I. Introduction
The global climate change and rapidly growing populations over the past decades have generated increasing demands for abundant, sustainable, and clean electric energy on a global basis. However, in most countries today, the increasing energy demand means an even heavier burden on the already overstressed, overaged, and fragile electricity infrastructure. In the U.S., for example, the average age of power-grid transmission lines is beyond 50–60 years [29]. Over the last 20 years, the electricity demand and consumption have increased continuously by 2.5% annually according to a U.S. Department of Energy report [8]. The increasing electricity demand, together with the complex and nonlinear nature of the electric power distribution network, have caused serious network congestion issues. The network congestion and safety-related factors have become the main causes of several major blackouts that happened in recent years. In addition to the overstressed situation, the existing power grid also suffers from the lack of pervasive and effective communications, monitoring, fault diagnostics, and automation, which further increase the possibility of region-wide system breakdown due to the cascading effect initiated by a single fault. Furthermore, the global increasing adaptation of renewable and alternative energy sources in the 21st century also introduced new issues, such as power-grid integration, system stability, and energy storage, which also need to be addressed as additional challenges.