Increasing utilization of the transmission grid requires new reliability criteria and comprehensive reliability assessment

Transmission grids are generally designed and operated in accordance with deterministic N-1 criteria. These have ensured adequate reliability in the past in large measure because of existing margins in the system. These margins have generally been sufficient to retain system reliability in the face of N-2 disturbances such as simultaneous outages of parallel lines caused by proximity or relay miss-operation. In the US, operating margins against events "beyond criteria" are being eroded by various means such as the use of parallel flow balancing devices, new generators placed to utilize the existing system, and pressure from transmission users to increase transfer limits. Flows on parallel paths are being equalized towards maximum capability making ever-larger numbers of N-2 events critical events, increasing the risk for serious trouble not recognized by present criteria. An effective means of recognizing and protecting against events not covered by N-1 criteria are needed.