  This paper proposed a new method to locate the source of forced oscillation
that involves resonance with natural oscillation modes. The new method is based
on comparing the oscillation mode shape of the forced oscillation with that of
the natural oscillation that the forced oscillation resonating with. The
location that has the largest angle difference between the forced oscillation
mode and the natural oscillation mode usually indicates the location of the
driving force in forced oscillations. Some examples in actual U.S. EI system
verified this approach.
