Abstract: This article analyzes the reasons why DCS systems play an important role in the process control of thermal power plants at present, and elaborates on some major problems existing in the practical application of DCS systems in process control. It also compares several DCS systems used in power plants in Zhejiang Province in some aspects. Keywords: Distributed Control System, DCS, Application Technology, Distributed Faults 1 Introduction DCS (Distributed Control System) has been used in the process control of thermal power plant generator units for more than ten years, and its application is increasing. Currently popular DCS systems include C&E's MOD-300, HONEYWELL's TDC-3000, FOXBORO's I/AS, Yokogawa's CENTUMXL, WESTINGHOUSE's WDPF, BAILEY's INFI-90, and SIEMANS' TELEPERM ME/XP. Looking at the systems provided by these major DCS manufacturers, there are some problems to varying degrees in their application, such as system reliability, real-time performance, and the flexibility and convenience of system configuration, all of which need further improvement. This article analyzes and discusses some problems existing in the practical application of DCS systems. 2 The original intention of DCS system The introduction of DCS system and its gradual development into the main body of process control are due to the following reasons: (1) Modern production process systems are becoming increasingly large-scale and complex, and the parameters that need to be detected and controlled are increasing in large numbers, making traditional instrument control systems inadequate and necessitating the search for alternative solutions. (2) Traditional instrument control systems usually use products from multiple manufacturers, resulting in a wide variety of spare parts required for process production, which consumes a lot of manpower and resources, and process production is quite dependent on these instrument manufacturers. This situation also needs to be changed. (3) The rapid development of digital circuit technology, especially the application of large-scale integrated circuit technology, has greatly improved the integration and yield, making it possible to use microprocessors extensively in process control in terms of cost. (4) The development of automation control theory, especially the theory of continuous system discretization and sampling theory, has greatly promoted the transformation of process control from traditional instrument control systems to DCS systems. (5) The development of communication theory and technology: In the process of extensive research on local area networks (LANs), communication theory and technology have been greatly developed and improved, which has had a significant impact on the development of DCS systems. (6) The development of computer application technology, especially the introduction of operating system software such as Microsoft's WINDOWS-95, has laid the foundation for the application of computers in process control systems. A good interface between computers and operators has been established, making it easier for operators without specialized computer knowledge to accept. (7) Computer centralized control systems have some inherent defects, such as concentrated faults. Therefore, huge costs are required to improve reliability. Centralized control systems require large-scale computers, which are expensive. In contrast, the microprocessors and microcomputers used in DCS systems are much cheaper, faults are relatively dispersed, and the microcomputers or microprocessors in DCS systems run in parallel. Compared with the serial operation of computers in centralized control systems, the processing speed is also greatly improved, thus having higher real-time performance. These are the key reasons why DCS systems are superior to centralized control systems and have developed rapidly. The above lists the main reasons why DCS systems have made a name for themselves in the field of process control. Now let's look at some of the original intentions for introducing DCS systems. DCS system is a computer (or microcomputer) control system relative to computer centralized control system. It was developed based on the research of computer local area network. Process control experts used the research results of computer local area network to turn the local area network into a network control system with high real-time and reliability requirements, and applied it to the field of process control. Experts call such a network control system DCS. The experts' original intention should be that such a control system brings at least the following benefits: (1) Fault dispersion. This is the biggest reason for launching DCS system. DCS system is to solve the fatal weakness of centralized control system, "fault concentration". The reason for fault dispersion is that DCS system uses a large number of microprocessors. Each microprocessor undertakes a small-scale (geographical) control task. The failure of a microprocessor will not affect the normal operation of the entire system. (2) Reduce the size of the control room or the length of the control panel. (3) Significantly reduce the cables required for the control system. (4) Significantly reduce the types and quantities of spare parts required for the control system. (5) Reduce the dependence of process production on instrument control equipment manufacturers and reduce the cost of training instrument control personnel. (6) It provides flexibility in the configuration of the control system, with convenient configuration and scalability. (7) It realizes the management of real-time process parameters and historical data, and provides performance calculation and equipment life calculation functions. This is something that traditional instrument control systems cannot match. 3 The actual capabilities of DCS systems that the market can provide at the present stage Based on the DCS systems such as BAILEY's INFI-90, C&E's MOD-300 and SIEMENSR's TELEPERM-ME/XP used in power plants in Zhejiang Province, the author's views are as follows: (1) Regarding fault distribution. It is believed that most DCS system manufacturers' current systems are not as "fault-distributed" as imagined in actual applications. In fact, due to some problems in the application technology of DCS systems, it is not uncommon for a thermal power plant unit generator unit controlled by a DCS system to be forced to shut down due to certain faults in the DCS system. This is better than traditional instrument control systems. Therefore, the closer a DCS system's structure is to a traditional instrument control system—that is, the more geographically dispersed the control tasks undertaken by microprocessors or multifunction controllers—the better it can achieve fault distribution. From this perspective, the SIE-MANS system performs well. One of its control cards (equipped with a microprocessor, capable of communicating with other cards or other subsystems such as operator stations and engineer workstations via a communication bus) only handles 2-4 open-loop control loops for motors or electric gates, or 1-2 closed-loop control loops. Furthermore, this card handles both logic control functions and related I/O functions. This approach is highly advantageous for fault distribution. However, other systems typically concentrate many process control tasks on a few multifunction control cards or microprocessors, and concentrate process control input/output (I/O) functions on I/O cards. Objectively, these systems concentrate process control faults rather than distribute them, seemingly contradicting the name "DCS." However, the SIE-MANS system also has problems; its Ethernet communication with the ME system still has some issues that need to be resolved. SIEMANS' TELEPERM-ME system is a very mature system. The ME system plus the ETHERNET network is the newly launched TELEPERM-EM/XP system (used in two 330 MW units of the fourth phase of the Taizhou Power Plant in Zhejiang). As it is a new system, many application software (especially those related to communication) will need to be tested over time. If these problems are solved, it will have a certain advantage in terms of fault distribution and other aspects such as manufacturing process. DCS, fault distribution, it seems that SIEMANS has taken the right path, because their system is closest to the traditional instrument control system. The control function is relatively distributed. To achieve fault distribution, the control function must be distributed. (2) Regarding the size of the control room and the length of the dial. All DCS systems can do this. However, compared with the traditional instrument control system, the size of the electronic room and the equipment are relatively increased. The actual benefits that DCS systems bring to us are not significant. (3) Regarding saving cables. Because the equipment and components used in the DCS system are still relatively delicate at this stage and have high requirements for the field environment, such as dust protection and air conditioning, and REMOTEI/O cannot be used extensively, the main equipment of the DCS system needs to be placed in a well-equipped electronic room, and a large number of field signals still need to be connected to the electronic room by cable. Therefore, compared with the traditional instrument control system, the number of cables has been reduced, but the benefits are limited. (4) Regarding the reduction of the types and quantities of spare parts. The types and quantities of spare parts have been reduced, and the number of instrument control equipment manufacturers that need to deal with them has also been reduced. (5) Regarding the reduction of the dependence of the unit operation on the instrument control equipment manufacturers and the reduction of the cost of technical training for instrument control personnel. Because the DCS system is not yet satisfactory in terms of application technology, the dependence of the unit operation on the DCS system manufacturer has not only not decreased but has increased, especially during the period when the unit is newly built and put into operation, the expert service sent by the DCS manufacturer to the construction site seems to have become indispensable during the commissioning of the unit. The number of engineering and technical personnel required to train in order to make good use of DCS systems has also increased, and therefore the training costs have also increased. (6) Regarding the flexibility of the control system structure, the ease of configuration and the scalability of the system. The flexibility of the system structure and the scalability of the system are indeed unmatched by traditional instrument control systems. The configuration of most DCS systems is also relatively convenient. However, there is still much work to be done in terms of online configuration functions for most systems. Many systems are configured offline, requiring programming on the engineering station, then compilation, and then downloading. And for some systems, this process is quite time-consuming, especially during commissioning. Regarding this issue, SIEMANS' system is quite unique. It not only has online configuration but also has a simulation function, which greatly facilitates the commissioning work. (7) Regarding the unique control functions provided by DCS systems, such as the management of historical and real-time data, performance calculation, etc. This is unparalleled by traditional instrument control systems. It is precisely because of these advantages of DCS that process control has been pushed to a new and higher level. In summary, the benefits brought by the DCS systems currently available on the market are far less than we imagined. Of course, the DCS system solves the problem that traditional instrument control systems are inadequate for process control in modern large-scale production. Compared with centralized computer control, faults are still distributed, and it has unique advantages in many aspects that are incomparable to traditional instrument control systems and centralized computer control systems. 4 Some problems in application The current DCS system still has the following problems in application technology: (1) Fault distribution. The current DCS system has not yet achieved true fault distribution. This is partly because the current devices are still relatively delicate, so the control system cannot be distributed geographically (as close as possible to the traditional instrument control system). The field conditions of industrial control are generally quite harsh. Dust, temperature and humidity are still difficult for the remote I/O cards provided by manufacturers at this stage to adapt to. For example, some I/O cards of Unit 1 of Beilun Power Plant can only be placed relatively centrally on site. During the commissioning, water ingress into these I/O cabinets has affected the normal operation of the entire system. Another example is that Unit 3 of Beilun Power Plant has set up remote I/O cabinets in the circulating pump room. The cables are certainly saved, but small air conditioners are set up on the I/O cabinets, so the cost is still not much saved. On the other hand, the control functions are not sufficiently decentralized in the design, including both the design and engineering of the DCS system itself. For example, in the MOD-300 system, its I/O is too centralized; I/O card failures can affect the normal operation of the system. Furthermore, the auxiliary equipment control of the unit is handled by a programmable logic controller (PLC) system, which communicates with the MOD-300 system via a gateway. This gateway becomes a bottleneck in the entire control system; a failure in this gateway will affect the normal operation of the unit. Another example is the system of Unit 3 at the Beilun Power Plant. The auxiliary equipment control of the unit is handled by its SCS subsystem. One MFP in the SCS subsystem has approximately 300 I/O cards, and each I/O card (16 points) is designed for different controlled objects, meaning that one controlled object requires several I/O cards. Therefore, a failure in one I/O card will affect the normal operation of several controlled objects, and repairing one controlled object will involve several I/O cards. In addition, an MFP contains too many controlled objects, and the failure of an MFP card will affect the normal operation of many control loops. A control system designed in this way will inevitably have high requirements for the reliability of the card or the system. References [1] and [3] both mention that some manufacturers are researching or have launched a system called FCS, which is a process control system composed of field intelligent instruments combined with fieldbus technology. Obviously, such a system is very close to the traditional instrument control system. Theoretically speaking, intelligent control instruments are placed near the controlled equipment, and the control functions can be completely distributed geographically. Therefore, the faults can also be completely distributed. However, in reality, given the current level of component production technology and the status quo of computer and communication technology, FCS may also face the same difficulties as DCS. First, FCS has to solve the problem of the very harsh industrial control field environment. Second, the field control equipment is spatially distributed and geographically dispersed. Connecting the fieldbus is not an easy task, and communication will also face the difficulty of too many nodes. (2) Real-time performance. Process control requires the control system to have high real-time performance. Due to limitations in communication and computer technology, current systems still need improvement in real-time performance. For example, in the early MOD-300 system, it took more than ten seconds from the operator issuing a command to seeing the feedback signal on the screen when operating a valve. The problem was solved after hardware upgrades and replacement with a new generation of chips. Some systems have slow screen opening speeds, requiring switching between several screens to operate a device, which is very inconvenient for operators in emergency situations. In addition, the communication time (or the response time of the DCS system) deteriorates the characteristics of the controlled object, causing a decrease in the quality indicators of the transition process of the closed-loop control system. (3) Convenience of configuration and necessary access protection. Regarding system configuration, most current DCS systems use computer window technology, which is generally convenient, but there are still areas for research in online configuration. Most systems are configured at the engineer station and then downloaded to the control subsystem or control card, which is usually very time-consuming, especially during debugging, where configuration modifications are frequent. A convenient and safe online configuration method needs to be sought. (4) Manufacturing process and installation process. One of the most important factors affecting the normal use of a DCS system is the manufacturing process and installation process. Sometimes, a small component with poor quality or poor contact can cause a major malfunction or accident. Some systems provide hardware with unsatisfactory manufacturing processes, and the provided cards and connectors are not reliable enough. Sometimes, the fault of some cards can be eliminated by unplugging and plugging them in or shaking them a few times. This is an issue that DCS manufacturers must pay close attention to. If the provided system cannot meet the reliability requirements in terms of manufacturing process, then no matter how attractive the principle is, it will be difficult to accept. On the other hand, there is the installation process, which is also an aspect that DCS system application technology needs to study. (5) Reliability and maturity of application software. Since the application practice of DCS systems is still in the process of continuous accumulation, many application software are not yet mature, and their reliability still needs to be tested over time. DCS system manufacturers should strengthen research in this regard to ensure that the DCS system software that has been put into use or is about to be put into use has sufficient reliability. 5 Conclusion Based on the reasons for the introduction of DCS and why DCS has become the mainstream in the field of process control, the above points out some problems in the application technology of DCS systems currently available on the market. These problems include: (1) related to fault distribution; (2) related to real-time performance; (3) related to the ease of configuration and necessary access control; (4) related to manufacturing and installation processes; and (5) related to the reliability of application software. These problems can be summarized as design problems, component manufacturing technology problems, and process problems. DCS systems should start from the actual needs of process control and address their own shortcomings, continuously improving and developing in terms of structure, components, and technology. This will enable DCS systems to achieve new breakthroughs in application technology. References: [1] Wang Changli, Luo An (eds.). Selection and Application of Distributed Control Systems. Tsinghua University Press, June 1996, 1st edition. [2] Wany H. Smeallie. Microprocessor-Based Distributed Control Systems in the Modern Power Plant. Babcock & Wilcox Operating & Maintenance Experience Conference, Oct. 1983. [3] Wang Jinquan, Fang Zhonghua, Zhong Weiyang. Current Status and Prospect of Industrial Control Systems. China Electric Power, 1998(4). About the author: Zhu Chunzhong (1953-), male, senior engineer, bachelor's degree, engaged in thermal power plant thermal automation.