I. Introduction
The power system consists of many system elements, such as generators, transformers, bus bars and transmission lines, which are protected by protection systems comprised of protective relays (PRs), circuit breakers (CRs) and communication equipment. The supervisory control and data acquisition (SCADA) system is equipped together with electric power systems. Fault diagnosis of power systems is a process of discriminating the faulted system elements by tripping of protective relays and circuit breakers. When a fault event occurs, it can lead to a large amount of alarm messages in SCADA system. The alarm messages must be analyzed by dispatchers according to their operating experiences in order to identify the faults. However, the received data is often incomplete and tripping of protective relays and circuit breakers is sometime uncertain. Therefore, fault diagnosis is a difficult and complicated task since circuit breakers may fail to operate the multiple faults with the incomplete and uncertain alarm messages.