Improved fitting of z-domain Frequency Dependent Network Equivalents for electromagnetic transient simulation

The complexity of modern power systems often makes it impractical to model it in its entirety for electromagnetic transient studies. Therefore, areas outside the immediate area of interest must be represented by some form of Frequency Dependent Network Equivalent (FDNE). The advantage of using z-domain fitting is that it can be directly implemented in a digital simulation program without any loss of accuracy. Fitting in the s-domain always requires "discretizing" a continuous system and the inherent approximations. This paper presents the formulation, investigates the effect of frequency range and accuracy of fitting at low frequencies. The importance of accurate low frequency fitting is highlighted. The frequency range and weighting issues are demonstrated by assessing the transient response of the Lower South Island of New Zealand test system.