## 1 Introduction  
The protection system is an important part of the smart substation, which mainly includes the merging unit, intelligent terminal, and protection device. It is of great significance for the safe operation of the whole power grid to detect the faults or abnormal conditions in the power system, so as to send an alarm signal, or directly isolate and remove the faulty part. Therefore, it is necessary to ensure the reliable operation of the protection system itself. Even if it fails, it should also quickly eliminate the fault and ensure its support for the normal operation of the power grid to the maximum extent. With the rapid development of smart grid technology, intelligent substations or digitally transformed smart stations have gradually become popular. Substations have the new features of intelligent equipment, data networking, and overall station informatization [1]. Compared with the traditional substation, the fault types and fault characteristics of its protection system have changed a lot, and it is necessary to continuously develop new fault diagnosis methods on the basis of traditional fault diagnosis methods [2]. Improvements in data acquisition, storage, and analysis in intelligent substations provide us with new ideas for developing new fault diagnosis methods. The function of the secondary system in the substation is more complete and the amount of monitoring information is larger, for example, the operation event record of the secondary system, the main station information, the key information of the protection device, the integrated alarm information, the network operation information, the device self-checking information, etc., which provide solid data support for the condition assessment and fault diagnosis of the substation. [PERSON] [3] analyzed the operation behavior of the secondary system of intelligent substation, and sorted out 18 core evaluation sub-items, including sampling accuracy, internal environment, self-checking state, protection start, input consistency, real-time input and output,operation time of the whole group, port flow, fault-free time, correct action, correct control, station-control layer communication, process-layer communication, port function, port performance, on-time receiving, and on-time output. A Big Data mining technology for intelligent substations was proposed by Reference [4]. By deeply mining the integrated alarm information, the device self-checking information, the link information of GOOSE and SV, the sampling value information provided data support for fault diagnosis. With regard to the research on the fault diagnosis of the protection system of the intelligent substation, Reference [5] proposed a method for evaluating the performance of secondary equipment in smart substations based on availability, dependability, and capability (ADC). The accuracy of its evaluation needs to be improved. Reference [6] proposed an online monitoring and fault diagnosis method of the secondary circuit of relay protection based on multi-parameter information. Through the monitoring and analysis of the SV, GOOSE, and manufacturing message specification (MMS) messages, the online state monitoring method, abnormal sampling value, switch monitoring, and abnormal alarm strategy for relay protection devices was proposed. At the same time, the typical alarm information of the protection device and secondary circuit when faults occur was collected, analyzed, and uploaded to construct a set of online monitoring and fault diagnosis systems for a secondary circuit of relay protection. References [7; 8; 9; 10] proposed a method of locating the secondary equipment fault based on the substation configuration description (SCD) of the intelligent substation. However, the actual workflow of intelligent substations is highly dependent on the configuration tools of integrators and manufacturers, and the difference in configuration tools leads to a poor standardization of the files. Therefore, there are still many shortcomings in the method of locating faults simply using SCD files.