## 3 State Evaluation and Fault Prediction of Protection System Based on Digital Twin Technology
### Fault Prediction of Protection System Based on Clustering Algorithm
#### 3.3.1 Common Fault Sets in Protection Systems  
By analyzing the historical operation data of a 220 KV intelligent substation, including historical SOE operation event records, alarm signal records, equipment self-test information, etc., and combining with the equipment manuals provided by the equipment manufacturers, the common fault sets of the protection system can be sorted. In order to improve the efficiency of the algorithm, the subsequent study does not involve the types of faults that can be obtained by simple inference based on the correlation between the characteristic quantities and the faults, such as the equipment power failure alarm. In this paper, the following nine faults that occur more frequently and have no simple correspondence with the characteristic quantities [32; 33; 34] are selected as the fault sets for studying the fault prediction of protection systems. 1. Merging Unit (MU) failure 1 Failure of the main DSP or sampling DSP module of the merging unit leads to abnormal signals of sampling and synchronization from the merging unit and abnormal DSP sampling of the protection device, resulting in abnormal sampling of the merging unit and out-of-step and invalid SV sampling of the protection device. 2. Failure of the input/output module of the merging unit leads to abnormal reception of the process layer signal and abnormal reception of the SV link signal by the protection unit, resulting in invalid sampling of the protection SV, error in the SV link, interruption of the MU/protection SV, interruption of the GOOSE of the merging unit, total alarm of the SV/GOOSE, etc., which will seriously lead to equipment lockout. Figure 6: Fault prediction process based on a clustering algorithm. Protection device failure. Its potential failure causes are mainly:  
* Failure of the protection device CPU, including the failure of the random memory RAM integrated in the CPU, the failure of the accessory module responsible for digital-to-analog signal conversion, the failure of the internal digital signal processor, etc., will lead to alarms of the DSP of the protection device directly connected to it, errors in configuration parameters, abnormal CPU, SV sampling being out of step, errors in self-storage self-test, etc., which will seriously lead to equipment lockout. * Failure of the line protection pilot channel, generally due to the pilot channel loss, fiber damage, etc., will lead to channel differential exit, pilot channel abnormalities, protection open-in and open-out communication interruption, and other alarms. * Failure of protection device input/output plug-in, including loose plug-in inserts, damaged interfaces or internal failures, resulting in alarms, such as SV/GOOSE total alarms, protection SV/GOOSE interruptions, etc., leading to equipment lockout in severe cases. * Failure of protection device GOOSE plug-in, including interface damage, poor contact, etc., resulting in the inability to receive GOOSE link signal, generating protection, merging unit, intelligent terminal GOOSE general alarm. * Intelligent Terminal Failure. Its potential failure causes are mainly:
* Failure of the I/O board of the smart terminal leading to communication blockage and thus many alarms of the information channel, such as GOOSE communication interruption and GOOSE general alarm. * Process level network failure. * GOOSE network fiber abnormalities, including line fouling, fiber damage, fiber head fouling, fiber flange quality is not qualified, fiber fusion splicing process is not good, etc. caused by channel interruption or optical power drop, optical intensity abnormal, bundle tube interruption, etc., resulting in optical port transceiver power being over the limit, GOOSE total alarm, intelligent terminal reported MU data abnormal, etc. * SV network fiber anomaly, same as, resulting in optical port transceiver power being over the limit, SV sampling error, SV link error, protection DSP sampling error, SV sampling invalid, SV link error, etc.