  Forced oscillation (FO) is a significant concern threating the power system
stability. Its mechanisms are mostly studied via linear models. However, FO
amplitude is increasing, e.g., Nordic and Western American FOs, which can
stimulate power system nonlinearity. Hence, this paper incorporates
nonlinearity in FO mechanism analysis. The multi-scale technique is employed in
solving the forced oscillation equation to handle the quadratic nonlinearity.
The amplitude-frequency characteristic curves and first-order approximate
expressions are derived. The frequency deviation and jumping phenomenon caused
by nonlinearity are discovered and further analyzed by comparing with linear
models. This paper provides a preliminary research for nonlinear FOs of power
system, and more characteristics should be further analysis in the near future.
