This paper suggests a mechanism for the dynamic performance of damping controllers in the condition of strong resonance. The mechanism explains theoretically how the variation of a control parameter can move the resonance point in such a way to stabilize or destabilize the coupled modes. As an application, this mechanism is applied to justify the performance of power system stabilizers (PSSs) in a 2-area 4- machine test system, in which an exciter mode and an inter-area mode interact near a strong resonance. It makes the performance of the PSSs on the stability of the inter-area mode become severely dependent on the place of the PSS and the position of operating point with respect to the resonance point. In this circumstance, the PSSs of one of the system areas destabilize the inter-area mode. Considering the proposed mechanism, the appropriate location of the PSS and its proper gain value are identified to obtain the maximum damping of inter-area mode at each of the operating points. In addition, it is shown that due to the strong resonance, conventional methods make incorrect placement of the PSS; however, by using the real part of speed participation factors, suitable machines are chosen to place the PSS. This index provides important information regarding the impacts of strong resonance on the performance of PSSs.

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