Table 2 summarizes the statistical information
concerning the post-contingency margins of 3 contingencies discussed
below, as well as their severity measured by 
. The first column describes the
contingency, the second column shows the number 
of relevant
states for this contingency among the 5000, while the third and fourth
(resp. fifth and sixth) columns indicate the mean and standard
deviation of the post-contingency margin (resp. severity) in the data
base.
Table 2: Statistics of post-contingency margins and severities
As a ground for comparison, the first line of Table 2 shows the mean and standard deviation of the pre-contingency margins computed for the 5000 states of the data base. We mention that the overall load level in the region shown in Fig. 5 varies between 5000MW and 7700MW, while the pre-contingency LPM in this region varies between 0 and 1750MW, with a mean of 756MW and a rather high standard deviation of 325MW, indicating the diversity of situations covered in the data base.
Figure 6: Scatter plots of pre- vs post-contingency margins (MW total
in the study region)
The scatter plots shown in Fig. 6, show the distribution of pre- vs post-contingency LPM in the data base. Each point represents an operating state, and its vertical distance from the diagonal is equal to the severity of the contingency for this state. Thus the farther the location of the cloud center below the diagonal the higher the mean severity of the contingency and the higher the spread of the points the more variable the severity from case to case.