Inertia (grid)
The kinetic energy stored in synchronously rotating generators and motors that resists sudden changes in grid frequency.
Grid inertia is the stored rotational energy of turbines, generators and large motors spinning in lockstep with the grid. When a large plant trips, this energy is released automatically within milliseconds, slowing the fall of frequency and buying time for frequency response to act.
The inertia constant H, in seconds, is stored kinetic energy divided by rated power. Typical values: steam turbines 4 to 9 s, gas turbines 3 to 7 s, hydro units 2 to 4 s. System inertia is usually expressed in GW·s or MW·s.
Wind turbines and solar PV connect through inverters and do not provide inertia inherently. As they displace synchronous plants, system inertia falls and the rate of change of frequency (RoCoF) after a disturbance rises. Great Britain's operator NESO has set a minimum inertia level around 102 to 120 GW·s and runs stability markets to procure it; the Irish grid caps non-synchronous penetration (SNSP), raised to 75% in 2023 (EirGrid).
Solutions include synchronous condensers (generators spinning without fuel, sometimes with flywheels), grid-forming inverters that provide synthetic or virtual inertia from batteries, fast frequency response from BESS, and RoCoF-tolerant protection settings (often 1 Hz/s).
Related terms
All terms →- Grid frequency
The rate at which alternating current changes direction on a power system, 50 Hz or 60 Hz, which reflects the instantaneous balance of supply and demand.
- Frequency response
The automatic change in generation or load that arrests and corrects a frequency deviation after a sudden imbalance on the grid.
- Grid-forming inverter
An inverter that sets its own voltage and frequency reference, behaving like a voltage source that can stabilise weak grids and provide synthetic inertia.
- Variable renewable energy (VRE)
Renewable generation whose output varies with weather and time of day, mainly wind and solar PV, and cannot be fully dispatched.
- Ancillary services
Services beyond bulk energy that system operators procure to keep the grid stable and secure, such as frequency reserves, voltage control and black start.
- BESS (Battery Energy Storage System)
A complete grid- or site-connected battery installation, including cells, battery management, power conversion, thermal management and controls.