  Power system oscillations under a large disturbance often exhibit distorted
waveforms as captured by increasingly deployed phasor measurement units. One
cause is the occurrence of a near-resonance condition among several dominant
modes that are influenced by nonlinear transient dynamics of generators. This
paper proposes an Extended Prony Analysis method for measurement-based modal
analysis. Based on the normal form theory, it compares analyses on transient
and post-transient waveforms to distinguish a resonance mode caused by a
near-resonance condition from natural modes so that the method can give more
accurate modal properties than a traditional Prony Analysis method, especially
for large disturbances. The new method is first demonstrated in detail on
Kundur's two-area system and then tested on the IEEE 39-bus system to show its
performance under a near-resonance condition.
