Typical Application of Fieldbus Technology in Substation Automation System
The substation automation system is developing in the direction of decentralization and networking. The substation has a special environment and needs a reliable, real-time, easy-to-operate communication network to connect the upper management network with the front-end measurement and control network. Fieldbuses, which are primarily used for low-level device communication, are perfectly suited for this application. The use of fieldbus instead of traditional serial communication not only fundamentally improves the performance of substation automation systems, but also makes the system reliable, flexible and easy to expand. This paper introduces the fieldbus concept, features and advantages of fieldbus in substation automation system and the introduction of several typical fieldbuses.
Key words: substation automation field bus FCS DCS network node IED equipment high-speed data communication network
0 Introduction Substation automation systems are based on computer technology and network communication technology. Throughout the development process of substation automation system, it can be roughly divided into three stages: Substation automation products with “centralized†structure began to appear in the early 1990s; “Layered and distributed†systems went to the market in the mid-1990s; The world of distributed "systems [1~3]. However, at present, whether it is a distributed distributed substation automation system at home or abroad, most of its components use serial communication methods (RS232, RS485, RS422 bus, etc.) [4]. Serial communication has a slow transmission rate, is susceptible to interference, has limited communication distance, and is inflexible in communication, so it cannot meet the requirements of a large amount of real-time data transmission.
The field bus is not only an open communication network, but also a fully distributed control system. As a contact link of intelligent devices (IEDs), the fieldbus connects intelligent devices hanging on the bus as network nodes into network systems, and further constitutes automation. The system realizes the integrated automation function of basic control, compensation calculation, parameter modification, alarm, display, monitoring, optimization and control integration [5]. The application of network methods, especially fieldbus technology, to solve the communication problems of substation automation systems has become a development trend.
1 Fieldbus concept Fieldbus (Fieldbus) was developed internationally in the late 1980s and early 1990s for field intelligent device interconnection communication networks in the fields of process automation, manufacturing automation, and building automation. As the basis of the factory's digital communication network, it communicates the relationship between the production process site and the control equipment and its relationship with higher control management. It is not only a grassroots network, but also an open, new fully distributed control system. This integrated technology with intelligent sensing, control, computer, digital communication and other technologies has attracted worldwide attention and become a hot spot in the development of automation technology, and will lead to profound changes in the structure and equipment of automation systems. As the most automated production department ever, the power system has become the front-end position for the application of fieldbus, especially in the substation automation system. Many internationally powerful and influential companies have developed fieldbus technologies and products to varying degrees. The working environment of the fieldbus device is at the bottom of the process equipment. As the factory equipment level basic communication network, it requires the characteristics of simple protocol, strong fault tolerance, good security and low cost. It has certain time certainty and high real-time performance. Requirements; also has a stable network load, mostly short frame transmission, frequent information exchange and so on. Due to the above characteristics, the field bus system has the characteristics of different upper layer high speed data communication networks from network structure to communication technology. The fieldbus system is generally referred to as the fifth-generation control system, also known as FCS-fieldbus control system. People generally refer to the PCS as the first generation of the Pneumatic Signal Control System before the 1950s, and the electric analog signal control system such as 4~20mA as the second generation. The digital computer centralized control system is called the third generation. The distributed distributed control system DCS since the middle of the decade is called the fourth generation. As a new generation control system, the fieldbus control system FCS breaks through the limitations of the DSC system using a communication-dedicated network. It adopts an open and standardized solution to overcome the defects caused by the closed system. On the other hand, the DCS The centralized and decentralized system of distributed systems has become a new fully distributed structure, and the control functions have been completely decentralized to the scene. It can be said that openness, dispersion and digital communication are the most prominent features of the fieldbus system.
2 The technical characteristics of the fieldbus a, the openness of the system. An open system means that the communication protocol is open, and devices of different manufacturers can be interconnected and exchange information. The fieldbus developer is committed to establishing a unified open system of the factory underlying network. Openness here refers to the consistency and openness of relevant standards, emphasizing consensus and compliance with standards. An open system that can be connected to any other device or system that adheres to the same standards. A bus-enabled fieldbus network system must be open, and the open system gives the user the right to system integration. Users can make products from different suppliers into random systems according to their own needs and objects.
b. Interoperability and interoperability. Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems. It can implement point-to-point, point-to-multipoint digital communication. Interoperability means that devices of similar performance from different manufacturers can be interchanged for interoperability.
c. Intelligent and functional autonomy of field devices. It distributes the functions of sensing measurement, compensation calculation, engineering quantity processing and control into the field equipment. The basic functions of automatic control can be completed only by the field device, and the running status of the equipment can be diagnosed at any time.
d, the high degree of dispersion of the system structure. Since the field device itself can complete the basic functions of automatic control, the field bus has formed a new architecture of a fully distributed control system. It fundamentally changed the system of distributed control system combining the existing DCS concentration and dispersion, which simplified the system structure and improved the reliability.
e. Adaptability to the on-site environment. Working at the front end of the field device, as the field bus at the bottom of the factory network, it is designed to reduce the transmission rate of the current serial communication, is susceptible to interference, the communication distance is limited, and the communication method is not flexible, so it cannot meet the requirements of a large amount of real-time data transmission.
The field bus is not only an open communication network, but also a fully distributed control system. As a contact link of intelligent devices (IEDs), the fieldbus connects intelligent devices hanging on the bus as network nodes into network systems, and further constitutes automation. The system realizes the integrated automation function of basic control, compensation calculation, parameter modification, alarm, display, monitoring, optimization and control integration [5]. The application of network methods, especially fieldbus technology, to solve the communication problems of substation automation systems has become a development trend.
1 Fieldbus concept Fieldbus (Fieldbus) was developed internationally in the late 1980s and early 1990s for field intelligent device interconnection communication networks in the fields of process automation, manufacturing automation, and building automation. As the basis of the factory's digital communication network, it communicates the relationship between the production process site and the control equipment and its relationship with higher control management. It is not only a grassroots network, but also an open, new fully distributed control system. This integrated technology with intelligent sensing, control, computer, digital communication and other technologies has attracted worldwide attention and become a hot spot in the development of automation technology, and will lead to profound changes in the structure and equipment of automation systems. As the most automated production department ever, the power system has become the front-end position for the application of fieldbus, especially in the substation automation system. Many internationally powerful and influential companies have developed fieldbus technologies and products to varying degrees. The working environment of the fieldbus device is at the bottom of the process equipment. As the factory equipment level basic communication network, it requires the characteristics of simple protocol, strong fault tolerance, good security and low cost. It has certain time certainty and high real-time performance. Requirements; also has a stable network load, mostly short frame transmission, frequent information exchange and so on. Due to the above characteristics, the field bus system has the characteristics of different upper layer high speed data communication networks from network structure to communication technology. The fieldbus system is generally referred to as the fifth-generation control system, also known as FCS-fieldbus control system. People generally refer to the PCS as the first generation of the Pneumatic Signal Control System before the 1950s, and the electric analog signal control system such as 4~20mA as the second generation. The digital computer centralized control system is called the third generation. The distributed distributed control system DCS since the middle of the decade is called the fourth generation. As a new generation control system, the fieldbus control system FCS breaks through the limitations of the DSC system using a communication-dedicated network. It adopts an open and standardized solution to overcome the defects caused by the closed system. On the other hand, the DCS The centralized and decentralized system of distributed systems has become a new fully distributed structure, and the control functions have been completely decentralized to the scene. It can be said that openness, dispersion and digital communication are the most prominent features of the fieldbus system.
2 The technical characteristics of the fieldbus a, the openness of the system. An open system means that the communication protocol is open, and devices of different manufacturers can be interconnected and exchange information. The fieldbus developer is committed to establishing a unified open system of the factory underlying network. Openness here refers to the consistency and openness of relevant standards, emphasizing consensus and compliance with standards. An open system that can be connected to any other device or system that adheres to the same standards. A bus-enabled fieldbus network system must be open, and the open system gives the user the right to system integration. Users can make products from different suppliers into random systems according to their own needs and objects.
b. Interoperability and interoperability. Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems. It can implement point-to-point, point-to-multipoint digital communication. Interoperability means that devices of similar performance from different manufacturers can be interchanged for interoperability.
c. Intelligent and functional autonomy of field devices. It distributes the functions of sensing measurement, compensation calculation, engineering quantity processing and control into the field equipment. The basic functions of automatic control can be completed only by the field device, and the running status of the equipment can be diagnosed at any time.
d, the high degree of dispersion of the system structure. Since the field device itself can complete the basic functions of automatic control, the field bus has formed a new architecture of a fully distributed control system. It fundamentally changed the system of distributed control system combining the existing DCS concentration and dispersion, which simplified the system structure and improved the reliability.
e. Adaptability to the on-site environment. Working at the front end of the field equipment, as the field bus on the bottom of the factory network, it is designed for working in the field environment. It can support twisted pair, coaxial cable, optical cable, RF, infrared, power line, etc., with strong resistance. The interference capability can be realized by two-wire system for power transmission and communication, and can meet intrinsic safety explosion-proof requirements.
3 The superiority of fieldbus in substation automation system Due to the above characteristics of fieldbus, especially the simplification of fieldbus system structure, the design, installation and commissioning of substation automation system to normal production operation and maintenance and repair are reflected. Superiority.
a. Save hardware and investment. Since the smart devices scattered in the front end of the bay device in the fieldbus system can directly perform a variety of sensing, control, alarm and calculation functions, the number of transmitters can be reduced, eliminating the need for separate controllers, computing units, etc. The DCS system's signal conditioning, conversion, isolation technology and other functional units and their complicated wiring are no longer needed. The industrial PC can also be used as a station-controlled operation workstation, which saves a large investment in hardware and can be reduced due to the reduction of control equipment. Reduce the footprint of the control room.
b. Save installation costs. The wiring of the fieldbus system is very simple. Since a pair of twisted pairs, a pair of optical fibers or a cable can usually be connected to multiple devices, the amount of cables, terminals, slots, and bridges is greatly reduced. The wiring design and the joint proofing are performed. The workload is also greatly reduced. When it is necessary to add on-site IED equipment, it is not necessary to add a new cable, and it can be connected to the original bus cable, which saves investment and reduces the workload of design and installation. According to the calculation data of typical test projects, the installation cost can be saved by more than 60%.
c. Save maintenance overhead. Because the field control equipment has the ability of self-diagnosis and simple fault handling, and sends relevant diagnostic maintenance information to the control room through digital communication, the operation and maintenance personnel can query the operation of all the interval layer equipment and diagnose the maintenance information, so as to analyze the cause of the fault early. And quickly eliminated. Maintenance downtime is reduced, and maintenance is reduced due to simplified system structure and simple wiring.
d, the user has a high degree of system integration initiative. Users are free to choose the equipment provided by different manufacturers to integrate the substation automation system. Avoid the choice of a certain brand of products that are “framed deadâ€, and will not be incompatible with the incompatible protocols and interfaces in the system integration, so that the initiative in the integration process of the substation automation system is completely in the hands of the users. .
e. Improve the accuracy and reliability of the substation automation system. Due to the intelligence and digitization of the fieldbus device, it fundamentally improves the accuracy of measurement and control and reduces the transmission error compared with the analog signal. At the same time, due to the simplified structure of the system, the equipment and wiring are reduced, and the internal functions of the field instrument are strengthened: the round-trip transmission of signals is reduced, and the operational reliability of the substation automation system is improved.
f. Easy to implement equipment expansion and product modification. Fieldbus-based substation automation systems can be easily expanded and changed. When the system is expanded, the new IED device can be connected to the existing fieldbus without adding any components or considering whether the fieldbus interface is matched. Modifications to functions and parameters can be made directly through fieldbus login and query. As a result, the cost of implementing a new control strategy or product retrofit is reduced.
4 several typical fieldbus a, HART
HART is an acronym for Highway Addressable Remote Transducer. A transitional temporary communication protocol developed by ROSEMOUNT in the United States in 1986. However, it is widely recognized and has become a de facto international standard.
The HART protocol uses the International Organization for Standardization (ISO) Open System Interconnection Model (OSI) as a reference, and uses the Layers 1, 2, and 7 of OSI, namely the physical layer, the data link layer, and the application layer. The physical layer uses the FSK technology based on the Be11202 communication standard, so it is possible to communicate by renting a telephone line.
â— The HART protocol uses FSK technology to superimpose a frequency signal on a 4~2 mA process analog signal. Logic 1 is 1200 Hz, logic 0 is 2200 Hz, and baud rate is 1200 bps. It successfully enables both analog and digital bidirectional signals to be simultaneously and without interference. Therefore, analog instruments, recorders, and analog controls can also be used when communicating with intelligent meters. The progressive implementation of digital performance (including digital process variables) is an ideal solution without retrofitting field instruments. This is a protocol that transitions from an analog system to a digital system.
â— Three types of missions are defined at the application layer. The first category is general commands, which are commands that all devices can understand and execute. The second category is general behavior commands, which provide functions that can be implemented in many field devices. The third category is special device commands to implement special features in some devices that can be unique to companies developing such devices. In addition, it provides users with a unified device description language DDL.
â—HART supports point-to-point, master-slave response, and multicast.
â— Direct communication distance: There is a shielded twisted pair single device 3000m, and multiple devices are 1500m away from each other. When only one power supply is used, 15 intelligent devices can be connected.
b, CAN
CAN is an abbreviation for Controller Area Network. This is a control local area network developed by BOSCH of Germany. It is a serial communication network with high reliability and support for distributed real-time control. It was originally a serial data communication protocol used for a large number of internal control instruments and data exchange between actuators. It has gradually evolved into control in other industrial sectors, including machinery manufacturing, CNC machine tools, medical devices, building management monitoring, and monitoring of substation automation equipment. The ISO/TC22 Technical Committee of the International Organization for Standardization has developed the international standard ISO/DIS1 1 898 (communication rate = 1 Mbps) and ISO/DIS 11519 (communication rate = 125 Kbps) of the CAN protocol. In the fieldbus, it is currently the only fieldbus that has been approved as an international standard. Chip manufacturers such as Intel, Philips, and Motorola all produce CAN bus chip products. In China, there are also HAPCAN series products, and the fully integrated substation integrated automation system DISA2 and DISA3 developed by domestic power system manufacturers adopt CAN bus. The internal bus of the ABB RTU560 in the automation system of the Zenith 220kV substation of Changsha Electric Power Bureau is also the CAN bus.
â— The CAN protocol implements layers 1, 2 and 2 of the ISO/OSI model. The physical layer defines all electrical characteristics during the transfer process; the target layer and transport layer include all the functions of the data link layer defined by ISO/OSI. The function of the target layer includes confirming which information is to be sent, confirming the information received by the transport layer and providing an interface for it. The functions of the transport layer include frame organization, bus arbitration, error detection, error reporting, error handling, and so on.
â— CAN can transmit and receive data in several ways, such as point-to-point, point-to-multipoint (grouping) and global broadcast. It can work in multi-master mode. Any node on the network can actively send information to other nodes in the network at any time. It is convenient to construct a multi-machine backup system.
â— Each node on the network can define different priorities to meet different real-time requirements. CAN adopts non-destructive bus arbitration technology. When two nodes simultaneously transmit information to the network, nodes with lower priority actively stop data transmission. Nodes with higher priority can continue to transmit data unaffected, effectively avoiding bus collisions.
â— The number of nodes on CANBUS, the theoretical value is 2000, and the actual value is 110. The direct communication distance is 10km/5Kbps, 40 m/ 1MKbps. The transmission medium is twisted pair and optical fiber.
â— CAN adopts short frame structure, has low interference probability, and adopts redundancy check CRC and other corrective measures to ensure extremely low information error rate. Moreover, it has an automatic shutdown bus function, and in the case of a serious error, it can be disconnected from the bus, so that other operations on the bus are not affected.
c, LONWORKS
LONWORKS is a local operation network, and LON is an abbreviation of LocalOperaTIon Network. In 1991, ECHERON Corporation of the United States launched the product. Its application range covers almost all areas of measurement and control applications, especially aviation/aerospace, building automation, energy management, substation monitoring and control equipment monitoring, factory automation, industrial process control, computer peripherals, electronic measuring equipment. LONWORKS has also been widely used in China's power system. For example, Sifang's CSC2000 substation integrated automation system uses the LONWORKS bus. In the Changsha Electric Power Bureau, there are six 110kV substations such as Xinkaipu, Guihua, Wuyi Te, Jingwanzi, Ansha and Zhentou.
â— LONWORKS communication protocol The LONTALK protocol follows the ISO/OSI reference model and provides all 7 layers of services defined by OSI. This is the only fieldbus that provides the full service in the fieldbus.
â— The core of LONWORKS is the Neuron (neuron) chip, which contains three 8-bit CPUs: the first CPU is the medium access control processor, which implements the first layer and the second layer of the L ONTALK protocol; the second CPU is The network processor implements the third to sixth layers of the LONTALK protocol; the third CPU is the application processor, implements the seventh layer of the LONTALK protocol, executes the code written by the user, and the operating system service invoked by the user code. The LON-WORKS neuron chip has been produced by Motorola and Toshiba.
â— Provide a set of development tools platform LON Builder and Node Builder. With this set of tools, users can use the neuron chip, LONTALK communication protocol and LONWORKS transceiver to easily and flexibly develop the systems and products they need.
â— The direct communication distance of LONWORKS is 2700m/78Kbps and 130 m/1.25Mbps; the number of nodes is 32000; The transmission medium is twisted pair, coaxial cable, optical fiber, cable, etc.
d, PROFIBUS
PROFIBUS is short for Process Field Bus. In 1987, the German Federal Ministry of Science and Technology concentrated 13 companies and 5 institutes to develop the German standard for fieldbus according to the ISO/OSI reference model. It was published in DIN19245 in April 1991 and officially became the German fieldbus standard, and then included the European standard EN50170. PROFIBUS has been widely supported. Only 47,000 systems were installed in Germany from October 1996 to January 1997. They have been widely used in the processing industry, process automation, intelligent buildings, substation automation systems and other fields. PROFIBUS has been widely used in power systems. For example, SIEMENS's SICAM and LSA substation automation systems use PROFIBUS. The Dongda Electric Power Bureau's Dongtang 110kV substation uses the LSA integrated automation system. Shanghai Yangshupu Power Plant and Wuqiangxi Hydropower Station dam flood gates are also controlled by PROFIBUS.
â— PROFIBUS omitted 3 to 6 layers according to the ISO/OSI reference model, adding a user layer. The first layer defines the physical transmission characteristics; the second layer defines the access protocol; the seventh layer defines the application function. PROFIBUS introduces functional modules (FB), object dictionary (OD) and device description language (DDL) at the user level, allowing users to perform complete internal operations on the device, thus enabling interoperability of devices.
5. Conclusion The fieldbus technology is applied to the substation automation system, which can fully meet the fast and efficient data communication requirements of the substation site, make the system more reliable, more open and lower cost, and greatly improve the overall level of the substation automation system.
references
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5. Yang Xianhui, "Fieldbus Technology and Its Application". Beijing: Tsinghua University Press. 1998
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