Distance protection in 150/60 kV transformer 60 kV feeders: two real blackout case studies

Correct setting of protection relays are of major importance to the reliable operation of the power system. The root cause of two consecutive blackouts is analysed and shown to origin from a combination of several minor errors which all can be related to an insufficient/wrong setting of the distance relays and their optional functions together with insufficient testing when putting into operation. The learning lessons are discussed and outlined in order to avoid future blackouts having similar origin.