Why testing digital relays are becoming so difficult! Part 3 advanced feeder protection

Abstract

The paper identifies more of the developing issues in relay testing - what has previously been well understood protection elements/functions used in designing feeder protection systems has become intertwined with modern complex programmable logic. The core issue is the same modern algorithms and combined protection element scheme logic that enables modern feeder protection to operate securely and reliably also prevents us from using legacy testing techniques we relied on for our “understood” elements. The paper compares various traditional single function protection relays to their modern replacements and illustrates how legacy testing becomes inadequate. This includes modern combinations of overcurrent, directional overcurrent, over/under voltage, and frequency elements. But also investigates the scheme logic employed: sympathetic trip, broken conductor, SOTF, BF, cold load pickup, CT/VT supervision, etc… Combine these with Distrbuted Generation and Distribution Automation deployments and today's feeder protection relay does not test at all like our legacy electromechanical devices. In some instances for today's feeder protection scheme only a network simulator tool can provide a resonable test case. The paper reviews these new challenges and explains why new testing methods are required.