Abstract-This paper presents a novel approach, which identifies a buffer zone around a given study system and replaces the external system with an equivalent. The aim is to obtain a reduced system, which is suitable for assessment of long-term voltage and frequency dynamics in the study system. First, the proposed methodology ensures that all components, which are impacting the voltage profile of the study system, are included in the buffer zone. For that purpose, the propagation of reactive power changes from the boundary of the study system into the external system are used in a modified depth-first search algorithm to identify the buffer zone. Second, the external system is replaced with equivalents accounting for inertia and primary frequency control effects. The proposed method was successfully tested on a modified version of the ENTSO-E Dynamic Study Model.

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