How Transmission Systems Deal with Failures

A power transmission system must be able to supply power reliably under all conditions of demand on the network:

• Summer peak load;
• Summer off-peak load;
• Winter peak load;
• Winter off-peak load;

The N-1 criterion expresses the ability of the transmission system to lose a linkage without causing an overload failure elsewhere. The N-2 criterion is a higher level of system security, where the system can withstand any two linkages going down. Details that accompany the N-1 and N-2 criteria give further information on the robustness of the system.

An N-2 safety criterion may, for example, involve additional feed in points from lower voltage networks to provide reserve supply, so the low voltage network itself acts as a power conduit. Additional criteria may include a requirement for load shedding - knocking off certain large power consumers to maintain supplies for the rest of the network and rescheduling of generation - bringing on generation units at short notice that normally would not be used.