## 1 Introduction
DT technology relies on digital carriers to grasp the operation of physical entities in real time by constructing a map of physical entities in virtual spaces [12]. In response to the problem of insufficient application of dynamic indicators in the status evaluation algorithm, the fast-sensing capability and data panoramic display function of Digital Twin can effectively capture various types of self-test and alarm information in equipment operation. To mitigate the problem of the hysteretic application of real-time operation information of the protection system, digital twin technology can digitize the obtained operation information into characteristic quantities and build digital models to grasp and predict the status of equipment in real time. After the state analysis and maintenance decision of the equipment, the real-time bi-direction data flow of Digital Twin can transmit the equipment status rating or maintenance results to help the digital model to make improvements. The data resource center of Digital Twin improves itself through the feedback of results, so it can continuously update the fault inference knowledge base and improve the inference rules during operation. The Digital Shadow can automatically obtain information about the physical object to simulate the real-time state of the entity, which is a one-way process. Digital Twin can realize two-way data interactions between the entity and the digital model in the virtual space, and the virtual model can pass commands and information back to the entity to achieve mutual control. The literature [13] develops a digital twin reference architecture consisting of Digital Twin Layers, App Layers, Cyber Layers, Digital Layers, Communication Layers, and Physical Layers in the context of Industry 4.0 and applies it to industrial cases. The literature [14] investigated the architecture, applications, and challenges in the implementation of digital twin technology with IoT capabilities. The literature [15] presents the challenges, applications and enabling technologies for AI, IoT and digital twin technology. The application of digital twin technology in power grids mostly remains theoretical at this stage. The literature [16; 17] explored the application of digital twin technology in the online analysis of power grids in terms of the structural system and operation mode. The literature [18; 19] analyzed from a feasibility point of view whether communication and computer technologies, such as communication, memory, and response speed were sufficiently complete to serve as technical support when digital twin technology was applied to power grids. Especially at the device management level, digital twin technology can realize a complete set of data collection, model establishment, and a model application system [20]. The literature [21] introduced a labor economics-based framework to achieve the satisfaction of the various interacting system entities, which provided a reference for framework studies. The literature [22] used digital twin technology as the basis for information interaction between distribution automation terminal devices and distribution master stations, predicting faults, and assisting distribution network operation and maintenance. In response to the problems mentioned above regarding the lack of application of digital twin technology in smart grids and the practical application of protection system state evaluation, this paper explores the application of digital twin technology in intelligent substation protection systems, and innovatively develops a five-layer architecture for digital twin protection systems. This provides a new path for the application of digital twin technology in intelligent substations. A workflow framework for protection system state evaluation and fault prediction is proposed, and a twin model of the protection system is constructed in the form of characteristic quantity sequence with the help of the bi-directional interactive data flow of the digital twin.