Islanding tests in medium voltage distribution systems with synchronous and non-synchronous generation

The increasing penetration of distributed generation is leading to a higher number of unintentional islands in distribution grids. Currently, islanded operation of distribution grids is not allowed, since it poses a threat to both maintenance crews and grid assets (because voltage and frequency may go out of range). This paper presents the results of intentional islanding tests of MV distribution grids. The tests were conducted in different zones of the distribution grid of a major Spanish distribution system operator (DSO), i-DE Redes Eléctricas Inteligentes S.A.U., with the aim of reproducing the unintentional islands. The tests performed have confirmed that distribution islands are formed and maintained over time, even without grid-forming elements. However, it has been determined that the stability of such islands depends on the boundary conditions of the islanded grid zone, such as the characteristics of the loads or the nature of the anti-island protections. In addition, the tests have shown that it is often difficult to obtain all the information about the boundary conditions (e.g., protection settings, load types, etc.).