In this forced oscillation case, the source of oscillations is at generator 77.
System model:
Generator 77 is in GENROU model.
All other generators are in GENCLS model.
Parameter D for all generators equals 4, except for generator 77 whose D equals 0.
All loads are in constant MVA model.
Small signal analysis:
All natural modes have reasonably good damping.
The system has a natural mode at 0.37Hz.
External force:
Forced oscillation is injected into excitation system as a sinusoidal signal at generator 77 with a frequency 0.37Hz to cause resonance.
The gain of the external force is tuned such that the largest peak-to-peak value among all generators' active power outputs is about 163MW.
Figure 1. Block diagram of the external force
Figure 2. WECC 179-bus power system