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Modal controllability, observability, and transfer function zeros play an important role in the selection of candidate locations and feedback signals for flexible ac transmission systems (FACTS)-based power swing damping controllers. This paper investigates location of zeros of the transfer function corresponding to FACTS devices and certain local feedback signals. Signals synthesized from locally measured signals are derived that can give good pole-zero separation. It is also shown that relative modal controllability of FACTS controllers at various locations can be directly inferred from relative modal observability in certain signals at those locations. These signals are identified for various FACTS devices. Case studies using detailed model are used to validate the results.