Smoothness of voltage stability surface with generator reactive power limit encountered

When constraints of generators are encountered, a stable power system can become unstable immediately and voltage collapse may occur. One of these constraints considered in the paper is the reactive power limit of generators. Based on an effective control strategy, the smoothness of the transfer limit surface of voltage stability is studied by characterizing each maximum loading point using the state of the generators. In general, the transfer limit surface is smooth without constrains. However, when two or more constraints are considered, the smoothness of a transfer limit surface is destroyed. The results obtained from a numerical example have proved these properties, which are useful for calculation of preventive controls and stability indices of voltage stability.