1. Overview of Fieldbus System

With the rapid development of control technology, computer technology and communication technology, digital technology is penetrating into the field equipment from the decision-making layer, management layer, monitoring layer and control layer in the industrial production process. Thus, fieldbus technology and its control system fieldbus control system (FCS) are produced. Since the advent of Fieldbus Technology in the late 1990s, it has created a new era of automatic control technology digital era. At present, fieldbus technology has been successfully applied in chemical, petrochemical, metallurgy, building materials, medicine and other industrial processes, and has achieved remarkable benefits.

In China, fieldbus has been used locally in thermal power plant unit control, but there is no example of using fieldbus control system in the whole plant. Therefore, we use fieldbus technology to control the boiler make-up water treatment, industrial wastewater treatment and circulating water treatment system (hereinafter referred to as the external water system of the main power house) with relatively low importance in a power plant, as an attempt and Research on the application of FCS.

2. Fieldbus Technology

According to IEC61158 standard of International Electrotechnical Commission, the data bus of digital, serial, bidirectional and multipoint communication between field device installed in manufacturing or production process area and automatic control device is called fieldbus. The control system composed of fieldbus and field intelligent equipment is called Fieldbus Control System FCS.

To measure whether a control system is a real fieldbus control system, FCS has three key points: core, foundation and essence. The core of FCS is bus protocol. Only the control system following fieldbus protocol can be called fieldbus control system; the basis of FCS is digital intelligent field instrument, which is the hardware support of FCS; the essence of FCS is the field of information processing, which is the embodiment of FCS system effectiveness. Fieldbus control system is a new control system with full computer, full digital and two-way communication. Digitalization and networking of field equipment level is the basis of power plant information management.

The starting point of fieldbus technology development is to provide users with open, interoperable, interchangeable and unified standard measurement and control products. The advantages of FCS based on fieldbus technology can be summarized as follows: interoperability, decentralization, reliability, accuracy, openness, economy and maintainability.

At the end of 1999, iectc65 (the 65th Standardization Committee responsible for industrial measurement and control) adopted 8 types of Fieldbus as IEC61158 international standard. Ten types of fieldbus standards (the Third Edition) were formulated after revision in August 2001
 

All kinds of fieldbus are developed for different application fields. It is impossible to use the same bus to solve all industrial control and process control.

PROFIBUS and FF Bus are suitable for continuous production process automation, such as petrochemical, chemical, electric power, metallurgy and so on. PROFIBUS and FF Bus are introduced as follows:

PROFIBUS Fieldbus

PROFIBUS is the abbreviation of process fieldbus. It is a fieldbus standard specified by IEC61158 and iec61784. PROFIBUS consists of three compatible parts, PROFIBUS-DP, Profibus-PA and profibus-fms.

PROFIBUS-DP is a high-speed and low-cost communication system, which is mainly used in unit level and field level communication of automation system. It defines the physical layer, data link layer and user interface according to ISO / OSI reference model. The transmission technology adopts RS-485 twisted pair cable and optical cable with baud rate from 9.6kbps to 12mbps. It supports single master or multi master system token transmission, and the maximum number of stations on the bus is 126. If twisted pair is used, the maximum communication distance is 1200m (the communication rate is 93.75kpbs), and it can be extended to 10km if repeater is used. With optical fiber, the distance can be extended to tens of kilometers and the communication rate can reach 12mbps.

Profibus-PA is specially designed for process automation. It can connect the transmitter and actuator on one bus, and provide intrinsic safety and bus power supply characteristics. Profibus-PA adopts the extended PROFIBUS-DP protocol, and there are also PA codes described by field devices. The communication medium is twisted pair, the transmission rate is 31.25kbps, and the distance can reach 1900m. Each segment has a maximum of 32 sites.

Profibus-fms defines the communication between master station and master station. According to ISO / OSI reference model, physical layer, link layer and application layer are defined. The application layer includes field message specification (FMS) and lower layer interface (LLI). The maximum communication rate is 12mbps (communication distance is less than 100m).

FF Fieldbus

FF fieldbus system consists of low speed H1 bus and high speed Ethernet H2. The transmission medium is twisted pair. H1 bus is mainly used in the production site, the speed is 31.25kbps, two-wire signal transmission and power supply to field equipment at the same time. H2 bus is mainly used in process control level, remote I / O and high-speed factory automation applications. It adopts 100Mbps high-speed Ethernet protocol.

Through the above introduction, we can compare PROFIBUS with FF Bus, FF is more suitable for continuous control (instead of 4 ~ 20mA analog), and PROFIBUS is more suitable for discrete control. PROFIBUS and FF Bus have been successfully applied in power plants at home and abroad.

3. Selection of Fieldbus for external water system of main power house of a power plant

Process characteristics of external water system of main workshop

Importance

The operation condition of each auxiliary system (workshop) is related to the normal operation of the whole power plant. Once a system has problems, it must be handled in time, actively contacted and reasonably adjusted, otherwise, the safe and economic operation of the whole plant will be affected.

Dispersion

Independent systems (workshops) are adopted for chemical make-up water treatment system, condensate polishing system, steam water sampling / chemical dosing system of unit, purified water treatment system, waste water treatment system, circulating water chlorination control system and domestic sewage treatment system, which are distributed throughout the plant and are far away from each other.

The discontinuity

Most of the auxiliary systems operate intermittently, that is to say, they can only be operated after certain requirements are met. When certain conditions are met, the system will stop running and wait for the next operation. Only the condensate polishing system needs long-term continuous operation.

Switching value control

Switch value control accounts for the vast majority of auxiliary system control. A large number of valves, solenoid valves and motors are subject to the logic control of interlocking conditions, which are mainly controlled by equipment status, valve switch, motor start-up and stop, pressure flow and material level limit. The control of analog quantity is mainly reflected in each dosing control link.

Operating environment conditions

The operation environment of each auxiliary system (workshop) is relatively complex. Some workshops often have large dust, a wide range of environmental temperature and humidity changes, some workshops have anti-corrosion requirements, and some workshops have explosion-proof requirements.

According to the control characteristics of the water system outside the main plant, combined with the characteristics and application fields of various fieldbus standards, PROFIBUS DP and FF fieldbus are selected to control the external water system of a power plant.

4. Fieldbus design scheme of external water system in main workshop of a power plant

The auxiliary network hardware of this project is ovation control system controller, PROFIBUS DP and FF Bus interface device of Emerson company. The following is a brief introduction to these two kinds of Fieldbus modules.

Ovation Foundation Fieldbus fieldbus module

Each ovation controller can connect up to 12 fieldbus FF modules. Each FF module can support two H1 fieldbus branches, and each H1 fieldbus branch can connect 16 field intelligent instruments supporting bus driving. Ovatiff fieldbus module can control the communication between intelligent devices and ovation controller. Ovationff fieldbus module provides a standard connection terminal, which uses a pair of shielded twisted pair cables to connect the field intelligent equipment, and the communication and power supply are realized through this pair of cables. Ovationff fieldbus module is connected with ovation controller through local I / O bus. This is equivalent to a super I / O module with many channels. The controller can use the data of this module in control logic, alarm and other applications just like other I / O data.

Ovation PROFIBUS DP fieldbus module

Each pair of controllers can support up to 12 PROFIBUS DP field bus modules, that is, 24 single master fieldbus branches or 12 redundant master fieldbus branches. PROFIBUS DP bus adopts master-slave communication mode. Each segment of Bus supports 32 slave devices, with a maximum of 126 devices when adding repeaters.

Ovationprofibus DP fieldbus module works as the main equipment of bus section. The module adopts standard RS485 transmission technology. This transmission technology, commonly known as H2, can be used in high-speed transmission, simple and cheap installation.

PROFIBUS Bus supports adding and deleting slave devices, which can be put into operation one by one without affecting the normal operation of other devices. In the future, the expansion of bus section will not affect the normal operation of the slave equipment. PROFIBUS DP technology is based on the bus with terminator, each connector can provide switchable terminator function. Ovation system usually sets this terminal at both ends of the bus section. After simultaneous interpreting the process signal from the DP interface module into the Ovation controller, it can be directly applied to the control strategy, just like the traditional direct connection I/O. Some PROFIBUS DP devices can provide diagnostic information, which can be displayed in a special diagnostic window or used by ovation system.

When the external water system of the main power house is determined to adopt the field bus, the ultrafiltration and reverse osmosis equipment have been invited for bidding, and the meters supplied by the factory do not have bus interface, so these meters are connected to the control system by hard wiring. Other equipment shall be designed according to the following principles:

Do signal of solenoid valve box of pneumatic valve, do signal of electric MCC control, do and di signal of electric valve adopt PROFIBUS-DP bus. Current AI signal, meter AI signal, electric control valve Ao, AI adopt FF Bus. The solenoid valve box of pneumatic valve adopts valve island, and its do signal is connected to the control system by field bus; MCC adopts intelligent motor controller, and do, Di and AI signals are connected to control system by field bus.

The control scheme of the system consists of two layers: the upper layer is the high-speed industrial Ethernet communication between the operator station and the controller; the field bus communication is adopted between the controller and the field equipment. Considering the importance of the external water system of the main power house and the large number of system equipment, five pairs of redundant ovation controllers are provided for the system, including 1 pair of boiler make-up water pretreatment system, 1 pair of boiler make-up water desalination treatment system, 1 pair of industrial wastewater + hydrogen storage and 2 pairs of circulating water treatment.

Taking one pair of controllers as an example, this paper describes the equipment connection form of FF Bus and PROFIBUS DP bus of each network segment and the type of bus cable used. FF Bus is a live bus, and the “chicken claw” topology structure is adopted in the field. Each intelligent field device is connected to the FF Bus communication junction box through the FF bus cable, and the junction box is connected to the FF power regulation module of the control cabinet through the FF Bus.

Install a field terminator in the junction box, which is clearly labeled to prevent inadvertent removal. Another terminal is installed in the power regulator on the host side. The junction box shall be installed locally and the shortest length from the equipment to the junction box shall be ensured as far as possible. FF cable model is Belden 3076f.

DP bus is non electrified bus, and the field equipment needs external power supply. The DP bus of the project adopts the bus topology structure. Each intelligent field device is connected to the DP bus through the dp9 bus connector (9-pin D-type connector, the third pin is positive and the eighth pin is negative). The socket part of the dp9 bus connector is installed on the equipment. A single device failure on the DP bus does not cause a bus failure. Terminal resistors are installed at the beginning and end of the bus. The beldeny47052 cable is used.

5. Advantages of Fieldbus Technology

(1) Compared with DCS, FCS can save a considerable number of isolators, terminal cabinets, I / O terminals, I / O cards, I / O files and I / O cabinets, as well as the area of electronic equipment room.

(2) The digital and intelligent field instruments and devices (such as sensors, actuators, etc.) not only replace the 4-20mA analog system, but also make the signals of many field equipment which need a lot of cables for point-to-point connection can be transmitted through one network cable, which can significantly reduce the use of cables, and correspondingly save a lot of cable tray and cable laying It also saves the cost of design, installation and maintenance.

(3) FCS System decentralizes the control function of the central controller to the intelligent field device, reduces the configuration and complexity of the controller, reduces the function and importance of the controller, and saves a lot of design and configuration workload.

(4) The control function and equipment management function are realized in the field equipment. Through digital communication, many information of the field equipment can be obtained in the control room, such as remote diagnosis, maintenance, configuration and identification, so that the maintenance and repair of these field equipment can be carried out on-line in a planned way. The richer and faster transmission of diagnostic data enhances the ability of pre maintenance.

(5) The accuracy is improved, and the precision will not be lost in D / A and a / D conversion.

(6) The system debugging is more flexible and convenient. The system can be divided into several parts for debugging according to the needs. The same method can also be used in the elimination of complex faults, so that the level of control system has crossed a new level.

6. Problems encountered in application of Fieldbus Technology in power plant

Fieldbus technology has a series of characteristics, the voice of building digital and information power plant is more and more high, but there are few projects put into use in power system. The problems encountered in the project include:

(1) There are many standards of fieldbus, so it is difficult to choose. In addition, various fieldbuses are developed for different application fields, such as FF and Hart, which have certain characteristics in continuous signal processing, while Profibus and INTERBUS have certain advantages in the processing of switching signal. It can be seen that fieldbus has its own limitations in the field of application.

(2) The specifications and varieties of intelligent field equipment supporting fieldbus standard are less, and the domestic equipment supporting bus standard is less, especially the intelligent equipment developed for power plant, which can not meet the needs of various specialties in power plant.

(3) The price of intelligent field devices supporting fieldbus standard is higher.

(4) The network connected to the bottom layer of fieldbus device does not support redundant structure.

(5) Because of the low transmission rate of fieldbus, it can’t meet the requirement of high control speed.

7. Conclusion

The application of FCS improves the level of field equipment information, especially in equipment diagnosis and operation and maintenance, shortens the construction period and saves investment. With the improvement of fieldbus technology and the improvement of thermal automation technology, as well as the further development and application of digital and intelligent field equipment, FCS will be more widely used in thermal power plants. With its open digital communication characteristics, FCS will play a greater role in the development of thermal automation technology in thermal power plants, and improve the automation level of thermal power plants to a new level, Speed up the realization of digitalization and informatization of power plant.

Editor in charge: GT

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