This paper analyzes the reasons why DCS system plays an important role in the field of process control in thermal power plants at present, and expounds the main problems existing in the practical application of DCS system in process control. Several DCS systems used in Zhejiang power plants are compared.

Keywords: distributed control system DCS application technology fault dispersion

1 Introduction

DCS(Distributed ControlSystem) system has been applied to the process control of generator sets in thermal power plants for more than ten years, and it is more and more widely used. At present, the more popular DCS systems are MOD-300 of C&E, TDC-3000 of Honeywell, I/As of FOXBORO, CENTUMXL of Yokogawa, WDPF of Westinghouse, Wang Xuwen -90 of Bailey and Teleperm Me/XP of Siemens. As far as the systems provided by these major DCS manufacturers are concerned, there are some problems in the application, such as system reliability, real-time, flexibility and convenience of system configuration, which need to be further broken through. This paper analyzes and discusses some problems existing in the practical application of DCS system.

2 the original intention of DCS system launch

The introduction of DCS system and its gradual development into the leading role in the field of process control are due to the following reasons:

(1) The modern production process system is becoming more and more complex, and the parameters that need to be detected and controlled are greatly increased. The traditional instrument control system is not competent, and it is bound to find another way.

(2) Traditional instrument control systems usually use products provided by many manufacturers, which leads to a wide variety of spare parts needed for process production and consumes a lot of manpower and material resources. Process production depends on these instrument manufacturers to a considerable extent. This situation also hopes to change.

(3) With the rapid development of digital circuit technology, especially the application of large-scale integrated circuit technology, the integration and yield have been greatly improved, which makes it possible to use a large number of microprocessors in process control.

(4) The development of automation control theory, especially the discrete theory and sampling theory of continuous system, has greatly promoted the development of process control from traditional instrument control system to DCS system.

(5) The development of communication theory and technology. In the process of a lot of research on LAN, communication theory and technology have been greatly developed and improved, which has a decisive impact on the development of DCS system.

(6) The development of computer application technology, especially the introduction of operating system software such as Microsoft Windows-95, has laid a foundation for the application of computers in process control systems. There is a good interface between computers and operators, which is acceptable for operators without computer expertise.

(7) Computer centralized control system has some inherent defects, such as fault concentration, so in order to improve the reliability, it needs huge expenses, and the centralized control system needs large computers, which is expensive. Relatively speaking, the microprocessors and microcomputers used in DCS system are much cheaper, and the faults are relatively scattered. Moreover, the microcomputer or microprocessor in DCS system runs in parallel, and the processing speed is much higher than that in serial operation of centralized control system, so it has higher real-time index, which is the key to the rapid development of DCS system than centralized control system.

The above lists the main reasons that can explain why DCS system will show its talents in the field of process control. Now let's take a look at some original intentions of launching DCS system.

DCS system is a computer (or microcomputer) control system relative to computer centralized control system. It is developed on the basis of the study of computer local area network, and is used by process control experts to turn local area network into a network control system with high real-time and reliability requirements, which is applied in the field of process control. Experts call such a network control system DCS. The original intention of experts should be that such a control system will bring at least the following benefits:

(1) Fault dispersion. This is the biggest reason for the introduction of DCS system, in order to solve the fatal weakness of centralized control system, "fault concentration". The reason of fault dispersion is that a large number of microprocessors are used in DCS system, each microprocessor undertakes a small-scale (regional) control task, and the fault of one microprocessor will not affect the normal work of the whole system.

(2) Reduce the size of the control room or control the length of the dial.

(3) A large number of cables required by the control system are reduced.

(4) The types and quantities of spare parts required by the control system are greatly reduced.

(5) Reduce the dependence of process production on instrument control equipment manufacturers and reduce the training cost of instrument control personnel.

(6) It provides the flexibility of the control system and has the advantages of convenient configuration and expansibility.

(7) Realize the management of real-time process parameters and historical data, and provide the functions of performance calculation and equipment life calculation. This is beyond the reach of traditional instrument control system.

3 The actual capacity of DCS system that can be provided by the market at this stage

Starting from the application of Wang Xuwen -90 of Bailey Company, MOD-300 of C&E Company and Teleperm-Me/XP DCS of Siemens Company in Zhejiang Power System Power Plant, the author's views are as follows:

(1) About fault dispersion. It is believed that the systems provided by most DCS system manufacturers at this stage are not as "fault-dispersed" as imagined in practical application. In fact, due to some problems in the application technology of DCS system, the generator of 1 thermal power plant controlled by DCS system was forced to stop due to some faults of DCS system. Compared with the traditional instrument control system, the latter seems to be superior to the former. Because of this, the closer the composition of DCS system is to the traditional instrument control system, that is, the more dispersed the control tasks undertaken by microprocessor or multifunctional controller geographically, the more dispersed the faults can be. From this point of view, SIEMAMS's system is very good. A control card (with a microprocessor, which can communicate with other cards or other subsystems such as the engineer workstation of the operator station through the communication bus) only undertakes 2 ~ 4 open-loop control loops of motors or electric doors, or 1 ~ 2 closed-loop control loops. The card not only undertakes logic control functions, but also I/O functions related to these logic controls. This is very beneficial to fault dispersion. However, other systems usually concentrate many process control tasks on a few multi-function control cards or microprocessors, and concentrate I/O functions of process control on I/O cards. These systems objectively concentrate the faults of process control, rather than disperse them, which seems to be contrary to the name "DCS". However, there are also some problems in SIE MAN system, and there are still some problems to be solved in the communication between its Ethernet and me system. Teleperm-ME system of Siemens is a very mature system. Me system and Ethernet are the latest Teleperm-em/XP systems (applied to two 330 MW units in the fourth phase of Taizhou Power Plant, Zhejiang Province). Because it is a new system, I believe many applications (especially those related to communication) will stand the test of time. If these problems are solved, there will be some advantages in fault dispersion and other aspects, such as manufacturing process. DCS, the fault is scattered, it seems that Siemens is on the right track, because their system is closest to the traditional instrument control system, and the control function is scattered. In order to realize fault decentralization, control functions must be decentralized.

(2) About 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 scale and equipment of the electronic room are relatively increased, and the actual benefits brought by DCS system are not significant.

(3) About saving cables. At present, the equipment and devices used in DCS system are relatively expensive, and the requirements for the field environment are also relatively high, such as dust prevention, air conditioning, and remote I/O can not be widely used. Therefore, the main equipment of DCS system needs to be placed in the electronic room with good conditions, and a large number of field signals still need to be connected to the electronic room through cables. Therefore, compared with the traditional instrument control system, there are fewer cables, but the benefits are limited.

(4) About reducing the types and quantities of spare parts. The types and quantities of spare parts are reduced, and the manufacturers of instrument and control equipment that need to be handled are also reduced.

(5) Reduce the dependence of unit operation on instrument and control equipment manufacturers, and reduce the technical training expenses of instrument and control personnel. Due to the unsatisfactory application technology of DCS system, the dependence of unit operation on DCS system manufacturers has not decreased, but increased, especially during the new construction and commissioning of the unit, and the expert service sent by DCS manufacturers to the construction site seems to be essential during the unit commissioning. In order to make good use of DCS system, the number of engineers and technicians who need training has also increased, so the training cost has also increased.

(6) Flexibility of the control system, convenience of configuration and expansibility of the system. The flexibility and expansibility of the system structure are really incomparable to the traditional instrument control system. The configuration of most DCS systems is also very convenient. However, most systems still have a lot of work to do in online configuration function. Many systems are configured offline, programmed at the engineer station, and then compiled and downloaded. And this process is time-consuming in some systems, especially during debugging. On this issue, Siemens' system has its own characteristics. Not only can it be configured online, but also the simulation function is designed, which brings great convenience to debugging.

(DCS system provides some unique control functions, such as management of historical data and real-time data, performance calculation, etc. This is unmatched by traditional instrument control system, and it is precisely because of these advantages of DCS that process control is pushed to a new and higher level field.

To sum up, the benefits of DCS system that can be provided by the market in application at this stage are far from what we imagined. Of course, DCS system solves the problem that traditional instrument control system is not competent for modern mass production process control. Compared with computer centralized control, the faults are still scattered, and it has its unique advantages in many aspects, which is incomparable to traditional instrument control system and computer centralized control system.

4. Some problems in application

There are still some problems in the application technology of DCS system at present:

(1) Fault dispersion. At present, the DCS system has not achieved real fault dispersion, on the one hand, because the current equipment is still relatively expensive, so the geographical dispersion of the control system can not be achieved (as close as possible to the traditional instrument control system). The field conditions of industrial control are generally harsh, and dust, temperature and humidity are still difficult to adapt to the remote I/O card provided by manufacturers at this stage. For example, some I/O cards of Beilun Power Plant 1 unit can only be placed relatively centrally in the field, and there have been cases where the water in these I/O cabinets affected the normal operation of the whole system during debugging. Another example is that Unit 3 of Beilun Power Plant has set up a remote I/O cabinet in the circulating water pump room. Of course, cables are saved, but because of the small air conditioner on the I/O cabinet, the cost is still not much saved. On the other hand, the control function is not decentralized in design, including the design of DCS system itself and engineering design. For example, the I/O of MOD-300 system is too concentrated, and the failure of I/O card will affect the normal work of the system. In addition, the auxiliary control of the unit is completed by the PLC system, and the communication between it and the MOD-300 system uses the gateway, which becomes a bottleneck of the whole control system. Once a fault occurs, it will affect the normal operation of the unit. Another example is the system of Unit 3 in Beilun Power Plant. The auxiliary control of the unit is completed by its SCS subsystem. One MFP of SCS subsystem is designed with 300 I/O cards, and one I/O card with 16 points is designed for different control objects, that is, one control object needs multiple I/O cards. Therefore, the failure of an I/O card will affect the normal work of several controlled objects, and one controlled object needs maintenance, which involves several I/O cards. In addition, an MFP contains too many controlled objects, and the failure of an MFP card will affect the normal work of many control loops. A control system designed like this inevitably requires high reliability of the card or system.

Literature [1] and literature [3] both talk about a system called FCS, which is a process control system composed of field intelligent instruments and field bus technology. Obviously, such a system is very close to the traditional instrument control system. Theoretically, the intelligent control instrument is placed near the controlled equipment, and the control function can be completely dispersed in the area, so the fault can also be completely dispersed. But in fact, judging from the technical level of component production and the present situation of computer technology and communication technology, FCS may also face the difficulties faced by DCS. First of all, FCS is to solve the problem that the industrial control site environment is very bad. Secondly, the field control equipment is distributed in space and geographically, so it is not easy to connect with the field bus, and communication will be too difficult.

(2) Real-time. Process control requires high real-time performance of the control system. Due to the limitation of communication technology and computer technology, the real-time performance of the current system needs to be improved. For example, in the early MOD-300 system, it took more than ten seconds to operate a valve from the time when the operator pressed the key to issue a command to the time when the feedback signal was seen on the screen. Later, after hardware upgrade and replacement of a new generation of chips, the problem was solved. In some systems, it is inconvenient for operators to switch several screens when operating a device because of the slow screen opening speed. In addition, the time required for communication (or the response time of DCS system) deteriorates the characteristics of the controlled object and reduces the quality index of the closed-loop control system in the transition process.

(3) Configuration convenience and necessary authority protection. As for system configuration, most DCS systems now adopt computer window technology, which is generally convenient, but there is still room for learning online configuration. Most systems are configured in the engineer station and then downloaded to the control subsystem or control card. Generally, this process is very time-consuming, especially during debugging, configuration modification is a frequent thing, so it is necessary to find a convenient and safe online configuration method.

(4) Manufacturing process and installation process. A very important factor affecting the normal use of DCS system is the manufacturing process and installation process. Sometimes it is because a small plug-in is of poor quality and poor contact that it leads to a major failure or accident. The hardware manufacturing process provided by some systems is not satisfactory, and the cards and connectors provided are not reliable enough. Some cards are faulty, and sometimes they disappear after a few plugs or shakes. This aspect is a problem that DCS manufacturers must attach great importance to. If the provided system cannot meet the reliability requirements technically, it is unacceptable in principle. On the other hand, it is installation technology, which is also an aspect to be studied in the application technology of DCS system.

(5) Reliability and maturity of application software. The application practice of DCS system is still in the process of continuous accumulation, and many application softwares are not very mature, and its reliability needs time to test. In this regard, the manufacturers of DCS system should strengthen research to make the DCS system software that has been put into use or will be put into use have sufficient reliability.

5 conclusion

Starting from the reasons why DCS is introduced and why DCS has become the mainstream in the field of process control, some problems in the application technology of DCS system that can be provided by the market at this stage are put forward. These problems are: (1) About the fault dispersion; (2) Real-time; (3) the convenience of configuration and necessary authority protection; (4) related to manufacturing technology and installation technology; (5) Reliability of application software. It can be summarized as a design problem, a component production technology problem and a process problem. Based on the actual needs of process control and aiming at its own shortcomings, DCS system should be continuously improved and developed in structure, equipment and technology. New breakthroughs have been made in the application technology of DCS system.

References:

[1] Changli Wang, edited by Luo An. Selection and application of distributed control system. Tsinghua University Publishing House, 1996, 1 version.

[2] Wany H. Smeallie. Distributed control system based on microprocessor in modern power plants. Babcock&Wilcox Conference on Operation and Maintenance Experience,1October 1983.

Wang Jinquan, Fang Zhonghua, Zhong Weiyang. Present situation and prospect of industrial control system. China Electric Power, 1998 (4).

I hope it helps you.