The N-1 criterion requires that, starting from a normal state with all N elements in service, the system can lose any one line, transformer, generator or busbar section without overloads, voltage violations, instability or loss of load beyond what is allowed. Planners and real-time operators run contingency analysis continuously against it. Tighter rules cover a second loss: N-1-1 for one that arrives after the first has been dealt with, N-2 for two at once.
If we take the Westside line out for maintenance we're no longer N-1 secure on the north import, so it waits until the new cable is in.
When a dispatcher says the system is 'not N-1', one more trip will cause real trouble, and routine switching gets postponed.
Also heard as
- N minus one
- single contingency criterion
Related terms
Load shedding
Deliberate disconnection of blocks of customer load to stop frequency or voltage from collapsing.
SCADA
Supervisory control and data acquisition: the system that lets a control room watch and switch remote plant.
Switching order
Written step-by-step sequence of operations used to isolate or restore equipment safely.