Case study: An adaptive underfrequency load-shedding system

Underfrequency (UF) schemes are implemented in nearly every power system and are deemed critical methods to avert system-wide blackouts. Unfortunately, UF-based schemes are often ineffective for industrial power systems. Traditional UF schemes are implemented in either discrete electromechanical relays or microprocessor-based multifunction relays. Individual loads or feeders are most commonly shed by relays working autonomously. The UF in each relay is set in a staggered fashion, using different timers and UF thresholds. Sometimes, dω/dt elements are used to select larger blocks of load to shed. Unfortunately, no traditional schemes take into account load-level changes, system inertia changes, changes in load composition, governor response characteristics, or changes in system topology. This paper explains an adaptive method that overcomes known UF scheme problems by using communication between remote protective relays and a centralized UF appliance. This method continuously keeps track of dynamically changing load levels, system topology, and load composition. The theory behind the improved scheme is explained using modeling results from a real power system.