1 Introduction In recent years, with the vigorous development of SIS systems in power plant information technology, the quality of maintenance, as an important component of SIS systems, has also been continuously improving. With the improvement of maintenance quality based on real-time information management, the requirements for the underlying process control of power plant information are also increasing. At the same time, in the process of DCS to FCS development, the advantages of FCS over DCS precisely meet the requirements of this maintenance strategy for the underlying process control system. This paper will specifically analyze the advantages of FCS over DCS in this direction based on the requirements of modern maintenance strategy technology, and from the perspective of maintenance strategy, discuss and prove from technical and economic aspects that the development of DCS to FCS technology is a trend. 2 Maintenance Strategy Maintenance strategy is the core issue of maintenance management. 2.1 Three Maintenance Methods (1) Time-based Maintenance Method This is a traditional maintenance method, in which the maintenance plan is arranged according to a fixed time cycle. It basically does not consider the maintenance status of the operating equipment, but only shuts down the equipment for maintenance according to the predetermined time. At present, this maintenance method is the most widely used in China, but at the same time, its maintenance cycle is the shortest. Therefore, this is a maintenance method with the most maintenance times per unit time. (2) Maintenance based on usage is a more advanced maintenance method compared to maintenance based on time cycle. It simply considers the operating status of the equipment, such as arranging maintenance according to the number of times the switch is opened and closed or the number of times the motor is started and stopped. However, such simple criteria are insufficient and incomplete for determining the true operating status of the equipment. Its maintenance cycle is longer than that of maintenance based on time cycle. (3) Maintenance based on equipment status is a method of judging equipment abnormalities and predicting equipment failures based on equipment status information provided by advanced condition monitoring and diagnostic technologies, and carrying out maintenance before the failure occurs. That is, maintenance plans are arranged and maintenance is carried out according to the health status of the equipment to prevent over-maintenance or under-maintenance to the greatest extent. It is a maintenance method with the longest maintenance cycle and is also the current direction of maintenance strategy development. 2.2 Two problems in the development of condition maintenance Condition maintenance is the direction of maintenance development and the focus of recent maintenance strategy development. Besides other issues such as the imperfect establishment of expert systems and the need for optimization of equipment mathematical models and neural network systems, condition-based maintenance also encounters the following two problems in practical applications in terms of process information and control: (1) Condition-based maintenance requires a large number of data collection points to predict the status of equipment, which requires the support of a large number of real-time I/O points. Currently, the collection of I/O points in power plants is often only suitable for remote monitoring of the basic operating status of equipment, which is far from meeting the requirements of condition-based maintenance. Only by collecting a sufficient number of I/O points, combined with other equipment status monitoring methods, and through the calculation of mathematical models in the SIS system, can the internal status of the equipment be accurately reflected and the next maintenance date be predicted. For example, for electric actuators, most domestic units currently only monitor the three sets of contacts: limit switch opening/closing and equipment fault. For condition-based maintenance, at least low voltage, torque opening, torque closing, and test position need to be added. Of course, in order to enable operators to better grasp the operating status of the equipment and maintenance personnel to have a deeper and more comprehensive understanding of the internal status of the equipment, other information such as local operation information and general equipment alarm information (usually including low power factor, current/voltage/load monitoring, rotor cage bar status, stator/rotor air gap characteristics, harmonic distortion, etc.) also need to be collected. (2) Condition maintenance requires more accurate and faster real-time I/O information. The current control system is more than enough for thermal control, but there is still a gap for the requirements of condition maintenance. For example, the measurement of superheated steam temperature and reheated steam temperature. Since superheated steam temperature and reheated steam temperature have a great impact on the unit efficiency, in order to improve the unit efficiency, the latest CCS optimization often controls the superheated steam temperature and reheated steam temperature very close to their critical values ​​by taking advantage of the sensitivity of the control system. This results in an increase in the number of times the superheated steam temperature and reheated steam temperature pulsation peak exceeds the temperature. The overheating of superheated steam and reheated steam has a significant impact on the safety of the unit. Therefore, from the perspective of condition-based maintenance and unit operation safety, higher requirements are placed on the accuracy and real-time performance of the amplitude and timing of superheater overheating. 3 Control System The two problems faced in process information and control are essentially due to the higher requirements that condition-based maintenance places on the power plant control system. The following is an introduction to the two control systems. 3.1 Introduction to the two control systems (1) DCS Control System The DCS control system is currently the most widely used control system in power plants. It is mainly a control system based on the acquisition, processing and command output of analog signals. The input analog signals are converted from analog to digital by I/O modules and sent to the central processing unit of the control unit for information processing, communication with the host computer, and display on the human-machine interface; the output commands enter the host computer through the human-machine interface, and then are sent to the central processing unit, and finally converted from digital to analog by I/O modules and sent to the field. Since DCS was put into commercial operation more than 10 years ago, it has developed into one of the most mature power plant control system products. (2) FCS control system It is a fieldbus-based control system and one of the current directions of DCS development. Under normal circumstances, the equipment in the same functional area communicates with the central controller, which is the master station, through the fieldbus. The basic information of the equipment is processed by the CPU on the local equipment and sent to the central controller in the form of digital signals through the fieldbus. After the central controller processes the signal, it sends the required information to the human-machine interface of the host computer for display. Similarly, the control command is sent from the host computer to the central controller through the human-machine interface. After the central controller processes it, it sends it to the CPU of the field equipment in the form of digital signals through the fieldbus to directly control the equipment. 3.2 Comparison of the adaptability of the two control systems in condition maintenance (1) The disadvantage of DCS control system in solving two problems of condition maintenance The two thermal problems encountered in condition maintenance are quite difficult for DCS. The essence of these two problems is to address the weaknesses of the widely used DCS. Problem 1: Condition maintenance requires more or even several times more I/O points, which is an economic and technical difficulty for DCS. From an economic perspective, the number of I/O points in a DCS (Distributed Control System) is closely related to the number of I/O modules, relays, cables, terminals, terminal cabinets, and the workload of system installation and commissioning. Therefore, the number of I/O points is generally linearly related to the price of a DCS. When the number of I/O points required for condition-based maintenance doubles, the price of the DCS also doubles. For power plants, from an economic standpoint, the exponential increase in investment in the DCS system for condition-based maintenance is unacceptable. From a technical perspective, the increase in I/O signals inevitably leads to strain on the internal resources of the DCS system, increasing the load on the central controller, bus, and other communication components, ultimately affecting the performance of the control system. For current DCS technology, the ultimate solution to this problem is simply to increase the number of control units and the number of internal buses. This not only increases hardware investment but also makes the system larger and more complex, resulting in more system failure points and increased maintenance workload. Question 2: Condition-based maintenance requires the DCS to provide more accurate and faster real-time I/O point information, while also demanding more accurate and faster delivery of control commands to the field. From the current DCS perspective, this means a desire for shorter CPU operation cycles, higher frequencies, and higher operational accuracy in control units; and larger and faster bus transmission volumes within the control system. For existing DCS systems, given a fixed total investment, accuracy and speed are always inversely proportional. That is, with a fixed number of control units within the DCS, a shorter central controller operation cycle leads to decreased operational accuracy; conversely, increased central controller accuracy necessitates a correspondingly longer operation cycle. Otherwise, simultaneously increasing the central controller CPU's operational accuracy and frequency would increase CPU load, leading to more control system failures and reduced availability. Therefore, it is difficult to simultaneously improve the accuracy and real-time performance of the DCS to meet the needs of condition-based maintenance without changing the total investment or reducing equipment availability. (2) Advantages of FCS in Solving Two Problems of Condition-Based Maintenance Regarding the issue of doubling the number of I/O points, FCS has a significant economic advantage. Because the underlying structure of FCS is completely different from DCS, the CPU of the central controller in the FCS control unit communicates directly with the CPUs of local devices via a fieldbus, replacing the complex communication process in the DCS system where analog signals communicate through local terminal cabinets, thermal cables, DCS wiring cabinets, DCS panel cables, I/O modules in the DCS, and cables within the DCS cabinet to the DCS central processor. Therefore, unlike DCS's pricing method based on the number of I/O points, FCS's pricing is based on the number of slave nodes, i.e., the number of local devices. The significant increase in I/O points during condition-based maintenance stems from the fact that, with the number of devices remaining constant, increasing the number of data collection points for each device allows for a more thorough understanding of its condition, which perfectly aligns with the advantages of FCS. Therefore, FCS can meet the requirements of a significant increase in I/O points during condition-based maintenance without increasing hardware investment or with only a small increase in hardware investment. Conversely, since the hardware structure of the FCS control system remains largely unchanged despite the increased number of I/O points required for condition-based maintenance, many hardware and software failures caused by increased system complexity can be avoided. For issues requiring higher real-time performance and accuracy, the FCS control system has unique advantages. The CPU of the FCS central control unit communicates directly with the CPUs on local devices via digital signals, eliminating the need for repeated digital-to-analog and analog-to-digital conversions during analog signal transmission and reception, as is required in DCS. This undoubtedly improves both the real-time performance and accuracy of signal propagation. For example, as shown in the attached table, the speed and accuracy of the FCS control section are more than double that of the DCS control section. 4. Product Examples Currently, many control system and information system manufacturers have developed numerous high-quality products that combine FCS with condition-based maintenance. Since Siemens has independently developed products in fieldbus, local devices, FCS control systems, and SIS information systems, and its systems exhibit good compatibility in commercial operation, a brief introduction will be given using Siemens configurations as an example. The entire system consists of the Siemens SIS product BFS++, the FCS product T-XP, the Profibus fieldbus, and local devices (such as SIPOS5). BFS++ is a Siemens SIS product. Condition-based maintenance is a component of BFS++, and the condition-based maintenance expert system runs on the BFS++ server. According to the needs of condition-based maintenance, data is transmitted from the T-XP control system to the SIS system BFS++ via the network management system XU. In the BFS++ server, based on information from the existing knowledge base and mathematical models combined with the transmitted data, the status of the equipment is fuzzy inferred. For example, the status of a circuit breaker can be determined by comparing the cumulative breaking current with the ultimate breaking current, and based on this, a fuzzy decision is made to determine the probability of needing maintenance. The cumulative breaking current is the integral of the breaking current over the number of breaks during switch operation, and the ultimate breaking current is the product of the rated breaking current and the full-capacity allowable breaking number. Based on user needs and in accordance with the overall maintenance strategy, in practice, this is ultimately simplified to a non-fuzzy expression of whether condition-based maintenance is needed. The T-XP control system, Profibus, and local fieldbus devices constitute the FCS control system, including the primary equipment. The T-XP control system is a DCS/FCS system with optional components. If I/O modules are configured in the control unit, and the central processing unit communicates with local devices via analog signals through the I/O modules, it becomes a DCS control system. If the IM308C communication module is configured in the control unit, and the central controller connects to local devices as slaves via the IM308C (as the master station in the Profibus bus system) and Profibus, it becomes an FCS control system. T-XP uses the FCS configuration. Profibus is a widely used fieldbus with the internationally open fieldbus standard EN50170. Profibus-DP, the most common type used in power plant control, is used for high-speed data transmission between distributed peripherals. It is an optimized, high-speed, and inexpensive communication connection designed specifically for communication between automatic control systems and distributed I/O at the device level. The orientation is based on the Open Systems Interconnection (OSI) model according to the ISO 7498 international standard. In T-XP FCS applications, the following parameter settings are generally used: Profibus-DP physical material is twisted-pair computer cable, transmission distance is 200m, transmission speed is 9.6kbps, transmission technology is RS485, IM308C is a Profibus Class 1 DP master station, and local devices are Profibus slave stations. Theoretically, a Profibus bus can have 127 nodes, but in practical applications, the actual number of slave nodes is generally in the double digits. Local fieldbus devices are local devices and instruments with Profibus fieldbus interfaces. Through the Profibus interface, they can easily connect to the Profibus fieldbus network. They communicate with the CPU in the FCS in the form of digital signals and simultaneously receive digital commands from the FCS to perform actions. For example, the commonly used SIPOS5 Profitron electric actuator has an RS485 Profibus-DP interface and a CPU for device management and communication. SIPOS5, acting as a slave station, communicates digitally with the IM308C communication module in the FCS system, which acts as the master station, via Profibus. The signals communicated between SIPOS5 and FCS can include not only conventional position feedback (AI), control commands (AO), limit switch open/close contact actions (BI), and torque switch open/close contact actions (BI), but also optional acquisition signals: FCS T-XP, Profibus fieldbus, and the fieldbus configuration of the SIPOS5 Profitron electric actuator. In the operation of the 200MW Unit 12 at Jiangsu Xinhai Power Generation Co., Ltd., this configuration demonstrates certain advantages in control quality compared to the DCS section of the same unit. Comparative experiments are shown in the attached table: Table : Comparison of Control Quality between FCS and DCS Traditional DCS Processing Fieldbus Profibus-DP Operation on CRT: 0.7s 0.3s Command and Feedback Interval Time Operation on CRT: 0.20% 0.05% Command and Feedback Accuracy Error As can be seen, without increasing hardware investment or even with minimal hardware investment, the real-time performance and accuracy of the FCS control system are improved to a certain extent. Simultaneously, according to the requirements of power plant condition-based maintenance, many signals related to the intrinsic performance of equipment can be acquired without increasing hardware, which greatly benefits the development of condition-based maintenance. At the same time, with the deepening of condition-based maintenance, some new BO and AO signals for equipment maintenance will emerge, and the output of these new signals can also be implemented in the same way. 5 Conclusion Currently, due to various reasons, the application of FCS in the field of power plant process control has progressed relatively slowly. However, with the development of other technologies, such as condition-based maintenance technology, the requirements for process control will gradually increase, thus gradually revealing the advantages of FCS over DCS, ultimately accelerating the development of DCS towards FCS.