This case contains single source, one unstable local mode and two poorly damped inter-area modes, where the oscillation source is generator 11.
System model:
All generators are in GENCLS model.
Parameter D for generators 6 and 11 equals 2 and -6, respectively.
Parameter D for all other generators equals 4.
All loads are in constant MVA model.
Disturbance:
A three-phase short circuit is added at bus 30 at 0.5s and cleared after 0.05s.
Small signal analysis:
The local mode at 1.63Hz has a negative damping of -0.54%.
The inter-area mode at 0.70Hz has a poor damping of 1.15%.
The inter-area mode at 0.46Hz has a poor damping of 2.22%.
Figure 1. WECC 179-bus power system