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The pu'pose of this paper is to discuss recent practical experience in external network modeling at a number of utility control centers.This paper presents a set of short notes that discuss w e n t practical experiences on external network modeling. The introduction provides a framework for the rest of the short notes by discussing network representation, modeling issues, unobservable internal models versus unreduced external models, and data for the unobservable network The first short note discusses the solution methods consisting of power flow, one pass SE, and two pass S E The remaining short notes discuss recent practical experience on external network modeling at a number of utility control ceqters. The last short note describes techniques for developing REI equivalents. INIRODUCnONAs on-line security assessment becomes more prevalent in utility control centers, the need for understanding the problems of external network modeling increases. This paper presents eight short notes that describe various aspects of making external network modeling work in utility control centers. The first short note discusses specific methods of external network solution techniques. The remaining seven short notes relate practical experience in modeling and solving external networks for real-time security assessment. The contributors represent a variety of utility network sizes from small to large. Likewise, the utility control centers reported in this paper have varyihg amounts of real-time power system data ranging from only tie-line flows to large quantities of real-time inter-utility data exchange. The techniques used by the contributors span a wide range, including REI equivalents and specialized linear programming algorithms.It will be observed that there is a wide diversity in external network representation and solution techniques in this report. This diversity is caused by differences in power system characteristics, availability of external network data, availability of the external network modeling techniques and software during the development/implementation of the control centers, and experience of the developer/implementer. There does not appear to be a single, universal external network modeling approach that works uniformly well across the spectrum of power systems reported in this paper. The diversity of approaches is a reflection of this fact. WM 164-4 PWRSby the IEEE Power System Engineering Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1993 Winter Meeting, Columbus, OH, January 31 -February 5, 1993. Manuscript submitted September 1, 1992; made available for printing November 30, 1992. A paper recommended and approvedIt should be understood that the issues of external network modeling occur only when network analysis functions, such as Contingency Analysis, Optimal Power Flow, and Dispatcher Training Simulator, are involved.Clearly, an external model is not needed for internal state estimation.Presentation and discussion of numerical test results are impractical UNREDUQZD MO...