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An overfrequency event is caused by loss of a large load or an exporting tie line (e.g. HVDC line). Displacement of conventional fossil‐fuel‐based conventional generators by IBRs such solar PV systems decreases system inertia. This exacerbates over frequency response of power systems. The typical behavior of power systems with different levels of stored kinetic energy during an overfrequency event is depicted in Figure 1.17 [11].

Systems with low inertia (e.g. 100 GWs) experience a higher “zenith” (peak frequency) subsequent to loss of a large load, than systems with higher inertia. The ROCOF is positive for all systems but higher for low inertia systems.

Overfrequency events have the following features [11]:

1 Lesser risk of system collapse as compared to underfrequency events.

2 Synchronous generators may provide an unexpected response due to sudden rise in frequency and may disconnect in some cases. Still, there is a low risk of cascaded loss of generation.

3 Less likelihood of disconnection of domestic loads.

Since there are no substantial adverse impacts on the system by overfrequency events, system operators are generally less concerned about these events. Overfrequency events are typically alleviated by reduction in power output from synchronous generators or IBRs.