This discussion has been titled "Interconnection Reliability".  The term reliability here is being used more in a generic sense rather than in a technical correct sense . We all know that power systems are planned and operated so that they are able to withstand a set of contingencies of predefined severity. This design approach  confers to the system "Security" but not "Reliability". Reliability refers to the probability that the system will not fail. Security instead refers to the whether the system is able to withstand contingencies of predefined severity. To calculate  reliability requires gathering failure statistics for the sytem components, enumerate all the possible states that the system can reside in and then checking each state for success or failure. The accumulation of the probabilities of the success states results in the probability that the system will not fail and hence the Reliability of the system. This is a monumental effort for for a system as large as any of our interconnections!  Instead, to evaluate the security of the system all we need is a Load Flow program, a Transient Stability Program, and a Dynamic Stability program, and then use these programs to simulate the predefined design contingencies, and if the system remains stable transiently and dynamically, and if the steady state conditions have acceptable voltage level and flows for all the design contingencies, the system Security is acceptable. See more formal definitions of Security and reliability in this website page "System Security Or System Reliability?".