How does a frequency sag of 0.5Hz relate to the percentage increase in load?

A frequency sag of 0.5Hz indicates a reduction in system frequency from the nominal level, which is typically around 60Hz in North America. When frequency drops, it is often indicative of an imbalance between supply and demand on the power system.

The relationship between frequency change and load increase can be derived from a standard frequency droop characteristic. Generally, a frequency decrease signifies that the load demand has increased beyond the available generation. The general rule of thumb is that every 1Hz drop in frequency corresponds to a load increase of about 5 percent.

In this case, a 0.5Hz sag suggests that the load has increased by approximately half of that threshold. Since a full 1Hz frequency drop correlates with a 5% increase in load, a 0.5Hz drop would correspond to a load increase of about 2.5%, which rounds to about 3%. However, depending on the specific context and interpretations of load frequency relationships, some considerations may yield a different approximation.

Keeping in mind the operational practices often referenced in NERC guidelines and how frequency deviations directly affect grid stability and operations, a 0.5Hz sag can lead to further operational responses and mitigations to keep the system balanced