## 1 Introduction  
With the rapid development of China's smart grid, intelligent substations use important secondary equipment such as merging units, intelligent terminals, and intelligent protection devices, etc. Intelligent substations can realize the digitalization of station-wide information, the networking of communication platform and the standardization of information sharing. The safe and stable operation of the current power grid is highly dependent on secondary systems such as protection, safety control, deregulation/load shedding, automation, and communication. Among them, protection systems, i.e., protection devices, merging units, intelligent terminals, and process layer networks, are the key to the safe operation of smart grids [1]. If a fault occurs in a certain section, it will lead to incorrect actions of protection, circuit breakers, etc., which in turn may lead to the expansion of the fault range and cause more damage. Therefore, it is important to grasp the real-time operation of the protection system and the operation trend in the future period, to enable the accurate assessment of equipment state and fault prediction, to ensure the timely detection of safety hazards, and to conduct effective targeted and preventive condition maintenance to ensure the safe and stable operation of the grid [2]. Most of the current research on protection system state monitoring and evaluation starts from the level of intelligent substations [3; 4; 5], and the state evaluation of secondary equipment is realized based on the monitoring information of intelligent stations [6; 7]. In the research on state evaluation related to protection systems, part of it focuses on the evaluation method research, such as the comprehensive fuzzy evaluation method combining the expert method and GI method [8], KPI-based relay protection operation evaluation index system [9], etc. Part of the research looks at relay protection reliability evaluation indexes as the primary object of its research, such as the misoperation rate, the rejection rate, the disturbance degree, completeness and other serviceability indexes [10]. Increasing attention is paid to economic indexes such as the protection reliability economic coefficient and the average annual economic loss in relay protection evaluation [11]. However, in practical application, there are still some difficulties and problems:  
1. The application of real-time operation information of protection systems, such as the operation event record (SOE), self-test information and numerous alarm information still remains in the diagnosis and location of equipment post-fault, and cannot evaluate the real-time state of characterized equipment, fault prediction and thus conduct preventive maintenance. 2. There are many types of secondary equipment faults, and there is no clear fault criterion, and there is no obvious correlation between fault characteristics; often a fault characteristic corresponds to multiple fault-types. Therefore, it is necessary to rely on a sufficiently rich historical database to summarize a reliable fault prediction inference knowledge base and complete inference rules. 3. In today's relay protection state evaluation, more static characteristic quantities such as serviceability indexes and economic indexes are used for evaluation, while less dynamic indexes such as operation information and alarm information are used, resulting in the evaluation results focusing on the management and regular maintenance of equipment, while it is difficult to grasp the equipment operation state in real time. In order to mitigate the problems and limitations described above, a system architecture based on digital twin technology could be introduced as a solution. Digital Twin (DT) is a simulation process that integrates multiple disciplines, physical quantities and dimensions, and is compatible with the current popular technologies such as intelligent sensors, big data, artificial intelligence, and so on.