Inertial response

Inertial response is a property of large synchronous generators, which contain large synchronous rotating masses, and which acts to overcome any immediate imbalance between power supply and demand for electric power systems, typically the electrical grid. Due to the ever existing power imbalance between mechanical power supply and electric power demand the rotational frequency of the rotating masses in all synchronous generators in the grid either speed up and thus absorb the extra power in case of an excess power supply, or slow down and provide additional power in case of an excess power demand. This response in case of a synchronous generator is built-in into the design and happens without any external intervention or coordination, providing the automatic generation control and the grid operator with valuable time (few seconds) to rebalance the system^[1] The grid frequency is the combined result of the detailed motions of all individual synchronous rotors in the grid, which are modeled by a general equation of motion called the swing equation.

In the US power systems, the grid operator is mandated to keep the frequency within a tight range, and can be financially responsible if the monitoring by the North American Electric Reliability Corporation detects a non-compliance. Furthermore, in order to protect the equipment, a portion of the load will be disconnected ("underfrequency load shedding", UFLS) if the frequency drops below a limit (59.5 Hz in most of the US, 59.3 Hz in Texas).^[2] When an unexpected supply disruption occurs (for example, a generator failure), the primary frequency response kicks in automatically - a sensor detects the lower frequency and adjusts the power of the prime mover accordingly. For a typical synchronous generator, this adjustment involves manipulation of the mechanical devices (valves, etc.) and thus takes time. During this time, the power grid has to rely on the accumulated inertia to slow down the decrease in frequency.^[3]