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Testing of anti-islanding protections for grid-connected inverters

pubblicazioni - Articolo

Testing of anti-islanding protections for grid-connected inverters

Francesco Groppi* International Conference on CLEAN ELECTRICAL POWER, Capri, 21-23 Maggio 2007 *CESI RICERCA Anti-islanding protections are most important in distributed generation – PV systems included – mainly because they prevent unintentional islanding when a main-loss occour. Nowadays, all photovoltaic inverters, as well most grid-connected inverters, have internal protections against islanding. Usually, these protections perform a disconnection from the grid when parameters go beyond over-voltage or under-voltage thresholds, as well as over-frequency or under-frequency thresholds. Furthermore, other methods – passive and active – for islanding detection, such as frequency derive, grid impedance and frequency shift, are often added to the previous ones. In spite of the importance that the intervention of anti-islanding protections act in every potentially dangerous circumstance, it is not easy to test on field thresholds, reliability and performances of these devices. Many inverters have an auto-check program that operates at start-up, but this function may hardly be defined a full test. In fact, metallic contacts are seldom taken into account and test details remain often unknown. 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 memorise results. From this activity a first prototype has been assembled. Functions, thresholds, times of intervention and allowable tolerances are taken from the prescriptions DK 5940 on grid connection of PV systems. This document was issued by Enel, the former electricity board in Italy, that is still a technical reference point for many other utilities. Therefore, the system is tailored on Italian needs, but set-up values may be easily changed to meet different specifications. The testing system, studied and developed by CESI RICERCA, may verify the four main thresholds of anti- islanding protections (under-voltage, over-voltage, under-frequency and over-frequency) and their times of intervention. Furthermore, a suitable methodology is used to assess and measure parameters of the frequency-derive protection.

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