Abstract: This article describes the system analysis process of a DC interconnection abnormality fault that occurred in a 500 kV smart substation. To gain a deeper understanding of such special DC interconnection faults, on-site drawing verification and detailed investigations were conducted to analyze the relationship between the control bus voltage and the short-circuit resistance during DC interconnection. The characteristics of DC interconnection abnormalities were identified, and preventive maintenance strategies and emergency response measures were summarized. These efforts significantly improved the reliability of the substation’s DC power supply monitoring system.