Islanding Detection in Power Systems

As the contribution of dispersed or distributed energy resources (DER) to the electric power production increases, the effects on the power system grow more important. As an example a critical situation may arise if protective relays trip a large part of the dispersed generation due to undervoltage at a short-circuit event. On the other hand it is crucial that the protection system acts correctly to protect life and property in other situations. These qualities are referred to as security and dependability. There are a number of different kinds of anti-islanding or loss-of-mains protections. Some are implemented in practice while others are still on a research level. This thesis provides an analysis of benefits and drawbacks of methods that are applied today. The ability of the different methods to avoid nuisance tripping and provide robust protection is investigated. To draw the attention to industrial experience, some cases are described where dispersed generations were nuisance tripped. Recordings from islanding events are presented and analyzed. Performance of present anti islanding protections is commented. A comparison of grid codes in Sweden, Denmark and Germany serves as an example of the situations in different countries. Today there are differences in the connection requirements depending on different designs of the distribution systems. These varying requirements lead to different demands on the islanding detection devices. The thesis summarizes the technical requirements on the islanding portion of the protection system. To compare the protection algorithms, they are exposed to challenging situations in a common simulation environment. In the simulation model, a DER-unit (induction generator) is connected to a typical distribution grid. The grid consists of two 20 kV feeders connected to a 130 kV network equivalent via a common bus bar and a transformer. Additional feeders are modeled with generic load and shunt capacitances. The concept of distributed energy resources is moving from being a local issue towards a system issue. The islanding protection devices being used in the future have to reflect this. The thesis aims at describing this process and ends with a list of possibly unresolved issues. (Less)