Why testing digital relays are becoming so difficult! Part 1

Abstract

This paper 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 the 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 like “constant test current” or “step change/state sequencing” to verify the set characteristic. Additionally, it makes a comparison of the commonly used test methods (constant current/constant voltage) focusing on common distance elements of Mho's and Quad's and how these now fall short in properly generating what the relay expects of a power system fault simulation. It identifies why these methods fall short, under what conditions they can and should be used and which lesser known test method should now be used to properly test all aspects of these distance characteristics.