## 2 Protection System Architecture Based on Digital Twin Technology  
Currently, the prevailing digital twin technology architecture generally includes a physical layer, a data layer, a model layer, a functional layer, and an application layer [23; 24]. The physical layer corresponds to the device entity. The data layer corresponds to the process of data sensing, acquisition, transmission, and processing. The model layer corresponds to algorithmic and data-driven models, and the functional layer is responsible for implementing specific functions such as description, diagnosis, early warning, and decision making. The application layer corresponds to the intelligent platform [18]. Based on the prevailing technical architecture today, this paper proposes a digital twin architecture and a working principle for protection systems consisting of five components: a physical device layer, a data transmission layer, a data-processing layer, a twin model layer, and an application management layer as shown in Figure 1.  
The main functions of each layer in the architecture of a digital twin protection system are shown in the Table 1.  
1. Physical device layer. The physical device layer includes all physical devices of the protection system. It includes intelligent microcomputer protection devices, merging units, intelligent terminals, and process layer network channels, etc. 2. Data transmission layer. The data transmission layer relies on the secondary monitoring system (SMS), which is responsible for the sensing and transmission of data. The secondary monitoring system is a global analysis system for secondary equipment developed by the State Grid Corporation in recent years to improve the management efficiency of the secondary system. It can realize the integrated modeling of secondary equipment and the comprehensive collection of operation information, and complete real-time monitoring, comprehensive alarm, online analysis, remote control and evaluation management of the secondary system, whose main station architecture is shown in Figure 2. Information on the operation status, strategy, and fixed value of all kinds of secondary equipment in the station is connected to the communication gateway through the station control layer network and sent to the main station. The construction of the secondary monitoring system provides an important technical support for the construction of the digital twin model of the protection system. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Layers** & **Main Function** \\ \hline Physical device layer & Constitute all physical devices of the protection system \\ \hline Data transmission layer & Carrying and transmission of bidirectional information data flow \\ \hline Data-processing layer & Processing of acquired secondary equipment operation data \\ \hline Twin model layer & Implementation of a specific algorithm for state evaluation and fault prediction of the protection system \\ \hline Application management layer & Decision-making and processing of algorithmic results, and assisting the operation and maintenance personnel to master the equipment situation \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main functions of each layer. Figure 1: Digital Twin Architecture and Working Principle of Protection System. Data-processing layer. Considering the actual operation of the secondary monitoring system, the data to be stored and managed in the database is massive after key information, such as protection action events, device status and monitoring data, pressure plate status, recording files, and fixed values are reported in real time. It is difficult to meet the functions of real-time rapid state evaluation and fault prediction by relying on traditional data management, so data mining, effective data selection and optimal data set selection are needed. 4. Twin model layer. The twin model layer is the core of the digital twin architecture.