‘(N-1) criterion’ is the rule according to which the elements remaining in operation within a Transmission System Operator's (TSO's) control area after occurrence of a contingency are capable of accommodating the new operational situation without violating operational security limits (Article 3(2)(14) of the Network Code on System Operation).
Network Code on System Operation provides for the rule that if after a contingency the transmission system is not compliant with the (N-1)-Criterion, the TSO must initiate remedial actions to recover compliance with the (N-1) Criterion as soon as reasonably practicable.
The (N-1)-Criterion is a long established and proven operational standard, which is common amongst TSOs (ENTSO-E NC OS Supporting Paper, p. 33).
Article 35 of the Network Code on System Operation
1. Each TSO shall assess the risks associated with the contingencies after simulating each contingency from its contingency list and after assessing whether it can maintain its transmission system within the operational security limits in the (N-1) situation.
2. When a TSO assesses that the risks associated with a contingency are so significant that it might not be able to prepare and activate remedial actions in a timely manner to prevent non-compliance with the (N-1) criterion or that there is a risk of propagation of a disturbance to the interconnected transmission system, the TSO shall prepare and activate remedial actions to achieve compliance with the (N-1) criterion as soon as possible.
3. In case of an (N-1) situation caused by a disturbance, each TSO shall activate a remedial action in order to ensure that the transmission system is restored to a normal state as soon as possible and that this (N-1) situation becomes the new N-Situation.
4. A TSO shall not be required to comply with the (N-1) criterion in the following situations:
(a) during switching sequences;
(b) during the time period required to prepare and activate remedial actions.
5. Unless a Member State determines otherwise, a TSO shall not be required to comply with the (N-1) criterion as long as there are only local consequences within the TSO's control area.
Network Code on System Operation, Article 3(2)(14), Article 35