The phenomenon of strong resonance that is noticed in systems with mode coupling is characterized by coincidence both of eigenvalues and eigenvectors. It is necessary to consider the effect of strong resonance on the coupled modes while designing a damping controller for a system. The work presented in this paper aims at investigating the behavior of two oscillatory modes in the neighborhood of strong resonance in power systems in the presence of a STATCOM damping controller. The modes that interact near the point of strong resonance are identified. The concept of multi-modal decomposition is applied to isolate the swing mode (SM) of interest. The relevant exciter mode (EM) is identified from the knowledge of participation factors. The method of perturbations suggested by Seiranyan to study the interaction of the eigenvalues near the point of strong resonance is modified suitably to make it applicable to systems with feedback controller. The illustrative examples include three- and four-machine systems.
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