AbstractTime‐domain approach examines the behaviour of the system, one determines whether stability has been maintained or lost. In contrast to the time‐domain approach, direct methods determine system stability based on energy functions. The basis of direct methods for the stability assessment of a system is knowledge of the stability region. During the last decade, many researches have thoroughly analysed the use of energy functions for the direct stability assessment of networks. Energy function analysis offers a different geometric view of voltage collapse. The Transient Energy Function, a technique based on Lyapunov stability theory and originally developed for direct stability analysis of power systems, has been successfully used as a voltage stability index for collapse studies. In this paper the simulation results are on the IEEE 173‐bus test system.