This paper introduces the application of state estimation to double bus double breaker distribution substations, which are typically used in the Korean power system, to improve the reliability of substation automation systems. In spite of the recent remarkable progress in the field of sensors and equipments, it is hard to detect device malfunctions or degraded devices automatically in the supervisory monitoring system. The object of the proposed system is not the state estimation itself, but its application to the supervisory detection of the malfunction or degradation of the devices. The supervisory monitoring process consists of a two step procedure-topology processing and normal state estimation. In a substation, the on-off states of the circuit breakers determine the topology. This paper mainly presents the topology processing expert system including the knowledge representation. The expert system is operated with a predefined period to identify and correct topological errors in the distribution substations. The proposed system is now in the field test stage in the Sanchung pilot substation and it will be applied to the next generation substation automation system in Korea.