N-1 security criterion
The operating rule that requires a power grid to withstand the loss of any single element without an outage or a limit violation.
The N-1 criterion is the ground rule of transmission grid operation: at every moment, the operator checks that losing any single line, transformer or generating unit would leave the grid within its limits. Generation and flows are dispatched accordingly, at a cost, since some capacity is held in reserve. The rule protects against a single contingency, by far the most common, but not against two losses in quick succession or against common-mode failures. That is why major blackouts almost always start with a second loss that occurs before the grid has been brought back within limits after the first. For an underwriter, the criterion explains the nature of blackout risk: a rare event whose frequency depends less on weather than on operating discipline, and which hits every insured in a region at once.
At 3:01 a.m. on September 28, 2003, the Swiss Lukmanier line tripped after a flashover to a tree; the shifted load brought down the San Bernardino line before the grid was back within limits, Italy separated from the European grid, and some 56 million people lost power.
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