RoCoF

ROCOF, short for rate of change of frequency, is a measure of how quickly an electrical grid's frequency is changing, expressed as dF/dt in Hz per second. In most power systems, nominal frequency is 50 or 60 Hz, and ROCOF indicates the dynamic response following a generation-load imbalance.

ROCOF is used in protection and control schemes to detect disturbances and trigger fast corrective actions before the frequency deviates too far. ROCOF relays monitor the slope of the frequency signal and trip, shed load, disconnect generation, or start islanding procedures when the rate of change exceeds predefined thresholds for a specified time window. Positive ROCOF occurs when frequency rises; negative ROCOF occurs when it falls.

Measurement involves high-resolution frequency estimates from devices such as phasor measurement units or protective relays. ROCOF is computed over a sliding window and often filtered to reduce noise and harmonics. The choice of window length and threshold values depends on grid operation and equipment characteristics.

ROCOF is particularly important in grids with high shares of intermittent renewables, where rapid swings can occur. It is also used in microgrids and islanding schemes to distinguish transient disturbances from sustained faults. However, ROCOF alone is not a complete protection scheme; it is typically used in combination with magnitude-based frequency protection and other measurements to avoid misoperations.