Online dynamic security assessment in an EMS

System stability analysis results have been part of electric utilities' operating guidelines for move than a decade. Today, these guidelines are compiled from offline stability studies conducted jointly with other utilities and independently by each utility. The offline study results are adjusted for actual generation patterns and network conditions, based on sensitivity analysis, and presented to operators through the energy management system (EMS). These stability limits tend to be conservative for normal conditions and may indeed be inaccurate following unusual system events. The offline study scope expands each year as more significant variables are identified. Limits derived from the offline studies also require extensive engineering analysis trying to anticipate conditions which may never occur. A better alternative is to compute these limits online, based on actual operating conditions. A dynamic security assessment (DSA) function implemented in an EMS environment would improve reliability, identify additional energy transaction potential and result in better use of engineering resources. This article presents DSA functional features and an implementation of an online DSA function under development at Siemens Power Systems Control under a collaborative sponsorship of EPRI and Northern States Power Company.