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In developing dynamic equivalents, the boundary between the study area and the external area is often demarcated on the basis of ownership considerations or geographic location. In an operations setting, when system conditions change, it might be necessary to adjust the existing boundary to accurately capture the dynamic characteristics of the system. This paper presents a systematic approach to predicting the changing patterns of generator slow coherency for different operating conditions and forming an appropriate study area boundary by including the critical generators in the external area that become strongly coherent with the study area. This approach is tested on a 5186-bus representation of a portion of the WECC system, and it has been shown that system topology changes have more influences on generator slow coherency behavior than load or generation changes. Test results also validate the efficacy of the developed procedure in determining the improved dynamic equivalents.