Abstract: This paper analyzes the application status of DCS remote I/O in power plants, describes the advantages of using remote I/O in power plants, and proposes the application scope of remote I/O. The development of the current bus technology, advantages and existing problems are analyzed, and the prospect of the field bus control system in the power plant is forecasted.
The most prominent feature of a decentralized control system is that it can achieve decentralized control functions, discrete physical locations, and centralized monitoring. However, since the introduction of DCS in China's 300MW or more units in the mid-1980s, people often only focused on using the DCS control function to disperse this feature. However, there are various practical applications of physical dispersion, which can obviously save investment and shorten the construction period. Concerns, thus failing to give full play to the huge economic benefits that DCS should generate.
1DCS Arrangement Situation and Status of Remote I/O Application At present, from the perspective of the DCS layout of 300 MW or more units that have been put into operation in China, most power plants have centralized the controller processing unit and I/O cabinet in the electronic room of the control building. The operator station is arranged in the unit control room, the data communication highway connects the controller and the operator station, the data communication cable is not in the control building, and all the I/O field cables are pulled to the electronic room. Centralized I/O cabinet. The advantage of this is that the protection level of the DCS cabinet is relatively low, and the operation and maintenance conditions are good. However, the control floor area is large and the cable investment is large. Distributing the controller processing unit cabinets to the site and interconnecting the controller processing unit cabinets with communication cables throughout the entire plant can save a lot of cables, but from the standpoint of the unit safety, people think that the risks are too great to be avoided. consider. Therefore, the controller processing unit cabinets are arranged relatively centrally, and the solution using remote I/O decentralized arrangement becomes the best choice.
The plant that uses remote I/O to monitor the whole plant and has actual performance is the 2X600MW unit of Zhejiang Beilun Power Plant Phase I project. The plant widely uses remote I/O in DAS, SCS, and BMS. Nearly a decade of experience has shown that remote applications are successful and that savings in cable investment are also significant. However, in the subsequent phase II project, the decentralized control system only retained the remote I/O of the circulating pump room, and the remote I/Os in the remaining main buildings were all cancelled. The main reason is that during the construction of the first phase, due to improper construction management, the remote I/O cabinet placed on the site was flooded and some of the card members were damaged. Therefore, in order to avoid the similar accident when determining the design principles of the second phase project. The occurrence has eliminated the remote I/O that is distributed in the main building.
In addition, from the perspective of other units that are currently being designed and put into production in China, except for the use of domestic data mining smart front-end devices such as “893-network†remote intelligent I/O products in the DAS, no remote I/O has been used for the entire plant. The control and monitoring application examples are also largely due to concerns about the security and reliability of remote I/O.
Under the situation in which the design unit emphasizes the control of project construction costs and optimized design, it is necessary for us to re-evaluate the advantages of DCS physical decentralization and application of remote I/O in today's civilized construction and management of power construction units.
2 Physical Isolation Solution Using Remote I/O 2.1 Remote I/O Application Scope According to the application scope of remote I/O, the following options are available:
Solution I. Apply remote I/O to the four systems of DAS, SCS, BMS, and MCS.
Although the MCS system has no examples of remote I/O at present in China, it should be fully applicable to MCS from the perspective of remote I/O security and reliability. Therefore, the author believes that pilot projects can be conducted in the project.
When equipment and system protection are also implemented using DCS, taking into account the safety of equipment and systems, signals involving the protection of boilers and turbine equipment should be directly introduced into the DCSI/O cabinets (I/O cabinets centrally arranged in the electronics room).
In order to reduce the number of spare parts and maintain the consistency of products, this proposal suggests that the remote I/O should use the products of the same DCS manufacturer as much as possible.
Solution Two, only remote I/O is applied to three systems: DAS, SCS, and BMS.
This program has mature use performance, such as the first phase of Beilun Power Plant. The principle of equipment and system protection is the same as that of scenario one.
Solution 3: Remote I/O is applied in the three systems of DAS, SCS, and BMS, but the DAS system adopts some domestic data mining smart front-end devices.
In order to control the cost, save equipment investment and obtain the best performance and price ratio, according to the requirements of the “Provisions on Certain Technical Issues for the Design of Distributed Control System of Unitary Units†in the No. 214 of the Electric Power Planning Institute (No. 1996) issued by the Electric Power Planning Institute. , "Boiler and turbine metal temperatures; generator coils, cores, hydrogen and cooling water temperatures; auxiliary bearing temperature, etc. should use domestic remote I/O channels with successful application experience", so consider using in DAS systems Domestic data mining smart front-end device.
The principle of equipment and system protection is the same as that of scenario one.
The fourth option is to use the domestic data mining intelligent front-end device only in DAS. The application scope is the same as that of Option 3. Other systems still use centralized I/O.
Comparing the above four schemes, we suggest adopting scheme 1 or scheme 3, or a combination of the two.
In the selection of DCS, products supporting field smart meters should be selected as much as possible so that DCS can use field device management software to manage on-site smart meters, such as on-line parameter tuning, online device operation status and diagnostic information, etc. Availability and maintainability.
The division of systems such as MCS, SCS, BMS, and DAS is not conducive to the full application of remote I/O resources. The author believes that in the case of full consideration of measurement points and equipment redundancy, DCS resource sharing should be achieved without artificially dividing the system, so remote I/O does not need to be set by the system.
2.2 Remote I/O physical decentralized deployment scheme To maximize the advantages of remote I/O physical decentralization, taking into account the environmental conditions of operation and maintenance, remote I/Os in the turbine room and boiler room can be arranged differently. the way. Due to the better conditions of the engine room, the remote I/O can be arranged as close as possible to the measuring point according to the principle of relative concentration of the area according to the arrangement of the maintenance equipment. The 0-meter, 6-meter, and operating layers can be arranged without separate compartments. The boiler room should be equipped with remote I/O cabinets between distributed electrical equipment rooms such as MCC due to the harsh environment. If there is no such condition, some small rooms can be set up under open air conditions to facilitate overhaul, but no air conditioning equipment needs to be configured. .
3 Advantages and Economic Analysis of Remote I/O 3.1 Remote I/O Design Features In recent years, most of the remote I/Os developed by DCS manufacturers have focused on improving their intelligence and ability to adapt to the environment. In order to adapt to the on-site industrial environment, such as: heat, temperature, impact, vibration, radio frequency interference, electromagnetic interference, water, dust, harmful substances, etc., DCS manufacturers have adopted special designs to prevent the remote I/O modules and cabinet structure from being prevented. Or reduce the impact of these factors. The following are some of the main design features of remote I/O:
1) The cabinet structure is fully sealed and the cabinet surface is treated with anti-corrosion.
2) The electronic module part is separated from the wiring unit and isolated. Some manufacturers, such as FOXBORO, even package each module individually without exposing the electronics.
3) The I/O module uses low-power components. The unique heat dissipation design makes it unnecessary for the I/O cabinet to be placed in an air conditioning environment. Some manufacturers' I/O cabinets have their own air conditioners.
4) The remote IlO uses serial communication mode to exchange data with the controller processing unit. The most important feature of the serial bus is that it is easy to isolate. In general, photoelectric isolation or transformer isolation technology is used in electrical. The communication standard uses RS-485 or similar serial bus. Communication media is shielded twisted pair, coaxial cable or fiber optic cable. According to need, the communication cable can be configured redundantly. The communication speed is greater than 100Kbps and the communication distance is over 1200M.
5) Intelligent remote I/O products can realize functions such as A/D conversion, cold-side temperature word compensation, engineering unit conversion, automatic range conversion, nonlinear compensation, upper and lower limit alarm, self-check, self-correction, self-diagnosis, and other functions Minimize the burden on the controller's processing unit.
The above design features do not represent the structural features of all DCS manufacturers' remote I/O products, but they are some basic design ideas.
The domestic data mining intelligent front-end device consists of three parts: the front end of the data acquisition, the network communication bus and the communication adapter.
The front end of the data acquisition is a device that completes the on-site signal conversion and processing, and generally has a concave channel or so. With independent power circuit, can work independently. Can realize the transformation of engineering units, more than limit alarm, data storage, event sequence recording, automatic range conversion, nonlinear compensation, thermocouple cold junction temperature compensation and other functions. Low-power chip, low heat dissipation, sealed with metal shell, with high temperature, waterproof, dust-proof and anti-jamming features, can be installed near the scene measurement and control points.
The network communication bus connects the data acquisition front-ends in parallel on the serial network bus to digitally communicate with the host (PC or workstation). An RS-485 or similar differential asynchronous serial half gg-r_ communication bus is commonly used as a network communication bus. The network communication bus has no redundant configuration. Devices connected to the network are strictly electrically insulated, leaving the network in an electrically "floating" state.
The communication adapter is an interface device that realizes mutual coupling between a PC or a workstation (or DPU of a DCS) and a network communication bus and completes the unified and coordinated management of the entire network system.
3.2 Advantages of using remote I/O The remote I/O, including the domestic data mining smart front-end device, has many advantages over the rack-mounted centralized I/O, which is mainly reflected in the following aspects:
1. Saving cable investment The remote I/O cabinet can be installed near the signal point on the industrial site. Therefore, signals in tens or hundreds of points in a certain area can only pass one pair or several pairs of shielded twisted pair cables, coaxial cables or optical cables. Decentralized control processing unit or PC communication. This means that the cable distance from the on-site monitoring point to the remote I/O will be significantly reduced compared to the centralized I/O cabinet. A 4500 MW unit (including electrical quantity), if I/3 uses remote I/O, the average point-by-point cable savings of 80-100 meters is estimated (the original cable length is calculated as an average of 120 meters per point) will save 120- 150 km cable, about saving cable X-27%.
2. Save on cable installation costs With centralized I/O, a large number of cables must be laid between the field and the electronics to transfer signals from the field to the DCS. Cabling must use cable trays or cable channels. In order to reduce the interference cable bridge, it should be set up in layers and be grounded or shielded. Infrastructure investment and installation projects are quite large. The 300MW unit cable tray usage amounted to 700 tons and the installation workload reached 4550 man-hours. With remote I/O, hundreds of signal cables entering the control building are replaced by several communication cables, so that a large number of cable trays can be replaced by several cable guides. According to preliminary estimates, cable bridges can save 20-25%.
3. Saving the investment in the electronic room of the control building As the electronic room of the control building will mainly contain controllers and some centralized control units for the centralized IlO cabinet, turbines and boilers, the area of ​​the electronic room can be greatly reduced. If 60% of the IlO cabinet is placed on site (discussed in detail before the scope of use of the remote I/O), the electronic room area will be reduced by 30-40%. After the area of ​​the electronic room is reduced, the investment in air-conditioning will also decrease accordingly.
4. System Reliability Improvements Centralized layout of I/O cabinets, due to the need to introduce signals into DCS systems over longer cables, has also introduced significant interference. If the I/O module is a common ground system, the probability of mutual crosstalk will increase exponentially. With the use of remote I/O, the signal can be processed nearby. Because the signal introduction cable is shortened and the interference introduced is small, compared with the centralized arrangement, the stability and measurement accuracy of the DCS are improved, and the damage rate of the module is reduced.
The remote I/O communication network adopts the serial bus, and there are strict electrical isolation measures between the network bus and the device. Therefore, even if a certain device is damaged due to high voltage, it will not affect the normal operation of the network and other devices. In the centralized I/O system, due to the parallel common ground structure, there is an electrical connection between the modules. Once a certain module has high voltage or strong interference, it will affect the operation of the entire system.
In addition, the remote I/O communication bus operates in an electrically “floating†manner, with a high degree of isolation and usually does not require a dedicated ground.
4 Technical Specifications of Remote I/O Requirements 4.1 Degree of Protection of Cabinets In closed steam room and boiler room, the degree of protection of remote I/O cabinets shall be IP54; the level of remote I/O cabinets placed outdoors shall be IP66, The surface is treated with anti-corrosion coating.
4.2 I/O module environment and performance requirements 1) Environmental conditions Operating temperature: 0-60°C,
Storage temperature: -40-+70°C
Relative humidity: 5-95% (non-condensing)
2) Performance requirements I/O channel isolation voltage: greater than 500VAC between channel and ground, channel and channel
Common mode rejection ratio: greater than 120db
Chain mode suppression ratio: greater than 60db
5 Field bus application prospects in power plants 5.1 Proposed fieldbus The practical application of decentralized control systems in the near-vice years of industrial process control, the characteristics and superiority of its digital control are increasingly known by people, such as measurement With high control accuracy, easy configuration, strong anti-interference ability, and easy maintenance, the distributed control system does not constitute a true all-digital control system. The signal transmission between the continuous control function of the DCS and the field instrument and control device is still based on analog signals, even if the digital communication is used between the current remote I/O distributed local area network and the control processor unit, and the remote I/O and Field instruments still retain analog signal transmission. The HART protocol superimposes digital information on analog signals and does not escape analog signals. In addition, the current control equipment has poor interchangeability and interoperability; the control system has a low level of management of field instruments and control devices; the larger the control scale, the greater the amount of control signal cables, the higher the installation and maintenance costs, and so on.
The rapid development of computer technology and network technology has promoted the structural revolution of industrial control systems. The control functions of computer control systems have penetrated into the lower layers of the production process, so smart meters and control devices have appeared. The field bus is the lowest level in the control system. The communication network is a local area network that directly relates to the production process. It connects the smart meters and control devices installed in the industrial field with the control devices set in the control room. It is a fully digital, serial, bidirectional, multivariable, multi-node communication. The internet. The fieldbus control system (FCS) is a system composed of many field intelligent devices that are interconnected with the control room interior master control system and man-machine interface. This is a true full decentralized, fully digital, fully open, interchangeable, and interactive system. Operational new production process control system.
5.2 Characteristics of Fieldbus Control System The fieldbus control system is based on fieldbus products and consists of a fieldbus network and on-site smart devices.
Due to the wide variety of fieldbus products at present and no real scale production. Therefore, we do not need to discuss which kind of field bus is more superior and more promising. We want to explore the possibility and reality of its application in power plants from the design ideas of fieldbus control systems. At the same time, because Foundation Fieldbus (DV) products dominate the field of process control, we use FF as the basis for discussion.
The FF Fieldbus is a practical technology and is an open, digital, multi-node communication technology for intelligent field devices and automation systems. It makes it possible to replace 4-20mA with digital signals; it makes it possible to unify the management and control of the field; it makes it possible to complete some basic control processes on site; it makes it possible to add non-control information to the equipment.
The on-site smart devices mainly include the intelligence of sensors, transmitters, actuators, and other on-site measurement and control units. They can intelligently process signals of the process equipment, including A/D conversion, digital filtering, automatic temperature compensation, and linear or nonlinear conversion. , also has PID adjustment, valve position compensation and other functions, and has a FF-compliant communication interface to achieve the exchange of information between on-site smart devices and on-site main control system, making it possible to manage the field devices.
FF field bus is divided into H1 low-speed bus and H2 high-speed bus. PF field bus communication medium uses shielded twisted pair, non-twisted multi-core cable or optical cable.
The H1 low-speed bus network is suitable for communication between instruments and actuators. Each segment connects 32 devices (using up to 240 repeaters), the transmission rate is 31.25Kbps, the communication distance is 1900 meters, and the topology can be a bus type. Or tree type. The H1 low-speed bus standard has been reviewed. The advantage of H1 is that it can achieve signal transmission and power supply through a pair of twisted pairs, and has intrinsic safety features. The H2 high-speed bus network is suitable for communication with multiple computers and actuators after centralized information, and is subject to unified management of the main control system. The H2 bus can integrate up to 32 H1 fieldbuses. The H2 transmission rate is a maximum of 1-2.5 Mbp. The communication distance is 500-750 meters. The topology is a bus type and can be configured redundantly.
The H2 bus standard has not been unified so far.
5.3 Advantages of Fieldbus Control System 1) True decentralized control and centralized management DCS is not truly decentralized in terms of function and structure. Functionally, it distributes the control functions to the control cards in the controller's processing unit. Each control card can control multiple circuits. From a physical point of view, the controller processing unit is generally centralized in the electronic room. Even if the remote intelligent I/O is used to bring the signal acquisition location close to the field signal, the I/O is relatively centralized. FCS differs from this in that it uses a completely decentralized control method, which spreads the control part to the site, and each control loop is completely implemented by distributed installation of field instruments. Of course, the complex control strategy also needs a higher-level controller to complete. At the same time, due to the intelligence of on-site instrumentation and equipment, FCS allows the control room to operate, adjust, and diagnose field instruments and devices and centralize information management through human-machine interfaces. The equipment management software can provide fault information, but also can provide the information needed for preventive maintenance and can help the user to analyze, identify the potential accident location in time before the accident, without requiring the operator to visit the site to inspect the fault and improve the system. Controllability and maintainability.
2 All-digital communication, system reliability, high accuracy In the field bus control system, from the transmitter's sensor to the control valve, the signal has always maintained the digital characteristics, so FCS is a purely digital system. The analog signal is replaced with a digital signal. Because of the high level of the digital signal, general noise interference is difficult to distort the digital signal in the FCS system. At the same time, the error detection function of digital communication can detect the bit error of digital signal transmission in the transmission. Therefore, all-digital communication has high precision and strong anti-interference ability, which greatly improves the accuracy and reliability of process control.
3 Investment Provinces and Short Construction Cycles A twisted-pair wire on the fieldbus can connect many instruments and devices, saving a lot of I/O, cables and bridges compared to DCS. At the same time, because the control function is dispersed to the field instruments, the control equipment between the electronic equipment in the control building will be greatly reduced, and the control floor area will also be greatly reduced. The installation workload is small, and the construction period will be greatly shortened.
4 System is open and interoperable The products of different manufacturers are interconnected, interchangeable, and interoperable. Spare parts will be greatly reduced. Interoperability means that field devices from different manufacturers can be used to replace field devices from another manufacturer that has failed. After the replacement, communications and functions can be restored to normal. The interoperability of field instruments is mainly achieved through the standardization of functional modules and their parameters. This requires that the description, parameter setting, and interconnection methods of the functional modules of different manufacturers must be open and unified, which is exactly what the Fieldbus Foundation 1V is committed to setting international standards.
5 The system is easy to configure and easy to expand Because the fieldbus defines the standard function modules, different manufacturers' equipments use the same or similar configuration methods. Therefore, there is no need to relearn and train configuration methods because of the different field instrument types or manufacturers. And programming languages.
When the system needs to be expanded, the new instrument can be connected to the existing field bus without adding any components, and only the function modules and parameters need to be set through the field bus.
6 A revolutionary fieldbus technology that will lead to thermal-control professional design will greatly reduce the amount of drawing design effort and shift to no-drawing design. The process of configuring the system hardware is the physical connection process of the fieldbus devices, and a large number of time-consuming control wiring diagram designs will no longer exist. Therefore, the design direction of thermal design for design institutes must be fundamentally adjusted. It must be turned to the design and development of application software. This is the only way to survive and develop thermal management professionals in design institutes. The personnel and knowledge structure of the thermal control professional must also adapt to the development requirements of the automatic control technology to make fundamental adjustments.
5.4 Field Bus Application Prospects in Power Plants 1) Fieldbus Technology Problems Currently Applied to Sub-Power Plants (1) The H1 field bus technology has been completed and the H2 field bus standard is under development. From the previous discussion, we know that the H1 fieldbus standard has no redundant configuration at present, and 32 field instruments can be connected to each H1 bus. Therefore, once the H1 bus fails, the impact is larger. If only a small number of field instruments are connected then the advantages of the field bus are lost.
(2) The process control of the power plant is a multivariable object control. Therefore, how to use the H1 field bus and realize the on-site signal measurement redundancy requirements and the complex control strategy needs further study.
(3) The fieldbus product series is not yet complete, and there is no point in the fieldbus products without a complete product series. The power plant has a lot of switch control, but so far there is no standard for switching equipment; and a large number of logic operations are not what a single field instrument can afford.
(4) The fieldbus control system lacks mature engineering experience. According to the market goal set by the Foundation Fieldbus in 1997, a small test system based on the U1 standard was provided by multiple suppliers in 1997 for training and technical confirmation; 1998 It began to be used for small-scale renovation projects; in 1999, it began to be applied to medium-to-large-scale engineering designs. More field equipment and software will be available for selection, and the It2 standard will be released.
2) The pilot project envisages that the power plant is an important production department involving the national economy and the people's livelihood. The design principle of the power plant is mature, safe, reliable and economical. From the perspective of the structure of the fieldbus, it is not developed for the application of the power plant. Therefore, the technology of applying the fieldbus control system in the power plant is not yet mature and needs to be gradually piloted.
To take advantage of fieldbus technology in a power plant, some of the issues mentioned above must be addressed. We believe that active application of field bus technology can effectively promote the solution of these problems.
Solution I. It is used together with the DCS. In the DAS part, the H1 field bus is connected to the smart transmitter for data acquisition. In the MCS part, the H1 field bus is used to connect the field intelligent controller and the actuator to replace part of the single-loop control system.
Solution II: It is used together with the PLC to use the H1 fieldbus to connect field smart transmitters, analysis instruments, and actuators to achieve data acquisition and control of auxiliary systems such as water treatment systems.
6 Concluding remarks Field bus control system has a long way to go in the full promotion of the power plant, and DCS remote I/O is mature in technology and has a successful application performance. Therefore, the use of remote I is strongly promoted in power plants. /O is the best way to control project cost and maximize the effectiveness of a decentralized control system.
The most prominent feature of a decentralized control system is that it can achieve decentralized control functions, discrete physical locations, and centralized monitoring. However, since the introduction of DCS in China's 300MW or more units in the mid-1980s, people often only focused on using the DCS control function to disperse this feature. However, there are various practical applications of physical dispersion, which can obviously save investment and shorten the construction period. Concerns, thus failing to give full play to the huge economic benefits that DCS should generate.
1DCS Arrangement Situation and Status of Remote I/O Application At present, from the perspective of the DCS layout of 300 MW or more units that have been put into operation in China, most power plants have centralized the controller processing unit and I/O cabinet in the electronic room of the control building. The operator station is arranged in the unit control room, the data communication highway connects the controller and the operator station, the data communication cable is not in the control building, and all the I/O field cables are pulled to the electronic room. Centralized I/O cabinet. The advantage of this is that the protection level of the DCS cabinet is relatively low, and the operation and maintenance conditions are good. However, the control floor area is large and the cable investment is large. Distributing the controller processing unit cabinets to the site and interconnecting the controller processing unit cabinets with communication cables throughout the entire plant can save a lot of cables, but from the standpoint of the unit safety, people think that the risks are too great to be avoided. consider. Therefore, the controller processing unit cabinets are arranged relatively centrally, and the solution using remote I/O decentralized arrangement becomes the best choice.
The plant that uses remote I/O to monitor the whole plant and has actual performance is the 2X600MW unit of Zhejiang Beilun Power Plant Phase I project. The plant widely uses remote I/O in DAS, SCS, and BMS. Nearly a decade of experience has shown that remote applications are successful and that savings in cable investment are also significant. However, in the subsequent phase II project, the decentralized control system only retained the remote I/O of the circulating pump room, and the remote I/Os in the remaining main buildings were all cancelled. The main reason is that during the construction of the first phase, due to improper construction management, the remote I/O cabinet placed on the site was flooded and some of the card members were damaged. Therefore, in order to avoid the similar accident when determining the design principles of the second phase project. The occurrence has eliminated the remote I/O that is distributed in the main building.
In addition, from the perspective of other units that are currently being designed and put into production in China, except for the use of domestic data mining smart front-end devices such as “893-network†remote intelligent I/O products in the DAS, no remote I/O has been used for the entire plant. The control and monitoring application examples are also largely due to concerns about the security and reliability of remote I/O.
Under the situation in which the design unit emphasizes the control of project construction costs and optimized design, it is necessary for us to re-evaluate the advantages of DCS physical decentralization and application of remote I/O in today's civilized construction and management of power construction units.
2 Physical Isolation Solution Using Remote I/O 2.1 Remote I/O Application Scope According to the application scope of remote I/O, the following options are available:
Solution I. Apply remote I/O to the four systems of DAS, SCS, BMS, and MCS.
Although the MCS system has no examples of remote I/O at present in China, it should be fully applicable to MCS from the perspective of remote I/O security and reliability. Therefore, the author believes that pilot projects can be conducted in the project.
When equipment and system protection are also implemented using DCS, taking into account the safety of equipment and systems, signals involving the protection of boilers and turbine equipment should be directly introduced into the DCSI/O cabinets (I/O cabinets centrally arranged in the electronics room).
In order to reduce the number of spare parts and maintain the consistency of products, this proposal suggests that the remote I/O should use the products of the same DCS manufacturer as much as possible.
Solution Two, only remote I/O is applied to three systems: DAS, SCS, and BMS.
This program has mature use performance, such as the first phase of Beilun Power Plant. The principle of equipment and system protection is the same as that of scenario one.
Solution 3: Remote I/O is applied in the three systems of DAS, SCS, and BMS, but the DAS system adopts some domestic data mining smart front-end devices.
In order to control the cost, save equipment investment and obtain the best performance and price ratio, according to the requirements of the “Provisions on Certain Technical Issues for the Design of Distributed Control System of Unitary Units†in the No. 214 of the Electric Power Planning Institute (No. 1996) issued by the Electric Power Planning Institute. , "Boiler and turbine metal temperatures; generator coils, cores, hydrogen and cooling water temperatures; auxiliary bearing temperature, etc. should use domestic remote I/O channels with successful application experience", so consider using in DAS systems Domestic data mining smart front-end device.
The principle of equipment and system protection is the same as that of scenario one.
The fourth option is to use the domestic data mining intelligent front-end device only in DAS. The application scope is the same as that of Option 3. Other systems still use centralized I/O.
Comparing the above four schemes, we suggest adopting scheme 1 or scheme 3, or a combination of the two.
In the selection of DCS, products supporting field smart meters should be selected as much as possible so that DCS can use field device management software to manage on-site smart meters, such as on-line parameter tuning, online device operation status and diagnostic information, etc. Availability and maintainability.
The division of systems such as MCS, SCS, BMS, and DAS is not conducive to the full application of remote I/O resources. The author believes that in the case of full consideration of measurement points and equipment redundancy, DCS resource sharing should be achieved without artificially dividing the system, so remote I/O does not need to be set by the system.
2.2 Remote I/O physical decentralized deployment scheme To maximize the advantages of remote I/O physical decentralization, taking into account the environmental conditions of operation and maintenance, remote I/Os in the turbine room and boiler room can be arranged differently. the way. Due to the better conditions of the engine room, the remote I/O can be arranged as close as possible to the measuring point according to the principle of relative concentration of the area according to the arrangement of the maintenance equipment. The 0-meter, 6-meter, and operating layers can be arranged without separate compartments. The boiler room should be equipped with remote I/O cabinets between distributed electrical equipment rooms such as MCC due to the harsh environment. If there is no such condition, some small rooms can be set up under open air conditions to facilitate overhaul, but no air conditioning equipment needs to be configured. .
3 Advantages and Economic Analysis of Remote I/O 3.1 Remote I/O Design Features In recent years, most of the remote I/Os developed by DCS manufacturers have focused on improving their intelligence and ability to adapt to the environment. In order to adapt to the on-site industrial environment, such as: heat, temperature, impact, vibration, radio frequency interference, electromagnetic interference, water, dust, harmful substances, etc., DCS manufacturers have adopted special designs to prevent the remote I/O modules and cabinet structure from being prevented. Or reduce the impact of these factors. The following are some of the main design features of remote I/O:
1) The cabinet structure is fully sealed and the cabinet surface is treated with anti-corrosion.
2) The electronic module part is separated from the wiring unit and isolated. Some manufacturers, such as FOXBORO, even package each module individually without exposing the electronics.
3) The I/O module uses low-power components. The unique heat dissipation design makes it unnecessary for the I/O cabinet to be placed in an air conditioning environment. Some manufacturers' I/O cabinets have their own air conditioners.
4) The remote IlO uses serial communication mode to exchange data with the controller processing unit. The most important feature of the serial bus is that it is easy to isolate. In general, photoelectric isolation or transformer isolation technology is used in electrical. The communication standard uses RS-485 or similar serial bus. Communication media is shielded twisted pair, coaxial cable or fiber optic cable. According to need, the communication cable can be configured redundantly. The communication speed is greater than 100Kbps and the communication distance is over 1200M.
5) Intelligent remote I/O products can realize functions such as A/D conversion, cold-side temperature word compensation, engineering unit conversion, automatic range conversion, nonlinear compensation, upper and lower limit alarm, self-check, self-correction, self-diagnosis, and other functions Minimize the burden on the controller's processing unit.
The above design features do not represent the structural features of all DCS manufacturers' remote I/O products, but they are some basic design ideas.
The domestic data mining intelligent front-end device consists of three parts: the front end of the data acquisition, the network communication bus and the communication adapter.
The front end of the data acquisition is a device that completes the on-site signal conversion and processing, and generally has a concave channel or so. With independent power circuit, can work independently. Can realize the transformation of engineering units, more than limit alarm, data storage, event sequence recording, automatic range conversion, nonlinear compensation, thermocouple cold junction temperature compensation and other functions. Low-power chip, low heat dissipation, sealed with metal shell, with high temperature, waterproof, dust-proof and anti-jamming features, can be installed near the scene measurement and control points.
The network communication bus connects the data acquisition front-ends in parallel on the serial network bus to digitally communicate with the host (PC or workstation). An RS-485 or similar differential asynchronous serial half gg-r_ communication bus is commonly used as a network communication bus. The network communication bus has no redundant configuration. Devices connected to the network are strictly electrically insulated, leaving the network in an electrically "floating" state.
The communication adapter is an interface device that realizes mutual coupling between a PC or a workstation (or DPU of a DCS) and a network communication bus and completes the unified and coordinated management of the entire network system.
3.2 Advantages of using remote I/O The remote I/O, including the domestic data mining smart front-end device, has many advantages over the rack-mounted centralized I/O, which is mainly reflected in the following aspects:
1. Saving cable investment The remote I/O cabinet can be installed near the signal point on the industrial site. Therefore, signals in tens or hundreds of points in a certain area can only pass one pair or several pairs of shielded twisted pair cables, coaxial cables or optical cables. Decentralized control processing unit or PC communication. This means that the cable distance from the on-site monitoring point to the remote I/O will be significantly reduced compared to the centralized I/O cabinet. A 4500 MW unit (including electrical quantity), if I/3 uses remote I/O, the average point-by-point cable savings of 80-100 meters is estimated (the original cable length is calculated as an average of 120 meters per point) will save 120- 150 km cable, about saving cable X-27%.
2. Save on cable installation costs With centralized I/O, a large number of cables must be laid between the field and the electronics to transfer signals from the field to the DCS. Cabling must use cable trays or cable channels. In order to reduce the interference cable bridge, it should be set up in layers and be grounded or shielded. Infrastructure investment and installation projects are quite large. The 300MW unit cable tray usage amounted to 700 tons and the installation workload reached 4550 man-hours. With remote I/O, hundreds of signal cables entering the control building are replaced by several communication cables, so that a large number of cable trays can be replaced by several cable guides. According to preliminary estimates, cable bridges can save 20-25%.
3. Saving the investment in the electronic room of the control building As the electronic room of the control building will mainly contain controllers and some centralized control units for the centralized IlO cabinet, turbines and boilers, the area of ​​the electronic room can be greatly reduced. If 60% of the IlO cabinet is placed on site (discussed in detail before the scope of use of the remote I/O), the electronic room area will be reduced by 30-40%. After the area of ​​the electronic room is reduced, the investment in air-conditioning will also decrease accordingly.
4. System Reliability Improvements Centralized layout of I/O cabinets, due to the need to introduce signals into DCS systems over longer cables, has also introduced significant interference. If the I/O module is a common ground system, the probability of mutual crosstalk will increase exponentially. With the use of remote I/O, the signal can be processed nearby. Because the signal introduction cable is shortened and the interference introduced is small, compared with the centralized arrangement, the stability and measurement accuracy of the DCS are improved, and the damage rate of the module is reduced.
The remote I/O communication network adopts the serial bus, and there are strict electrical isolation measures between the network bus and the device. Therefore, even if a certain device is damaged due to high voltage, it will not affect the normal operation of the network and other devices. In the centralized I/O system, due to the parallel common ground structure, there is an electrical connection between the modules. Once a certain module has high voltage or strong interference, it will affect the operation of the entire system.
In addition, the remote I/O communication bus operates in an electrically “floating†manner, with a high degree of isolation and usually does not require a dedicated ground.
4 Technical Specifications of Remote I/O Requirements 4.1 Degree of Protection of Cabinets In closed steam room and boiler room, the degree of protection of remote I/O cabinets shall be IP54; the level of remote I/O cabinets placed outdoors shall be IP66, The surface is treated with anti-corrosion coating.
4.2 I/O module environment and performance requirements 1) Environmental conditions Operating temperature: 0-60°C,
Storage temperature: -40-+70°C
Relative humidity: 5-95% (non-condensing)
2) Performance requirements I/O channel isolation voltage: greater than 500VAC between channel and ground, channel and channel
Common mode rejection ratio: greater than 120db
Chain mode suppression ratio: greater than 60db
5 Field bus application prospects in power plants 5.1 Proposed fieldbus The practical application of decentralized control systems in the near-vice years of industrial process control, the characteristics and superiority of its digital control are increasingly known by people, such as measurement With high control accuracy, easy configuration, strong anti-interference ability, and easy maintenance, the distributed control system does not constitute a true all-digital control system. The signal transmission between the continuous control function of the DCS and the field instrument and control device is still based on analog signals, even if the digital communication is used between the current remote I/O distributed local area network and the control processor unit, and the remote I/O and Field instruments still retain analog signal transmission. The HART protocol superimposes digital information on analog signals and does not escape analog signals. In addition, the current control equipment has poor interchangeability and interoperability; the control system has a low level of management of field instruments and control devices; the larger the control scale, the greater the amount of control signal cables, the higher the installation and maintenance costs, and so on.
The rapid development of computer technology and network technology has promoted the structural revolution of industrial control systems. The control functions of computer control systems have penetrated into the lower layers of the production process, so smart meters and control devices have appeared. The field bus is the lowest level in the control system. The communication network is a local area network that directly relates to the production process. It connects the smart meters and control devices installed in the industrial field with the control devices set in the control room. It is a fully digital, serial, bidirectional, multivariable, multi-node communication. The internet. The fieldbus control system (FCS) is a system composed of many field intelligent devices that are interconnected with the control room interior master control system and man-machine interface. This is a true full decentralized, fully digital, fully open, interchangeable, and interactive system. Operational new production process control system.
5.2 Characteristics of Fieldbus Control System The fieldbus control system is based on fieldbus products and consists of a fieldbus network and on-site smart devices.
Due to the wide variety of fieldbus products at present and no real scale production. Therefore, we do not need to discuss which kind of field bus is more superior and more promising. We want to explore the possibility and reality of its application in power plants from the design ideas of fieldbus control systems. At the same time, because Foundation Fieldbus (DV) products dominate the field of process control, we use FF as the basis for discussion.
The FF Fieldbus is a practical technology and is an open, digital, multi-node communication technology for intelligent field devices and automation systems. It makes it possible to replace 4-20mA with digital signals; it makes it possible to unify the management and control of the field; it makes it possible to complete some basic control processes on site; it makes it possible to add non-control information to the equipment.
The on-site smart devices mainly include the intelligence of sensors, transmitters, actuators, and other on-site measurement and control units. They can intelligently process signals of the process equipment, including A/D conversion, digital filtering, automatic temperature compensation, and linear or nonlinear conversion. , also has PID adjustment, valve position compensation and other functions, and has a FF-compliant communication interface to achieve the exchange of information between on-site smart devices and on-site main control system, making it possible to manage the field devices.
FF field bus is divided into H1 low-speed bus and H2 high-speed bus. PF field bus communication medium uses shielded twisted pair, non-twisted multi-core cable or optical cable.
The H1 low-speed bus network is suitable for communication between instruments and actuators. Each segment connects 32 devices (using up to 240 repeaters), the transmission rate is 31.25Kbps, the communication distance is 1900 meters, and the topology can be a bus type. Or tree type. The H1 low-speed bus standard has been reviewed. The advantage of H1 is that it can achieve signal transmission and power supply through a pair of twisted pairs, and has intrinsic safety features. The H2 high-speed bus network is suitable for communication with multiple computers and actuators after centralized information, and is subject to unified management of the main control system. The H2 bus can integrate up to 32 H1 fieldbuses. The H2 transmission rate is a maximum of 1-2.5 Mbp. The communication distance is 500-750 meters. The topology is a bus type and can be configured redundantly.
The H2 bus standard has not been unified so far.
5.3 Advantages of Fieldbus Control System 1) True decentralized control and centralized management DCS is not truly decentralized in terms of function and structure. Functionally, it distributes the control functions to the control cards in the controller's processing unit. Each control card can control multiple circuits. From a physical point of view, the controller processing unit is generally centralized in the electronic room. Even if the remote intelligent I/O is used to bring the signal acquisition location close to the field signal, the I/O is relatively centralized. FCS differs from this in that it uses a completely decentralized control method, which spreads the control part to the site, and each control loop is completely implemented by distributed installation of field instruments. Of course, the complex control strategy also needs a higher-level controller to complete. At the same time, due to the intelligence of on-site instrumentation and equipment, FCS allows the control room to operate, adjust, and diagnose field instruments and devices and centralize information management through human-machine interfaces. The equipment management software can provide fault information, but also can provide the information needed for preventive maintenance and can help the user to analyze, identify the potential accident location in time before the accident, without requiring the operator to visit the site to inspect the fault and improve the system. Controllability and maintainability.
2 All-digital communication, system reliability, high accuracy In the field bus control system, from the transmitter's sensor to the control valve, the signal has always maintained the digital characteristics, so FCS is a purely digital system. The analog signal is replaced with a digital signal. Because of the high level of the digital signal, general noise interference is difficult to distort the digital signal in the FCS system. At the same time, the error detection function of digital communication can detect the bit error of digital signal transmission in the transmission. Therefore, all-digital communication has high precision and strong anti-interference ability, which greatly improves the accuracy and reliability of process control.
3 Investment Provinces and Short Construction Cycles A twisted-pair wire on the fieldbus can connect many instruments and devices, saving a lot of I/O, cables and bridges compared to DCS. At the same time, because the control function is dispersed to the field instruments, the control equipment between the electronic equipment in the control building will be greatly reduced, and the control floor area will also be greatly reduced. The installation workload is small, and the construction period will be greatly shortened.
4 System is open and interoperable The products of different manufacturers are interconnected, interchangeable, and interoperable. Spare parts will be greatly reduced. Interoperability means that field devices from different manufacturers can be used to replace field devices from another manufacturer that has failed. After the replacement, communications and functions can be restored to normal. The interoperability of field instruments is mainly achieved through the standardization of functional modules and their parameters. This requires that the description, parameter setting, and interconnection methods of the functional modules of different manufacturers must be open and unified, which is exactly what the Fieldbus Foundation 1V is committed to setting international standards.
5 The system is easy to configure and easy to expand Because the fieldbus defines the standard function modules, different manufacturers' equipments use the same or similar configuration methods. Therefore, there is no need to relearn and train configuration methods because of the different field instrument types or manufacturers. And programming languages.
When the system needs to be expanded, the new instrument can be connected to the existing field bus without adding any components, and only the function modules and parameters need to be set through the field bus.
6 A revolutionary fieldbus technology that will lead to thermal-control professional design will greatly reduce the amount of drawing design effort and shift to no-drawing design. The process of configuring the system hardware is the physical connection process of the fieldbus devices, and a large number of time-consuming control wiring diagram designs will no longer exist. Therefore, the design direction of thermal design for design institutes must be fundamentally adjusted. It must be turned to the design and development of application software. This is the only way to survive and develop thermal management professionals in design institutes. The personnel and knowledge structure of the thermal control professional must also adapt to the development requirements of the automatic control technology to make fundamental adjustments.
5.4 Field Bus Application Prospects in Power Plants 1) Fieldbus Technology Problems Currently Applied to Sub-Power Plants (1) The H1 field bus technology has been completed and the H2 field bus standard is under development. From the previous discussion, we know that the H1 fieldbus standard has no redundant configuration at present, and 32 field instruments can be connected to each H1 bus. Therefore, once the H1 bus fails, the impact is larger. If only a small number of field instruments are connected then the advantages of the field bus are lost.
(2) The process control of the power plant is a multivariable object control. Therefore, how to use the H1 field bus and realize the on-site signal measurement redundancy requirements and the complex control strategy needs further study.
(3) The fieldbus product series is not yet complete, and there is no point in the fieldbus products without a complete product series. The power plant has a lot of switch control, but so far there is no standard for switching equipment; and a large number of logic operations are not what a single field instrument can afford.
(4) The fieldbus control system lacks mature engineering experience. According to the market goal set by the Foundation Fieldbus in 1997, a small test system based on the U1 standard was provided by multiple suppliers in 1997 for training and technical confirmation; 1998 It began to be used for small-scale renovation projects; in 1999, it began to be applied to medium-to-large-scale engineering designs. More field equipment and software will be available for selection, and the It2 standard will be released.
2) The pilot project envisages that the power plant is an important production department involving the national economy and the people's livelihood. The design principle of the power plant is mature, safe, reliable and economical. From the perspective of the structure of the fieldbus, it is not developed for the application of the power plant. Therefore, the technology of applying the fieldbus control system in the power plant is not yet mature and needs to be gradually piloted.
To take advantage of fieldbus technology in a power plant, some of the issues mentioned above must be addressed. We believe that active application of field bus technology can effectively promote the solution of these problems.
Solution I. It is used together with the DCS. In the DAS part, the H1 field bus is connected to the smart transmitter for data acquisition. In the MCS part, the H1 field bus is used to connect the field intelligent controller and the actuator to replace part of the single-loop control system.
Solution II: It is used together with the PLC to use the H1 fieldbus to connect field smart transmitters, analysis instruments, and actuators to achieve data acquisition and control of auxiliary systems such as water treatment systems.
6 Concluding remarks Field bus control system has a long way to go in the full promotion of the power plant, and DCS remote I/O is mature in technology and has a successful application performance. Therefore, the use of remote I is strongly promoted in power plants. /O is the best way to control project cost and maximize the effectiveness of a decentralized control system.
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