Use of Petri-nets to describe and verify protection system models for time domain simulations

The assessment of critical system states via intensive power system simulation is more and more common especially for stability studies. The Dynamic Security Assessment (DSA) is one way to investigate the stability of a power system by simulating different contingencies. But one is mostly missing, the models for secondary equipment. To integrate protection systems in the simulation environment, the models have to be verified and tested, if they behave like the original relay or algorithm. Petri nets are one possibility to describe the discrete-event behaviour of protection relays like pickup, trip and reset. The Petri net model can be as detailed as needed to check the behaviour of the simulation model. This article describes different Petri nets, which include also special functionalities like power swing blocking.