In this forced oscillation case, the source of oscillations is at the governor of generator 79.
System model:
All the generators are in GENCLS model.
Parameter D for all generators equals 4.
All loads are in constant MVA model.
Small signal analysis:
All natural modes have reasonably good damping.
External force:
Forced periodic, rectangular and symmetric signal is injected into the governor of generator 79.
This forced signal creates the spectrum of odd harmonics 0.2, 0.6, 1.0, 1.4 Hz ... with the lowest frequency 0.2Hz.
The gain of the external force is 0.02 p.u.
None of these harmonics create resonance conditions with the natural modes.
At the source, the largest MW magnitude is at the lowest frequency and decreases for higher frequency harmonics. This picture became more complicated for other generators. For example, the largest MW magnitude of oscillations at generator 77 is observed at f=1Hz and is 61MW, while the magnitude for f=0.2Hz is only 26MW.
Figure 1. Block diagram of the external force
Figure 2. WECC 179-bus power system