Why testing digital relays are becoming so difficult! Part 2

This paper is part two of the subject title and identifies some of the developing issues in testing what has here to fore been well understood protection elements we commonly used in designing modern protection systems. The core issue is the complexity of today´s combined protection element scheme logic that enables these “understood” protection elements to function more securely and reliably - but also prevent us from using legacy testing techniques to prove and verify the defined characteristic. Additionally, it makes a comparison of the commonly used test methods (step change and ramping) focusing on common differential elements of many bus, generator, and especially transformer relays and how they now react based on their algorithms and logic. It identifies why these test methods fall short, under what conditions they should and should not be used and what techniques are now required to properly test today´s relays.