The basic objective of this paper is to present the relevant methodological and practical aspects of forming the unified external network equivalent. This equivalent consistently respects the effects of primary voltage and frequency control of external power system, including the real possibilities and limitations, as well as the effects of possible action of under frequency load shedding or under frequency protection of generator. The first practical experience in the application of the equivalent has been gained on an example of the synchronous parallel operation of the 2nd UCTE Synchronous Zone. The results obtained show a very good accuracy of unified external network equivalent formed and also show the very high speed of steady-state security assessment in context of real interconnection.