Abstract In the field of substation automation, the development of intelligent electrical systems, especially the emergence of mechatronic equipment such as intelligent switches and photoelectric current and voltage transformers, is ushering in a new digital stage for substation automation technology. This paper discusses the characteristics, structure, and functional division of digital substation automation systems. Keywords Substation automation, digitalization, intelligentization Substation automation technology has reached a certain level after more than ten years of development. In the transformation and construction of urban and rural power grids in China, not only have medium and low voltage substations adopted automation technology to achieve unmanned operation, but also a large number of new automation technologies have been adopted in the construction of ultra-high voltage substations of 220kV and above. This has greatly improved the modernization level of power grid construction, enhanced the possibility of power transmission and distribution and grid dispatch, and reduced the total cost of substation construction, which has become an indisputable fact. However, technological development is endless. With the increasing maturity of technologies such as intelligent switches, photoelectric current and voltage transformers, online status monitoring of primary operating equipment, and substation operation training simulation, as well as the development and application of high-speed computer networks in real-time systems, there will inevitably be a profound impact on existing substation automation technology, and fully digital substation automation systems are about to emerge. 1. Characteristics of Digital Substation Automation Systems 1.1 Intelligent Primary Equipment The signal circuits and controllable operation drive circuits of primary equipment are designed using microprocessors and optoelectronic technology, simplifying the structure of conventional electromechanical relays and control circuits. Digital programmable controllers and digital common signal networks replace traditional wire connections. In other words, conventional relays and their logic circuits in the substation secondary circuits are replaced by programmable logic controllers, and conventional high-voltage analog signals and control cables are replaced by optoelectronic digital signals and optical fibers. 1.2 Networked Secondary Equipment Conventional secondary equipment in the substation, such as relay protection devices, anti-misoperation interlocking devices, measurement and control devices, remote control devices, fault recording devices, voltage and reactive power control, synchronizing operation devices, and the developing online status monitoring devices, are all designed and manufactured based on standardized, modular microprocessors. All connections between devices utilize high-speed network communication. Secondary equipment no longer has redundant I/O field interfaces found in conventional functional devices. Through the network, true data sharing and resource sharing are achieved, and conventional functional devices become logical functional modules. 1.3 Automated Operation Management System The automated operation management system for substations should include paperless statistical recording of power production operation data and status records; automated hierarchical and distributed data exchange; immediate fault analysis reports to identify the causes of faults and propose fault handling suggestions when substation malfunctions occur; and automatic issuance of substation equipment maintenance reports, transforming conventional "periodic maintenance" into "condition-based maintenance." 2. Structure of Digital Substation Automation System In the field of substation automation, the development of intelligent electrical systems, especially the emergence of intelligent switches, photoelectric transformers, and mechatronic equipment, has ushered in a new digital stage for substation automation technology. In high-voltage and ultra-high-voltage substations, the I/O units of protection devices, measurement and control devices, fault recording devices, and other automatic devices, such as A/D converters, optical isolation devices, and control operation circuits, will be separated and integrated as part of the intelligent primary equipment. Conversely, the digital sensors and digital control circuits of intelligent primary equipment replace the I/O parts of conventional relay protection devices, measurement and control devices, etc.; while in medium and low voltage substations, the protection and monitoring devices are miniaturized and compacted, and installed completely on the switch cabinet, realizing the electromechanical integration design of the substation. The structure of the digital substation automation system can be divided into two categories in terms of physical structure, namely intelligent primary equipment and networked secondary equipment; in terms of logical structure, it can be divided into three levels. According to the definition of the IEC6185A communication protocol draft, these three levels are respectively called "process layer", "bay layer" and "station control layer". 2.1 Process layer The process layer is the interface between primary equipment and secondary equipment, or the process layer refers to the intelligent part of intelligent electrical equipment. The main functions of the process layer are divided into three categories: (1) real-time electrical quantity detection of power operation; (2) status parameter detection of operating equipment; (3) operation control execution and drive. (1) Real-time electrical quantity detection of power operation. Similar to the traditional functions, it mainly involves the detection of current, voltage, phase and harmonic components. Other electrical quantities such as active power, reactive power and electrical energy can be calculated by the equipment in the bay layer. The difference from the conventional method is that the traditional electromagnetic current transformer and voltage transformer are replaced by photoelectric current transformer and photoelectric voltage transformer; the traditional analog quantity acquisition is replaced by the direct acquisition of digital quantity. The advantages of this are strong anti-interference performance, good insulation and anti-saturation characteristics, and miniaturization and compactness of the switching device. (2) Online detection and statistics of the status parameters of the operating equipment. The equipment in the substation that needs to be monitored for status parameters mainly includes transformers, circuit breakers, disconnectors, busbars, capacitors, reactors and DC power supply systems. The online detection content mainly includes data such as temperature, pressure, density, insulation, mechanical characteristics and working status. (3) Execution and driving of operation control. The execution and driving of operation control includes transformer tap adjustment control, capacitor and reactor switching control, circuit breaker and disconnector opening and closing control, and DC power supply charging and discharging control. The execution and driving of process layer control is mostly passive, that is, it acts according to the control instructions of the upper layer, such as receiving the trip command of the bay layer protection device, the switching command of voltage reactive power control, and the remote control opening and closing command of the circuit breaker. When executing control commands, it is intelligent, can distinguish the authenticity and rationality of commands, and can control the precision of the upcoming actions. It can enable the circuit breaker to close in a fixed phase and open in a selected phase. It can achieve the closing and opening of the circuit breaker at the selected phase angle, and requires the operation time to be limited within the specified parameters. For example, the synchronous operation of vacuum switches requires that the switch contacts close when there is zero voltage and open when there is zero current. 2.2 The main functions of the bay layer equipment are: (1) to summarize the real-time data information of the process layer of this bay; (2) to implement the protection control function of primary equipment; (3) to implement the operation interlocking function of this bay; (4) to implement the operation synchronization and other control functions; (5) to have priority control for data acquisition, statistical calculation and control command issuance; (6) to have the communication function of connecting the upper and lower layers, that is, to simultaneously and at high speed complete the network communication function with the process layer and the station control layer. When necessary, the upper and lower network interfaces have dual-port full-duplex mode to improve the redundancy of the information channel and ensure the reliability of network communication. 2.3 Station Control Layer The main tasks of the station control layer are: (1) to collect real-time data information of the entire station through a two-level high-speed network, continuously refresh the real-time database, and log in to the historical database on time; (2) to send relevant data information to the dispatch or control center according to the established protocol; (3) to receive relevant control commands from the dispatch or control center and transfer them to the bay layer and process layer for execution; (4) to have online programmable whole-station operation interlocking control function; (5) to have (or be equipped with) local monitoring and human-machine communication functions within the station, such as display, operation, printing, alarm, and even multimedia functions such as images and sound; (6) to have online maintenance, online configuration, and online parameter modification functions for the equipment in the bay layer and process layer; (7) to have (or be equipped with) automatic substation fault analysis and operation training functions. 3. Network Selection in Digital Substation Automation System The network system is the lifeline of the digital substation automation system. Its reliability and the speed of information transmission determine the availability of the system. In conventional substation automation systems, the information acquisition and protection algorithm operation of a single protection device are generally carried out under the control of the same CPU. This makes the entire process of synchronous sampling, A/D conversion, calculation, and output of control commands fast and simple. However, in a fully digital system, the information sampling, protection algorithm formation, and control command generation are completed collaboratively by multiple CPUs on the network. Controlling the synchronization of sampling and the rapid output of protection commands is a complex problem. The most basic condition is network adaptability, and the key technologies are improving network communication speed and developing suitable communication protocols. Using conventional fieldbus technology may not meet the technical requirements of digital substation automation. Currently, Ethernet has emerged as a powerful force and has entered the field of industrial automation process control. Embedded Ethernet control and interface chips with a fixed OSI seven-layer protocol and a speed of up to 100MHz are widely available. It is feasible for both levels of the digital substation automation system to adopt 100MHz Ethernet technology. 4. Major Problems in the Development of Digital Substation Automation Systems Research on digital substation automation systems is developing gradually from the bottom up across the three levels. The current research focuses on the process layer, such as the research and development of technologies and equipment like intelligent switching equipment, photoelectric transformers, and condition detection. Foreign countries have already accumulated some mature experience, and domestic universities, research institutes, and relevant manufacturers have invested considerable manpower in research and development, achieving substantial progress in some areas. However, the main problems currently are: (1) the need to strengthen professional collaboration during research and development; for example, the research on intelligent electrical appliances requires collaboration among mechanical, electrical, and optical professionals; (2) deficiencies and improvements in materials and devices; and (3) weak links in testing equipment, testing methods, inspection standards, especially EMC (electromagnetic interference and compatibility) control and testing. 5. Conclusion This paper discusses the characteristics, structure, and development of a digital substation integrated automation system. Digital substation automation is a systematic project; to achieve full digital substation automation functionality, many technical problems still need to be addressed. The author believes that in the near future, digital substation automation systems will experience a period of vigorous development.