Real-Time Calculation of Power System Loadability Limits

Determination of loadability limits in real-time is essential for the effective and efficient utilization of the power system network particularly in an open access environment. However, determining these limits in real-time in an energy management system (EMS) has been extremely difficult and off-line studies, time consuming simulations, and the operator\´s knowledge and experience, have been the basis for determining the limits especially when taking into account stability consideration. Therefore, accurate and reliable estimates of the limits in real-time based on the current system state are highly desirable. In this paper, we describe the theoretical basis for an approach that has been successfully installed and proven in several EMS systems. The approach relies on several distinct theoretical concepts: criterion for assessing the steady-state stability limit, the development of a radial equivalent network to which the steady-state stability limit criterion can be applied, a procedure for determining the "distance" of the current operating state from the stability limit (hence finding the available margin of loading), evaluating the effect of contingencies, and determining the "weak" flow gates (links) in the network. Field experience in NOS BiH (ISO Bosnia and Herzegovina) is also presented.