Smart grids will be regarded as a dual compound network composed of power network and cyber network, called Cyber-power System (CPS). Studying the cyber-power relationship, especially the effects of cyber on the whole system, is of theoretical and engineering importance. In this paper, CPS studies were reviewed first, and then a method based on Inclusion-exclusion theory was proposed, which made elements disjoint and obtained the minimal path set for an assessment. By putting forward a simple model and precise calculation, we conclude that the optimized network topology is useful for analyzing the information transfer capability of smart grids. The results may provide theoretical guidelines for the construction of power grids, especially for optimizing the placement of smart grids.