NERC´s transmission availability data system and analysis

On a NERC-wide average basis, the automatic transmission outage rate has improved from 2008 to 2010. Considering both automatic and non-automatic outages, 2010 records indicate transmission element availability percentage exceeds 95%. Unexpectedly, almost one-third of all sustained, automatic outages are dependent or common mode events. Though a number of protection systems are intended to trip three or more circuits, many events go beyond their design basis. In addition, a number of multiple outage events were initiated by protection system misoperations. These events, which go beyond their design expectations and operating procedures, represent a tangible threat to reliability. A deeper review of the root causes of dependent and common mode events, which include three or more automatic outages, is a high priority for NERC and the industry. More focus into this problem would improve the design basis assumed for reliable operation of the system.