Definition
Rate of Change of Frequency (RoCoF) measures how quickly an AC grid's frequency moves immediately after a generation-load imbalance, expressed in hertz per second (Hz/s). It is a key vital sign of grid stability: the larger the disturbance and the lower the system's inertia, the steeper the RoCoF. RoCoF is inversely proportional to total online inertia, so as synchronous machines are displaced by inverter-based generation, the same disturbance produces a faster, sharper frequency slope.
Why it is dangerous
High RoCoF can trip protective relays prematurely and trigger cascading disconnections before slower control systems can react. Grid codes therefore set RoCoF withstand limits, and operators care intensely about keeping enough inertia — or synthetic inertia — online to keep the slope manageable after the loss of a large generator or interconnector.
Relevance to inverter-dominated microgrids
A small, off-grid or behind-the-meter system has very little physical inertia, so its RoCoF after a load or generation step can be severe. This is precisely why grid-forming inverters with synthetic inertia and fast droop response matter: they arrest the frequency excursion in milliseconds, keeping a mining or compute microgrid stable where a passive system would collapse.
Related terms include synthetic inertia and the grid-forming inverter.
In Simple Terms
Rate of Change of Frequency (RoCoF) measures how quickly an AC grid’s frequency moves immediately after a generation-load imbalance, expressed in hertz per second (Hz/s).…