  The post-contingency loadability limit (PCLL) and the secure operating limit
(SOL) are the two main approaches used in computing the security margins of an
electric power system. While the SOL is significantly more computationally
demanding than the PCLL, it can account for the dynamic response after a
disturbance and generally provides a better measure of the security margin. In
this study, the difference between these two methods is compared and analyzed
for a range of different contingency and load model scenarios. A methodology to
allow a fair comparison between the two security margins is developed and
tested on a modified version of the Nordic32 test system. The study shows that
the SOL can differ significantly from the PCLL, especially when the system has
a high penetration of loads with constant power characteristics, or a large
share of induction motor loads with fast load restoration. The difference
between the methods is also tested for different contingencies, where longer
fault clearing times are shown to significantly increase the difference between
the two margins.
