It is time consuming to use the entire network to conduct setting calculation for local area. The way to narrow down the grid size for setting calculation becomes an important subject. Based on the complex network theory, the weighted power network model is formed. The buffer subsystem for setting calculation consists of the coordination region and constraint region. After determining the scope of setting calculation, we include the adjacent branches of boundaries and its end nodes in the coordination region according to remote backup protection. Furthermore, the shortest electrical path is used to determine the minimum ring network of boundaries and coordination branches. Meanwhile the substation scope is determined by the self-impedance variation rate of boundary nodes. The constraint region consists of the smallest ring structures and substation scope. Under these, the depth first search and backtracking technique are applied to form the buffer subsystem automatically.