N minus one (N-1) is used in the electricity industry as a model for resilience. The formula expresses the concept that the grid should be capable of experiencing the outage of a single transmission line, cable, transformer or generator (the ‘minus one’ in question) without the loss of electricity supply.

N-1 informs resilience policy everywhere, from high-voltage transmission to low-voltage networks. But it may need updating for the energy transition as the grid becomes more complicated. Either way, the integration of distributed energy resources (DERs), such as large-scale wind and solar power plants, along with domestic sources of energy generation and storage, poses new challenges for protection and control engineers.

We have seen vivid examples of this in recent months in some European countries, which have experienced widespread blackouts, leaving businesses, schools, traffic systems and government buildings without power for extended periods. Climate change and the electrification of consumption, along with growth in power generation from distributed (mostly renewable) sources, are exerting pressure on power networks, their components, and traditional grid management.

“Recent events in Europe have demonstrated that electrical grids must evolve, adopting new strategies and technologies to meet new challenges,” points out Paul Bancroft, executive power system consultant specialising in grid code compliance and power system protection, Siemens Grid Software. “On the protection front, an increase in energy from renewable sources can translate into potential issues caused by system inertia related to a very high level of power electronic-based generation at the time.”