Any OPF (optimal power flow) tool used in on-line minimum loss calculation requires an accurate representation of the external or unobservable parts of the network. This paper proposes an optimization model of both internal and external network represented explicitly. Although requiring only external network data, any available information about the external state may be used to improve its response. A general methodology for the performance analysis of equivalent OPF models is also proposed. On one level, the robustness of equivalent models for the production of accurate control actions in the internal system is analyzed. Accurate control actions are those that are as close as possible to the controls obtained through a hypothetical OPF solution of the complete system. On a second level, the capacity to react accurately, when such control actions are set in the internal system, is investigated. The proposed model and the Extended Ward equivalent have been tested through the presented methodology. The proposed model has been capable to take into account the information related to the optimization "status" (under optimization or not) of external system. This is an important information since it affects considerably the internal control action calculation. This information cannot be incorporated in the reduced equivalent models. Computational test results with the Brazilian South-Southeast 810 bus real system are presented.