Abstract: The DC system is an important part of the substation to provide power for the relay protection device, circuit breaker operation, all kinds of signal circuits, accident lighting and so on. The battery is the core component of the DC system. When the AC input of the DC system disappears or the DC charging module fails, the battery provides DC power for the equipment to ensure the stable operation of the substation for a period of time. This paper analyzes the causes of a 10 kV distribution line fault that causes the DC bus voltage in substation to be abnormal, leading to the failure of multiple protection measurement and control devices, and finds out the hidden causes of battery internal resistance abnormalities, providing reference for substation abnormal accident analysis and daily safety operation and maintenance.