0 Introduction In rural areas of China, the power infrastructure is generally underdeveloped, which seriously affects the momentum of economic development and restricts the further development of the national economy. Accelerating the technological transformation of rural power grids is a necessary condition for achieving the goal of equal pricing for urban and rural power grids. Rural power grids have numerous substations, and their distribution equipment is outdated, with many technologies dating back more than 20 years, no longer meeting the requirements of power system development. Emerging integrated automation systems for substations based on microcomputer technology have become relatively mature technologies after nearly 10 years of development and application in China, and are being used more and more widely. How to effectively construct a high-efficiency, low-cost microcomputer integrated automation system that conforms to the design specifications of the power industry and reflects the level of technological development is a topic worthy of discussion. [b]1. Structurally Distributed Substation Integrated Automation System[/b] With the development of technology, the bottleneck of processor processing power that previously affected high functional integration has been alleviated. More and more substations have adopted a model that combines RTU functions and protection functions into one, typically a structurally distributed substation integrated automation system. The distributed structure system adopts an object-oriented automation equipment construction model. Specialized automation equipment (AIOs or AIO devices) is manufactured to monitor the real-time operation status of different power facilities, based on their characteristics and requirements. Different AIOs are connected via a serial data bus to form a complete integrated automation system. Functionally, the different AIO devices have no electrical connection other than communication with each other. Therefore, the distributed structure automation system has flexible scalability, reliability, and maintainability, representing an important direction for the development of integrated power plant automation systems. 1.1 System Design Concept and Technical Performance 1.1.1 Characteristics and Requirements of Internal System Communication Because substations are environments with strong electromagnetic interference, including power supply lightning strikes and ground potential difference interference, the communication environment is harsh. Since all equipment in the substation operates continuously, changes in parameters during accidents need to be quickly reflected in the dispatching system. Therefore, the internal communication of the integrated substation automation system should have the following characteristics and requirements. (1) Requirements for substation communication networks: a. Fast real-time response capability. Power industry standards have strict real-time performance indicators for system data transmission, so the network must have sufficient speed to ensure data real-time performance; b. High reliability; c. Excellent electromagnetic compatibility performance; d. Hierarchical structure. This is determined by the hierarchical distributed structure of the entire system. (2) Requirements for information transmission response speed: Different types and characteristics of information require different transmission times, which are specified in detail in the "Technical Specification for Design of Power System Dispatch Automation". Therefore, the network should be able to determine the priority of various types of data transmission. (3) Requirements for information transmission time between and within each layer The “Technical Specification for Design of Power System Dispatch Automation” stipulates that the minimum time for information to be transmitted to the dispatching master station is 1-2 seconds. Therefore, the maximum transmission time within the substation cannot exceed 1 second. The specific transmission time allocation requirements are as follows: a. Equipment layer and bay layer 1-100ms; b. Each module within the bay 1-100ms; c. Between each bay unit in the bay layer 1-100ms; d. Between the bay layer and the substation layer 10-1000ms; e. Between each device in the substation ≥1000ms; f. Between the substation and the dispatching control center ≥1000ms. 1.1.2 Implementation of field communication (1) For substations with simple structures, field communication is mostly point-to-multipoint mode. Communication based on RS-485 bus is a commonly used method, applied to serial communication between a single CPU and multiple CPUs. For situations involving more than 32 AIO devices or requiring further improvement in data access speed, multiple master-slave communication subnets can be constructed, with each subnet managing a portion of the AIO devices. Then, the communication master stations of different subnets are connected using parallel communication technology to form a complete automation system. (2) For complex internal communication in substations, Lonworks fieldbus can be used. Fieldbus is based on microcomputer-based intelligent field instruments, realizing a fully distributed, fully digital, intelligent, bidirectional, multi-variable, multi-point, and multi-station communication network between field instruments and control systems and control rooms. It provides network services according to the International Organization for Standardization (ISO) and the Open Systems Interconnection (OSI), with high reliability, good stability, strong anti-interference ability, fast communication speed, low cost, and low maintenance cost. Due to the characteristics of Lonworks, it is more suitable for large-scale substation integrated automation systems. Each AIO device has two relatively independent functions, namely RTU function and protection function. AIO devices are classified according to their application objects (power facilities). The RTU and protection functions of AIO devices vary depending on the requirements of the application objects. To better construct a comprehensive power plant automation system based on AIO devices, some technical factors should be given priority when designing AIO devices. 1.1.3 System Time The power plant real-time monitoring system based on AIO devices has given priority to time function: the maximum time difference between all AIO devices in operation (excluding the moment of startup) does not exceed 1ms. The time-based functions are mainly reflected in: (1) Time stamp function: all real-time data, including analog power data, switch status information and protection action records, are accompanied by time stamp information. The time stamp error of real-time data within the same AIO device does not exceed 1ms; the time stamp error of real-time data within the same system (referring to AIO devices sharing the same communication channel) does not exceed 2ms; (2) Time setting function: AIO devices support the scheduled event function, that is, the function of specifying the expected event to occur backward, which lays the foundation for implementing synchronous measurement and other functions of the system. 1.1.4 Protection function It can construct a complete protection scheme for power plants of 35kV and below, and meet the safety requirements of the primary equipment of the power plant. The main protection functions include: two-stage proportional braking characteristic transformer differential main protection; current-type transformer backup protection with composite voltage start-up; three-stage current line protection with direction selection and voltage interlocking, and three-phase primary automatic reclosing function and accelerated tripping function with no-voltage or synchronization detection functions; for fast-acting protection, the action time is ≤30ms; for protection with delay, the delay error is ≤±30ms or 0.3%, the relative error of the amplitude of the protection action is ≤±5%, and the margin of the action return coefficient (compared to 1) is not less than 5%; based on the above three types of protection, a full range of supporting protection equipment for medium and low voltage substations will be developed. 1.1.5 Measurement Function Measurement data bound to time information can be transmitted to the host computer through communication means. The transmission rate depends on the supported communication protocol, but the AIO device should be able to support a data access cycle of at least 0.2s. The measured data includes: providing 3-phase 4-voltage measurement function with a measurement accuracy of 0.2 class; providing 3-phase 4-current measurement function with a measurement accuracy of 0.2 class; providing one frequency measurement function with a measurement accuracy of ±0.01Hz; providing three-phase four-wire active/reactive power measurement and power accumulation function with a measurement accuracy of 0.5 class; and providing 1-2 channels of isolated direct electrical quantity measurement function with a measurement accuracy of 0.2 class. 1.1.6 Remote Signaling Function The remote signaling quantities of the power station are divided according to the main equipment object of the power station. The corresponding protection measurement AIO equipment is responsible for monitoring and detection, and transmission, collection, and summarization are performed through a serial bus. The response time of the automatic device to the remote signaling quantities should meet the requirements of the power industry standards. Due to the limitation of data transmission rate of the transmission means, we classify all remote signaling quantities into two categories according to their importance: switching quantities and status quantities, defining different response times and monitoring methods for each, in order to ensure that the system has both a sufficiently fast response time and comprehensive monitoring functions. Each AIO series device is equipped with 16 remote signaling monitoring channels, of which 6 are defined as switching quantities and 10 are defined as status quantities; among the 10 status quantities, 4 can be defined as pulse quantities to connect to power pulse devices. For equipment such as transformers that are equipped with both main protection and backup protection, there can be 32 remote signaling monitoring channels. 1.1.7 Remote Control and Adjustment Functions All AIO series devices support 8 independent output controls, of which 4 are used to support the closing/opening operation of DL, and the other 4 are reserved for backup, which can be considered for implementing remote control and adjustment functions. 1.1.8 Other related communication functions (1) Parameter setting function: Provides communication setting function for all field data; (2) Data sampling function: Provides data sampling function for any logical channel starting at any backward specified time (time resolution not less than 2ms) for 10 cycles, with a sampling interval not less than 24 points/cycle; (3) The data storage of power failure protection can save at least the 4 most recent accident records for query; 1.1.9 Communication protocol As an example, the data packet format of serial bus data communication in a substation integrated automation system can be defined as follows. [img=345,81]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hndl/2002-4/51-1.jpg[/img] DA Command Type Information Length Text Checksum Substation: SA Command Type Information Length Type Status Text Checksum DA/SA is the destination/source address number, ranging from 1 to 32. Additionally, 99H is defined as the broadcast address number, used for common operations such as system time synchronization; the command type can be one of FOH to FFH; the information length is the frame byte length minus 2, i.e., the data length excluding the station number and checksum byte; the text is the valid information part being transmitted; some commands may have no text; the type is the type of substation AIO device calibrated by SA; the status reflects the operating status of the substation AIO device calibrated by DA/SA, and can be used by the master station to determine subsequent communication strategies. 1.2 Substation Integrated Automation System Based on AIO Devices This system can be constructed in a centralized panel configuration, where protection/measurement AIO devices for different power facilities (referred to as objects) are placed in one or several panel cabinets; or it can be constructed in a separate arrangement, where AIO devices for different objects are arranged near the objects according to the principle of proximity. AIO devices are connected via a serial data communication bus. 1.2.1 AIO Device Design Schematic Diagram (see Figure 1). [img=357,181]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hndl/2002-4/51-2.jpg[/img] 1.2.2 Composition of the Substation Integrated Automation System Based on AIO Devices (see Figure 2). Due to the characteristics of transformer protection and monitoring, at least two AIO devices (referred to as device A and device B) are required for the measurement and protection of a double-winding transformer. One device is used for the main protection of the transformer (i.e., differential protection, device A), which completes the measurement of electrical quantities on the high-voltage side of the transformer and performs related control such as high-voltage side DL; the other device is used for the backup protection of the transformer (device B), which completes the measurement of electrical quantities on the low-voltage side of the transformer and performs related control such as low-voltage side DL. 1.2.3 Microcomputer Protection Measurement Scheme for Double-Coil Transformer Based on AIO Device [img=360,187]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hndl/2002-4/51-3.jpg[/img] Since different types of AIO devices are only designed with one complete DL control output circuit, neither the main protection AIO device nor the backup protection AIO device can simultaneously and directly control the two DLs on the high/low voltage side, while the protection action requires simultaneously tripping the two DLs on the high/low voltage side. To solve this contradiction, both the main protection AIO device and the backup protection AIO device (all AIO devices) are equipped with two dedicated auxiliary input and output channels each, which quickly and automatically coordinate to trip the two DL switches on the high voltage side and the low voltage side through a principle similar to manual tripping. Specifically, for substation integrated automation schemes requiring double tripping, when the main protection device A operates, it not only trips the high-voltage side's DL (Diverterless Detection and Disconnection), but also activates a specific input of device B through a backup output contact, thereby tripping the low-voltage side's DL; the reverse is also true. 2. Conclusion: The distributed substation integrated automation model is technically feasible, structurally simple and reliable, and flexible in its construction scheme. Furthermore, the AIO (Automated Identification and Control) equipment automation system has advantages such as good scalability and high maintainability. With the further development of computer technology and communication technologies such as Wide Area Networks (WAN), Fiber Distributed Data Interface (FDDI), Integrated Services Data Network (ISDN), and Frame Relay, the distributed substation integrated automation scheme will be increasingly widely and deeply applied, representing a very important development direction for substation integrated automation. **References** 1. Liu Jian, Ni Jianli, Deng Yonghui. Distribution Automation System. China Water Resources and Electric Power Press, January 1999. 2. General Technical Conditions for Remote Terminal Blocks. GB/T 13729-92. 3. Technical Conditions for AC Sampling Remote Terminal Blocks. DL/T630.1997. 4. Compilation of National Standards for Electronic Product Design Specifications. China Standards Press.