Testing the Tester, Common Pitfalls Testing Microprocessor Based Relays

Microprocessor based relays have advanced algorithms in them to provide enhanced security against transients and other conditions that cannot actually exist on a real power system. This means that test methods should mimic real power system conditions, otherwise the relay may not operate or operate in an unpredictable manner for unrealistic conditions that are presented by a test set. This has given rise to the common phrase "test the tester". This paper will show common mistakes that are made when testing Microprocessor based relays with unrealistic power system conditions. The paper also explains why these conditions are unrealistic, and explains how typical relay algorithms respond to these conditions. Unrealistic tests to be explored will include: a step change in frequency for under-frequency testing, absence of pre-fault conditions when testing distance elements, change in voltage phase when testing distance elements, improperly set zero sequence compensation factors in test set software, and improper phase direction on bus protection.