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Modern power systems are very complex and to model them completely is impractical for electromagnetic transient studies. Therefore, areas outside the immediate area of interest must be represented by some form of frequency-dependent network equivalent (FDNE). One of the motivations for investigating z-domain fitting is that it can be directly implemented in a digital-simulation program without any loss of accuracy as it is already a discrete formulation. Fitting in the s-domain always requires 'discretising' a continuous system and the inherent approximations. The formulation for developing 1- and 2-port frequency-dependent equivalents of the AC system using z-domain rational-function fitting of the frequency response is presented and its use illustrated. This 1- and 2-port FDNE have been applied to the New Zealand lower South Island AC power system. The electromagnetic transient package PSCAD/EMTDC is used to assess the transient response of the 1- and 2-port FDNE developed under different conditions (linear load, fault and nonlinear loading). The study results have indicated the robustness and accuracy of land 2-port FDNE for electromagnetic transient simulation.