often added in commercial equipment. In spite of the importance of anti-islanding protections, it is not easy to test on field thresholds, reliability and performances of these devices. CESI RICERCA undertook the study and the development of a system that simulates a grid whose parameters may vary in a suitable way to cause the protections trip and to display results. From this activity a first prototype has been assembled. The testing system may verify the four main thresholds of anti-islanding protections (under-voltage, over-voltage, under-frequency and over-frequency) and their operation times. Furthermore, a suitable methodology is used to assess and measure parameters of the rate of frequency change protection.