What is (local) inertia?
Inertial Response is a property of large rotating masses, especially large synchronous generators. These rotating masses have as an important property that they resist sudden changes in rotational frequency, they cannot instantly move faster or slower but need a certain time to accelerate or decelerate. Thanks to this property, these rotating masses can overcome the immediate an inevitable imbalance between power supply and demand for electric power systems, and this way help to keep the grid stable and balanced.
The local inertia provides an image of the distribution of these rotating masses over a synchronously interconnected power system. Because power delivered by the inertia of a synchronous generator needs to be transported to the place where the disturbance takes place, it is important that the inertia in the grid is well distributed to keep power flows as local as possible.