Voltage stability limit of electric power systems with generator reactive power constraints considered

This paper investigates the smoothness of the transfer limit surface, or the loadability surface, of power systems. The reactive power output constraints of generators are taken into consideration. A new methodology employed for the investigation is based on characterizing each maximum loading point by the state of the generators (PV or PQ). Then the transfer limit surface is investigated through careful observation of nose curves. In those special cases with only one constraint, the transfer limit surface is smooth. However, in the more general case of multiple constraints, the transfer limit surface is nonsmooth. These properties have been confirmed in a numerical example using a system with two constraints and three load parameters.