Synchronous generators connected to distribution networks can sustain an electric island after contingences. Islanding occurs when a portion of the distribution system becomes electrically isolated from the remainder of the power system, yet continues to be energized by distributed generators connected to the isolated system. Failure to trip islanded generators can lead to a number of problems to the generators and the connected loads. Thus, the current industry practice is to disconnect all distributed generators immediately after the occurrence of islands. Typically, a distributed generator should be disconnected within 100 to 2000 ms after loss of main supply. To achieve such a goal, each distributed generator must be equipped with an islanding detection device, which is also called anti-islanding device. The most common devices used for this purpose are the under/over frequency relays, under/over voltage relays and their variations. These relays have very low cost and are widely available. They are the first choice for anti-islanding protection. Unfortunately, these relays are not totally reliable due to their inherent limitations. If the active/reactive power imbalance in an island is small, it will take some time for the islanded system to exhibit detectable frequency or voltage change. As a result, the relays will not be able to provide anti-islanding protection in a timely manner. The corresponding system operating conditions are called non-detection zones of the relays. Therefore, this research project has as objective to develop a methodology for risk assessment of non-intentional islanding of distributed generators composed by synchronous machines taking into consideration anti-islanding relays based on voltage and frequency. Since the performance of these relays depends on the active and reactive power imbalance, the idea is to develop a index based on such imbalances and on the concept of critical active and reactive power imbalance.
News published in Agência FAPESP Newsletter about the scholarship: