Evaluation of Failure behavior of Service Aged Electromagnetic Protective Relays Thesis

Protection systems in medium voltage grids consist for the largest part of protective relays of the electromechanical kind. The first series of this kind of protective relays were introduced in the late 40s. Later kinds (static and numerical) were introduced in the early 70s. For years, these electromechanical protective relays have been serviced to an “as new” condition with a time based maintenance policy (preventive maintenance). This is no longer the case. The influence of no longer servicing to an "as new" condition and the ageing of these relays on the reliability of these relays has not been investigated. For the investigation of failure behavior of electromechanical protective relays, a new evaluation method has been developed. This method is a qualitative method based on Failure Mode Effect and Criticality Analysis