Abstract: Fieldbus is a system developed in recent years for bidirectional serial multi-point digital communication between microcomputer-based measuring devices in production environments. This paper introduces the structural characteristics and technical performance of fieldbus systems, as well as the specific methods and application prospects of using fieldbus to construct scheduling automation systems. Keywords: Fieldbus; Monitoring; Scheduling; Automation 1. Structure of Fieldbus Systems Fieldbus is a bidirectional digital communication protocol for interconnecting field automation devices and their control systems. A fieldbus system can be viewed as a distributed system composed of digital communication devices and monitoring devices. In fact, a fieldbus is a computer network, where each node is an intelligent device, as shown in Figure 1. [IMG=Schematic diagram of fieldbus control system architecture]/uploadpic/THESIS/2008/1/2008010809290281734Q.jpg[/IMG] Figure 1 Schematic diagram of fieldbus control system architecture 1.1 Network Communication To support information transmission between devices, the Fieldbus Foundation system architecture defines a three-layer communication system, including: (1) Physical layer defines how signals are sent; (2) Data link layer defines how the network between devices is shared and scheduled; (3) Application layer defines the message format. 1.2 Functional Module Definition The general format of the three major elements: parameters, algorithms, and event responses. 1.3 Object Dictionary and Device Description To support the standardization of functional modules, the Fieldbus Foundation defines two tools—object dictionary and device description language—for defining and describing network-visible objects of application processes. 1.4 Network Management The Fieldbus Foundation system architecture defines a network agent for each device. The network agent can provide configuration management, function management, and error management capabilities. 1.5 The system management kernel performs the following functions: allocating device addresses, scheduling the execution of functional modules, and clock synchronization. 2. Technical Characteristics of Fieldbus Fieldbus technology is the result of the development of 3C (Computer, Control, Communication) technology from the control layer to the process equipment field. The characteristics of fieldbus technology are as follows: 2.1 Fully Digital Communication Traditional distributed control systems (DCS) are "semi-digital" systems, with many I/O modules receiving or sending 4-20 mA analog signals; while fieldbus control systems (FCS) are "purely digital" systems, with fully digital signal transmission, improving signal reliability and accuracy, and enabling error detection and correction functions. 2.2 Interoperability and Interoperability Interoperability refers to the ability to transmit and communicate information between interconnected devices and systems, while interoperability means that devices with similar performance from different manufacturers can be interchanged. 2.3 Complete Decentralization DCS is a "semi-decentralized" system, using one or more "control units" to control multiple loops. Its control components are distributed across several control boards, each with multiple loops. Fieldbus adopts a completely distributed architecture, with all control units distributed across the field. Control loops are implemented by field devices, while fieldbus also allows for operation and adjustment via digital communication in the control room. 3. Application of Fieldbus in Dispatch Automation With the continuous development of the power grid, the amount of information requiring monitoring and control is rapidly increasing. Timely and accurate assessment of the power grid's operating status to ensure its safe and economical operation is a pressing concern for leaders at all levels. The dispatch automation system includes the plant/station end system, the dispatch end system, and the communication system. 3.1 Plant/Station End System Composition The plant/station monitoring and control system consists of front-end intelligent units, industrial control computers, network communication management adapter cards, dedicated modems, and corresponding software. Its system structure is shown in Figure 2. [IMG=Hardware Structure Diagram of Plant/Site Fieldbus Monitoring and Control System]/uploadpic/THESIS/2008/1/20080108092907808616.jpg[/IMG] Figure 2 Hardware Structure Diagram of Plant/Site Fieldbus Monitoring and Control System The front-end intelligent unit performs the following tasks: on the one hand, it completes the measurement of electrical parameters such as current (A phase, B phase, C phase), voltage (A phase, B phase, C phase voltage, AB phase voltage, BC phase voltage, CA phase voltage), three-phase/single-phase active power, three-phase/single-phase reactive power, three-phase/single-phase positive active energy, three-phase/single-phase negative active energy, three-phase/single-phase positive reactive energy, three-phase/single-phase negative reactive energy, three-phase/single-phase power factor, and frequency of each line; on the other hand, it also completes the detection and control of the status of each switch. The front-end intelligent unit adopts an Intel R80C196KC 16-bit system architecture, mainly including acquisition and control logic, analog signal processing, digital input/output processing, communication processing, system monitoring and protection, real-time clock, and power supply. To improve system reliability and anti-interference capabilities, all digital logic processing is integrated into a single high-density, ultra-large-scale programmable logic array. To ensure reliable operation of the front-end intelligent unit in harsh electromagnetic environments, in addition to perfecting circuit design, component selection, wiring layout, and shielding isolation as much as possible, the operating status of the front-end intelligent unit should be monitored at all times to ensure no misoperation or data loss in the event of unexpected faults. It should be noted that the front-end intelligent unit of this system can be installed in an outdoor terminal box (right below the PT and CT). The substation-side industrial control computer is used to display the main electrical wiring and its operating status, record various data, display various curves and charts, provide alarms for abnormalities or accidents, and forward and receive electronic information. Generally, a single industrial control computer is sufficient for all the above tasks in a substation. In power plants, additional industrial control computers can be added as needed for monitoring points. The network communication management adapter card is inserted into the expansion slot of the industrial control computer to realize high-speed data exchange between the front-end intelligent unit and the industrial control computer, and to manage the network. The modem in this system is a dedicated modem for the power system. It completes the data communication between the substation and the dispatching terminal, and its data transmission format strictly follows the 91CDT protocol. 3.2 Dispatch Terminal System Composition The dispatching terminal monitors, controls and manages the entire system. It consists of an industrial control computer, a modem, a multi-channel communication interface device, and a large-screen display. Its system structure is shown in Figure 3. [IMG=Dispatch Terminal System Structure Diagram]/uploadpic/THESIS/2008/1/2008010809291463744B.jpg[/IMG] Figure 3 Dispatch Terminal System Structure In Figure 3, the modem receives the modulated signal from the carrier unit. At the same time, the commands issued by the dispatching terminal can be modulated by the modem and sent to the carrier channel, and then sent to the substation. The multi-channel communication interface completes the data communication between multiple digital signals and the industrial control computer at the substation. 3.3 Data exchange between the power plant and the dispatching terminal is transmitted through a data channel. Currently, the power system has carrier channels, microwave channels, fiber optic channels, etc. For carrier channels and microwave channels, a dedicated modem of the power system is required at both ends of the channel. According to the author's experience, the modems at both ends of the same channel should be from the same manufacturer and of the same model. 4 Advantages of dispatching automation based on basic fieldbus The use of a real-time monitoring and control system for power plants and substations based on fieldbus, and the dispatching automation system formed therefrom, can avoid the use of a large number of cables to transmit the secondary electrical parameters of PT and CT to the central control room, reduce the error caused by line loss, improve measurement accuracy, and replace the central control room, thus saving land area and infrastructure investment. Compared with the traditional substation monitoring and control system, this system has the following characteristics: (1) The front-end intelligent unit can be installed in the CT or PT terminal box to process the collected data locally and transmit it digitally. (2) The network is composed of two twisted-pair cables, with low failure rate and high communication baud rate. (3) It solves the voltage drop problem between CT/PT and the central control room in traditional monitoring systems, and has high measurement accuracy. (4) It has strong anti-interference ability and can operate under harsh outdoor conditions. (5) It has a simple structure, small size, and is easy to install and maintain. 5 Conclusion With the continuous expansion of the power grid and the rapid development of computer measurement and control technology, the unmanned and minimally staffed substation model has been promoted in the national power system. In particular, for the transformation of urban and rural power grids, a large number of monitoring, control and protection equipment for electrical parameters can be placed at (near) outdoor terminal boxes, without the need to build a separate control room. Decentralizing monitoring and protection equipment is a trend in power grid development and is also the current model in developed countries such as the United Kingdom and the United States. By reducing the land area of ​​substations and saving infrastructure investment and cable costs, the transformation cost will be greatly reduced. In addition, the direct result of completely abandoning long-distance measurement signal cables is a significant improvement in measurement accuracy, anti-interference ability and reliability. The fieldbus-based monitoring and control system will be widely used in power plants and substations of the power system, and together with data channels and dispatch terminal monitoring systems, it will constitute a new generation of dispatch automation system. References: [1] Chen Hong, Tang Hongru, Zeng Wei. Technical research on fieldbus-type intelligent instruments in substations. China Electric Power, 1997, 8. [2] Pang Yanbin, Li Lingqi, Cheng Wei, Zhao Enping. Network integration of fieldbus and DCS. Measurement and Control Technology, 1999, 9.