Building very high reliability into the design and manufacture of relays

Protection relays are essential devices in detecting power system faults, instructing circuit breakers when to trip. It is thus essential that the relays offer the level of dependability (assured trip operation for an in-zone fault) and security (stability when no trip operation is required) demanded in power system applications. The lecture investigates typical processes within the design cycle, and manufacturing operations, which seek to ensure compliance. The lecture recaps the evolution of protection relay technologies, from electromechanical to numerical, highlighting possible failure modes and setting/commissioning errors. Special focus is given on modern numerical (digital) relays, presenting the typical hardware and software build which together create the functional device. A case study design cycle is outlined, showing how the process can be controlled. Verification and validation testing, certification/approval testing, and regression test concepts are introduced, especially focusing on real-time digital simulator testing. Manufacturing issues assuring reliability are introduced, right from component sourcing strategies through the serial production stages. The pros and cons of other test/inspection philosophies are presented, such as accelerated testing (eg. heatsoaking).