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The ROCOF is a measure of how quickly the frequency changes following a sudden imbalance between generation and load [9]. ROCOF at any point on the frequency response curve is the tangential line at that point. However, it is usually calculated as the change in frequency over a short period of time, immediately after the sudden loss of generation.

The initial ROCOF after generation loss at t = 0 s is typically calculated as:

(1.5)

A high ROCOF indicates that the system frequency can potentially fall to a level where Under Frequency Load Shedding (UFLS) may be initiated. ROCOF is used to determine the approximate time required to reach the UFLS threshold. This helps systems planners and operators to provide adequate amount of frequency response within this period to prevent load shedding. It is noted that as soon as the frequency starts to decline, several factors come in to play such as inertial response, load response, and other nonlinear actions which may alter the actual time to reach UFLS [9].