Islanding Detection in Power Systems

Abstract: 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.