Frequency response

Frequency response is the sequence of automatic and manual actions that restore grid frequency after a disturbance such as the loss of a large generator or interconnector.

• Inertial response (0 to a few seconds): rotating masses release kinetic energy, limiting the rate of change of frequency (see inertia).
• Primary response (seconds to 30 s): generator governors and fast resources raise output in proportion to the deviation. In Continental Europe this is Frequency Containment Reserve (FCR), about 3,000 MW sized for the loss of two large nuclear units.
• Secondary response (30 s to 15 min): automatic generation control returns frequency to nominal and restores scheduled flows (aFRR).
• Tertiary reserves replace the depleted secondary reserves.

Primary response is governed by droop: a 4% droop means the unit moves from zero to full output over a 4% frequency change (2 Hz at 50 Hz).

NERC measures frequency response in MW per 0.1 Hz; the Eastern Interconnection obligation is roughly 1,000 MW/0.1 Hz. With falling inertia, operators have introduced faster products: Great Britain's Dynamic Containment acts within 1 second and is supplied almost entirely by batteries; ERCOT runs Fast Frequency Response at 0.5 s.