In large scale power systems

the specially defined transmission section could be used to measure the security levels of actual system operating states. However

existing approaches for the automatic discovery of a transmission section do not properly consider the critical branch with the minimum security margin

and hence the accuracy of the security level evaluation cannot be guaranteed. Given this background

a new method of automatically discovering transmission sections

with the critical branch taken into account

is developed for assisting system operators to find more reliable and accurate control strategy. First

a set of transmission sections is obtained by employing the network topology and real-time power flow. Next

a repeated power flow based approach is used to evaluate the maximum transmission capability of each transmission section

and at the same time the critical branch is identified. Then

the branches in a transmission section are adjusted to ensure the critical branch is included

and a control strategy could thus be obtained for ensuring the security of the power system concerned. The effectiveness of the proposed method is demonstrated by Guangdong provincial power system.