Summary form only given. Protection security assessment of power grids becomes an important task in the course of a competitive energy business and distributed power generation based on renewable resources. The analysis of past blackouts studied by the North American Electric Reliability Council (NERC) shows that protection relays are involved in about 75% of all major disturbances. One reason for that is the non adequate adaption of the protection systems to the changing network conditions. In particular the relay hidden failures causes relay malfunctions and unintended supply interruptions in the past.