Abstract: A chain differential protection device is developed, by which the range of fault points and output optical signals to drive the audible and visual alarms from the local current collector can be accurately obtained,which is convenient for maintenance personnel to find the fault points and reduce the time of defect elimination.The multi-stage differential protection realized by large-area differential and small-area differential can effectively avoid the misoperation of the protection device caused by human errors or the failure of the device itself, which improves the operation reliability of the protection device.