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Critical inertia is defined as the minimum level of system inertia necessary to ensure that frequency responsive reserves have sufficient time to be deployed and prevent the operation of the first stage of UFLS after the largest credible contingency [9]. Critical inertia in an interconnected system is also referred as “inertia floor” for that system.

Critical inertia is dependent on various factors, such as the amount of fast frequency response (FFR) and primary frequency response (PFR) available in the power system, size of critical contingency, the thresholds for initiating UFLS, etc. The critical inertia can be reduced if (i) FFR from loads or other mechanisms can be provided faster, (ii) the thresholds for UFLS are lowered, or (iii) the size of the largest contingency is made smaller.