In this forced oscillation case, the source of oscillations is at generator 79.
System model:
Generator 79 is in GENROU model.
All other generators are in GENCLS model.
Parameter D for all generators equals 4, except for generator 79 whose D equals 0.
All loads are in constant MVA model.
Small signal analysis:
All natural modes have reasonably good damping.
External force:
Forced oscillation is injected into excitation system as a rectangular wave at generator 79 with a fundamental frequency at 0.4Hz.
Figure 1. Block diagram of the external force
Figure 2. WECC 179-bus power system