Online state estimation is concerned with computing solutions of the basic load-flow problem every few minutes using online data telemetered periodically to the energy control center. An external equivalent representation will be necessary to perform online contingency analysis. Without an external equivalent model the uses of online state estimation will be limited to the monitoring of voltage levels, phase angles, line flows, and network topology. In the basic load-flow problem the input/demand variables describe steady-state behavior of the system. In generalized state estimation there is no clear-cut distinction between the processing of logical and analog data since network topology processing may include local, substation level, state estimation, whereas when state estimation is performed for the whole network, parts of it can be modeled at the physical level. The conventional states of bus voltages and angles are augmented with new state variables.