“…The efforts of the first group lead to efficient algorithms that can obtain approximated solutions by exploiting structures of the power grid, e.g. hierarchical analysis [34], multigrid [24,16,30,38,39,7], random walks [26,19], Poisson solver [28], frequency domain methods [35,13], and model order reduction techniques [31,17,18]. The second group [2,37,27,8,33,32,4,36], which attracts a lot of interests recently due to the demand to accurately characterize the static and dynamic behaviors of power grids [20,21], utilize iterative solvers, especially preconditioned conjugate gradient (PCG), to further improve the solution accuracy.…”