Out of step protection in modern power networks

In the paper we concentrate on so called ´asynchronous operation´ of power systems, which may characterize the operation of individual generating units as well as their groups (power plants) connected in parallel with the power system, or even the operation of two interconnected power systems. Asynchronous operating conditions are a fact in modern power systems and should be eliminated as fast as possible in order to keep their stronger parts in normal operation. Introduction of pole slip protection on tie lines proved to be one of the best solutions. This requires comprehensive stability studies under different operating conditions and in many cases also excellent cooperation of experts from different utilities, and even different countries.