Direct Ranking for Voltage Contingency Selection

It is now commonly accepted that for security assessment of a power system, the most efficient and practical strategy is to deal with the problem in two stages. First in contingency selection, those potentially critical contingency cases are ranked by the severity of their impacts on a system. Then in contingency analysis, detailed AC power flows are applied only to the most dangerous cases appearing on top of the ranked list. In the past decade, the problem of ranking outage cases in the order of decreasing severity has attracted a lot of intensive studies, and many efficient and reliable algorithms were developed. But most of these techniques can only be applied to MW limit security problems. On the other hand, voltage problems were also found to be a very important aspect of security assessment. It has become the target of many research projects. Because the MVAR-voltage problem involves a much more complicated model than the MW-angle problem, significant obstacles were experienced in developing ranking algorithms for voltage security monitoring, making it an area where further study is still required. Almost all contingency ranking algorithms employ a scalar performance index to represent a global severity of voltage disturbance after a contingency event. The scalar performance index can be viewed as a weighted distance in voltage space measuring the post contingency voltage profile against specified voltage limits.