North American Electric Reliability Corporation (NERC) Practice Exam

An UFLS (Under-Frequency Load Shedding) plan is designed to protect the power system by automatically disconnecting loads when the frequency drops below a certain threshold, indicating a potential imbalance between supply and demand. For an UFLS plan to be effective, it must operate under specific conditions where the frequency is low and there is a need to reduce demand quickly to stabilize the system.

In the case of over-frequency combined with loss of load, the core principle behind the UFLS mechanism does not apply. The UFLS system becomes ineffective when the frequency is rising or normal (over-frequency) because the primary purpose of the UFLS is to respond to under-frequency conditions. During over-frequency, the system may already be experiencing a surplus of power, leading to less demand rather than a deficiency that requires immediate load reduction. Therefore, using UFLS in this scenario would not achieve the desired outcome of balancing supply and demand, as the system is operating under conditions it was not designed to address.

This clearly delineates why over-frequency scenarios undermine the effectiveness of an UFLS strategy, emphasizing that the conditions warranting UFLS activation must be directly aligned with under-frequency occurrences.