Network islanding — A real event

One of the main nightmare scenarios regarding Distributed Generation is islanding. In that event, the electrical Utility ceases to have control over the frequency and voltage levels in the island. However, it is still responsible for the quality of service rendered to its clients. To prevent the occurrence of islanding a protection function is used. In the Portuguese case only frequency protection is directly used for this purpose. Despite having anti-islanding protection in all DG an islanding situation still occurred in Portugal during the year of 2011. Due to a fault a substation was disconnected from the network, however, the DG connected to that substation did not trip forming an island in which voltage was maintained at a low level (90%Vn) and frequency at normal levels. The DG feeding the substation was comprised of two thermal generation plants. These DG are connected to a factory and are able to form an island with the factory´s loads, however, the islanding “mode” should only be activated in the case of a disconnection of the DG from the network. In this paper the events leading to the network disconnection of the substation are presented with the aid of disturbance records. The transient evolution of voltage and frequency immediately after the fault is shown. The estimated behaviour of the DG during the islanding situation, and its elimination is also presented. Finally, several comments regarding data acquisition necessary for “post-mortem” analysis of DG related islanding situations are made. EDP has managed to persuade the government and most DG to accept changes to law regarding the necessity of information gathering at the DG facility.