  Large power systems are complex and real-time modeling of the grid for
electromagnetic simulation (EMT) studies is impractical. In general, there are
methods that reduce large power system into an equivalent network that requires
less computational resource, while preserving electromechanical (low frequency)
and high frequency behavior of the original system. This can be achieved by
modeling the area not of interest (external area) as a combination of Transient
Stability Analysis (TSA) type phasor model equivalent and Frequency Dependent
Network Equivalent (FDNE). TSA retains electromechanical behavior, whereas FDNE
retains high frequency behavior of the original power system. To this effect,
this paper introduces a method of developing FDNE based on an online recursive
least squares (RLS) identification algorithm in z-domain, and modeling of
reduced power systems as FDNE and as a combination of TSA and FDNE using
real-time digital simulators.
