In the 1980s, DCS (Digital Distributed Control System) began to enter the field of power plant automation control. Due to its positive effects on safety and economic benefits, which were unparalleled by any previous control system, DCS was widely used in power plants. Currently, all generating units above 300MW, whether domestically produced or imported, use DCS. Even 200MW and 100MW units are being retrofitted using DCS, and some self-owned power plants' 25MW and 12MW thermal power units also use DCS systems. So, can the fieldbus control system (FCS), which only became practical in the 1990s and is the newest type of control system in the field of automation, also be accepted and widely adopted by power plants like DCS? 1. The most significant difference between FCS and DCS According to the International Electrotechnical Commission (IEC) 1158 definition: a digital, serial, multi-point communication data bus between field devices installed in the manufacturing or process area and automatic control devices in the control room is called a fieldbus. A fully digital control system based on fieldbus is called a fieldbus control system (FCS). That is, fieldbus is an open, fully digital, bidirectional, and multi-station communication system used between field instruments and control room systems. The specific comparison between DCS and FCS is as follows: (1) DCS system is a large system with powerful controllers and a very important role in the system. The data highway is also the key to the system, so the overall investment must be completed in one step, and it is difficult to expand afterward. FCS has more thorough decentralization of functions, information processing is field-based, and the widespread use of digital intelligent field devices makes the controller function and importance relatively weaker. Therefore, FCS system has a low investment starting point and can be used, expanded, and put into operation at the same time. (2) DCS system is a closed system, and products from different companies are basically incompatible with each other. FCS system is an open system, and users can choose various devices from different manufacturers and brands to connect to the fieldbus to achieve the best system integration. (3) The information in DCS system is all formed by binary or analog signals, and D/A and A/D conversion is required. The FCS system is fully digital, eliminating the need for D/A and A/D conversion, and is highly integrated and high-performance, improving accuracy from ±0.5% to ±0.1%. (4) The FCS system can incorporate PID closed-loop control functions into transmitters or actuators, shortening the control cycle. Currently, it can be increased from 2-5 times/s in DCS to 10-20 times/s in FCS, thereby improving regulation performance. (5) The DCS system can control and monitor the entire process, and perform self-diagnosis, maintenance, and configuration. However, due to its fatal weakness, its I/O signals use traditional analog signals, making it impossible to remotely diagnose, maintain, and configure field instruments (including transmitters, actuators, etc.) on the DCS engineer station. The FCS system adopts fully digital technology, and digital intelligent field devices send multi-variable information, not just single-variable information, and also have the function of detecting information errors. The FCS system adopts a bidirectional digital communication fieldbus signal system. Therefore, it can remotely diagnose, maintain, and configure field devices (including transmitters, actuators, etc.). The FCS system has advantages that the DCS system cannot match. (6) Due to the on-site information processing of the FCS system, compared with the DCS system, it can save a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files and I/O cabinets. It also saves space and floor area for I/O devices and device rooms. Some experts believe that it can save 60%. (7) For the same reason as (6), the FCS system can reduce a large number of cables and cable trays for laying cables. It also saves design, installation and maintenance costs. Some experts believe that it can save 66%. Regarding points (6) and (7), it should be added that the investment saving effect of adopting the FCS system is undeniable. However, whether it reaches 60% to 66% as some experts say still needs to be objectively evaluated. Although these figures appear in many articles, the author believes that this is the result of mutual excerpting. The original source of these figures has not yet been found. Therefore, readers should be cautious when citing these figures. (8) FCS is simpler to configure than DCS. Due to its standardized structure and performance, it is easy to install, operate, and maintain. (9) Key points for the design and development of FCS for process control. This point is not a comparison with DCS, but only to illustrate the key issues that should be considered in the design and development of FCS for process control or for simulating continuous processes. 1) The intrinsic safety and explosion-proof function of the bus is required and is of paramount importance. 2) The changes in basic monitoring such as flow rate, level, temperature, and pressure are slow and have a hysteresis effect. Therefore, node monitoring does not require a fast electronic response time, but requires complex analog quantity processing capabilities. This physical characteristic determines that the system basically adopts a centralized polling system between master and slave, which is technically reasonable and economically advantageous. 3) The physical principle of measuring parameters such as flow rate, level, temperature, and pressure is classical, but the sensors, transmitters, and controllers should develop towards digital intelligence. 4) As an FCS system developed for continuous processes and their instrumentation, the focus should be on improving the design of the low-speed bus H1. 2. Practical Application of FCS in Thermal Power Plants The Fieldbus Control System (FCS) is a new type of control system, and its introduction to China is relatively recent. Currently, its application in thermal power plants is still in the stage of localized use. Example 1: The Cegelec Alspa P320 system (see Figure 1) is used in the automatic control of the second phase (2×360MW) units of the Huaneng Luohuang Power Plant (4×360MW) in China. This system has 31 dual-redundant WorldFIP networks, 16 monitoring and control stations, 32 redundant PLCs, and processes 18,000 I/O points and 50,000 control data points. The system uses twisted-pair media, with redundant operator stations and backup stations. The typical system response time is 50ms. The second phase of the Huaneng Luohuang Power Plant is a complete import of 2×360MW coal-fired generating units from the French GEC ALSTOM Group. The boiler, turbine, and generator were all supplied by STG. The power generation control system, fully operational by the end of 1998, is the ALSPA P320 control system, developed and launched in the mid-1990s by CEGELCE of France. It implements key functions such as the unit's Data Acquisition System (DAS), Closed-Loop Control System (MCS), Sequential Control System (SCS), and Burner Management System (BMS). The turbine's Digital Electro-hydraulic Control System (DEH) uses the MICROREC control system provided by GEC ALSTOM STG of France. The various parts of the ALSPA P320 control system communicate with each other via a standard network, facilitating communication with other control components. The ALSPA P320 has three main networks: the LOCAFIP fieldbus network (WorldFIP), using the FIP standard (UTEC64+601607), used to link input/output modules to the P320's C370 controller. The F900 data bus (WordlFIP) is used for data exchange between C370 controllers and communication between the C370 controller and CENTRALOG. F900 is a high-speed data transmission network based on the IEEE FIP standard (UTEC64+601607) for industrial LANs. The CONTRONET control network uses Ethernet technology for data exchange between the centralized control layer CENTRALOG database and operator workstations. CONTRONET complies with the IEEE 803.3 LAN standard. Through the INTERENT standard protocol, the P320 can perform long-distance communication for remote maintenance and large-scale power grid control. Example 2: The Yangling Gas Turbine Power Plant in Shaanxi Province, located in the Yangling Agricultural High-tech Development Zone, uses the Simatic PCS7 control system manufactured by Siemens, Germany, which follows the Profibus fieldbus protocol, as the centralized control system for the turbine, boiler, and electrical systems. The system configuration diagram is shown in Figure 2. The system is configured with three pairs of redundant CPU-41H controllers. Each pair of redundant controllers connects to a number of ET200M remote I/O expansion racks and I/O modules via a redundant Profibus-DP fieldbus (latest data transfer rate up to 12 Mbit/s). The remote I/O stations, composed of these racks and modules, are located near the field and communicate with the redundant controllers in the main control building via the Profibus-DP bus. The system comprises 32 ET200M remote racks, divided into 8 remote I/O stations according to the process flow, placed in 8 different locations throughout the plant. The furthest remote I/O station—the deep well pump room remote I/O station—is approximately 3.4 km from the main plant area, while the distances between the other remote I/O stations are within 200 m. Communication between the controller and each remote I/O station is accomplished via the Profibus-DP fieldbus. Within the main plant area, the Profibus-DP fieldbus uses twisted-pair cable for transmission. Data communication between the remote I/O station at the deep well pump station and the main plant area uses optical fiber as the transmission medium, with both ends connected to the Profibus-DP fieldbus via photoelectric conversion interfaces. Due to the use of Profibus-DP fieldbus technology, I/O cabinets were deployed on-site, enabling bidirectional data exchange between the gas turbine peripheral equipment, waste heat boiler, steam turbine auxiliary equipment, circulating water pump station, integrated water pump station, deep well pump station, plant electrical system, 110kV substation, and other systems with the main control ring server, achieving centralized monitoring of all systems in the plant. The automatic system's availability rate reached 100%, and the system was put into commercial operation in August 2001. Example 3: Sichuan Guang'an Power Plant successfully connected the boiler feedwater control system with the condensate polishing control system and other auxiliary workshop control systems by adopting L2-DP network technology that follows the Profibus fieldbus protocol, thereby improving the labor productivity and system reliability of the auxiliary workshop control in the thermal power plant. 3. Effects of FCS in Local Applications in Thermal Power Plants First, let's clarify the three key points of FCS: (1) Core: The core of the FCS system is the bus protocol, i.e., the bus standard. That is to say, only control systems that follow the fieldbus protocol can be called fieldbus control systems. (2) Foundation: The foundation of the FCS system is digital intelligent field devices. Digital intelligent field devices are the hardware support of the FCS system. (3) Essence: The essence of the FCS system is the fieldization of information processing. This is the manifestation of the effectiveness of the FCS system. Let's analyze the examples of FCS application in thermal power plants mentioned above. In Example 1, the ALSPA 320 control system of Huaneng Luohuang Power Plant uses the WorldFip fieldbus protocol for both its Local Fip and F900 networks, and Ethernet technology for its CONTRONET control network. This is a very typical fieldbus architecture. In Example 2, the Simaeic PCS7 control system of Yangling Gas Turbine Power Plant uses the Profibus-DP fieldbus for communication between the controller and each remote I/O. Data communication between controllers and between the controller and the server is accomplished through a redundant ring-type industrial Ethernet. In both examples, the communication networks follow the fieldbus protocol, meaning they both contain the core components of a fieldbus control system. However, another common feature is that neither uses digital intelligent field devices; they still use analog measurement elements and actuators, lacking field control functionality. Without the hardware support of the fieldbus control system, on-site information processing cannot be achieved. In other words, the key features of fieldbus—reduced system investment costs, reduced operating expenses, and improved operation and management—are not fully realized. Example 3 is a successful example of the local application of fieldbus in thermal power plants. 4. Application Prospects of FCS in Thermal Power Plants (1) Fieldbus control system is the latest type of control system. It is a new type of control system that is fully computerized, fully digital, and bidirectionally communicative. Fieldbus technology has brought about a revolution in the field of automation and represents the development direction of automation. Digital communication is a trend and an inevitable trend of technological development. Theoretically speaking, bidirectional digital communication fieldbus signal control technology will bring tangible benefits to the safe and economical operation of thermal power plants and improve management level. This is incomparable to any control system used in power plants in the past. (2) As the core part of the fieldbus control system, the bus protocol has been successfully running in the communication network of the thermal power plant control system. This has not only eliminated many doubts that people had before, but also laid a good foundation for the promotion and application of fieldbus control system in thermal power plants. (3) Fieldbus control system can play its greatest role in the network control of auxiliary workshop control systems in thermal power plants that are mainly based on sequential control and use PLC (Programmable Logic Controller) as hardware (see Example 3). The PLC is connected to the high-speed bus as a station, giving full play to the advantages of PLC in handling switching quantities. The application of fieldbus in this field has been successful, which will be a preferred solution for the moderate centralized control policy of auxiliary workshops of thermal power plants in the future. （4） As there are still very few types of digital intelligent field devices that can meet the control requirements of thermal power plants, the theoretical benefits of fieldbus cannot be fully realized. Therefore, the time is not yet ripe for the comprehensive adoption of typical fieldbus control systems in large units. （5） At present, control systems like those in Examples 1 and 2 are a transitional control system to FCS. They retain the powerful controllers and I/O modules of the DCS system, while also following the fieldbus protocol in the communication network. We call this system a digital distributed control system that follows the fieldbus protocol in the direction of communication and data transmission, and temporarily refer to this system as FDCS. ...