According to a good rule of thumb, how much will frequency sag for every percentage increase in load?

The principle behind this rule of thumb relates to the relationship between system load and frequency. In a power system, frequency is a measure of the balance between power supply and demand. When load increases, unless there is a corresponding increase in generation, the frequency will drop as the system struggles to meet the new demand.

Specifically, a common rule of thumb states that for every 5% increase in load, the frequency is expected to sag by approximately 0.5 Hz. This is a simplistic model used in reliability coordination and operational planning, providing a quick way for operators to gauge how much frequency may decline in response to changing loads.

Understanding this relationship is crucial, as frequency stability is vital for the reliable operation of electrical grids. A drop in frequency can indicate that the system is becoming overloaded, which may lead to larger issues such as equipment overloading or cascading failures if not addressed promptly.

The other options provide different increments of load increase that would not align with the established understanding of frequency response; hence, they do not accurately reflect the rule of thumb regarding frequency sag for load increases.