Power system electromagnetic transient studies require that a small part of the electrical network be modelled in detail. The rest of the system is represented by a network equivalent taking into account the frequency dependence of the system components (FDNE). With the purpose of getting a FDNE, this paper presents a procedure for including high-order rational models in EMTP-type simulation programs based on Norton equivalent circuits. This approach has the advantage of using few conductance branches compared to those obtained by fitting any particular circuit structure.In relation to the work carried out, this paper presents the following issues: a) a procedure for calculating the admittance matrix of the rest of the system as a function of frequency b) this matrix is synthesized in the form of multi-terminal π-equivalent circuits, whose branches are fitted by rational functions in the pole-residue form c) each partial fraction is converted into a differential equation in time domain. The trapezoidal rule of integration is applied to that differential equation, resulting in a conductance in parallel with a history term current source. Considering all the partial fractions corresponding to a branch, a Norton equivalent circuit is obtained for that branch.This paper also shows how the procedure is implemented in three different ways in an EMTP-type program, along with the validation of such approach through simulations in an electric utility 500 kV transmission system.
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