ufls

Under Frequency Load Shedding (UFLS) is an automatic protection scheme used in electrical power systems to preserve stability and prevent widespread outages when system frequency falls below predefined levels. It works by rapidly reducing the total load in a controlled manner so that generation can regain balance with demand, helping to avoid cascading failures and further loss of generation.

UFLS schemes are typically organized into multiple stages or blocks. Each stage has a frequency threshold and an associated time delay; if the system frequency remains below a threshold for the duration specified, a predetermined amount of load is shed. As the frequency falls further, additional stages may operate, shedding larger portions of load. UFLS can be implemented at substations locally, by feeder protections, or through centralized control that coordinates actions across a wider area. Modern implementations often use a mix of local sensing and wide-area communication to ensure coordinated response and to minimize unnecessary shedding.

Key design considerations include accurate and fast frequency measurement, reliable communications, and robust protection logic to prevent misoperation. UFLS must balance the need to arrest frequency decline with the impact of shedding on customers, critical facilities, and regional reliability. Coordination with other protection schemes, such as under-voltage load shedding and generation-based protections, is essential. In many systems, UFLS is complemented by demand response programs and generation shedding as part of an integrated ancillary service strategy. The specific thresholds, block sizes, and timing are region-specific and tailored to the local generation mix and load profile.