Power and obstacles of field bus control system application in power plants

Abstract: The application of fieldbus control system in the industrial field is a new beginning. With the emergence of various sensors and transmitters, a new fieldbus control system has begun a new era. The article mainly introduces the principle, characteristics of the FCS and its application in power plants.

I. INTRODUCTION With the rapid development of control technology, computer technology and communication technology, digitalization as a trend has been permeating field devices from the decision making, management, monitoring and control layers of the industrial production process. With the advent of fieldbus, digital communication technology quickly occupied the last field of analog signals in industrial process control systems. A fully digital, fully open, interoperable new control system - fieldbus control system is coming to us. The emergence of fieldbus control systems represents the beginning of a new era in the field of industrial automation and will have a profound impact on the development of this field.

Fifty years ago, the process control system used a 3 to 15 psi pneumatic signal standard, the so-called first generation process control architecture (PCS). In the 1960s, the wide application of 4~20mA analog signals led to the second-generation process control architecture (ACS). Around the 1970s, digital computers entered the field of process control and there was a centralized control, the third generation process control architecture (CCS). After entering the 1980s, a distributed control system, known as the Fourth Generation Process Control Architecture (DCS), was created due to the widespread use of microprocessors. DCS systems widely used in industrial production today employ a decentralized architecture, patented network support, and analog field signals. Although it overcomes the disadvantages of the CCS system, such as its complex structure, poor reliability, and poor real-time performance, due to the closed nature of the patented communication network and the limitations of on-site analog signal transmission, people have to re-examine the rationality of the DCS architecture. With the emergence of various smart sensors, transmitters and actuators, the demand for digitization to the field, control functions to the field, and equipment management to the field is increasing. A new type of process control system architecture—Fieldbus Control System FCS It has already been presented to us.

Second, the classification of fieldbus control system Fieldbus control system as the fifth generation of process control system structure is still in the development stage, a variety of different fieldbus control system emerges in an endless stream, its system structure varies, and some in accordance with the fieldbus system The conceptual design of the structure of the new control system, some of the existing DCS system on the expansion of the field bus functions. For ease of discussion, we will focus on monitoring, control, and field levels. The management level, decision level, etc. above the monitoring level are not considered. Therefore, we can divide the FCS into three categories: one is a two-layer FCS composed of field devices and human-machine interfaces, and the other is a three-layer FCS composed of field devices, control stations, and human-machine interfaces; There is also an FCS that is augmented by DCS fieldbus interface modules.

1. Two-layered FCS

A two-layered FCS is shown in Figure 1. It is composed of two parts, a field device and a human-machine interface. The field devices include various smart meters that conform to fieldbus communication protocols. For example, fieldbus transmitters, converters, actuators, and analytical instruments. Since there is no separate controller in the system, the control functions of the system are all completed by the field device. For example, conventional P1D control algorithms can be implemented in fieldbus transmitters or actuators. Human-machine interface devices generally have an operator operating station and an engineer workstation. The operator operating station or engineer workstation exchanges information with field devices through the fieldbus interface card and fieldbus located in the machine. The information exchanged between the human machine interfaces or between the higher layer devices is implemented through the high speed Ethernet HSE. High-speed Ethernet can also be connected to field devices that require high-speed communications, such as programmable logic controllers (PLCs). The low-speed fieldbus can also be connected to a high-speed fieldbus via a gateway to exchange information with human-machine interface devices or other high-level devices via a high-speed fieldbus.
The structure of this fieldbus control system is suitable for controlling the production process with a relatively small scale, a relatively independent control loop, and no need for complex coordination and control functions. In this case, the control function provided by the field device can meet the requirements. Therefore, in the system structure, the control station in the traditional sense is cancelled, and the control function of the control station is decentralized to the site, simplifying the system structure. But the problem is that it is inconvenient to deal with coordination problems between control loops. One solution is to implement the coordinated control function on the operator's operation station or other high-level computers; another solution is to use the fieldbus interface card. On the realization of some coordinated control functions.

2. Three-layered FCS

The FCS with a three-layer structure is shown in FIG. It consists of three layers: field equipment, control station and human-machine interface. The field devices include various smart sensors, transmitters, actuators, converters, and analytical instruments that conform to the fieldbus communication protocol; the control station can complete basic control functions or coordinate control functions and execute various control algorithms; human-machine interface Including: operator operating station and engineer workstation, mainly used for the monitoring of the production process and the configuration, maintenance and overhaul of the control system. The functions of the rest of the system are the same as those described above, so we will not go into details.

Although the structure of this fieldbus control system retains the control station, the functions realized by the control station are quite different from the traditional DCS. In the traditional DCS, all control functions, whether it is the PID operation of the basic control loop or the coordinated control function between the control loops, are realized by the control station. However, in the FCS, the basic control functions of the lower layer are generally realized by field devices, and the control station only completes the coordination control or other advanced control functions. Of course, if necessary, the control station itself can fully implement the basic control functions. This will allow users more flexible choices. The FCS with a three-layer structure is suitable for use in more complex industrial processes, especially those where the control loops are closely related, require coordination of control functions, and production processes that require special control functions.

3. Field bus control system augmented by DCS
Fieldbus as an advanced field data transmission technology is infiltrating into various fields in emerging industries. Manufacturers of DCS systems are also using this technology to improve existing DCS systems. They attach fieldbus interface modules to the I/O bus of the DCS system and extend several fieldbuses through fieldbus interface modules. Then connected to the on-site smart devices via fieldbus. As shown in Figure 3.
This fieldbus control system evolved from DCS. Therefore, some features of DCS are inevitably retained. For example, the I/O bus and high-level communication network may be proprietary communication protocols of DCS manufacturers, and the system is less open. The configuration of the fieldbus device may require special configuration equipment and configuration software, that is to say that the field device cannot be configured on the original engineering workstation of the DCS. This type of system is more suitable for further expanding the application field bus technology in the existing DCS system of the user, or modifying the analog I/O in the existing DCS system, improving the overall performance of the system and the maintenance and management level of the field device.

Third, the power plant bus fieldbus control system fieldbus control system is a fully digital control system, its application will make the power plant automation into a new era. It is not difficult for us to see its huge potential for application through the characteristics of fieldbus control systems, and the tremendous impetus for the process of enterprise automation.

(I) Characteristics of fieldbus control system Distributed decentralized FCS system reflects the idea of ​​"information concentration and control decentralization" better than DCS system. Compared with traditional DCS, FCS shows a more complete dispersion. It can be composed of field devices to form an autonomous control loop. The field control equipment has a high degree of intelligence and functional autonomy, can complete most of the basic control functions, and can diagnose the operation of the equipment at any time. DCS only realizes the control and decentralization of the control station level, while FCS realizes the control and decentralization of the field control equipment level.

2. Reliability Compared to DCS, the number of FCS barriers, isolators, terminal cabinets, I/O modules, field cables, and terminals is greatly reduced. Therefore, the chance of downtime caused by equipment or wiring failure is greatly reduced; FCS The digital signal transmission in the field signal greatly improves the anti-interference ability of the system. The structure of the FCS is simplified compared to the DCS, and some of the DCS control stations are omitted, and the operation stations are directly connected to the field instruments. These factors all increase the reliability of the FCS.

3. The introduction of precision into the fieldbus eliminates errors in data transmission during analog communication and improves transmission accuracy. There are three causes of errors in the analog communication method: errors in the A/D conversion in the input channel, errors in the analog signal transmission, and errors in the D/A conversion in the output channel. With fieldbus, conversion errors and transmission errors can be eliminated. The field bus uses digital signals to transmit data. The difference between digital signal transmission and analog signal transmission is that the former does not generate errors in the signal transmission process. The data in the field bus is transmitted in a digital state and does not require A/D or D/A conversion. Therefore, no conversion error will occur. Introducing the fieldbus eliminates the above three errors and improves the transmission accuracy.

In addition, FCS uses a thorough decentralized control method. The control tasks are executed simultaneously by microprocessors in different fieldbus devices, and parallel distributed processing between control loops is realized. The control speed can be increased by 2 to 10 times than DCS. Can achieve higher control accuracy and control speed.

4. The open fieldbus control system is open. It is consistent and open to relevant standards and emphasizes consensus and compliance with standards. The communication protocol is unanimously disclosed. The information exchange between devices of different manufacturers can be realized, and an open interconnection system in the field of automation can be constructed through a field bus. Openness determines its interoperability and interoperability. Interoperability refers to the transmission and communication of information between interconnected devices and systems. Interoperability means that devices of similar performance from different manufacturers can be replaced with each other. The field bus, which is the bottom layer of the plant network, also has strong adaptability to the environment and has strong anti-interference ability.

5. Economics FCS saves more hardware than DCS due to changes in the FCS architecture. The use of FCS can greatly reduce safety barriers, isolators, I/O modules, field cables, and terminals, which saves I/O devices and the cabinets and space in which these devices are installed, while significantly reducing installation costs.

Using fieldbus technology, multi-meter interconnection, multivariable detection and multivariable transmission can be realized, which can reduce the number of field devices and reduce the cost of the control system.

6. The serviceability of the maintenance FCS is much better than that of the DCS. The diagnostic scope of DCS's fault diagnosis system can only reach the I/O module level. In the DCS, the working condition of the field device can hardly be understood completely; while the FCS fault diagnosis scope can reach the field device level, in the FCS, the technical personnel. On the engineer station, you can understand the working status of all field devices, adjust the zero point and range, and check the product model, technical specifications, manufacturing plant, and installation and maintenance records. In a sense, FCS has completely eliminated the “on-site blind spot” of DCS, significantly improving the maintainability of the control system.

(B) Fieldbus control system is the need for enterprise information integration After China's accession to the WTO, companies will join the international economic cycle, and face more fierce market competition. In order to gain a firm foothold in the ever-changing market situation, enterprises must attach importance to information integration technology. In the composition of enterprise information, not only market information at the top level, management information and monitoring information at the middle level, but also on-site information at the bottom level are required. Currently widely used DCS cannot effectively obtain on-site information and has become an obstacle for enterprises to implement information integration. With the continuous development of network technology, the upper layer of the decentralized control system will be integrated with the Internet, and the lower layer will use fieldbus communication technology to extend the communication network to the site. Finally, a three-network converged network architecture consisting of a wide-area network-based Internet Internet, a LAN-based enterprise network Intranet, and a fieldbus-based Infranet is formed. Relying on such a network architecture, market information, management information, monitoring information, and on-site information needed by enterprises can be integrated more effectively.

(C) Fieldbus control system is the need for modern enterprise management The modernization of enterprise management contains quite rich content. It not only requires personnel management, financial management, production management, but also equipment management, operation management, maintenance management and so on. The latter management tasks require a lot of on-site information. Unfortunately, DCS cannot provide this information. The fieldbus control system can store a large amount of on-site information in the field device. This information can include various operation and maintenance records, equipment adjustment records, environmental parameter records, and even record the sensor's installation position and handling precautions. . There is no doubt that this will greatly improve the management level of field equipment, and lay the necessary foundation for the implementation of equipment condition maintenance, reduction of spare parts and implementation of total quality management.

(IV) Fieldbus control system is the enterprise's comprehensive automation The integrated automation of an enterprise consists of three aspects: basic automation, management automation and decision automation. In the past, we paid more attention to basic automation. Management automation still remains in a narrow range and at a low level, and decision automation is almost blank. The emergence of fieldbus control systems has promoted the integration, exchange and interaction of on-site information, monitoring information, management information and market information. Integrate basic automation, management automation, and decision automation organically to achieve seamless integration of the three. It can better realize the optimal operation and optimal scheduling of the enterprise, and can support the correct decision-making of the enterprise in a wider range, and create more economic benefits for the enterprise. Obviously, fieldbus communication technology is a key technology for improving the level of basic automation and realizing comprehensive automation for enterprises.

IV. Features of Thermal Power Plant Process Control System Thermal power plant thermal process is a highly complex process that includes the chemical energy of steam to steam, and the thermal energy of steam to the mechanical energy of steam turbine. Its control system is the same as the general process control system. In comparison, the main features are as follows.

1. Highly complex process control system The highly complex nature of the thermal process control system in a power plant determines that it must adopt hierarchically distributed control systems, control concepts for decomposition and coordination. Between the boiler and steam turbine main control system, each subsystem of the boiler or each subsystem of the steam turbine, and the control loops in each subsystem require complex coordination and control functions.

2. Switching quantity control is closely related to analog quantity control. In thermal power plant process control systems, the relationship between switch quantity control system and analog quantity control system is very close. Almost all analog quantity control systems inevitably contain switch quantity control. . Similarly, the switching control system also more or less contains some analog detection or control.

3. High-speed switching and analog control systems require high-speed switching or analog control systems in some thermal process control systems in power plants. For example: Event Sequence Recording System (SOE), Turbine Digital Electrohydraulic Control System (DEH), etc. The former requires a resolution of 1 ms, and the latter requires a control period of about 10 ms, which imposes high requirements on the control system.

4. High-reliability control system The power plant is a continuous production process and requires highly reliable control systems, especially the protection system of the unit, often adopting a 1:1 or 3 redundant system to meet the reliability requirements of the system. .

V. Obstacles to the application of fieldbus control systems in power plants Although there are many advantages in the fieldbus control system, its application in power plants, especially in large-scale generating units, is still subject to the following obstacles.

1. The standard unified communications technology is the core technology of the fieldbus control system. Adopting a unified international standard is undoubtedly the user's expectation and the right choice. Although the International Electrotechnical Commission has already published the international standard IEC61158 for fieldbus, this standard contains eight fieldbuses. It is not a unified standard, which brings difficulties for users to select fieldbus.

2. Coordinated control of complex systems Field devices in fieldbus control systems can implement basic control functions, but how to achieve coordinated control of complex systems is still a matter of consideration. In a two-layer FCS system, coordination control can only be implemented in the process control interface card (PCI). However, PCI cards in some fieldbus control systems cannot perform control functions. Even though PCI cards can implement control functions, people still have concerns about its reliability.

3. The integration of the switch system and the analog system Most of the fieldbus devices currently implement analog control functions. Although some companies have developed fieldbus devices with switch control functions, two types of control must be implemented simultaneously in one fieldbus device. The function is still very difficult.

4. Most of the fieldbus devices in high-speed switching and analog control systems cannot realize the high-speed switching and analog control systems required by power plants. These control systems still use traditional dedicated controllers.

5. Fieldbus Physical Layer Redundancy Currently, the underlying fieldbus connecting fieldbus devices cannot be redundant. People are skeptical about the reliability of the underlying fieldbus. This situation is particularly serious in power plants.

6. Lack of plant-specific fieldbus equipment Although the series of fieldbus products and high speeds have been continuously introduced, for example, fieldbus equipment certified by the Fieldbus Foundation FF has approached 100, some plant-specific fieldbus equipment are still not available. Suitable product.

VI. Possible Solutions As a new control system architecture, the fieldbus control system is in a stage of perfect development. Although the thermal process control system of the power plant is special, some special requirements have been put forward for the fieldbus control system. These requirements are not difficult to solve through the further development and research of the fieldbus control system.

The problem of coordinated control of the system can be achieved by setting up a controller at the middle level, that is, using a three-layer field bus control system; in addition, the FCS extended from the DCS can naturally be coordinated by the original controller. control function.

Now some companies have developed fieldbus devices with logic control functions, or added field bus interfaces on existing programmable logic controllers to solve the problem of mixed control of the switch quantity and analog quantity.

At present, although there is no high-speed switching and analog control system that can be used for SOE and DEH, field bus technology with millisecond-level time synchronization has matured. The emergence of such products can be said to be just around the corner.

With regard to the redundancy of the fieldbus physical layer, it is now included in the R&D program. In the near future, there will be a series of field control devices with redundant fieldbuses. At that time, concerns about the reliability of the fieldbus will no longer exist.

In short, with the continuous deepening of fieldbus control system research and development work, a variety of fieldbus devices will continue to emerge, and will certainly meet the full needs of power plant automation.

7. Conclusion Fieldbus control system, as a new generation of control system, is being gradually applied in the industrial process control in China. Its control area and control scale are continuously expanding. At present, field buses have been used in more than a dozen power plants such as Wangting, Anshan, Yeli, Jinsu, Laibin, Junliangcheng, Huoping and Lingao in China. Recently, the country's largest fieldbus control system project has been started at Zhuzhou Smelter. The system has a total I/O points of 1765 points, 98 control loops, 11 H1 cards, 13 workstations, and 4 controllers. It can be predicted that with the continuous development of fieldbus technology, the fieldbus control system will become the mainstream of the 21st century industrial process control system.