I. Introduction
Modern power system is one of the most complex artificial systems in the world. It is operated to maintain a continuous process of electricity generation, transmission, and distribution. However, the operation of a power system is inevitably exposed to all kinds of disturbances and faults, such as short-circuit of a transmission line or an unexpected generator outage. Depending on the severity of the disturbances and faults, the power system may lose its security, which can result in catastrophic consequences, such as wide-spread blackouts.