Inertia certificates - possibilities to guarantee sufficient inertia in the power system

The instantaneous reserve is of decisive importance for maintaining frequency stability in AC systems. Currently, it is almost exclusively provided by directly connected synchronous generators, whose inertia resists changes in rotational speed and, thus, limits frequency gradients after imbalances between the electrical power supply into and consumption from the network. It is to be expected that the rotating mass available for the power system will decrease in the course of the increasing spread of generation systems connected via power electronics. The resulting higher frequency gradients pose a threat to the frequency stability of the interconnected system. Inertia certificates are a way of quantifying the contribution of generating plants to the provision of instantaneous reserves in the interconnected system and of assigning the instantaneous reserve a price that corresponds to its value for system operation. They enable a fair balance between those systems that inherently provide instantaneous reserves due to their physical mode of operation and those that, due to their principle, have no or only low inertia. In addition, by assigning and compensating for the provision of the instantaneous reserve, the cost-intensive retrofitting of such low-inertia and no-inertia generators can be avoided.