**Po Hu \* and Lily Lee \***

School of Electrical Engineering and Automation, Wuhan University, Wuhan 430072, China **\*** Correspondence: phu@whu.edu.cn (P.H.); 2014102070020@whu.edu.cn (L.L.)

Received: 9 December 2019; Accepted: 18 January 2020; Published: 21 January 2020

**Abstract:** The propagation of cascading failures of modern power systems is mainly constrained by the network topology and system parameter. In order to alleviate the cascading failure impacts, it is necessary to adjust the original network topology considering the geographical factors, construction costs and requirements of engineering practice. Based on the complex network theory, the power system is modeled as a directed graph. The graph is divided into communities based on the Fast–Newman algorithm, where each community contains at least one generator node. Combined with the islanding characteristics and the node vulnerability, three low-degree-node-based link-addition strategies are proposed to optimize the original topology. A new evaluation index combining with the attack difficulty and the island ratio is proposed to measure the impacts on the network under sequential attacks. From the analysis of the experimental results of three attack scenarios, this study adopts the proposed strategies to enhance the network connectivity and improve the robustness to some extent. It is therefore helpful to guide the power system cascading failure mitigation strategies and network optimization planning.

**Keywords:** power systems; complex network theory; Fast–Newman algorithm; link-addition strategy; cascading failures
