For many years, protection engineers have applied local breaker-failure protection to high-voltage (HV) and extrahigh-voltage (EHV) systems with electromechanical relays and solid-state relays. These breaker-failure schemes were designed within the limits of the prevailing technology, such as inaccurate timers with overtravel and slow-reset overcurrent elements. The development of microprocessor-based relaying, digital input filtering, and low-burden CT secondary circuits allows easier breaker-failure protection application with measurable gains in performance.
Héctor J. Altuve, Michael J. Thompson, and Joe Mooney, Advances in Breaker-Failure Protection
The SEL-501 Dual Universal Overcurrent Relay provides two complete and independent groups of protection functions in one compact unit. The unit contains Relay X and Relay Y, each having separate optoisolated inputs, output contacts, and three-phase current inputs.