Abstract: This paper introduces the characteristics and functions of fieldbus technology. Combining the characteristics and requirements of hydropower plant unit control, it discusses the hardware and software design of a hydropower plant unit control system based on the Profibus-DP fieldbus. Keywords: Fieldbus; Profibus-DP; Hydropower plant; PLC; Unit control 1 Fieldbus Profibus-DP and its main characteristics Fieldbus is a serial, digital, multi-point, bidirectional communication data bus installed in the production process area between field devices, instruments, and automatic control devices and systems in the control room. In other words, fieldbus is a network system and control system that uses individual, distributed, digital, and intelligent measurement and control devices as network nodes, connected by a bus to exchange information and jointly complete automatic control functions. Fieldbus control system (FCS) is a new generation of industrial automation control system characterized by information digitization, decentralized control, and open interoperability. It represents the development of information technology, intelligence, digitization, and networking towards the field. The essence of FCS is the field-based processing of information, possessing openness, interoperability, interchangeability, integrability, high reliability, ease of maintenance and management, reduced investment, reduced operating costs, and enhanced field-level information integration capabilities. It overcomes the shortcomings of traditional industrial process control systems, such as high investment, low transmission accuracy and anti-interference performance, closed systems, poor integrability, and difficulty in installation, maintenance, and management. Fieldbus is one of the key devices in FCS, with Profibus (Process Fieldbus) and FF (Foundation Fieldbus) being the most representative. Profibus consists of three compatible parts: Profibus-DP, Profibus-PA, and Profibus-FMS. Profibus-DP (Decentralized Periphery) is an optimized, high-speed, and inexpensive communication connection designed specifically for communication between automated control systems and distributed I/O device levels. It can replace 24VDC or 4-20mA signal transmission. Profibus-DP uses a physical layer, data link layers (layers 1 and 2), and a user interface for high-speed data transmission at the field layer. The master station periodically reads input information from the slave station and periodically sends output information to the slave station. Furthermore, Profibus-DP provides the non-periodic communication required by intelligent field devices for configuration, diagnostics, alarm handling, and determination of parameters for complex equipment during operation. The basic functions and features of Profibus-DP are as follows: (1) Long-distance high-speed communication: baud rate from 9.6Kbit/s to 12Mbit/s; maximum distance of 100m at 12Mbps and 200m at 1.5Mbps, which can be extended by repeaters. (2) Distributed structure: token passing between master stations and master-slave transmission between master and slave stations. Each segment can have up to 32 stations, and the segments can be connected by connectors, up to 126 stations. (3) Easy to install: because RS-485 transmission technology is simple, the laying of twisted-pair cables does not require professional knowledge. The bus structure makes the installation and unloading of one station not affect the normal operation of other stations. The system installation can also be carried out separately, and the later installed system will not hinder the earlier installed system. (4) Diagnostic function: the extended diagnostic function of Profibus-DP can quickly locate faults. Diagnostic information is transmitted on the bus and collected by the master station. (5) Open communication network. (6) Reliability and protection mechanism. Each system includes three types of equipment: (1) Level 1 DP master station, i.e., central controller (such as PC, PLC), which can exchange information with other stations. (2) Level 2 DP master station, which refers to configuration equipment, programmer, or operation panel, to complete data reading and writing, system configuration, fault diagnosis, etc. of each station. (3) DP slave station, which refers to peripheral equipment (such as PLC, distributed I/O, driver, sensor, actuator, etc.) for collecting and sending input and output information. 2 Advantages of Profibus-DP fieldbus for hydropower plant unit control In recent years, although the development of computer monitoring system for hydropower plants in China has been relatively fast, due to the weak foundation, the functions implemented are still relatively simple, far behind those of developed countries. In particular, hydropower stations with few or no staff are only in the initial stage, far behind countries such as the United States and France that have basically achieved unmanned operation. With the development of field control technology, the importance of fieldbus has been increasingly valued, fieldbus products are increasing, and fieldbus control systems are increasingly being applied in the field of automatic control. Applying fieldbus technology to hydropower plant control can greatly improve the automation level of hydropower plant production processes, gradually narrow the gap between my country's hydropower plant monitoring technology and that of foreign countries, and thus catch up with advanced countries. Specifically, the application of fieldbus to hydropower plant unit control has the following advantages: (1) It realizes the full openness of the system. Open automation systems are an inevitable trend in the development of fieldbus; (2) It realizes true distributed control. By delegating control functions to the field, each control loop is fully implemented by distributed field intelligent instruments, enhancing the localization of system data processing; (3) It realizes fully digital communication. This greatly improves the system's anti-interference and detection accuracy; (4) It is safe, reliable, flexible in configuration, and easy to use. Fieldbus makes full use of the standard "functional modules" specified by the user layer to configure the system, making the configuration operation of control strategies very simple; (5) It has high economic benefits. The hydropower plant monitoring system composed of fieldbus shares a single data bus to transmit information, with rich bus media, saving cables and control equipment, etc. 3. Implementation of the Hydropower Plant Unit Control System The basic task of the automatic control of hydropower plant units is to realize the logical control and monitoring of unit operation and auxiliary equipment such as oil, gas, and water by means of automation devices, thereby realizing the automation of the power generation process and the overall plant automation. The unit control system we are talking about here mainly realizes the functions of automatic start-up, various operating condition switching, normal shutdown, and fault shutdown of the hydro-turbine generator unit. The basic principles of hydropower plant control system design are: (1) Advanced technology. Make full use of advanced technologies in the field of automation and the hierarchical distributed fully open structure. (2) Practicality. The main goal of functional configuration and equipment selection is to meet the actual needs of the production process. (3) Reliability. High reliability industrial control equipment should be selected, and redundancy technology and modular technology should be adopted. (4) Economy. Pay attention to the performance-price ratio of the system to achieve satisfactory returns on investment. (5) Operability. It can perform routine automatic operation or automatic operation with different operating permissions. (6) Maintainability. The system adopts a modular structure. When the equipment fails, only the corresponding module needs to be replaced. (7) High degree of automation. The overall design is based on the "unmanned operation and minimal staffing" model. (8) Good adaptability. The system should be able to adapt to the requirements of the hydropower plant site environment. (9) User-friendly human-machine interface. It conforms to the operating habits of hydropower plant operators. 3.1 Hardware composition According to the characteristics and requirements of the hydropower plant monitoring system, the system structure is designed as shown in Figure 1. Each function is independent of each other and is implemented by different parts combined with corresponding software programs. [align=center] Figure 1 System structure diagram Figure 2 Hardware composition[/align] According to the system structure diagram, we can have a variety of solutions, but through comprehensive consideration, the hardware structure is designed as shown in Figure 2. The following is a detailed description of each component: (1) The main station adopts the Siemens S7-300 series CPU315-2DP PLC as the central controller, which runs the entire control program and realizes information exchange with other stations. The secondary master station uses an Advantech industrial PC, including an engineering station and an operator station, both connected to the bus via a CP-5613DP card. The engineering station configures and programs the entire system; the operator station operates, runs, and monitors the entire system, but cannot modify configuration data or system programs, and also functions as a server on the local area network. The touchscreen uses a Siemens TP170A, connected to the MPI communication port of the S7-300 PLC via MPI, and is installed in the field to enable simple operation of the S7-300 and display of relevant operating parameters. (2) There are various forms of slave stations. The ET200M slave station IM-153 has an analog input/output module; the slave station S7-200PLC CPU224 is connected to the bus through the EM277 DP interface. The S7-200PLC can run its own program, connect I/O input/output, and exchange information with the S7-300PLC; intelligent instruments and intelligent modules are directly connected to the bus. Their CPUs work in parallel with the PLC's CPU, occupying very little time of the host, which is beneficial to improving the PLC's scanning speed and completing special functions. Of course, more slave stations can also be connected. (3) The local area network is formed by the operator station as the server through network cards, switches, and other computers. Clients can monitor the operating status, view reports, print, etc., but cannot operate the operating system. From the perspective of the entire components, the master station realizes the monitoring of the system, the slave stations realize the data acquisition and drive the corresponding actuators to complete the control actions, and the local area network, together with the operator station, realizes the information management function. Therefore, the hardware composition is consistent with the functional structure of the system. 3.2 Software Solution The software part includes hardware configuration, control program development and configuration monitoring system. (1) Hardware Configuration After the hardware is connected, the hardware configuration and network configuration are performed according to the hardware parameters. Hardware configuration is the foundation of system implementation. The so-called "hardware configuration" is to simulate the real PLC hardware system, install the CPU, power supply and signal module equipment on the corresponding rack, and set and modify the parameters of the PLC hardware module. When the user needs to modify the parameter address of the module, needs to set up network communication, needs to connect the distributed peripheral to the master station, or remove a device from the master station, hardware configuration must be performed. This system uses the SIMATIC S7-300's supporting software package STEP 7 V5.2 to complete hardware configuration, parameter setting, communication testing, system debugging, etc. (2) Control program programming software includes STEP7 for S7-300 PLC and MicroWIN32V3.1 for S7-200 PLC. It is easy to use, and online debugging is intuitive and convenient. It has three programming forms: ladder diagram (LAD), statement list (STL) and function chart (FBD), which can be switched and combined at will. First, set variables, allocate I/O addresses, and edit symbol table. Then, draw the control flow diagram according to the unit control sequence and requirements. Finally, compile the ladder diagram control program. The programming methods of S7-300 and S7-200 are very similar. They exchange data through the V area. (3) Monitoring system configuration monitoring software adopts Siemens' industrial control configuration software SIMATIC WinCC. WinCC includes functions such as a graphical designer, alarm logger, tag logger, report designer, global scripting, control center, and user management. It integrates control technology, human-machine interface technology, graphics technology, database technology, and network technology, and contains controls for real-time data reports, historical data reports, real-time curves, historical curves, and report queries, providing engineers with an excellent secondary development platform. Communication between WinCC and the S7300 PLC is also based on a physical connection via the PROFIBUS-DP bus. Using WinCC, we can easily configure a user-friendly, convenient, and powerful human-machine interface for the unit control system. The Siemens dedicated touchscreen configuration tool Protool V6.0 is used to configure the TP170A. The method is similar to WinCC, but much simpler. The touchscreen is generally placed at the control site to achieve simple control and display. 3.3 The final step in the design of the system is the operation and debugging. Two methods are used here: (1) Software simulation debugging STEP7 software itself has a simulation software component, and WinCC also has a variable simulator. It can simulate the input of some quantities and observe the program execution or the real-time situation of the monitoring screen in a timely manner. This debugging method is simple and easy to implement and can be carried out at any time. The disadvantage is that hardware-related problems cannot be found. (2) Hardware debugging Connect various sensors to the system in the laboratory, simulate the actual field situation, and debug. Most problems can be eliminated. If it is to be more perfect, it is best to conduct on-site debugging. 4 Conclusion This system adopts PROFIBUS-DP fieldbus network technology to realize distributed control. The network speed is fast, the reliability is high, the openness is good, and the anti-interference ability is strong, which brings convenience to installation, debugging and equipment maintenance, and improves production efficiency and management level. This network system has a high performance-price ratio and can be expanded to various forms and scales according to user requirements. The gradual adoption of PROFIBUS-DP technology by various hydropower plants to transform traditional monitoring systems will surely make a great contribution to China's hydropower industry. (Economic benefits of RMB 750,000) The innovation of this paper: PROFIBUS-DP fieldbus network technology has been widely used in industry, but its application in hydropower plants is still in the trial stage. In view of this situation, this paper designs a hydropower plant unit control system based on PROFIBUS-DP fieldbus. References [1] Liu Lei, Gao Jun. Application of fieldbus technology in equipment monitoring system [J]. Microcomputer Information, 2006, 7-1: 73-74. [2] Yang Xianhui. Fieldbus technology and its application [M]. Beijing: Tsinghua University Press, 2002. [3] Yu Di, Wang Wei. Application of STU hydropower plant computer monitoring system [J]. Electric Power System and Automation, 1999 (1): 14-17.