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.

The main objective of this paper is to analyze the behaviour of a pair of oscillatory modes of a power system, essentially a swing mode and an exciter mode, that Dass near strong resonance. The method of prturbations suggested by Seiranyan [I] to describe the behaviour of a pair of eigenvalues in the neighbourhood of a multiple point has been suitably modified to make it applicable to systems with feedback controllers. In the case of multimachine systems, the swing mode of interest is isolated by making use of the concept of modal transformation.The knowledge of participation factors helps in identifying the exciter mode that interacts with the swing mode. The illustrative examples comprise a Single Machine connected to Infinite Bus (SMIB) system and a 3machine system with STATCOM supplementary modulation controller.

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